Recent Advances in Lower Carboniferous Geology
Geological Society Special Publications Series Editor A. J. FLEET
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Recent Advances in Lower Carboniferous Geology
Geological Society Special Publications Series Editor A. J. FLEET
G E O L O G I C A L SOCIETY SPECIAL P U B L I C A T I O N NO. 107
Recent Advances in Lower Carboniferous Geology
EDITED BY PETER STROGEN, IAN D. SOMERVILLE & GARETH LL. JONES Department of Geology, University College, Dublin, Ireland
1996 Published by The Geological Society London
THE G E O L O G I C A L SOCIETY The Society was founded in 1807 as The Geological Society of London and is the oldest geological society in the world. It received its Royal Charter in 1825 for the purpose of 'investigating the mineral structure of the Earth'. The Society is Britain's national society for geology with a membership of around 7500. It has countrywide coverage and approximately 1000 members reside overseas. The Society is responsible for all aspects of the geological sciences including professional matters. The Society has its own publishing house, which produces the Society's international journals, books and maps, and which acts as the European distributor for publications of the American Association of Petroleum Geologists, SEPM and the Geological Society of America. Fellowship is open to those holding a recognized honours degree in geology or cognate subject and who have at least two years' relevant postgraduate experience, or who have not less than six years' relevant experience in geology or a cognate subject. A Fellow who has not less than five years' relevant postgraduate experience in the practice of geology may apply for validation and, subject to approval, may be able to use the designatory letters C Geol (Chartered Geologist). Further information about the Society is available from the Membership Manager, The Geological Society, Burlington House, Piccadilly, London WlV 0JU, UK. The Society is a Registered Charity, No. 210161. Published by The Geological Society from: The Geological Society Publishing House Unit 7 Brassmill Enterprise Centre Brassmill Lane Bath BA1 3JN UK (Orders: Tel. 01225 445046 Fax 01225 442836)
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Contents
Preface
Mineralization, hydrocarbons and diagenesis JOHNSTON, J. D., COLLER, D., MILLAR, G. & CRITCHLEY, M. F. Basement structural controls on Carboniferous-hosted base metal mineral deposits in Ireland SHEARLEY, E., REDMOND, P., KING, M. & GOODMAN, R. Geological controls on mineralization and dolomitization of the Lisheen Zn-Pb-Ag Deposit, Co. Tipperary, Ireland HOLLIS, C. & WALKDEN, G. The use of burial diagenetic calcite cements to determine the controls upon hydrocarbon emplacement and mineralization on a carbonate platform, Derbyshire, England VEALE, C. & PARNELL, J. Metal-organic interactions in the Dinantian Solway Basin, UK: inferences for oil migration studies Carbonate buildups and Waulsortian mud-mounds PICKARD, N. A. H. Evidence for microbial influence on the development of Lower Carboniferous buildups AHR, W. M. & STANTON, R. J. JR. Constituent composition of Early Mississippian carbonate buildups and their level-bottom equivalents, Sacramento Mountains, New Mexico KIRKBY, K. C. & HUNT, D. Episodic growth of a Waulsortian buildup: the Lower Carboniferous Muleshoe Mound, Sacramento Mountains, New Mexico, USA JEFFERY, D. L. & STANTON, R. J. JR. Biotic gradients on a homoclinal ramp: the Alamogordo Member of the Lake Valley Formation, Lower Mississippian, New Mexico, USA SOMERVILLE, I. D., STROGEN, P., JONES, G. LL. & SOMERVILLE, H. E. A. Late Vis6an buildups at Kingscourt, Ireland: possible precursors for Upper Carboniferous bioherms RODRiGUEZ, S. Development of coral reef-facies during the Vis6an at Los Santos de Maimona, SW Spain Siliciclastic rocks MORENO, C., SIERRA, S. & SAEZ, R. Evidence for catastrophism at the FamennianDinantian boundary in the Iberian Pyrite Belt MAGUIRE, K., THOMPSON, J. & GOWLAND, S. Dinantian depositional environments along the northern margin of the Solway Basin GRAHAM, J. R. Dinantian river systems and coastal zone sedimentation in northwest Ireland Carbonate platforms and ramps RIZZI, G. & BRAITHWAITE, C. J. R. Cyclic emersion surfaces and channels within Dinantian limestones hosting the giant Navan Zn-Pb deposit, Ireland HORBURY, A. D. & ADAMS, A. E. Microfacies associations in Asbian carbonates: an example from the Urswick Limestone Formation of the southern Lake District, northern England
vii
1 23 35
51
65 83 97 111
127 145
153 163 183
207 221
vi
CONTENTS
GALLAGHER, S. J. The stratigraphy and cyclicity of the late Dinantian platform carbonates in parts of southern and western Ireland KELLY, J. G. Initiation, growth and decline of a tectonically controlled Asbian carbonate ramp: Cuilcagh Mountain area, NW Ireland STROGEN, P., SOMERVILLE, I. D., PICKARD, N. A. H., JONES, G. LL. & FLEMING, M. Controls on ramp, platform and basinal sedimentation in the Dinantian of the Dublin Basin and Shannon Trough, Ireland VANSTONE, S. The influence of climatic change on exposure surface development: a case study from the Late Dinantian of England and Wales
Basinal facies GURSKY, H.-J. Siliceous rocks of the Culm basin, Germany BELKA, Z., SKOMPSKI, S. & SOBON-PODGORSKA, J. Reconstruction of a lost carbonate platform on the shelf of Fennosarmatia: evidence from Vis~an polymictic debrites, Holy Cross Mountains, Poland NAYLOR, D., SEVASTOPULO, G. D. & SLEEMAN, A. G. Contemporaneous erosion and reworking within the Dinantian of the South Munster Basin REES, J. G., CORNWELL, J. D., DABEK, Z. K. & MERRIMAN, R. J. The Apedale Tufts, North Staffordshire: probable remnants of a late Asbian/Brigantian (Pla) Dinantian volcanic centre Faunas, floras and biostratigraphy MAKHLINA, M. KH. Cyclic stratigraphy, facies and fauna of the Lower Carboniferous (Dinantian) of the Moscow Syneclise and Voronezh Anteclise RUKINA, G. A. Sequence biostratigraphy of the Tournaisian-Lower Vis~an rocks of the Russian Platform JONES, G. LL. & SOMERVILLE, I. D. Irish Dinantian biostratigraphy: practical applications LEBEDEV, O. A. Fish assemblages in the Tournaisian-Vis~an environments of the East European Platform IVANOV, A. The Early Carboniferous chondrichthyans of the South Urals, Russia HARPER, D. A. T. & JEFFREY, A. L. Mid-Dinantian brachiopod biofacies from western Ireland SMITH, J. A palynofacies analysis of the Dinantian (Asbian) Glenade Sandstone Formation of the Leitrim Group, northwest Ireland Index
239 253 263
281
303 315
331 345
359 365 371 387 417 427 437
449
Preface To those of us who were students during the 1950s in the UK and Ireland the term Dinantian conjured up a picture of the Avon Gorge at Bristol, the great limestone uplands of Ingleborough in Yorkshire or the Burren of County Clare. To those who have since wandered into the petroleum industry the Lower Carboniferous was, and largely remains economic basement. Within this volume both viewpoints are, we hope, shown to be both parochial and commercially erroneous! The first European Dinantian Environments (EDE) Conference was held in Manchester in 1984. Speakers and attendees at the EDE II Conference, held at University College Dublin in September 1994, upon which this volume is based, came from points as far apart as western Ireland, Russia to the foot of the Ural Mountains, and the southern tip of Iberia. So much for parochialism. Almost all European countries with significant Dinantian strata were represented- Russia, Poland, Germany, Belgium, Spain, the United Kingdom, and Ireland. There was a most welcome addition to EDE - from the USA, as a result of which a new title for this decennial meeting has to be found! Many of the great steps forward in geology during the nineteenth century were made by geologists who had travelled widely and had seen rocks of very different character. These were demonstrably, because of the increasing success of palaeontologists in creating reliable biozonation schemes, of the same age. Today, despite the ease of modern travel, we are all too busy teaching, administrating and trying to keep up with our brighter research students to "travel extensively. This volume has attempted to remedy this situation arising from a relatively informal meeting of researchers from far-flung areas who are working on rocks of this one age. Those of us who have also worked on Lower Palaeozoic rocks are always struck by the relative ease with which correlations can be made, in some cases along the entire length of the Caledonian belt. Yet the younger Variscan (Hercynian) belt seems poorly-understood by comparison: to date there has been no overall, agreed synthesis of the Variscan of Europe. In part this is due to Mesozoic and Tertiary cover, but it is mainly the result of the intricate yet subtle nature of Late Palaeozoic, especially Carboniferous, events. The reasons for this complexity are but vaguely understood: compare, for example, the well-argued but contradictory structural interpretations of the Variscan orogen by Matthews (1984) and Matte (1986).
The very nature of the Variscan foldbelt is still a matter of debate. A major key to greater understanding of the evolution of this huge part of western Europe is a fuller knowledge of the stratigraphy and palaeontology of its Carboniferous rocks. The Dinantian is crying out for an even more refined biozonation to help us to sort out this complexity. In this matter we can only envy workers in the Jurassic for example! Apart from their intrinsic geological interest rocks of Dinantian age are of very considerable economic value, not least as hosts to major ZnPb-Cu-Ba deposits in Ireland, and Au-FeS2 deposits in the Iberian Pyrite Belt. The term 'Irish-type' is now applied world-wide to a major class of Zn-Pb deposits first described in the 1960s and 1970s from Ireland where, as in Iberia, exploration still continues apace. Apart from Navan, Europe's largest and one of the world's great Zn/Pb mines, further large mines are coming on stream at Galmoy and Lisheen in Tipperary. These and earlier discoveries have stimulated extensive exploration drilling. We would like here to acknowledge the many mineral exploration companies who have made drill cores available for academic research to workers from Ireland, the UK, the USA and elsewhere who are concerned with the origin of these enigmatic but valuable orebodies, and with matters Carboniferous in general. Research into the origin of these ores involves many sub-disciplines within geology: mineralogy, geochemistry, sedimentology, biostratigraphy and structural geology. It also incorporates a great deal of academic research from earlier years, raising once again the perennial question: what is 'purely academic' research, and what is its value? How much modern academic research will one day come to economic fruition? All of the modern Irish deposits have been located within a few hundred metres of the surface, and the search for deeper orebodies, not detectable by today's geophysical techniques, will require the erection of much more refined models for the origin and siting of these orebodies. Further, the Upper Palaeozoic rocks of Europe will increasingly become the target of the oil and gas explorationist. The wealth of academic data on Dinantian r o c k s - their sedimentology, biostratigraphy, tectonics and basin evolution- while no guarantee of success, will be an invaluable tool to exploration. The first section of the volume is concerned with the economic geology of the Dinantian, especially the mineralization and diagenesis of
From STROGEN, P., SOMERVILLE,I. D. & JONES, G. LL. (eds), 1996, Recent Advances in Lower Carboniferous Geology, Geological Society Special Publication No. 107, pp. vii-ix.
viii
PREFACE
Dinantian rocks in Ireland and Britain. It emphasizes the importance these limestones play as host rocks for major Zn-Pb deposits. In particular, two papers focus on the role of syn-depositional faulting and tectonic controls on mineralization pathways. There are also studies of the timing of oil and gas generation and migration through these rocks. The second section is more academic, and focuses upon the highly active field of research into Dinantian carbonate buildups. It highlights recent advances in our knowledge of the sedimentology, facies and fauna of Dinantian mud-mounds both in Europe and in North America. It is now becoming generally accepted that microbial activity had a major influence on buildup development and that many mudmounds are dominated by matrix peloidal fabrics. New work on the Muleshoe Mound, New Mexico has demonstrated that 'Waulsortian' buildups could form in higher energy environments than previously suspected and were subject to episodic erosional events which may have been controlled by relative changes in sea-level. Many late Vis6an buildups are now known to have greater skeletal diversity than earlier Waulsortian mounds and could form rigid frameworks aided by microbialite cementstone fabrics. The significance of calcareous algae in buildups is now becoming better understood, both as skeletal contributors and as water depth and photic indicators. The nature and scale of siliciclastic environments in the Dinantian are described and interpreted in the third section. Potential clastic reservoirs seem to be of limited extent and clearly subject to strong tectonic control. They appear in places to be stacked against basement margin faults and these sandstones may be important as vertical migration pathways; there is abundant evidence presented in the first section that this has occurred in some basins. In the fourth and fifth sections the controls on the formation and evolution of carbonate platforms, ramps and basins are discussed. In late Dinantian platform successions subtle changes detected by microfacies analysis and changes in faunas have been widely noted by many authors. The possible role of eustatic control on these changes has been widely discussed. At the same time acknowledgement is made of the equal and locally greater importance of tectonic influence on the evolution of platforms and ramps. Sequence stratigraphy for the Dinantian is still in its infancy as a result of the interplay of these two very different controls on sedimentation. Basinal
calciturbidites, pelagic carbonates and cherts, and the palaeooceanographic significance of the latter are also discussed. Reworked platform materials in Dinantian basinal sequences are described and their significance assessed; foundered platforms, no longer visible at surface, have been detected by these means. Finally, two papers deal with Dinantian vulcanicity, a topic that surely deserves more attention in the future. The final section on fauna and biostratigraphy emphasizes the importance of accurate dating in the correlation of Lower Carboniferous sections, which underpins all syntheses of basinal evolution and the timing of major tectonic pulses, transgressions and regressions. The problem of facies control on faunas and of distinct biozonation schemes for platform and basinal facies is apparent from many of the earlier papers. Several papers here highlight the advances made in micropalaeontological studies, particularly on biostratigraphic refinement using different fossil groups in an integrated approach. Also an increasing contribution is being made by research on microvertebrates (fish and sharks) and microproblematica, especially in successions where diagnostic taxa are sparse or absent. The editors record their thanks to the Geological Society and all the staff of the Geological Society Publishing House. They would also like to express their gratitude to the many sponsors of the conference and the ongoing support for Dinantian research from the mining industry in Ireland. The editors would also like to express their thanks to the following individuals who reviewed one or more of the papers for this volume, and helped to improve its quality: Tony Adams, John Ashton, Ron Austin, Fernando Barriga, Adrian Black, Paul Brenckle, Paul Bridges, Geoff Clayton, Jerry Davies, Jaraslov Dvorak, Howel Francis, Stephen Gallagher, John Graham, Pete Gutteridge, Stephen Habesch, James Hein, Hans-Georg Herbig, Ken Higgs, Murray Hitzman, Andy Horbury, Dave Hunt, Dave Johnston, John Kelly, Ben Kennedy, Martin Laloux, Oleg Lebedev, Alan Lees, Marie Legrand Blain, J. M. Leistel, Michael Lipiec, Kelly Maguire, Jim Marshall, John Miller, Ian Mitchell, Dave Mundy, Dave Naylor, John Nudds, Bernard Owens, Eva Paproth, Mike Philcox, Neil Pickard, Eddy Poty, Tony Ramsay, Pat Redmond, Robert Riding, Nick Riley, Pat Shannon, Andy Sleeman, Bob Stanton, Roger Suthren, Sue Turner, Brian Turner, Greg Webb, Brian Williams, Sally Young and Jiri Zidec.
PREFACE Finally, it is with great sorrow that the editors record that the senior author of the first paper of this volume, and also one of our reviewers, Dave Johnston, was killed in early October 1995 at the age of 36 in an accident while on fieldwork on coastal cliffs in the West of Ireland. Dave was an undergraduate of Trinity College Dublin until 1980 and completed his PhD at Monash, Melbourne in 1984, when he returned as a lecturer in the Department of Geology at Trinity. He has written extensively on the structural geology of Ireland and more recently had particular interests in the structural control on economic mineralization in the Irish Carboniferous. He will be greatly missed for his good company, his enthusiasm, and his enormous ability for generating stimulating ideas and models.
ix
References MATTE, P. 1986. Tectonics and Plate Tectonics model for the Variscan Belt of Europe. Tectonophysics, 126, 329-374. MATTHEWS, S. C. 1984. Northern margins of the Variscides in the North Atlantic region; comments on the tectonic context of the problem. In: HUTTON, D. H. & SANDERSON, D. J. (eds) Variscan Tectonics of the North Atlantic Region. Geological Society, London, Special Publication, 14, 71-85. Peter Strogen Ian D. Somerville Gareth L1. Jones
Basement structural controls on Carboniferous-hosted base metal mineral deposits in Ireland J. D. J O H N S T O N l*, D. C O L L E R 2, G. M I L L A R 2 & M. F. C R I T C H L E Y 2
l Geology Department, Trinity College, Dublin 2, Ireland 2 ERA-Maptec, 5 South Leinster Street, Dublin 2, Ireland Abstract: Explorationists have long recognized the importance of structural control of Irish
base metal deposits, but the kinematics and timing of the structures relative to the mineralization are subject to wildly differing interpretations. This paper presents a combination of structural, magnetic and gravity data that show that the fundamental structural controls are easterly to northeasterly-trending basement (Caledonian) structures which have been reactivated in dextral transtension by northeasterly to north-northeasterly extension during the Dinantian. In general, in the north of the country the ore-controlling faults dip south, while in the south of the country they dip north. The structures have evolved through a temporal kinematic history of early normal faulting, later oblique slip, and some show evidence of later reverse movements. Part of this evolution may reflect burial history, although it also reflects the transition from Dinantian transtension to Variscan (Hercynian) compression. The bulk of the mineralization appears to post-date the normal faulting, and pre-date Variscan compression. Mineralization is thought to be postcompactional and could have occurred during the dextral transtension, although some of the sulphides could post-date most of the transtensional movement. The Lower Carboniferous of Ireland is host to a large number of base metal deposits (Fig. 1). These range in size from small occurrences up to the Navan deposit of more than 70 million tonnes (Mt) of 12% combined Z n - P b (Table 1). Vein-hosted copper ores in red beds are abundant in the south of the country, while further north Zn predominates in carbonatehosted deposits. Metal ratios vary regionally, with Cu and Pb decreasing and Zn increasing northwards (Johnston 1996). This suggests that all the mineralization may be generated by the same regional phenomenon. All of the major deposits occur in the hanging walls of normal faults. Copper deposits at Aherlow, Mallow and Ballinalack are in compressional structures, and no faulting has been documented at Courtbrown or Carrickittle. These latter cases may reflect limited exploration, rather than absence of faults. Generally, individual faults trend between ESE (Silvermines) and NE (Keel). Local faults with lengths of hundreds of metres, controlling ore lenses (ESE at Silvermines and E at Lisheen), are made up of en echelon segments defining ENE to NEtrending fault zones many kilometres long, which are inferred to be expressions of *Dave Johnston lost his life whilst on fieldwork in Mayo during October 1995. His co-authors would like to pay tribute to Dave's great contribution to structural geology around the world over the last two decades.
controlling basement structures. The local faults, although generally clockwise of the trend of the zones, show a larger variation in orientation than the zones themselves. Although the empirical relationship is well known, only a few studies of the structural settings of the deposits have been published (Moore 1975; Coller 1984; Reilly 1986; Shearley et al. 1992, this volume). Russell (1968, 1972) and Russell & Haszeldine (1992) invoke N-S geofractures to explain the mineralization in the Irish Midlands. However, as pointed out by Leeder (1988), there is almost no field or geophysical evidence for the existence of such structures. Rather, both field and geophysical evidence suggest that reactivated Caledonian (E to NE-trending) basement structures were the key in focusing mineralizing fluids. There is extensive hydrothermal alteration around these faults where they cut Devonian and Carboniferous rocks. Alteration is less extensive in the basement, but may be observed at Navan. In this study, both structural and petrographical observations and magnetic and gravity data are used to locate basement structures. On the basis of fault and mineralized vein geometries, most of the deposits are seen to lie in dextral transtensional settings (oblique extension with a component of clockwise rotation), which are the result of northeasterly extension of east to northeast-trending basement structures. Each mineral deposit has been further localized by
From STROGEN, P., SOMERVILLE,I. D. & JONES, G. LL. (eds), 1996, Recent Advances in Lower Carboniferous Geology, Geological Society Special Publication No. 107, pp. 1-21.
2
J. D. JOHNSTON E T AL.
200
7
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•
SUB- AND W A ~ R T I A N
["1
HOSTED Zn-Pb-Ba
DISCORDANT Zn - Pb AND Ba DEPOSITS EXTENDING ABOVE WAULSORTIAN
Fig. 1. Location map showing main mineral deposits and stratigraphical provinces. National Grid is in kilometres.
specific secondary structures: termination zones, bends in faults, or intersections of second or third order shears with the primary basement structures. There is abundant evidence (feeder veins, alteration zones, changing metal ratios, fluid inclusions) that hot brines came up these fault zones and deposited metals in the hanging-wall carbonates. Most of the deposits in the northern half of the Midlands lie in south-dipping faults,
while those in the southern half lie in the hanging walls of north-dipping faults. Generally, the mineralized structures are en echelon and leftstepping, trending clockwise of the underlying basement structure. The displacement prior to the mineralization is predominantly dip-slip, indicating that the ore-controlling structures are extensional, and it is postulated here that these are rooted in dextral transtensional shears in the basement.
BASE METAL
Table 1.
MINERAL
DEPOSITS, IRELAND
Tonnages and grades of the main Irish sediment-hosted base metal deposits
Deposit
Host
Tonnage (Mt)
Zn+Pb (Wt%)
Zn (Wt%)
Pb (Wt%)
Navan Aherlow Mallow Gortdrum Tatestown Oldcastle Keel Abbeytown Clougherboy Ballyvergin Moyvoughly N'town-Cashel Lisheen Silvermines Tynagh Galmoy Magcobar Ballinalack Rickardstown Courtbrown Garrycam Carrickittle Harberton Boston Hill Allenwood
N N N N N N N N N N N N W W W W W W W W W W S S S
>70 6.0 4.2 3.8 3.6 3.0 1.9 1.1 0.34 0.15 0.13 ? >20.0 17.7 9.9 6.7 5.0 3.5 3.5 1.0 1.4
Arcente Member ¢0
_
.2 ~ . ~ ~
~-.~
~>
~
TierraBlanca _=~ Member
~
"~
6
anchoralislatus
SU
U. typicus
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L. typicus
Nunn
'~ =.
Member
Alamogordo Member ~
Andrecito Member
""
-
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-
:" 't -- ., ",1-~-'
2
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isosticha-
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Fig. 10. Distribution of components of Waulsortian depth phases modified from Lees & Miller (1985, 1995). Phase D is delineated by the presence of micritization, interpreted to extend to a depth of 220 m. Depth limits of other phases are based on thicknesses of strata below the occurrence of micritization containing the diagnostic components.
ALAMOGORDO RAMP BIOTIC GRADIENTS within Belgian Waulsortian mounds are depicted in Fig. 10 (Lees & Miller 1985, 1995). These phases have proven valuable in the analysis of Waulsortian mounds not only in Europe but in North America (Lees & Miller 1985; Ahr 1989a), and, to the extent that water depth can be determined during mound growth, are useful in estimating magnitudes and fluctuations in subsidence and sea level during sedimentation. The bathymetric interpretation of Waulsortian phases depends on the delineation of the 'photic zone', based on the lowest occurrence of micritization, which is assumed to be caused by algae and to occur as deep as 220 m in modern environments (Fig. 10). The depth values inferred for deeper phases are based on the stratigraphic distance of each phase below the lowest occurrence of micritization. Thus, the lowest occurrence of micritization is the lower limit of Phase D (220 m); the lowest occurrence of plurilocular foraminifers, stratigraphically 30 m below the limit of micritization, is the lower limit of Phase C (250m); and the lowest occurrence of hyalosteliid spicules, 60 m below the limit of micritization, is the lower limit of Phase B (280 m). Uncertainties in this procedure include the extent to which micritization is uniquely caused by algae, and thus is indicative of the photic zone (see Lees et al. 1985), and the possibility that depth values based on sediment thicknesses are in error as a result of compaction, diagenesis, subsidence, uplift, and sea-level fluctuation during deposition. Aoujgaliids in the Waulsortian phases have been considered to be sponges because of their poor association with the < 110 m limit for green algae. If aoujgaliids are actually red algae (stacheiins), however, then they could have extended as much as 140m deeper than the l l 0 m limit, to 250m, which would provide a more dependable depth value than 280 m for the first appearance of hyalosteliid sponges. Although the assemblages in the Alamogordo Ramp and in the Waulsortian mounds are similar, depth inferences for the two entities differ for a number of reasons. Firstly, the criteria and procedures differ in the two studies. Secondly, differences in age and location may significantly affect the biota. Most of the Waulsortian mounds that have been studied previously (Lees & Miller 1985; Lees et al. 1985) are European, and younger than the Alamogordo Member. Whereas the earlier phases of the European mounds are Tournaisian and may be contemporaneous with the Alamogordo Member based on conodont zones, the later,
123
shallower phases in the European mounds range across several conodont zones and are younger than the Alamogordo Member. Furthermore, the different floral and faunal realms to which North America and Europe belonged might help to explain the differences in components (e.g. Mamet 1992; Ross 1981). Closely tied to this point is the fact that first appearances of many components and boundaries between phases in the European mounds closely parallel conodont zones, whereas the Alamogordo Member assemblages within each bed are contemporaneous. Lastly, the samples from the Alamogordo Member are from level-bottom strata rather than from mounds, and thus may reflect significantly different environments (Ahr & Stanton 1994, this volume). For example, several taxa that are typically restricted to the shallower spectrum of Waulsortian mounds (gastropods and sponges) are ubiquitous on the Alamogordo Ramp. Interestingly, Ahr & Stanton (1994) found that, in the Waulsortian mounds of the Sacramento Mountains, spicules are more common in the deeper mound growth phase, and gastropod abundance is not significantly different between level-bottom and mound. This may signify differences in preservation or habitat between European and North American Waulsortian facies. It may also indicate that some factor discouraged or inhibited the habitation of the early stages of European mounds by sponges or gastropods. Hennebert & Lees (1991) described the biotic gradients on a Dinantian ramp in southwest England that contains largely the same components as those recognized in the Waulsortian mounds by Lees et al. (1985) and Lees & Miller (1985). The goal of Hennebert & Lees (1991) was to demonstrate the effectiveness of correspondence analysis in recognizing general trends in grain components within a depositional system, especially where there is a limited number of samples. Their data yield a relay of components correlated with a depth gradient across a ramp. This relay is similar to the distribution of components on the Alamogordo Ramp, but more detailed comparison is not possible because of marked differences in the quality of geographic and temporal control and in the analytical procedures used.
Conclusions The Alamogordo Member of the Lake Valley Formation is exposed in the Sacramento Mountains of south-central New Mexico in
124
D. L. JEFFERY & R. J. STANTON JR.
continuous outcrop for 32 km. Individual beds in the Member extend the full length of the transect, which represents a dip profile of a homoclinal ramp. Criteria for determining palaeobathymetry are established by determining gradients of biotic components and component assemblages on the ramp. Assemblages on the ramp, from deepest to shallowest, are characterized by: I
crinoid/echinoderm debris + fenestellid hash + ostracodes; II Assemblage I+stacheiins, rare salebrids and commonly abundant sponge spicules; III Assemblage II +plurilocular foraminifers; IV Assemblage III + green algae.
Depth limits for these assemblages are based on the occurrences of green algae in Assemblage IV and probable red algae (stacheiins) in Assemblage II, and on the probable maximum depth of occurrence for these algae of l l 0 m and 250m, respectively. As a result, Assemblage IV lived in water less than 110m deep, and Assemblage II in water less than or equal to 250 m deep. Using these depth values and distances along the ramp of assemblage limits, the slope of the Alamogordo Ramp was about 0.5°; the shallowest depth, at the north end of the study area, was about 110m, and the water depth estimate for the ramp-basin floor transition in the southern portion of the study area was about 410 m. Sea-level fluctuations during deposition of the Alamogordo Member are recorded by the lateral shifts in assemblages from bed to bed within the Alamogordo Member. The lower portion of the Alamogordo Member (beds 1-7) represents a transgressive systems tract. The middle portion of the Alamogordo Member (beds 8-10 and 12) represents maximum flooding and the onset of highstand systems tract deposition. The upper portion of the Alamogordo Member (beds 11, 13, 14) represents a highstand systems tract and shallowing. The assemblages on the Alamogordo Member ramp are similar to those in the phases in the Waulsortian mound at Furfooz, Belgium, and depth interpretations are also similar (Lees et al. 1985; Lees & Miller 1985, 1995). Differences are explained by differences in the mound v. ramp habitats, in nomenclature, in the systematic placement of problematic taxa, and in the choice of bathymetric criteria used. Comparison of the biotic gradient on the Alamogordo Member ramp with that on the Dinantian ramp of southwest England (Hennebert & Lees 1991)
is more generalized because of the broad spatial and temporal distribution of samples there, and because bathymetric inferences there are only relative. The excellent exposures and continuous bedding of the Alamogordo Member in the Sacramento Mountains provide a tightly constrained temporal and spatial framework within which quantitative as well as relative bathymetric interpretations can be made. The biotic gradients and assemblages in the Alamogordo Member provide depth criteria for recognizing palaeobathymetry of level-bottom strata, and provide a standard for the comparison of levelbottom and adjacent mound biotas and developmental histories. Support from the Paleontological Society, the Texaco Philanthropic Foundation, the Texas A&M University Department of Geology and Geophysics, and the Ray C. Fish Professorship are gratefully acknowledged. We thank W. Ahr, S. Bachtel, J. Miller, N. Pickard and G. Webb for their stimulating discussions and valuable reviews of the manuscript.
References
AHR, W. M. 1973. The carbonate ramp: an alternative to the shelf model. Transactions of the Gulf Coast Association of Geological Societies Annual Convention, 23, 221-225. 1989a. Comparative sedimentology of Waulsortian reefs at Waulsort, Belgium and Alamogordo, New Mexico. Geological Society of America Annual Meeting Abstracts with Programs, 21, A292. - - 1 9 8 9 b . Sedimentary and tectonic controls on the development of an Early Mississippian carbonate ramp, Sacramento Mountains area, New Mexico. Society of Economic Paleontologists and Mineralogists Special Publication, 44, 203-212. 81, STANTON, R. J. JR. 1994. Comparative sedimentology and palaeontology of Waulsortian mounds and coeval level-bottom sediments of the lower Lake Valley Formation (Lower Mississippian) in the Sacramento Mountains (New Mexico, USA). Abhandlungen der Geologischen Bundesanstalt, Wien, 50, 11-24. - & -1996. Constituent composition of Early Mississippian carbonate buildups and their levelbottom equivalents, Sacramento Mountains, New Mexico. This volume. BERGQUIST, P. R. 1978. Sponges. University of California Press, Berkeley & Los Angeles. BOGUSH, O. I. and BRENCKLE, P. L. 1982. Salebridae- a new family of uncertain affinity from the Lower Carboniferous of the USSR and USA: In: YUFEREV, O. V. (ed.) Stratigraphy and Palaeontology of the Devonian and Carboniferous. Akademiya Nauk SSSR. Sibirskoe Otdelenie. Institut Geologii i Geofiziki. Trudy 483, 103-118.
ALAMOGORDO
R A M P BIOTIC G R A D I E N T S
BOLTON, K. LANE H. R. & LEMONE, D. V. (eds) 1982. Symposium on the Environmental Setting and Distribution of the Waulsortian Facies. E1 Paso Geological Society and the University of Texas at E1 Paso. BRENCKLE, P. 1977. Mametella, a new genus of calcareous red algae (?) of Mississippian age in North America. Journal of Paleontology, 51, 250-255. BRIDGES, P. H. & CHAPMAN, A. J. 1988. The anatomy of a deep-water mudmound complex to the southwest of the Oinantian platform in Derbyshire, UK. Sedimentology, 35, 139-162. BURCHETTE, T. P. & WRIGHT, V. P. 1992. Carbonate ramp depositional systems. Sedimentary Geology, 79, 3-57. BYRD, T. M. 1985. Facies Analysis of the Caballero Formation and the Andrecito Member of the Lake Valley Formation in the Northern Sacramento Mountains, Otero County, New Mexico. MSc Thesis,Texas A&M University. DUNHAM, R. J. 1962. Classification of carbonate rocks according to depositional texture. In: FRIEDMAN, G. T. (ed.) Classification of Carbonate Rocks. American Association of Petroleum Geologists Memoir, 1, 108-121. FLOGEL, E. 1982. Microfacies Analysis of Limestones. Springer Verlag, Berlin. GUTSCHICR, R. C., & SANDBERG, C. A. 1983. Mississippian continental margins of the conterminous United States. In: STANLEY, D. J. & MOORE, G. T. (eds) The Shelfbreak: Critical Interface on Continental Margins. Society of Economic Paleontologists and Mineralogists Special Publication, 33, 79 96. HENNEBERT, M. & LEES, A. 1991. Environmental gradients in carbonate sediments and rocks detected by correspondence analysis: examples from the Recent of Norway and the Dinantian of southwest England. Sedimentology, 38, 623 -642. HOOK, J. E., GOLUBIC, S. & MILLIMAN, J. D. 1984. Micritic cement in microborings is not necessarily a shallow-water indicator. Journal of Sedimentary Petrology, 54, 425-431. JAMES, N. P. & GINSBURG, R. N. 1979. The seaward margin of Belize barrier and atoll reefs. International Association of Sedimentologists Special Publication, 3, 1-161. JEFFERY, D. L. & STANTON, R. J. JR. 1993. Extensive and continuous parallel bedding as evidence for stable level-bottom deposition on the distal portion of a ramp. Geological Society of America Annual Meeting Abstracts with Programs, 25, A160. LANE, H. R. 1982. The distribution of Waulsortian facies in North America as exemplified in the Sacramento Mountains of New Mexico. In: BOLTON, K., LANE, H. R. & LEMONE, D. V. (eds) Symposium on the Environmental Setting and Distribution of the Waulsortian Facies. E1 Paso Geological Society and the University of Texas at El Paso, 96 114.
--,
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SANDBERG, C. A. & ZIEGLER, W. 1980. Taxonomy and phylogeny of some Lower Carboniferous conodonts and preliminary standard post-Siphonodella zonation. Geologica et Paleontologica, 14, 117-164. LAUDON, L. R. & BOWSHER, A. L. 1949. Mississippian formations of southwestern New Mexico. Geological Society of America Bulletin, 60, 1 88. LEES, A. 1982. The paleoenvironmental setting and distribution of the Waulsortian facies of Belgium and southern Britain. In: BOLTON, K., LANE, H. R. & LEMONE, D. V. (eds) Symposium on the Environmental Setting and Distribution of the Waulsortian Facies, El Paso Geological Society and the University of Texas at El Paso, 1-16. & MILLER, J. 1985. Facies variations in Waulsortian buildups, 2: Mid-Dinantian buildups from Europe and North America. Geological Journal, 20, 159 180. & -1995. Waulsortian Banks. In: MONTY, C., BOSENCE, D. W. J, BRIDGES, P. H. & PRATT, B. (eds) Carbonate Mud-Mounds: their Origins and Evolution. International Association of Sedimentologists Special Publication, 23, 191-271. --, HALLET, V. & HIBO, D. 1985. Facies variation in Waulsortian buildups, 1: a model from Belgium. Geological Journal, 20, 133-158. MAMET, B. 1991. Carboniferous calcareous algae. In: RIDING, R. (ed.) Calcareous Algae and Stromatolites, Springer-Verlag, Berlin, 370-451. 1992. PalOog6ographie des algues calcaires marines carbonif~res. Canadian Journal of Earth Sciences, 29, 174-194. MEYERS, W .J. 1975. Stratigraphy and diagenesis of the Lake Valley Formation Sacramento Mountains. In: PRAY, L. C. (ed.) Mississippian ShelfEdge and Basin Facies Carbonates, Sacramento Mountains in Southern New Mexico Region. Dallas Geological Society, 45-66. 1977. Chertification in the Mississippian Lake Valley Formation, Sacramento Mountains, New Mexico. Sedimentology, 24, 75-105. MILLER, J. & GRAYSON, R. F. 1982. The regional context of Waulsortian facies in northern England. In: BOLTON, K., LANE, H. R. & LEMONE, D. V. (eds) Symposium on the Environmental Setting and Distribution of the Waulsortian Facies. El Paso Geological Society and the University of Texas at E1 Paso, 17-33. PRAY, L. C. 1961. Geology of the Sacramento Mountains escarpment, Otero County, New Mexico. New Mexico Bureau of Mines and Mineral Resources Bulletin, 35, 1-144. RAILSBACK, L. B. 1993. Original mineralogy of Carboniferous worm tubes: evidence for changing marine chemistry and biomineralization. Geology, 21,703-706. ROSS, J. R. P. 1981. Biogeography of Carboniferous ectoproct bryozoa. Palaeontology, 24, 313-341. SWINCHATT, J. P. 1969. Algal boring: a possible depth indicator in carbonate rocks and sediments. Geological Society of America Bulletin, 80, 1391-1396.
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D. L. J E F F E R Y & R. J. STANTON JR.
TABACHNICK, K. R. 1991. Adaptation of the hexactinellid sponges to deep-sea life. In: REITNER, J. & KEUPP, H. (eds) Fossil and Recent Sponges, Springer-Verlag, Berlin, 378-386. VACHARD, D. 1980. T&hys et Gondwana au Pal~ozo~que Sup6rieur, les donnees Afghanes; biostratigraphie, micropal6ontologie, palbog6ographie. Doc. et Trav. Igal, Paris, 2, 1-464.
1991. Parathuramminides et moravamminides (Microproblematica) de l'Emsien Sup+rieur de la Formation Moniello (Cordilleres Cantagriques, Espagne). Revue de Palkobiologie, 10, 255-299.
Late Vis~an buildups at Kingscourt, Ireland: possible precursors for Upper Carboniferous bioherms I A N D. S O M E R V I L L E ,
PETER
& H. E. A N N E
STROGEN,
GARETH
LL. J O N E S
SOMERVILLE
Dep a rtmen t o f Geology, University College Dublin, Belfield, Dublin 4, Ireland Abstract: Two late Vis6an (Asbian-early Brigantian) buildup complexes occur in the Kingscourt Outlier in Ireland, near the top of the Mullaghfin Formation, a shallow-water, grainstone unit. These massive buildups at Ardagh and Cregg accumulated on the margins of a carbonate platform bordering a deep-water basin. Both have a buildup facies of finegrained, peloid-rich, algal lime mudstones and wackestones, interbedded with coarser grained intraclastic, skeletal packstones and grainstones (interbuildup facies). Microbial structures are well developed in the buildup facies, principally domal stromatolites, thrombolites and oncoidal fabrics of cyanophytes (Ortonella and Girvanella) with encrusting foraminifers (Aphralysia and Tetrataxis). Algal structures include rhodoliths (Solenopora) and fragments of Ungdarella and stacheiids, with less abundant chlorophytes (Koninckopora); in the interbuildup facies, Koninckopora is more abundant. Near the top of the Ardagh buildup is an unusual development of phylloid algal boundstone composed of the possible ancestral coralline red alga Archaeolithophyllum. This boundstone also contains encrusting bryozoans and foraminifers, and directly overlies abundant Brigantian in situ fasciculate rugose corals. This upper unit appears to be associated with a rapid shallowing event, which stimulated the development of a waveresistant rigid framework. Laterally, bedded intraclastic skeletal packstones and grainstones, with thin interbedded clays and shales, pass into the lower part of the massive Ardagh buildup, but appear to onlap and drape the upper part of the buildup. Buildup clasts occur within the interbuildup facies and proximal flank facies, indicating synsedimentary cementation. Late Vis6an buildups of the Ardagh type highlight an important period of diversification of colonial rugose corals and calcareous algae associated with the development of widespread shallow-water carbonate platforms worldwide. This allowed the emergence of new algal groups, particularly the red algae (Archaeolithophyllum and Ungdarella) and the palaeoberesellid green algae (Kamaenella), which subsequently dominated Upper Carboniferous wave-resistant bioherms.
The early Carboniferous marks a period of recovery after the late Devonian reef collapse, with buildups dominated by mud-mounds (West 1988). Recent analysis of Lower Carboniferous bioherms (Webb 1994), however, has shown that their distribution and succession was not the result of evolution from a single primitive 'reef' community, but controlled by regional tectonostratigraphic settings and environments of deposition. Mud-mounds are particularly well developed in the late Tournaisian (Courceyan and early Chadian in the British Isles, equivalent to the early Mississippian of North America) where they are referred to as Waulsortian buildups (Lees 1964; Cotter 1965; Miller & Grayson 1972, 1982; Sevastopulo 1982; Lees et al. 1985; Miller 1986; Bridges & Chapman 1988; Somerville et al. 1992b). However, in the late Vis6an (Asbian and Brigantian Stages, equivalent to the late Mississippian of North America) many buildups had rigid skeletal, cementstone
or microbialite frameworks (Parkinson 1957; Schwarzacher 1961; Orme 1971; Mundy 1980, 1994; Bancroft et al. 1988; Webb 1994). Although fenestrate bryozoans, crinoids and hyalosteliid sponges are locally abundant in Waulsortian mud-mounds, none are deemed to be important in reef construction (West 1988). Many workers consider that in the absence of a recognizable framework they accumulated by microbially-induced precipitation of lime mud. In the early Vis+an (late Chadian and Arundian), Waulsortian-type mud-mounds developed sporadically (Kelly & Somerville 1992; Somerville et al. 1992a), but it was not until the late Vis6an (Asbian and Brigantian) that a second major development of buildups occurred (Nevill 1958; Ramsbottom 1969; Stevenson & Gaunt 1971; Bridges et al. 1995; Gutteridge 1995). Many of these later buildups occurred in shallower water depositional environments than the Waulsortian mud-mounds, which were initiated in deeper
From STROGEN, P., SOMERVILLE,I. D. & JONES, G. LL. (eds), 1996, Recent Advances in Lower Carboniferous Geology, Geological Society Special Publication No. 107, pp. 127-144.
128
I. D. SOMERVILLE E T A L .
water distal ramp settings (Lees 1982; Lees & Miller 1985; Lees et al. 1985; Somerville et al. 1989, 1992b). This paper presents data from two late Vis6an buildup complexes in the Kingscourt area at Ardagh and Cregg, Co. Meath, Ireland (Fig. 1). The massive buildup complex at Ardagh is exposed in a large working quarry, which reveals the internal structure and geometry of the buildup and its relationship to enclosing bedded limestones. The buildup complex at Cregg, 8 km south of Ardagh, is a natural exposure, showing one main buildup with two smaller satellites.
Geological setting and history of research The buildups at Ardagh and Cregg were first reported by Jackson (1955) who noted that the mud-mounds had a specialized fauna of
brachiopods, bivalves, gastropods and goniatites, but a paucity of colonial rugose corals. He considered that the buildup complex at Cregg (his Lower Ardagh 'Reef' Limestone) was older than that at Ardagh (his Upper Ardagh 'Reef' Limestone), but the buildup complexes are now considered to be approximately coeval (Strogen et al. 1995). They occur near the top of the Mullaghfin Formation (Fig. 2), a 500m thick, shallow-water crinoidal-algal-intraclastic packstone/grainstone unit with pseudobreccias and palaeokarstic horizons. New coral and foraminiferal/conodont evidence (Strogen et al. 1995) has established that the two buildups are of late Asbian age, with the top of the Ardagh buildup being early Brigantian. Both buildup complexes represent accumulation at the margin of the shallow-water Ardagh Platform, which passes south into the deeper water basinal facies of the Lucan and Loughshinny Formations (Fig. 3).
LONGFORD - DOWN
Borehole / / Fault Permian & Triassic
m
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~ . Loughshinny ~ j ~us.
~ Basin ~Ma,ahide
Fig. 1. Location of the late Vis6an buildups at Ardagh and Cregg within the Kingscourt Outlier (adapted from Strogen et al. 1995).
LATE VISEAN BUILDUPS, IRELAND STRATIGRAPHY Jackson George
1955
etal.
1976
OF THE KINGSCOURT S t r o g e f l et at. 1 9 9 5
MOYNALTY SYNCLINE
ALTMUSH SHALES BALLINAVORAN z CHERTS ~ TRANSITION BEDS z :,~-UPPERARDAGH?: (3 ~:~>,','REEF' LST.?~,~;: m E IN~T~ER-'REEF' BEDS RATHGILLAN BEDS: z :::LowER ARDAGH'I ~ ?,'I '.VllaUaVUm)I/vuavutv~I "uoetue)t •Slold zouosqe/oouosoad oldm!s o.xe s]uauodtuoo [elZp~lS-UOU ~ql jo lSOlA[ utunloo qoeo jo op!s pueq lq~!a )q:l uo Jnooo s~Iead u~etu oql ]eql os '(lr) luepunqe pue '(E) uotutuo3 '(i~) o.xea '([) luasqe ~u!luasoada.~ '.lr-I tuo.~j )i-~:)s l-eUopep,ea~ e uo pol-eJzlSnll] oae mo!q polooIOS o q I "(uope3o 1 Joj L 7~ g s~.t~ oos) so!ogjoql!l pug sluouodtuo3 u! uo!le!aeA [eo tlaoA oql ~u!nxoqs ~a.xen~) q~epJ v le saqouaq .~addn pug oipp!tu 'Jo~o I oql jo ~o[oql!'-1 "1~"~!~I
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Methods of study Systematic sampling of both buildups was undertaken to examine the vertical and lateral variation in lithofacies and fauna. A vertical sampling interval of approximately 2 m was used in both buildup complexes. Sampling of the adjacent proximal bedded limestones at Ardagh was also carried out. Approximately 100 thin sections and polished slabs were collected from Ardagh (70 from the buildup and 30 from the bedded limestones) and 75 thin sections and polished slabs from Cregg. However, only petrographical detail from the much larger and more complete Ardagh buildup complex is presented here. A semi-quantitative log of the entire Ardagh complex was compiled (Fig. 4), based on visual estimates of allochem distribution in oriented thin sections of samples taken, in a composite vertical profile from base to top, encompassing measured sections in the three quarry bench faces and natural scarps. A separate study of the conodont faunas from the buildups and associated bedded limestones is being carried out by H.E.A.S. and has provided further faunal, palaeoecological and sedimentological data. All thin sections and fossils are housed in the UCD Geology Department collection.
131
Ardagh buildup complex Location This buildup complex is well exposed at Ardagh (N 835955), 3 km east of the town of Kingscourt. Quarry activity currently utilizes three benches cut into a prominent knoll-like hill (Fig. 5). In the quarry the mud-mound facies is at least 200m wide and over 300m long, but extends for a further 225 m to the southwest beyond the quarry. The complex is over 95 m thick, with neither the lower nor upper contacts exposed. Field mapping indicates that the buildup complex forms an elongated domiform ridge trending NE-SW and is bounded to the east by a normal fault of similar trend, where Namurian shales are downthrown against the buildup. The northwestern margin of the buildup is exposed in the west face of the middle bench. There, thickly bedded limestones with thin clay interbeds dipping 30-45 ° NW abut abruptly (and higher up drape over) massive buildup facies (Fig. 6).
Lithofacies The buildup complex is exposed in stratigraphic order, in the lower, middle and upper benches (Figs 4, 7). Note that thicknesses cited in the text
Fig. 5. Oblique aerial view of Ardagh Quarry looking southwest showing the low domiform topographic expression of the buildup complex. 1, lower bench and roadside scarp; 2, middle bench; 3, upper bench and eastern scarp; m, massive mud-mound; b, bedded limestones; F, Fault.
132
I. D. S O M E R V I L L E E T AL.
Fig. 6. View of the west face of the middle bench at Ardagh looking SW, showing the contact of bedded limestones (b) dipping to the NW and the massive pinnacle (m) of mud-mound facies (see Fig. 5). Note the thinning of the thick limestone beds towards the buildup as indicated by the continuity of the thin interbedded clay and shale horizons, and the onlap and drape of the upper bedded limestones over the buildup (between localities 10 and 12 in Fig. 7).
Metres i
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50
\ 100
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~
,
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~3,a ARDAGH QUARRY (Quarry faces as at Jan. 1994)
Fig. 7. Plan of Ardagh Quarry showing the three working benches and localities referred to in the text.
LATE VISEAN BUILDUPS, IRELAND refer to measured heights above the exposed base of the buildup complex (see Fig. 4).
Lower bench. The lowest 2m of the exposed buildup in the roadside scarp east of the quarry (Loc. 1, Fig. 7) are coarse, fossiliferous packstones (coquinas) with abundant brachiopods, bivalves and gastropods (Fig. 8a). The succeeding 35 m is a uniform peloid-rich wackestone/packstone interval (Fig. 8b) with rare concentrations of skeletal material and scattered solitary corals. Stromatactoid cavities with radiaxial fibrous (RFC) and bladed calcite cements (Fig. 8c) are developed rarely (e.g. at 16m), but small irregular spar-filled cavities (Fig. 8b) occur throughout.
Middle bench. Approximately 35 m of massive limestone are exposed in the west face of the middle bench (Loc. 12, Fig. 7) overlying strata of the lower bench with a 2 m exposure gap. Between 39-48m (Fig. 4) are similar peloid-rich wackestone/packstone buildup facies to those of the lower bench, but with more abundant skeletal material, especially calcareous algae (Fig. 9h). However, between 48-67m the complex is dominated by mostly massive interbuildup facies consisting of (i) coarse-grained, intraclastic, skeletal packstones/grainstones rich in algae (Figs 8d, e), brachiopods and gastropods, (ii) finegrained well-sorted peloidal, skeletal packstones and (iii) intraclastic limestone breccias (e.g. at 49m) (Fig. 8f). The latter contain clasts of buildup facies, skeletal grainstones and exotic black, wackestone clasts. Colonial rugose corals are first recorded at 51.5 m, whilst small heterocorals (Hexaphyllia) are recorded only in the interval between 54.7-57.7 m. Upper bench. The upper 10 m in the 35 m thick middle bench overlap with the lowest strata in the upper bench (see Fig. 4). Nineteen metres of buildup complex (including the 10 m of overlap) is exposed in the SE corner of the upper bench (Loc. 14, Fig. 7) and in the natural scarp east of the quarry. Most of the section is in the same peloidrich wackestone buildup facies as in the lower bench, except here very large cavities are developed (up to a metre in width) filled with laminated, peloidal geopetal sediment and RFC and blocky spar cements. The matrix in the upper buildup facies is noticeably finer and has a volumetrically lower proportion of skeletal debris compared with the lower buildup facies (except for a distinctive thin skeletal grainstone bed at 73 m) (see Fig. 4). Macrofauna is locally
133
conspicuous (Loc. 13, Fig. 7), consisting of large brachiopods (Gigantoproductus maximus), solitary corals (Axophyllum, Dibunophyllum bipartitum and Siphonophyllia siblyi) and scattered colonial rugose corals (Siphonodendron and Lithostrotion). However, above 82 m, fasciculate corals are extremely abundant consisting of large in situ colonies (Figs 10, 11) up to 2 m in diameter, with corallites coated with RFC cements in a lime mudstone matrix. The uppermost beds are locally dolomitized. Approximately 10 m are exposed in natural sections above the upper bench (Locs 15 & 16, Fig. 7). These exposures are mostly in interbuildup facies and contain scattered coquinas rich in bivalves, brachiopods and gastropods, and one oolitic horizon.
Biota, non-skeletal components and fabrics Lower bench. Foraminifers are present in almost all samples, although typically in small numbers, except near the base and at 10m (Fig. 4). Similarly, kamaenids (septate tubular microproblematica), which have been referred to the Chlorophyta (dasycladacean palaeoberesellids) by Mamet & Roux (1974), Skompski (1987) and Deloffre (1988), are present in most samples (Fig. 8d) with peak abundance at 22.5 m. Other algae, notably Koninckopora (dasycladacean green alga) and aoujgaliids (problematical red algae - Rhodophyta of Petryk & Mamet 1972; Mamet & Roux 1977; Chuvashov & Riding 1984), which are otherwise very rare in this part of the buildup, have similar peaks in abundance at 22.5m. Cyanophytes (e.g. Ortonella/Girvanella) are fairly common in lower samples but are absent above 18m. Mirroring the development of cyanophytes are Aphralysia-like domes, which were considered encrusting foraminifers (Belka 1981). Thrombolites (see Aitken 1967) and cyanophyte oncoids are present in the same interval, but absent above 20m. Fragments of fenestrate bryozoans are found in the majority of samples (Fig. 8h), but sponge spicules are very rare, occurring in only three samples, two of which are in reworked clasts (Fig. 8h). In many samples large, well-rounded, micritic intraclasts (Fig. 8a) are conspicuous. They are not bored or encrusted and do not show evidence of a former skeletal fabric that may have been micritized. Middle bench. Foraminifers are most abundant, and red and green algae are more diverse and abundant. Within the interbuildup facies between 48-58m (Fig. 4), Koninckopora (Fig.
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©
LATE VISEAN BUILDUPS, IRELAND 9a), aoujgaliids (Fig. 9b) and Ungdarella (red algae of Maslov 1956, 1962; Petryk & Mamet 1972; Chuvashov & Riding 1984) occur in most samples, although Ungdarella (Figs 8d, 8e &9c) is not known from lower buildup levels. Cyanophytes (Ortonella/Girvanella) are abundant in the lower part but absent above 51.5 m. Aphralysia-like domes and oncoids occur in this interval, especially at 51.5m, and unusual encrusting foraminifers similar to Calcitornella, encrusting Tetrataxis and Ungdarella (Fig. 9c) occur at 59-61m. Fenestrate bryozoans and sponge spicules are notably absent in most samples.
Upper bench. Foraminifers are relatively sparse in this part of the buildup, apart from the lowest 2 m (65-67 m) and in the beds above the quarry face (Fig. 4). Calcareous algae are much less abundant than in the middle bench, with kamaenids and aoujgaliids in only a few samples. Ungdarella is very rare, whilst Koninckopora is completely absent throughout this upper section. Cyanophytes and cryptalgal fabrics are also very rare except at 73.5 m (Fig. 9d). Near the top of the section (83-85m) however, there is a profusion of phylloid algae resembling the ancestral red coralline alga Archaeolithophyllum (Figs 9e, f). It is associated with Aphralysia, Girvanella, corals, sponge spicules (Fig. 9g) and encrusting bryozoans (Fistulipora and Tabulipora). This crustose and foliaceous phylloid alga has branches and nodes, and formed vertical as well as horizontal sheets which sheltered geopetal cavities, similar to those illustrated by Reid (1986). Unfortunately, the internal structural detail of this alga was obliterated during diagenetic alteration and there is little evidence of cellular structure under cathodoluminescence or ultraviolet light.
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Bedded strata adjacent to the Ardagh buildup Lower bench. The bedded limestones by the crusher (loc. 3) are coarse-grained and thickbedded ( 3 - 5 m thick), well-sorted crinoid-rich intraclastic grainstones with abundant foraminifers and calcareous algae (Fig. 8g). All samples from these beds contain the diagnostic Asbian foraminifer Vissariotaxis which is not recorded elsewhere in the quarry. Rhodophytes are very abundant and include Aoujgalia, Stacheoides (Fig. 8g) and Ungdarella, with rare Solenopora. Chlorophytes are also abundant including Kamaena and Kamaenella (Fig. 9h), but Koninckopora is conspicuously absent from all the samples. Middle bench. A similar intraclastic-skeletal packstone/grainstone lithofacies is present, although here packstones are dominant. On the east face close to the hopper (Locs 5 & 6) are very coarse-grained, intraclastic crinoidal rudites with large crinoid stems (proximal flank facies). As in the lower bench the bedded limestones have abundant Ungdarella and aoujgaliids, but here Koninekopora is common. Cyanophytes are absent. Fenestrate bryozoans are also absent in 14 of 15 samples. Foraminifers are common, especially Eostaffella and bilaminar palaeotextulariids. In the west face (Loc. 9) the limestones are interbedded with a blue-green shale (Fig. 12), which thickens from 10 cm to 4 m over a lateral distance of 10m before terminating against a fault. The shale appears to fill an irregular top of the underlying limestone, which may represent a palaeokarst surface associated with temporary emergence. Palaeokarst surfaces have been described elsewhere from late Asbian platform limestones (see Strogen et al. 1995). At Loc. 10 (west face), limestone beds (up to 10m
Fig. 8. Thin section photomicrographs of lithofacies in Ardagh Quarry. (a) Coarse-grained skeletal packstone (basal coquina) rich in bivalves, brachiopods, ostracodes, crinoids and well-rounded micrite intraclasts (i). Base of roadside scarp, Loc. 1. Scale bar 3 mm. (b) Peloid-rich wackestone buildup facies with irregular spar-filled cavities lined with thin layer of inclusion-rich RFC cements and blocky sparry calcite. Lower bench, 18.5m, Loc. 1. Scale bar 3 ram. (c) Stromatactoid cavities with a thick layer of inclusion-rich, zoned RFC cements (z) and clear, voidfilling blocky cements (b). Buildup facies, lower bench, 16 m Loc. 1. Scale bar 3 mm. (d) Interbuildup skeletal packstone with crinoids and calcareous algae Ungdarella(u) and Kamaena (k). Middle bench, 57.7m, Loc. 12. Scale bar 1.5 ram. (e) Interbuildup skeletal packstone rich in Ungdarella(u). Middle bench, 18.7 m, Loc. 12. Scale bar 1.5 mm. (f) Limestone conglomerate (intraclastic-skeletal rudstone) with clasts of buildup facies, crinoids and Koninckopora (k). Interbuildup facies, middle bench, 48.1 m, Loc. 12. Scale bar 3 mm. (g) Coarse-grained crinoidal-algal-intraclastic grainstone with abundant Stacheoides (s), Aoujgalia (a) and crinoids (c). Proximal bedded facies, lower bench, Loc. 3. Scale bar 3 mm. (h) Hexactinellid sponge spicules (h) within a micrite clast embedded in a mosaic of sparry calcite cement containing fragments of fenestellid bryozoans. Lower bench, 37 m, Loc. 1. Scale bar 1.5 mm.
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Fig. 10. In situ fasciculate colonial rugose coral (Siphonodendron) > 1.25 m high, from the top of the upper bench (Loc. 14). Scale bar 25 cm.
thick) thin towards the massive buildup and have primary depositional dips of 20-30 ° (Fig. 5). These very thick limestone beds are separated by very thin (5-10 cm) green, possibly bentonitic clay bands, the lowest of which is overlain by a thin fenestral micrite. Calcretes and rhizocretions were not recognized in the limestones beneath the clay bands.
Upper bench. Bedded limestones are only present in the N W corner (Loc.17) and appear to form flank beds to the massive buildup (Fig. 5). These fine-grained, well-sorted peloidal packstones contain abundant spar-filled cavities, are poor in foraminifers and devoid of calcareous algae.
Interpretation of buildup complexes Although the base of the > 9 5 m thick Ardagh buildup is not exposed, the lowest 2 m, a coarsegrained skeletal packstone rich in brachiopods, bivalves and gastropods may be interpreted as a possible 'shell bank' or stabilizing skeletal platform on which cyanophytes and calcareous algae formed the pioneer colonizers. As such it could represent the stabilization and colonization stages of Walker & Alberstadt's (1975) ecological succession. This 'basal' coquina may have created a 'local high' on the sea-bed, providing a site preferential for cyanophyte colonization and subsequent vertical accretion of the buildup.
Fig. 9. Thin section photomicrographs of biota from the buildup complex at Ardagh Quarry. (a) Algal grainstone rich in Koninckopora (k). Interbuildup facies, middle bench, 51.5m, Loc. 12. Scale bar 1.5mm. (b) Algal-rich packstone with Ungdarella (u), Koninckopora (k) and Epistacheoides (e). Interbuildup facies, middle bench, 51.5 m, Loc. 12. Scale bar i .5 mm. (c) Wackestone buildup facies with encrusting Tetrataxis (T), Ungdarella (u) and encrusting foraminifer similar to Calcitornella (c). Middle bench, 59 m, Loc. 12. Scale bar 1.5 mm. (d) Coarsegrained, intraclastic-skeletal grainstone with large domiform oncoid with irregular concentric cryptalgal laminae. Interbuildup facies, upper bench, 73.5 m, Loc. 14. Scale bar 1.5 mm. (e) Peloid-rich Archaeolithophyllum boundstone showing horizontal and vertical orientation of phylloid algal sheets. Note branching (b), nodes (n) and cavities (c) beneath horizontal sheets. Buildup facies, upper bench, 83.2 m, Loc. 14. Scale bar 3 mm. (f) Detail of Archaeolithophyllum boundstone showing Archaeolithophyllum (a), intimately associated with Aphralysia (A), Fasciella (F) and Girvanella forming black specks. Buildup facies, upper bench, 83.5 m, Loc. 14. Scale bar 1.5 ram. (g) Wackestone with abundant sponge spicules (mostly in transverse section), with two long spindles in longitudinal section. Note large spar-filled cavity lined with zoned RFC cement. Buildup facies, upper bench, 84m, Loc. 14. Scale bar 3 mm. (h) Algal grainstone rich in fragments of branching Kamaenella (k). Interbuildup facies, middle bench, 41.2m, Loc. 12. Scale bar 3mm.
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Fig. 11. in situ fasciculate colonial rugose coral ('Koninckophyllum') from the top of the upper bench (Loc. 14). Hammer head is 17 cm long. A prolonged period of cyanophyte growth and lime mud entrapment followed, forming a cohesive meshwork of Girvanella and Ortonella thalli with locally developed Solenopora rhodoliths in a peloid-rich matrix, typical of many shallow water Vis6an platform buildups (see Somerville et al. 1992a; Pickard this volume). Buildup growth may have initiated in a moderately deep (>20m) and protected part of
the platform margin, below fair-weather wavebase and at lower light intensities. This is suggested by the abundance of fenestrate bryozoans, the lack of wave and current structures, the absence of mud winnowing and dasycladacean algae (e.g. Koninckopora) in the lower part of the buildup (cf. Horbury & Adams this volume). However, despite the abundance of fenestrate bryozoans, many are fragmentary with
Fig. 12. Wedge-shaped shale unit (outlined by dashed line) thickening rapidly towards the fault plane (F), west face of middle quarry (Loc. 9).
LATE VISEAN BUILDUPS, IRELAND few intact fronds, which may be a result of bioturbation or transportation. This may also explain in part the paucity of large stromatactoid cavities, which are locally abundant in Waulsortian buildups (Phases A and D of Lees & Miller 1985) where fenestellids form support for cavity roofs. Nonetheless, there are abundant small, irregular-shaped, spar-filled cavities developed throughout the buildup. These cavities may be lined with a thin layer of RFC cements similar to those recorded in north Co. Dublin by Somerville et al. (1992a). Although kamaenids occur in the buildup facies, they may not be as reliable a shallow water-depth indicator as suggested by Skompski (1987), because they are also known from the deeper water Waulsortian buildups of the Dublin Basin (Lees & Miller 1985; Strogen et al. 1990; Somerville et al. 1992b). However, the abundance of Kamaena and Kamaenella, often fragmented, in the laterally equivalent bedded limestones and proximal flank facies is indicative of shallow water environments (see Adams et al. 1992). Occasionally, rapid shallowing events occurred as evidenced by skeletal packstones (e.g. at 22m and 29m) rich in kamaenids, Koninckopora and aoujgaliids, and accompanied by abundant foraminifers on the buildup crest. Buildup facies accumulated at Ardagh for much of the lower 37m before there was a gradual change to interbuildup facies (39-43 m and especially 48-58m). At these levels there is a profusion of foraminifers, chlorophytes (especially Koninckopora) and rhodophytes (Ungdarella and stacheiids), suggesting a much shallower water depth, well within the photic zone. Breccia horizons containing clasts of buildup facies, skeletal grainstones derived from locally bedded limestones and exotic, black wackestone clasts occur within the interbuildup facies. The black, fine-grained limestone clasts may represent a similar facies to the Deer Park Formation (a slightly younger platform limestone unit to the NW of Ardagh Quarry; Fig. 3), but they possibly were derived from the coeval quieterwater platform facies, which developed leewards of the buildup and is no longer exposed. The presence of buildup facies clasts in the breccia, which are clearly recognizable by their peloidal wackestone fabric and spar-filled cavities with geopetal sediment, indicates that the lower part of the buildup complex experienced synsedimentary cementation. Following this period of interbuildup facies deposition, cyanophytes were reestablished and
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a second major phase of lime mud accumulation occurred (forming the top of the middle bench and most of the upper bench). However, the contact between the buildup and interbuildup facies is poorly defined at outcrop. The matrix in the upper buildup facies is noticeably finer and has a lower proportion of skeletal debris compared with the lower buildup facies. Conversely, macrofauna is more conspicuous in the upper buildup facies. The absence of Koninckopora in this section may signify a return to greater water depths of buildup development associated with a sea-level rise. However, irrespective of water depth, the absence of Koninckopora may mark its final demise, as the genus became extinct close to the Asbian/ Brigantian boundary (Somerville & Strank 1984; Somerville et al. 1992c; Jones & Somerville this volume). The top of the buildup is Brigantian in age based on its coral faunas (Strogen et al. 1995). A rapid relative sea-level fall occurred near the top of the buildup complex, which exerted a significant control on faunal and floral diversification in the top of the buildup. This is witnessed by the sudden profusion of abundant in situ rugose coral colonies, which acted as baffles, overlain directly by Archaeolithophyllum, encrusting bryozoans and foraminifers (Aphralysia and Tetrataxis). The resulting bindstone fabric is characterized by cavities and corallites lined with RFC cements. This is an ecological community replacement and may represent the diversification stage of Walker & Alberstadt (1975). Unfortunately, the topmost part of the complex is no longer preserved. The ancestral coralline red alga Archaeolithophyllum, which forms crustose and foliaceous sheets several centimetres in length in the buildup, first occurs in the late Vis6an (see Wray 1964, 1977; Belka 1981). Belka (1981) noted that Aphralysia can be found encrusting Archaeolithophyllum and is itself encrusted by Girvanella in algal-foraminiferal mud-mounds. Moreover, of all the Lower Carboniferous calcareous algae, two in particular (the encrusting foliaceous Archaeolithophyllum and the ramose Ungdarella) became cosmopolitan reef formers in the Upper Carboniferous (Wray 1977; Mamet 1991). Recent studies (James et al. 1988) have suggested that Arehaeolithophyllum may be a peyssonelid with an aragonitic wall structure which has been subsequently diagenetically altered. This may explain the lack of cellular detail in the specimens from the Ardagh buildup. Wray (1964, 1977) has suggested that Archaeolithophyllum from Upper Carboniferous
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(Pennsylvanian) bioherms may have formed algal banks similar to those of the Recent Lithothamnium (Bosence 1985), with individual species of Archaeolithophyllum adapted to the more turbulent-water crests of banks. This may be analogous to the Ardagh buildup where Archaeolithophyllum becomes locally abundant just below the top. However, the alga never assumes the volumetrically important role of dominance in the buildup, which forms the culminating stage of the ecological succession model of Walker & Alberstadt (1975). It is inferred that this last stage is associated with a rapid shallowing event. The bedded limestones immediately to the NW of the buildup in the middle bench, although containing clasts of reworked platform limestones, do not contain recognizable buildup derived fragments. The lowest bedded limestones in the middle bench abut the buildup, but higher bedded limestones drape it. Unfortunately there is, as yet, insufficient precision in the foraminiferal/conodont biostratigraphy to differentiate late Asbian and early Brigantian faunas within the peripheral bedded limestones, or between the bedded limestones and the massive buildup facies (see Jones & Somerville this volume). Thus at present it is not possible to demonstrate the nature of the contact, or to determine if there is onlap of an exhumed mound or lateral passage from buildup to bedded flank beds. The buildup complex at Cregg does not show any contact between bedded limestones and massive buildup facies. However, a similar vertical sequence of buildup-interbuildupbuildup facies at Ardagh is recognized in the largest mud-mound at Cregg. At Cregg, as at Ardagh, the top of the buildup complex is not exposed. However, limited field data suggests that the main Cregg buildup was rapidly buried by terrigenous mud with interbedded graded limestone turbidites of the basinal Loughshinny Formation, which crops out immediately to the west. The latter facies is associated with a marine transgression that resulted in the drowning of the southern margin of the Ardagh Platform in the late Asbian-early Brigantian (Fig. 3; Strogen et al. 1995).
Discussion It is generally agreed that in the Lower Carboniferous (Dinantian) there was a major hiatus in reef construction following the collapse of large framework-building metazoans in the late Devonian (James 1984; West 1988). As
Newell (1972) remarked 'this was a time of crisis for reef-building'. The result was that initially in the Dinantian a slow period of recovery was characterized by the development of deeperwater Waulsortian mud-mounds that lack any recognizable organic framework. The prevailing theory on their construction is that they resulted from microbial activity (Lees & Miller 1985, Somerville et al. 1992b). A similar pattern is evident in the role of calcareous algae as reef builders (Chuvashov & Riding 1984). Many algal groups that were present in the Cambrian to Devonian showed a marked decline or became extinct at the end of the Devonian. New algal groups (e.g. KamaenaDonezella group, Archaeolithophyllum group and Ungdarella-Stacheia group) that evolved in the Lower Carboniferous later became important reef-builders in the Middle and Upper Carboniferous (Westphalian-Stephanian) (Chuvashov & Riding 1984; West 1988). Algae are rarely reported as a common reef constituent in the Dinantian, although members of the Renalcis group (a possible cyanophyte; Pratt 1984), which ranges back to the Cambrian (Chuvashov & Riding 1984), occur in some Vis6an bioherms and mud-mounds (Adams 1983; Webb 1989; Somerville et al. 1992a, c). In the early Carboniferous the dominant metazoans in Waulsortian mud-mounds were crinoids, fenestrate bryozoans and sponges (mostly as spicules). Occasionally at the top of these mud-banks and in flank beds, or within interbank facies (in areas of shallower water deposition), chlorophytes (Koninckopora, A tractyliopsis and kamaenids) and cyanophytes (Girvanella) are recorded (Jones et al. 1988; Somerville et al. 1992b). For the most part, calcareous algae are absent from Waulsortian mounds because of their presumed greater (subphotic) water depth (Lees & Miller 1985). Calcareous algae are likely to be present only in mud-banks that have Phase D (shallowest water-depth) components, such as many of the Waulsortian mounds in the Dublin Basin (Somerville et al. 1992b) and slightly younger early Vis~an Waulsortian buildups in NW Ireland (Kelly & Somerville 1992). VisOan bioherm f r a m e w o r k s
In the late Vis6an and Namurian, significant but subtle changes occurred in buildup construction, most notably in the volumetric decline of bryozoans and the rise of calcareous algae and corals in forming frameworks in bioherms and mud-mounds. However, bryozoans (fenestrates,
LATE VISEAN BUILDUPS, IRELAND trepostomes and fistuliporids), and cyanophytes in some cases, still occurred as in situ meshwork or framework builders (Dix & James 1987; Bancroft et al. 1988; Christopher 1990; Lauwers 1992). In the early Vis6an (late Chadian and Arundian), cyanophytes began to emerge as important binders and bafflers of sediment, and formed wave-resistant 'cryptalgal' structures (domal stromatolites and thrombolites) and oncoids. These were constructed by meshworks of Girvanella, Ortonella and Renalcis, assisted by framebuilders such as solenoporids, encrusting foraminifers (Aphralysia and Tetrataxis, interpreted by Cossey & Mundy (1990) as a limpet-like epiphytic encruster), encrusting bryozoans (Fistulipora) and tabulate corals (Adams 1984; Fang & Hou 1987; Somerville et al. 1992a). In the later Vis6an, colonial rugose corals, calcareous algae and non-skeletal microbialite structures are reported (similar to those at Ardagh) (Mundy 1980, 1994; Webb 1989, 1994), and may have been the dominant biotic component in buildups and platform margins bound by syndepositional marine cements (Horbury 1992). The importance of corals in Vis6an bioherms has until recently been unrecognized. Most workers (James 1984; Chuvashov & Riding 1984; West 1988) have commented on the dominance of Waulsortian-type mud-mounds and the absence of well-developed coral reef and algal reef communities, which characterized late Devonian reefs. Vis6an coral reefs referred to by some earlier workers (Johnson 1959; Caldwell & Charlesworth 1962) have subsequently been reinterpreted as coral biostromes. However, Webb (1989, 1990, 1994), in a detailed study of the late Vis6an Lion Creek bioherms from Queensland, Australia, observed that they had a rigid coral-algal framework. The coral-algal boundstone core facies consists of a framework of colonial rugose ('Siphonodendron' and 'Orionastraea') and tabulate (Multithecopora) corals in growth position. The coral colonies are up to a metre in diameter and occur within a microbialite matrix in buildups up to 15m thick. These structures are comparable with the top of the Ardagh buildup. A very large atoll reef over 20km across in southern Japan ranges in age from early Carboniferous to Permian (Ota 1968). In the late Vis6an and Middle Carboniferous period of reef development, colonial rugose (Siphonodendron) and tabulate (multithecoporan) corals formed reef flat communities, assisted by chaetetids (sclerosponges), encrusting bryozoans, encrusting foraminiferans, red algae and cyanophytes (Sugiyama & Nagai 1990, 1994).
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Rodriguez (this volume) has also recognized Upper Vis6an reefs in Spain composed of Siphonodendron and tabulate colonial corals with red algae.
Conclusions (1) Two, massive, late Asbian buildup complexes at Ardagh and Cregg, dominated by cyanophytes and calcareous algae, were developed on a shallow-water carbonate platform close to the margin of a deeper water basin. Buildup relief is difficult to assess but is probably low; nowhere can bedded limestones proximal to the buildup be seen to interfinger with the massive limestones. The bedded limestones at Ardagh appear to thin against, onlap, and drape the buildup, and are coeval with or post-date buildup accumulation. Furthermore, there is no evidence of reef talus in the bedded limestones adjacent to the buildup. However, in the interbuildup facies at Ardagh, one interval of limestone breccia contains a variety of limestone clasts, some of which are of buildup type, whereas others appear to represent more distant sources. (2) Both buildup complexes show vertical variation in lithofacies involving an alternation of fine-grained, peloid-rich algal wackestone/ packstones (buildup facies) and coarse-grained, intraclastic skeletal packstone/grainstones (interbuildup facies). Small, irregular, spar-filled cavities are common in the buildup facies, but particularly near the top of the Ardagh complex, larger cavities contain numerous generations of geopetal sediment. (3) Cyanophyte structures are frequently developed in both buildups and appear to form meshworks and frameworks. However, near the top of the Ardagh buildup, Archaeolithophyllum boundstone facies is associated with thickets of colonial rugose corals, sponges, encrusting bryozoans and foraminifers. For the most part, however, bryozoans and sponges are notably subordinate components and colonial corals are extremely rare. Increasing metazoan diversity towards the top of the Ardagh buildup may represent an ecological community replacement. This is associated with a rapid shallowing event and the development of a wave-resistant skeletal framework of colonial corals and red algae, as the buildup grew into the surf zone. Indication of shoaling is provided by a rare oolitic horizon near the top of the Ardagh buildup. (4) The late Vis6an Ardagh and Cregg buildup complexes contain a complex suite of lithofacies and biofacies with vertical variation in
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textures, microfacies and biota typically related to alternations of buildup and interbuildup facies. This may reflect the vertical stacking or coalescence of smaller discrete mud-banks, and obscure internal geometries. However, the second major phase of buildup facies at Ardagh may be related to a deepening event associated with an early Brigantian transgression, as seen in the abrupt passage from massive bedded, pale grey limestones of the Mullaghfin Formation to dark grey, cherty and argillaceous limestones of the Deer Park Formation. (5) These late Vis6an buildups are more advanced than the earlier Waulsortian (late Tournaisian-early Vis6an) mud-mounds. Cyanophytes are important in providing initial meshworks which acted as baffles, and later under more favourable conditions formed rigid frameworks when associated with other metazoans, principally colonial corals, sponges, encrusting bryozoans, foraminifers, and specialized red algae such as Archaeolithophyllum. These late Vis6an buildups thus appear to represent an important transition in the evolution of Carboniferous reefs: from microbe-dominated Waulsortian mud-mounds in the early Carboniferous, to conspicuous reef-framework buildups of the late Carboniferous. Moreover, it is very significant that some of the algal flora that first emerged in the late Vis6an as contributors to buildup frameworks (e.g. Archaeolithophyllum, Ungdarella and Kamaeneila) later formed the dominant component of Upper Carboniferous bioherms. We would like to thank Roadstone Provinces Ltd, Castleblayney and the quarry manager J. Glynn for permission to visit Ardagh Quarry. We are also indebted to G. Webb and A. Horbury for their invaluable critical reviewing of the paper.
References ADAMS, A. E. 1983. Lower Carboniferous Renalcis from Cumbria. Proceedings of the Yorkshire Geological Society, 44, 327-331. - - 1 9 8 4 . Development of algal-foraminiferal-coral reefs in the Lower Carboniferous of Furness, northwest England. Lethaia, 17, 233-249. - - , HORBURY, A. D. & RAMSAY, A. T. S. 1992. Significance of palaeoberesellids (Chlorophyta) in late Dinantian sedimentation, UK. Lethaia, 25, 375-382. AITKEN, J. D. 1967. Classification and environmental significance of cryptalgal limestones and dolomites with illustrations from the Cambrian and Ordovician of Southwestern Alberta. Journal of Sedimentary Petrology, 37, 1163-1187.
BANCROFT, A. J., SOMERVILLE, I. D. & STRANK, A. R. E. 1988. A bryozoan buildup from the Lower Carboniferous of North Wales. Lethaia, 21, 51-65. BELKA, Z. 1981. The alleged algal genus Aphralysia is a foraminifer. Neues Jahrbuch ffir Geologie und Palaontologie Monatshefte, 1981, 257 266. BOSENCE, D. 1985. Preservation of coralline-algal reef frameworks. Proceedings of the Fifth International Coral Reef Congress, Tahiti, 6, 623-628. BRIDGES, P. H. & CHAPMAN,A. J. 1988. The anatomy of a deep water mud-mound complex to the southwest of the Dinantian platform in Derbyshire, UK. Sedimentology, 35, 139-162. , GUTTERIDGE, P. & PICKARD, N. m. H. 1995. Environmental setting of Carboniferous mudmounds in NW Europe. In: MONTY, C. L. V., BRIDGES, P. H., PRATT, B. & BOSENCE,D. W. J. (eds) Carbonate Mudmounds-Origin and Evolution. Special Publication of International Association of Sedimentologists, 23, 171-190. CALDWELL, W. G. E. & CHARLESWORTH, H. A. K. 1962. Vis6an coral reefs in the Bricklieve Mountains of Ireland. Proceedings of the Geologists' Association, 73, 359-382. CHRISTOPHER, C. C. 1990. Late Mississippian Girvanella-bryozoan mud mounds in southern West Virginia. Palaios, 5, 460-471. CHUVASHOV, B. & RIDING, R. 1984. Principle floras of Palaeozoic marine calcareous algae. Palaeontology, 27, 487 500. COSSEY, P. J. & MUNDY, D. J. C. 1990. Tetrataxis, a loosely attached limpet-like foraminifer from the Upper Palaeozoic. Lethaia, 23, 311-322. COTTER, E. 1965. Waulsortian-type carbonate banks in the Mississippian Lodgepole Formation of central Montana. Journal of Geology, 73, 881-888. DELOFFRE, R. 1988. Nouvelle taxonomie des algues dasycladales. Bulletin de Centres Recherche Exploration Production Elf Aquitaine, 12, 165-217. DIX, G. R. & JAMES, N. P. 1987. Late Mississippian bryozoan/microbial build-ups on a drowned karst terrain: Port au Port Peninsula, western Newfoundland. Sedimentology, 34, 779-793. FANG, S. & HOU, F. 1987. Tatangian (Carboniferous) bryozoan-coral patch reef in Langping of Tianlin, Guangxi. 11th International Congress of the Stratigraphy and the Geology of the Carboniferous, Beijing 1987, Abstracts, 1, 167-168. GEORGE, T. N., JOHNSON, G. A. L., MITCHELL, M., PRENTICE, J. E., RAMSBOTTOM, W. H. C., SEVASTOPULO, G. D. & WILSON, R. B. 1976. A Correlation of Dinantian Rocks in the British Isles. Geological Society of London Special Report No. 7. GUTTERIDGE, P. 1995. Late Dinantian carbonate mud mounds of the Derbyshire carbonate platform. In: MONTY, C. L. V., BRIDGES, P. H., PRATT, B. & BOSENCE, D. W. J. (eds) Carbonate Mudmounds- Origin and Evolution. Special Publication of International Association of Sedimentologists, 23, 289-307.
LATE VISEAN B U I L D U P S , I R E L A N D HORBURY, A. D. 1992. A late Dinantian peloid cement stone-palaeoberesellid buildup from North Lancashire, England. Sedimentary Geology, 79, 117-137. & ADAMS, A. E. 1996. Microfacies associations in Asbian carbonates: an example from the Urswick Limestone Formation of the sothern Lake District, northern England. This volume. JACKSON, J. S. 1955. The Carboniferous Succession of the Kingscourt Outlier with Notes on the PermoTrias. PhD Thesis, University of Dublin. JAMES, N. P. 1984. Reefs. In: WALKER, R. G. (ed.) Facies Models. Geoscience Canada Reprint Series 1 (2nd edn), 229-244. --, WRAY, J. L. & GINSBURG, R. N. 1988. Calcification of encrusting aragonitic algae (Peyssonneliacea): implications for the origin of Late Palaeozoic reefs and cements. Journal of Sedimentary Petrology, 58, 291-303. JOHNSON, G. m. L. 1959. The Carboniferous stratigraphy of the Roman Wall district in western Northumberland. Proceedings of the Yorkshire Geological Society, 32, 83-130. JONES, G. LL., & SOMERVILLE, I. D. 1996. Irish Dinantian biostratigraphy: practical applications. This volume. & STROGEN, P. 1988. The Lower Carboniferous (Dinantian) of the Swords area: sedimentation and tectonics in the Dublin Basin, Ireland. Geological Journal, 23, 221-248. KELLY, J. G. & SOMERVILLE, I. D. 1992. Arundian (Dinantian) carbonate mudbanks in North-West Ireland. Geological Journal, 27, 221-242. LAUWERS, A. 1992. Growth and diagenesis of cryptalgal-bryozoan buildups within a midVis~an (Dinantian) cyclic sequence. Annales de la Soci~t~ G~ologique de Belgique, 115, 187-213. LEES, A. 1964. The structure and origin of the Waulsortian (Lower Carboniferous) 'reefs' of west-central Eire. Philosophical Transactions of the Royal Society London, 247B, 483-531. --1982. The palaeoenvironmental setting and distribution of the Waulsortian facies of Belgium and southern Britain. In: BOLTON, K., LANE, H. R. & LEMONE, D. V. (eds) Symposium on the Environmental Setting and Distribution of the Waulsortian Facies. E1 Paso Geological Society and University of Texas at E1 Paso, 1-16. & MILLER, J. 1985: Facies variation in Waulsortian buildups. Part 2: Mid-Dinantian buildups from England and North America. Geological Journal, 20, 159-180. --, HALLET, V. & HIBO, D. 1985. Facies variation in Waulsortian buildups. Part 1: A model from Belgium. Geological Journal, 20, 133-158. MAMET, B. 1991. Carboniferous calcareous algae. In: RIDING, R. (ed.) Calcareous Algae and Stromatolites. Springer-Verlag, Berlin, 370-451. -& Roux, A. 1974. Sur quelques algues tubulaires scalariformes de la Tethys Pal6ozoique. R~vue de Micropaleontologie, 17, 134-156.
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& 1977. Algues rouges devoniennes et carbonif~res de la Tethys Occidentales, quatrieme partie. R~vue de Micropaleontologie, 19, 215-266. MASLOV, V. P. 1956. Calcareous algae of the USSR. Transactions of Akadamie de Sciences USSR Geological Institute, 160, 1-301 [in Russian]. - - 1 9 6 2 . Fossil red algae of the USSR. Transactions of Akadam& de Sciences USSR Geological Institute, 53, 1-221 [in Russian]. MILLER, J. 1986. Facies relationships and diagenesis in Waulsortian mudmounds from the Lower Carboniferous of Ireland and Northern England. In: SCHROEDER, J. H. & PURSER, B. H. (eds) Reef Diagenesis. Springer Verlag, Berlin, 311-335. & GRAYSON, R. F. 1972. Origin and structure of the Lower Vis~an "reef" limestones near Clitheroe, Lancashire. Proceedings of the Yorkshire Geological Society, 38, 607 638. & 1982 The regional context of Waulsortian facies in northern England. In: BOLTON, K., LANE H. R. & LEMONE, D. V. (eds) Symposium on the Environmental Setting and Distribution of the Waulsortian Facies. El Paso Geological Society and University of Texas at E1 Paso, 17-33. MUNDY, D. J. C. 1980. Aspects of the Palaeoecology of the Craven Reef Belt (Dinantian) of North Yorkshire. PhD Thesis, University of Manchester. --1994. Microbialite-sponge-bryozoan-coral framestones in Lower Carboniferous (Late Vis6an) buildups of northern England (UK). In: BEAUCHAMP, B., EMERY, A. F. & GLASS, D. J. (eds) Pangea." Global Environments and Resources. Canadian Society of Petroleum Geologists Memoir, 17, 713-729. NEVlLL, W. E. 1958. The Carboniferous knoll-reefs of east-central Ireland. Proceedings of the Royal Irish Academy, 59B, 285-303. NEWELL, N. D. 1972. The evolution of reefs. Scientific American, 226, 54-65. ORME, G. R. 1971. The D2-P1 'reefs' and associated limestones of the Pin Dale-Bradwell Moor area of Derbyshire. Comptes Rendues 6th International Congress in Stratigraphy and Geology of the Carboniferous, Sheffield 1967, 3, 1249-1262. OTA, M. 1968. The Akiyoshi Limestone Group: A geosynclinal organic reef complex. Bulletin of the Akiyoshi-dai Science Museum, 5, 1-44. PARKINSON, D. 1957. Lower Carboniferous reefs of northern England. Bulletin of the American Association of Petroleum Geologists, 41, 511-537. PETRYK, A. m. & MAMET, B. 1972. Lower Carboniferous algal flora, southwestern Alberta. Canadian Journal of Earth Sciences, 9, 767-802. PICKARD, N. A. H. 1996. Evidence for microbial influence on the development of Lower Carboniferous buildups. This volume. PRATT, B. R. 1984. Epiphyton and Renalcis: diagenetic microfossils from calcification of blue-green algae. Journal of Sedimentary Petrology, 54, 948-971. -
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RAMSBOTTOM, W. H. C. 1969. Reef distribution in the British Lower Carboniferous. Nature, 222, 765-766. REID, P. R. 1986. Discovery of Triassic phylloid algae: possible links with the Paleozoic. Canadian Journal of Earth Sciences, 23, 2068-2071. RODRIGUEZ, S. 1996. Development of coral reef-facies during the Vis6an at Los Santos de Maimona, SW Spain. This volume. SCHWARZACHER, W. 1961 Petrology and structure of some Lower Carboniferous reefs in Northwestern Ireland. Bulletin of the American Association of Petroleum Geologists, 45,1481-1503. SEVASTOPULO, G. D. 1982. The age and depositional setting of the Waulsortian Limestones in Ireland. In: BOLTON, K., LANE H. R. & LEMONE, D. V. (eds) Symposium on the Environmental Setting and Distribution of the Waulsortian Facies. El Paso Geological Society and University of Texas at El Paso, 65-79. SKOMPSKI, S. 1987. The dasycladacean nature of Late Palaeozoic palaeoberesellid algae. Acta Geologica Polonica, 37, 21-31. SOMERVILLE, I. D. & STRANK, A. R. E. 1984. The recognition of the Asbian/Brigantian boundary fauna and marker horizons in the Dinantian of North Wales. Geological Journal, 19, 227-237. -, PICKARD, N. A. H., STROGEN, P. & JONES, G. LL. 1992a. Early to mid-Visban shallow water platform buildups, north Co. Dublin, Ireland. Geological Journal, 27, 151-172. , STRANK, A. R. E. & WELSH, A. 1989. Chadian faunas and flora from Dyserth: Depositional environments and palaeogeographic setting of Vis6an strata in northeast Wales. Geological Journal, 24, 49-66. , STROGEN, P. & JONES, G. LL. 1992b. MidDinantian Waulsortian buildups in the Dublin Basin, Ireland. Sedimentary Geology, 79, 91-116. , & --1992c. Biostratigraphy of Dinantian limestones and associated volcanic rocks of the East Limerick Syncline. Geological Journal, 27, 201-220. STEVENSON, I. P. & GAUNT, G. D. 1971. Geology of the country around Chapel en le Frith. Memoir of the Geological Survey of Great Britain.
STROGEN, P., JONES, G. LL. & SOMERVILLE,I. D. 1990. Stratigraphy and sedimentology of Lower Carboniferous (Dinantian) boreholes from west Co. Meath, Ireland. Geological Journal, 25, 103-137. - - , SOMERVILLE,I. D., JONES, G. LL. & PICKARD,N. A. H. 1995. The Lower Carboniferous (Dinantian) stratigraphy and structure of the Kingscourt Outlier, Ireland. Geological Journal, 30, 1-23. SUGIYAMA, T. & NAGAI, K. 1990. Growth forms of Auloporidid corals in the Akiyoshi Limestone Group, southwestern Japan: Palaeoecological studies of reef-building organisms in the Akiyoshi organic reef complex. Bulletin of the Akiyoshi-dai Science Museum, 25, 1-25. - & 1994. Reef facies and palaeoecology of reef-building corals in the lower part of the Akiyoshi Limestone Group (Carboniferous) Southwest Japan. Courier Forschungsinstitut Senckenberg, 172, 231-240. WALKER, K. R. & ALBERSTADT, L. P. 1975. Ecological succession as an aspect of structure in fossil communities. Palaeobiology, 1,238-257. WEBB, G. E. 1989. Late Vis~an coral-algal bioherms from the Lion Creek Formation of Queensland, Australia. Comptes Rendu l l th International Congress of the Stratigraphy and the Geology of the Carboniferous, Beijing 1987, 3, 282-295. - - 1 9 9 0 . Lower Carboniferous coral fauna of the Rockhampton Group, east-central Queensland. In: JELL, P. A. (ed.) Devonian and Carboniferous Coral Studies. Association of Australasian Palaeontologists Memoir 10, 1-167. - - 1 9 9 4 . Non-Waulsortian Mississippian bioherms: a comparative analysis. In: BEAUCHAMP, B., EMERY, A. F. & GLASSS, D. J. (eds) Pangea: Global Environments and Resources. Canadian Society of Petroleum Geologists Memoir 17, 701-712. WEST, R. R. 1988. Temporal changes in Carboniferous reef mound communities. Palaios, 3, 152-169. WRAY, J. L. 1964. Archaeolithophyllum, an abundant calcareous alga in limestones of the Lansing Group (Pennsylvanian), southeastern Kansas. Kansas Geological Survey, 170, 1-13. --1977. Calcareous algae. Developments in Palaeontology and Stratigraphy. Elsevier, Amsterdam.
Development of coral reef-facies during the Vis~an at Los Santos de Maimona, SW Spain SERGIO
RODRIGUEZ
Departamento de Paleontologia, Facultad de Ciencias Geoldgicas, Universidad Complutense de Madrid, 28040 Madrid, Spain Abstract: The Siphonodendron Limestone at Los Santos de Maimona Basin is regarded as a reef structure built mainly by rugose corals. This unit is composed of biogenic marls and biostromal limestones containing abundant rugose corals and brachiopods, and frequent calcareous algae, tabulate corals, foraminiferans, bryozoans, echinoderms, ostracodes and molluscs. It is present throughout the basin, which is 12 km long and 3 km wide, but the thickness, development of the framework and distribution of organic components varies from SE (seaward) to NW (landward). It is 40 m thick in the SE, but its thickness reaches only 6 m in the NW. The unit shows a vertical evolution of lithological facies, from biogenic marls at the bottom up to biostromal limestones at the top. The main environmental factors controlling the development of the organic framework are the tidal regime, minor subsidence pulses and periodic storms. Arguments in favour of the reefal origin are structural (distribution, development and relationships between different facies of the Siphonodendron Limestone) and ecological (distribution and relationships of building organisms, environmental indicators, etc.). Objections to the reefal hypothesis (absence of reef crest and talus, low diversity, problematical wave resistance, biostromal nature of the upper beds, storm layers) are discussed. Building structures by corals, brachiopods and calcareous algae are briefly described.
Carboniferous rugose corals are not usually regarded as reef-builders. They were described as the main components of reef mounds or bioherms (Pareyn 1959; Sutherland & Henry 1977; Webb 1989) or as secondary components of reef complexes (Adams 1984), but very rarely as the main builders of reef complexes (Ota 1968). In fact, many authors deny the existence of true ecological reefs (or reef complexes) during the Carboniferous (Heckel 1974; James, 1979, 1983; Copper 1988, 1994) or at least during the Early Carboniferous (West 1988). Nevertheless, Lower Carboniferous true reef complexes have been described from America (Schenk & Hatt 1984), the UK (Adams 1984) and Japan (Sugiyama & Nagai 1994), and at least in the last case, rugose corals are the main component. The Siphonodendron Limestone at Los Santos de Maimona (SW Spain) seems to be an additional example of the building capability of rugose corals during the Carboniferous. The Los Santos de Maimona Basin is located in the Ossa-Morena region, near Zafra (Badajoz, SW Spain, Fig. 1). It is small and elongated in a N W - S E orientation, coinciding with the main Hercynian structures of the region, and limited by tear faults with rotational movement (Odriozola et al. 1983). It comprises Upper Vis~an (Asbian) rocks of varied characteristics, which have been subdivided into eight lithostratigraphic units
numbered from 0 to 7 (Sfinchez et al. 1988, 1991; Falces 1991; Rodriguez et al. 1992). Units 1, 3, 4 and 6 are basically carbonates. The purpose of this contribution is to describe the coral reef-facies of Unit 1, here interpreted as a wave-resistant reef structure developed during the early Asbian. Unit 1 is composed of biostromal and bioelastic marls and limestones containing abundant colonies of the rugose coral genus, and consequently it was named the Siphonodendron Limestone. Large brachiopods, calcareous algae and tabulates participate in the framework of the biostromal levels (Rodriguez et al. 1994). Several sections containing Unit 1 were studied (Albuera River, E1 Almendro, Guadajira River, El Portezuelo, Navafria, E1 Torre6n and Los Santos Hill). Some of these sections illustrate the lateral variations of the Siphonodendron Limestone (Fig. 2). The thickness, development of the framework and distribution of organic components vary largely from SE to NW.
Lithofacies Marly biogenic beds in the lower part of the southern sections (Lower Member; Rodriguez et al. 1992, 1994) are composed of heterogeneous colonies of Siphonodendron and syringoporoids,
From STROGEN, P., SOMERVILLE,I. D. & JONES, G. LL. (eds), 1996, Recent Advances in Lower Carboniferous Geology, Geological Society Special Publication No. 107 pp. 145-152.
146
S. RODRiGUEZ foraminiferans and calcareous algae are abundant in the bioclastic levels. Ooids are also common.
. ......
The hypothesis
t II~:(LI~,I e~
A~o
z -~
L,StOA
AB
Fig. 1. Location map of the Los Santos de Maimona Basin, with positions of the studied sections. AB, Albuera River; AL, El Almendro; GU, Guadajira River; PT, El Portezuelo; TO, El Torre6n; NA, Navafria; SS, Los Santos Hill. Carboniferous outcrops are indicated by vertical haching in the general map of the Ossa-Morena region.
red and blue-green algae and large gigantoproductids growing in a rigid organic framework (Fig. 2). The dominant microfacies are bafflestones and packstones, but grainstones, bindstones and framestones are not uncommon. Intercolonial spaces are filled mainly with micrite and bioclasts (rugose and tabulate corals, bryozoans, brachiopods, etc.; see Rodriguez et al. 1992). The upper part of the southern sections and almost the whole of the northern sections (Upper Member; Rodriguez et al. 1992, 1994) are composed of biostromal limestones built mainly by corals (bafflestones). Several bioclastic levels (packstones, grainstones), where the Siphonodendron colonies are less developed, occur in the whole area, but more frequently in the northern sections (Rodriguez et al. 1992, 1994). Fragments of corals, brachiopods, bryozoans, bivalves, gastropods, ostracodes, trilobites, echinoderms,
The Siphonodendron Limestone was interpreted by Rodriguez et al. (1994) as the reef-flat of a fringing reef with the open sea towards the south and the land towards the north. The reef was developed on terrigenous sediments that filled the Los Santos Basin during the Middle Vis6an. Carbonate sedimentation occurred only when the water depth was shallow enough to allow the building organisms and carbonate producers to develop at the beginning of the Upper Vis6an (Sfinchez et al. 1988; Rodriguez et al. 1994). The basal, marly levels at the Los Santos Hill and Torre6n sections, which constitute the main reef framework, developed rapidly and arose from the sea bottom to the surface (Fig. 3a, b). The framework began to develop on a very characteristic bed of red algae (Rodriguez & S~nchez-Chico 1995) which provided the corals with a solid substrate on which to grow (Fig. 4). In this part of the reef, coral colonies reach up to 1 m high. When the framework reached the low tide level, vertical growth was substituted by the extensive development of the reef-flat, which is present in most sections (Fig. 3c). Pulses of subsidence in the Los Santos Basin continued, allowing the subsequent development of biostromal beds in the upper part of the sections. After each subsident pulse the growth of the gigantoproductids and the Siphonodendron colonies began again. Periodic storms disturbed the development of the reef so that some beds composed of coral shingle are present in the upper part of the Los Santos Hill, Torre6n and Navafria sections. Fragments are larger and less reworked in the Los Santos Hill section, situated at the south end of the Siphonodendron Limestone outcrops. Northern sections (Guadajira, E1 Almendro and Albuera river) show scarce development of coral colonies, which are replaced by bryozoans and other small colonial organisms.
The basis for recognition as a reef The basis for recognizing the Siphonodendron Limestone as a reef are largely discussed in Rodriguez et al. (1992, 1994), and Rodriguez & Sfinchez-Chico (1995), and will be simply
VISEAN CORAL REEF-FACIES, SW SPAIN
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147 SE
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Los Santos hill
Fig. 2. Spatial distribution of the studied sections of the SiphonodendronLimestone (Unit 1) at Los Santos de Maimona. The base of the unit is placed in the southern sections at a calcareous bed composed of solenoporacean algae. The boundary between the two members (subunits) is placed at the level where the limestones become dominant. Buildup facies of the lower member are replaced by terrigenous facies in the northern sections. summarized here. The arguments can be divided into two groups; those related to structure, and those related to ecology.
Structure The lower marly, coral member constitutes a lenticular sedimentation unit, 3 km long and up to 20 m thick, which influenced the deposition in adjacent areas (Fig. 2) and that of subsequent units. Also, the thickness of the upper part of the unit decreases to the north (20 to 6 m, Fig. 2), and cycles of gigantoproductid and coral growth are also reduced in thickness.
Ecology The marly, coral member at the lower part of the southern sections (Los Santos Hill, Navafria, Torre6n) is almost totally composed of building organisms surrounded by bioclastic mud (Rodriguez & Sfinchez-Chico 1995). Coral colonies usually grow on ramose solenoporoid red algae (Parachaetetesjohnsoni, Pseudochaetetes, Fig. 4). The most abundant facies in the upper part are biostromal limestones. These biostromes are mainly composed of fasciculate corals which
reach the same growth-level in each bed, sometimes developing typical 'microatoll' structures (Fig. 6), described by Rodriguez & S~nchez-Chico (1995). Surrounding sediments are mainly packstones to wackestones, lacking primary sparry calcite. When corals are firmly established the structure is wave-resistant (but not storm-resistant; Fig. 5). The building organisms decrease to the north, where the proportion of accessory building organisms increases. Vagile organisms are also more abundant to the north (or at least most usually entirely preserved). Storm layers occur throughout the basin at the top of the sections (Fig. 5). The sandy bioclastic debris is proportionally more abundant to the north, but the size of the fragments is larger to the south. Subaerial exposure features occur at the top of many sequences in all studied sections and are regarded as having originated during exceptional low tides or temporary sea-level falls.
The objections There are several objections to the theory that the Siphonodendron Limestone is a true reef complex. Some arise from the accepted concept of a reef, others were posed and explained in Rodriguez et
148
S. RODRiGUEZ
Sea level Stabilitation ~ S l o w
subsidence
Sea level
Rapid reef growth to reach file sea level
Slow subsidence
Sea level
(a)
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Incipient reef growth
Fig. 3. Stages of development of the Siphonodendron Limestone. Subsidence is progressivelygreater to the south.
al. (1994); others will be briefly explained here. Further objections were posed by colleagues during the VI Fossil Cnidaria Symposium in Mtinster in 1991, leading the author to revise and study again some aspects of the Siphonodendron Limestone, which are described in Rodriguez & Sfinchez-Chico (1995) and here.
The concept of a reef Detailed discussions on this concept are included in Lowenstam (1950), Heckel (1974), Walker (1974) and Longman (1981) and will not be repeated here. Definitions given by Heckel (1974) include: 'a buildup that displays: (1) evidence of (a) potential wave resistance or (b) growth in turbulent water which implies wave resistance; and (2) evidence of control over the surrounding environment'. The relief, organic buildup and influence on adjacent areas has
already been detailed (Rodriguez et al. 1992, 1994, and here). It only remains to prove that the Siphonodendron Limestone was a waveresistant structure and, in fact, many of the structures present in the upper part of the unit show evidence of this. Some beds at Navafria, Los Santos Hill and El Torre6n sections show clastic material bordering large coral colonies. In addition, some colonies with some parts that are broken away, rest in life position. Nevertheless, some layers are composed of broken colonies mixed with many other bioclasts. They are regarded as storm beds such as those from many present-day reefs (Mather & Bennett, 1984). The lower part of the reef, present in El Torre6n and Los Santos Hill sections, is not required to be a wave-resistant structure because it developed at depth, and the wave-base reached the corals there only sporadically. In any case, the evidence of the development of building organisms growing in turbulent water allow us to infer that the unit was wave-resistant.
VISEAN C O R A L R E E F - F A C I E S , SW SPAIN
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, I
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:i Fig. 4. Stages of buildup development in the base of the lower member at El Torre6n section. The figure shows the top of one cycle (1, growth of Siphonodendron colonies) and another complete cycle (2). (a) Bioclast (corals, brachiopods) accumulation. (b) Solenoporaceae buildup. (c) Bioclast (corals, bryozoans, gastropods, ostracodes) accumulation and gigantoproductid growth. (d) Buildup of rugose (Siphonodendron) and tabulate (Syringopora, Pleurosiphonella) corals. Note the dense packing of corallites. No vertical exaggeration.
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Fig. 5. Stages of buildup development of the upper biostromal beds (1) and storm beds (2, 3) at the top of Los Santos Hill section. Three cycles of unequal development are included in the picture (1 3). The general pattern comprises a bioclastic accumulation level (a), a gigantoproductid-growth level (b), a coral (Siphonodendron, Syringopora, Pleurosiphonella) buildup (e) and a reworking phase (d). Cycle 1 shows the normal sequence. The reworking phase is so strong in Cycle 2 that coral colonies were moved and broken (but not brachiopods, which consequently should be cemented to substratum). Coral colonies are almost intact (but overturned), and brachiopods are completely broken in cycle 3; consequently, the reworking phase was probably very strong but short-lived. Red crusts are present over and below this bed, showing temporary subaerial exposure. No vertical exaggeration.
150
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Fig. 6. Stages of buildup development in the upper biostromal beds, at E1 Torre6n section. Cycle 1 shows a similar structure to the cycle 2 of Fig. 5. Cycle 2 shows a great horizontal development of the coral colonies (c). The presence of red crusts at the upper surface suggest a temporary subaerial exposure that forced the corals to grow in that way (microatoll growth). The vertical growth later continued (c~), when the relative sea-level rose. No vertical exaggeration.
The reef crest and talus The reef crest and talus are not preserved. In fact, there is no outcrop of the Siphonodendron Limestone southwards from Los Santos Hill, which should be the nearest locality. One possible reason is that the reef crest was totally removed by recent erosion. Another possibility is that a true reef crest never existed, and the reef flat finished with a fore-reef talus. The relief of the bioconstructed beds is small, and such talus should also be small. This possibility agrees with the model described by Cloud (1952) for fringing reefs with slow subsidence. The absence of massive framebuilders may be justified by a moderate level of energy (with the exception of occasional storm episodes). Two facts support this hypothesis: no massive colonies were found in the biostromal layers, but also not in the storm layers, where fragments of the reef crest should be found. Abundant coral debris, possibly belonging to the suggested fore-reef talus, was found in the southern slope of Los Santos Hill. However, this hypothesis is not confirmed because both pieces of evidence are circumstantial.
Bioherms and biostromes Usually, only bioherms are regarded as true reefs. Nevertheless, the upper beds of the
Siphonodendron Limestone are typically biostromal. These beds represent the reef-fiat developed on the buildup and built by corals, algae and gigantoproductid brachiopods. In a very broad sense the Upper Member is also a buildup (6 m thick at Albuera and El Almendro sections, 20 m thick at E1 Torre6n section).
Low diversity One of the most prominent characteristics of coral reefs is a very high diversity of biota (Heckel 1974). In contrast, some cases of bioherms composed of only one or two species have been described (e.g. Rich 1969; Sutherland 1984; M6ndez-Bedia et al. 1994). The diversity of the main builders in the studied example at Los Santos de Maimona is low; only two species of rugose corals (Siphonodendron martini and Siphonodendron irregulate), two tabulate genera (Syringopora and Pleurosiphonella) and one gigantoproductid species (Gigantoproductus aft. semiglobosus) are found. Some red algae (Parachaetetes johnsoni and Pseudochaetetes) play an important role at the beginning of the buildup. Such diversity increases if we take into account secondary builders and associated fauna and flora. At least four species of brachiopods
VISEAN CORAL REEF-FACIES, SW SPAIN (Martinez-Chacdn & Legrand-Blain 1992), 16 foraminiferans (Comas-Rengifo et al. 1992), 12 rugose corals and three tabulate corals (Rodriguez & Falces 1992, 1994), and 13 of calcareous algae (Sfinchez-Chico et al. 1995) have been described. Dental plates and scales of fishes were also described (Soler-Gij6n & Rodriguez 1991). In addition, many species of bryozoans, bivalves, ostracodes, trilobites, echinoderms, and so on, are present in the bioclastic beds. The number of species increases if we consider the components without preservation potential. S t o r m beds
The main objection posed to recognizing the Siphonodendron Limestone as a reef is the presence of storm beds, which are common in the Upper Member. It is claimed that if the framework is wave-resistant, storms should affect little or none of the structure of the reef. However, many actual reefs show periodic disturbance of growth because of storms. Many of them actually show coral shingle beds, and even shingle cays (Mather & Bennett 1984). Periodic storm disturbance does not destroy the reef, but provides a high quantity of material for the reef-flat and back-reef areas.
Conclusions The Siphonodendron Limestone shows many characteristics of a reef (following definitions by Heckel 1974 and Longman 1981) built mainly by corals. It is an exception to the usually accepted idea that rugose corals did not build reefs during the Carboniferous. Some objections remain unanswered, or the explanations are unconvincing (absence of reef crest, low diversity of main builders), but arguments in favour of recognizing it as a reef largely overcome these objections. This study is included in the research project PB910083, supported by the Spanish DGICYT. I am grateful to W. J. Sando, I. D. Somerville, E. Poty and J. R. Nudds for their useful comments on the manuscript.
References ADAMS, A. E. 1984. Development of algal-foraminiferal-coral reefs in the Lower Carboniferous of Furness, northwest England. Lethaia, 17, 233-249. CLOUD, P. E. JR. 1952. Facies relationships of organic reefs. Bulletin of the American Association of Petroleum Geologists, 36, 2125-2149.
151
COMAS-RENGIFO, M. J., RODRiGUEZ, S. & SANCHEZ, J. L. 1992. Foraminiferos. In: RODRiGUEZ, S. (ed.) And~isis Paleontoldgico y Sedimentoldgico de/a cuenca Carbonlfera de Los Santos de Maimona (Badajoz). Coloquios de Paleontologia, 44, 145-157. COPPER, P. 1988. Ecological succession in phanerozoic reef ecosystems: is it real? Pa/aios, 3, 136-152. 1994. Reefs under stress: the fossil record. Courier Forschungsinstitut Senckenberg, 172, 87-94. FALCES, S. 1991. Cartografia y Paleontologla de la Cuenca Carbonifera de Los Santos de Maimona." Corales Solitarios de la Fauna de Cyathaxonia. PhD Thesis, Complutense University of Madrid. HECKEL, P. H. 1974. Carbonate buildups in the geological record: a review. In: LAPORTE, L. F. (ed.) Reefs in Time and Space. Society of Economic Paleontologists and Mineralogists Special Publication, 18, 90-154 JAMES, N. P. 1979. Reefs. In: WALKER, R. G. (ed.) Facies Models. Geoscience Canada Reprint Series, 1, 121-132. 1983. Reef environment. In: SCHOLLE, P. A., BEBOUT, D. G. & MOORE, C. H. (eds.) Carbonate Depositional Environments. Association of American Petroleum Geologists Memoir, 33, 345-440. LONGMAN, M. W. 1981. A process approach to recognizing facies of reef complexes. In: TOOMEY, D. F. (ed.) European Fossil Reef Models. Society of Economic Paleontologists and Mineralogists Special Publication, 30, 9 40. LOWENSTAM, H. A. 1950. Niagaran reefs of the Great Lakes Area. Journal of Geology, 58, 430-487. MARTiNEZ-CHACON, M. L. & LEGRAND-BLAIN, M. 1992. Braquibpodos. In: RODRiGUEZ, S. (ed.) Andlisis Paleontoldgico y Sedimentoldgico de la cuenca Carbonifera de Los Santos de Maimona (Badajoz). Coloquios de Paleontologia, 44, 91-144. MATHER, P. & BENNETT, I. 1984. A Coral Reef Handbook. A Guide to the Fauna, Flora and Geology of Heron Island and Adjacent Reefs and Cays. The Australian Coral Reef Society, Brisbane. MI~NDEZ-BEDIA, I., SOTO, F. & FERN~,NDEZ-MARTiNEZ, E. 1994. Devonian reef types in the Cantabrian Mountains (NW Spain) and their faunal composition. Courier Forschungsinstitut Senckenberg, 172, 161-184. ODRIOZOLA, J. M., PEON, A., VARGAS, I., GARROTE, A. & ARRIOLA, A. 1983. Mapa Geoldgico de Espaffa a escala 1.'50.000. Hoja 854; Zafra. Instituto Geoloegico y Minero de Espafia (2nd edn), Madrid. OTA, M. 1968. The Akiyoshi Limestone Group: a geosynclinal organic reef complex. Bulletin Akiyoshi-dai Science Museum, 5, 1-44. PAREYN, C. 1959. Les rdcifs Carbonifdres du Grand Erg occidental. Bulletin de la Soci~t~ G~ologique de France, 711, 347-364. RICH, M. 1969. Petrographic analysis of Atokan carbonate rocks in central and southern Great Basin. American Association of Petroleum Geologists Bulletin, 53, 340-366.
152
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RODRiGUEZ, S. & FAECES, S. 1992. Corales Rugosos. In: RODRiGUEZ, S. (ed.) Andlisis Paleontol6gico y Sedimentol6gico de la Cuenca Carbonifera de Los Santos de Maimona (Badajoz). Coloquios de Paleontologia, 44, 159-218 & 1994. Coral distribution patterns in the Los Santos de Maimona Lower Carboniferous Basin (Badajoz, SW Spain). Courier Forschungsinstitut Senckenberg, 172, 193-202. -& SJ~NCHEZ-CHICO, F. 1995. Corales rugosos y algas calc~treas de la secci6n de E1 Torre6n (Viseense, Badajoz). Coloquios de Paleontologia, 46, in press. , FAECES, S., ARRIBAS, M. E., DE LA PElqA, ~I. m., COMAS-RENGIFO, M. J. & MORENO-EIRIS, E., 1992. Descripci6n litoestratigrfifica y aspectos sedimentol6gicos de las unidades. In: RODR|GUEZ, S., (ed.) Andlisis Paleontol6gico y Sedimentol6gico de la Cuenca Carbonifera de Los Santos de Maimona (Badajoz). Coloquios de Paleontologia, 44, 49-90. , ARRIBAS, M. E., FAECES, S., MORENO-EIRIS, E. & DE LA PENA, J. 1994. The Siphonodendron Limestone of Los Santos de Maimona Basin: development of an extensive reef-flat during the Vis6an in Ossa Morena, SW Spain. Courier Forschungsinstitut Senckenberg, 172, 203-214 S~,NCHEZ, J. L., COMAS-RENGIFO, M. J. & RODRIGUEZ, S. 1988. Estudio estratigr/lfico de los materiales carbonatados del Carbonifero de Los Santos de Maimona (Badajoz, SO de Espafia). Comunicaciones del segundo Congreso de Geologla de Espa~a, 1, 197-200. & 1991. Foraminiferos del Carbonifero inferior de Los Santos de Maimona (Badajoz, SO de Espafia). Boletin de la Real Sociedad Espa~ola de Historia Natural (secci6n de Geologia), 86, 101-147. S~,NCHEZ-CHICO, F., MAMET, B., MORENO-EIRIS, E. & RODRiGUEZ, S. 1995. Algas calcfireas del Viseense de Los Santos de Maimona (Badajoz, SO de Espafia). Revista Espa~ola de Micropaleontologia, in press. -
-
SCHENK, P. E. & HATT, B. L. 1984. Depositional environment of the Gays River reef, Nova Scotia, Canada. In: GELDSETZER, H. H. J. (ed.) Atlantic Coast Basins. Compte Rendu, Ninth International Congress on Carboniferous Stratigraphy and Geology, 3, 117-130. SOLER-GIJON, R. & RODRiGUEZ, S. 1991. Estudio de un resto de Bradiodonto (clase Chondrictyes) del Viseense de Los Santos de Maimona (Badajoz, SO de Espafia). Coloquios de Paleontologia, 43, 101-114. SUGIYAMA, T. & NAGAI, K. 1994. Reef facies and paleoecology of reef-building corals in the lower part of the Akiyoshi Limestone Group (Carboniferous), Southwest Japan. Courier Forschungsinstitut Senckenberg, 172, 231-240. SUTHERLAND, P. K. 1984. Chaetetes reefs of exceptional size in Marble Falls Limestone (Pennsylvanian), central Texas, Paleontographica Americana, 54, 543-547. - - & HENRY, T. W. 1977. Carbonate platform facies and new stratigraphic nomenclature of the Morrowan series (Lower and Middle Pennsylvanian), northeastern Oklahoma, Geological Society of America Bulletin, 88, 425-440. WALKER, K. R. 1974. Reefs through time: a synoptic review. In: Principles of Benthic Community Analysis; Notes for a Short Course: Sedimenta IV. Comparative Sedimentology Laboratory, University of Miami, 8, 1 20. WEBB, G. E. 1989. Late Vis6an coral-algal bioherms from the Lion Creek Formation of Queensland, Australia. Compte Rendu 11th Congr~s International Congress de Stratigraphie et de Geologie du Carboniferd, 3, 282-295. WEST, R. R. 1988. Temporal changes in Carboniferous reef mound communities. Pataios, 3, 152-169.
Evidence for catastrophism at the Famennian-Dinantian boundary in the Iberian Pyrite Belt CARMEN
MORENO,
SONIA SIERRA & REINALDO
SAEZ
Departamento de Geologia, Universidad de Huelva, 21819 Palos de la Frontera, Huelva, Spain Abstract: Terrigenous shelf sedimentation during the Devonian created a homogeneous basin in the Iberian Pyrite Belt. This shallow south-Iberic basin changed into a mosaic of horsts and graben at the Famennian-Dinantian boundary and subsequent evolutionary history can be explained in terms of locally increased rates of subsidence. Deposits commonly related to highly energetic processes characterize this change (fan-deltas, sediment gravity flows, rapid basin-shallowing). These resulted from convulsive/ catastrophic events related to the Bretonic phase of the Hercynian Orogeny. Marker beds due to convulsive/catastrophic geological events are a valuable correlation tool and a precise key to the analysis of many sedimentary basins. These events can be preserved in the geological record in a variety of ways, generally characterized, by evidence of high transport energy for short periods of time. Identification of 'event beds' is not always easy, but analysis of facies and particularly their accurate interpretation in relation to background sedimentary processes is usually a successful means of identification. Some recent papers dealing with sediment gravity flow deposits, fan-deltas and other coarse-grained deltas are examples of this type of interpretation (Ethridge 1985; Galloway 1985; Kleverlaan 1987; Pilkey 1988; Leigh & Hartley 1992; Mastalerz & Wojewoda 1993). Until recently, the world-famous massive sulphide deposits of our study area, the Iberian Pyrite Belt (IPB) in SW Spain, have absorbed most of the geological research, neglecting other geological features such as those described in this study. The aim of this paper is to show that the sedimentary features of deposits at the Famennian-Dinantian boundary in the IPB are produced by episodic/catastrophic events related to the Bretonic phase of the Hercynian Orogeny, which controlled basin compartmentalization. Knowledge of the morphology of the Dinantian basin of the IPB, which was inherited from older structures, could in turn be a key to understanding the genesis and distribution of Dinantian massive sulphides of the IPB.
Riotinto, Neves-Corvo, Aljustrel, Tharsis and Aznalcollar (Fig. 1). The IPB occupies the southwestern corner of the Iberian Peninsula, extending from Seville, in Spain, to the Atlantic Ocean south of Lisbon, Portugal, in an arcuate belt about 230 km long and 40 km wide. The IPB is a part of the South Portuguese Zone (Julivert et al. 1974) which has been interpreted as a tectonostratigraphic terrane sutured to the Iberian Massif during the middle Carboniferous (Quesada 1991). The sedimentary record of the IPB consists of Devonian and Carboniferous rocks whose conspicuous features include intense Dinantian magmatic activity and the abundance of huge massive sulphide deposits. The accepted stratigraphic sequence for the IPB was established by Schermerhorn (1971) and comprises three main units: the Phyllite-Quartzite (PQ) Group, the Volcanic-Siliceous Complex (VSC) and the Culm Group (Fig. 1).
The Phyllite-Quartzite (PQ) Group This comprises a detrital sequence of shales and sandstones. Limestone lenses near the top of the succession have provided conodonts and other fossils of middle to late Famennian age (Van den Boogaard & Schermerhorn 1975). The extent at depth of the group remains unknown, although estimates of thickness indicate several thousand metres. Sedimentary facies of'the PQ represent marine deposition on a shallow platform.
The Volcanic-Siliceous Complex (VSC) Geological framework The IPB is one of the oldest mining districts in the world. It is characterized by giant and supergiant massive sulphide deposits, including
This complex contains the economically important massive sulphide and manganese deposits. The VSC is late Famennian to Vis~an in age, and consists of a heterogeneous group of rocks
From STROGEN, P., SOMERVILLE,I. D. & JONES, G. LL. (eds), 1996, Recent Advances in Lower Carboniferous Geology, Geological Society Special Publication No. 107, pp. 153-162.
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The Culm Group
Named the Baixo Alentejo Flysch Group in Portugal (Oliveira et al. 1979), the Culm Group is a thick Upper Carboniferous succession of shales, litharenites and rare conglomerates which overlies the VSC in the IPB. The estimated thickness for this group exceeds several thousands of metres. The Culm Group represents the in fill of a rapidly subsiding basin, mostly by
turbidite sediments coming from at least two source areas: the IPB proper, and the Ossa Morena Zone (Moreno 1993).
Structure and metamorphism
Rocks of the IPB were deformed and regionally metamorphosed in the Asturian phase of the Hercynian Orogeny, from late Vis6an to Westphalian-D times. Three stages of deformation have been recognized in the IPB. The first, D1, generated regional structures and was accompanied by lowgrade regional metamorphism, whereas D2 and D3 only modified the D1 structures. Both deformation and metamorphism seem to increase in intensity from southwest to northeast (Ribeiro & Silva 1983; Munhfi 1990). However, the apparent increase in metamorphic grade could also be related to differential erosion levels from NE to SW. The structure of the IPB has been interpreted in terms of a thin-skinned foreland thrust-andfold belt (Silva et al. 1990; Quesada 1991). Deformation (D1) generated asymmetric folds verging to the SW, which often show transposed bedding on their short limbs, mimicking the structural features of a thrust-belt. Folding was accompanied by development of a penetrative foliation which shows sinistral transection of the
FAMENNIAN-DINANTIAN CATASTROPHISM
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axial planes. This transection is related to a wrench component of non-coaxial strain during folding (Silva 1983). The largest Devonian outcrops in the IPB lie in the cores of two major antiforms, the Puebla de Guzmfin and Valverde del Camino anticlines. Our study area comprises a part of both structures (Figs 1, 2) and therefore may be considered to be representative of the whole IPB. Two of the main mining districts of the region (Sotiel-Coronada and Tharsis) also lie in the study area.
Sedimentary facies and environments
The Devonian PQ Group is the lowest deposit of regional extent in the IPB. Its sedimentary and petrographical characteristics are similar along the whole IPB, both in Spain and Portugal. It comprises a monotonous detrital sequence of shales and sandstones with a slightly variable lutite/arenite ratio, always >1 except in the upper part of the Group. Thicknesses of single sandstone beds range from 5 to 40 cm. Sedimentary structures such as grading, parallel lamination, planar cross-bedding and bimodal crossbedding are common. Bioturbation is also commonly observed; Skolithos, Nereites and Lophoctenium trails have been identified (Van
den Boogaard 1967). Petrographically the sandstones are quartzarenites and quartzwackes according to the classification of Pettijohn et al. (1972). The total thickness of the group is not known. Rocks of PQ Group were deposited on a shallow marine platform, probably storm-dominated, but a detailed sedimentological study of the complete PQ Group is still to be made. The depositional style near the top of the PQ Group displays a major break from the sedimentary homogeneity described above. Lutite/ arenite ratios show a rapid decrease, and the former Devonian shaly series (with sandy intercalations) changes into a mainly sandy sequence. Moreover, patches of exotic sediments appear among these sandstones, so that the top of the PQ Group exhibits a mosaic of apparently unrelated facies. Four facies associations typify this upper part of the PQ Group: (1) clastic shallow-marine facies association; (2) delta facies association; (3) facies of sediment gravity flow deposits; (4) calcareous shallow-marine facies association. All four facies associations lie at the same stratigraphic level and represent the Famennian Dinantian boundary in the IPB, although in the study area (Fig. 2) calcareous shallowmarine facies are poorly represented. Small, sparsely distributed, calcareous lenses do occur, but the poor outcrop makes them unsuitable for detailed study.
156
C. MORENO ET AL.
Clastic shallow-marine facies Sedimentary rocks of the clastic shallow-marine facies form tabular bodies of sandstones varying in thickness from 3 to 100 m, which commonly form topographic highs. These tabular bodies are built up by interlocking stacked bedforms that vary in size from 1-10 m in length and 0.11 m in height. No internal structures have been observed, but these bedforms are interpreted as megaripples and sandwaves. Overlying these are tabular beds of quartzite ( O m 400
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punctuated by conglomerate deposition, which represents tectonic pulses in the continental source. The clast-supported conglomerates represent the infilling of rapidly prograding channels cutting across the sand bars. Mud-supported conglomerates suggest sporadic deposit of mudflows (probably of flash-flood type). The delta facies association represents a smallsize delta complex with an apical feeder system (type A of Postma 1990). This kind of system is
characterized by radial spreading of sediments and 'commonly by ephemeral, unconfined streams involving mass-flows, [these] are relatively small in radius and form along a basinmargin fault-scarp' (Postma 1990). We suggest that the preserved remains of the whole system represents the submarine portion of the fandelta with a shoal-water profile. The portion corresponding to the 'fan' is absent, and has been either eroded or tectonically displaced, so
FAMENNIAN-DINANTIAN CATASTROPHISM that its behaviour was probably similar to the Mediterranean fan-deltas described by Bardaji et al. (1990).
Facies of gravity flow deposits Rocks of this facies crop out throughout the study area, although the best exposures are near Sotiel village and mine (Fig. 2). Extensive outcrops are more than 3km long and 50m thick. They are formed of large-scale matrixsupported conglomerates different from those found in the delta sediments. These conglomerates are poorly-sorted, with angular monogenic clasts of quartzarenite ranging in size from granules to boulders (0.5-18cm across). Clasts show internal sedimentary structures such as parallel and cross-lamination, and wave ripples; texturally and mineralogically they are similar to quartzites from the shallow-marine facies associations. These clasts are isolated in a shaly matrix which comprises 85-90% of the whole rock. In some cases, bodies of matrix-supported conglomerates are over 50 m thick. At first sight they appear to have no recognizable internal stratigraphy, but the vertical distribution of clasts (Fig. 4) seems to show a crude stratification. Individual layers reach a maximum of 3 m in thickness. Also present, enclosed in a shaly matrix, are large isolated boulders of quartzarenite up to 4m in diameter, and slumped layers of quartzarenite sequences similar to the sandy shallow bar facies described above. This facies is interpreted as the result of sediment gravity-flow deposition, i.e. by dense and highly viscous flows exhibiting plastic behaviour. These are cohesive debris flows (Middleton & Hampton 1976; Lowe 1982; Stow 1986; Pierson & Costa 1987) coming from intrabasin highs and deposited by flow-freezing mechanisms (Fraser & Suttner 1986). Both the physical characteristics of these flows and the thicknesses of the deposits point to the low spreading capability and high competence of the mass flows (Nemec & Steel 1984; Porebski 1984; Nemec 1990), involving a high angle of slope to begin and maintain movement (Campbell 1989; Bjorlykke 1989). These mega-debris flow deposits are thicker than most found in other ancient successions (Leigh & Hartley 1992). The lithological and geometric characteristics of the clasts (granules to boulders, and even slumped beds) suggest an intrabasinal sediment source from positive topographic features on the basin floor. The thickness
159
of mass-flow units indicates that huge volumes of debris were carried down in each single failure event. The failure surfaces in the marine platform had to be deep enough to involve sediments that were at least partly lithified, suggesting a major role for seismic activity in the genesis of these sediment gravity-flow deposits.
Palaeogeographical inferences Terrigenous shelf sedimentation during the Devonian created the homogeneous PQ Group of the IPB. However, this style of sedimentation was interrupted near the Devonian-Carboniferous boundary by highly energetic sedimentary processes, resulting in rapid lateral changes in both facies and thicknesses. These were preserved in the local geological record as facies and facies associations caused by catastrophic events related to large-scale slope failures. This modification of sedimentary style at the top of the Devonian sequence indicates that the broad, continuous and gently-sloping PQ basin changed into a set of sub-basins with different subsidence rates, and intervening horsts. Such a transformation could occur by rifting owing to either a continental extensional tectonic episode or within a strikeslip regime. Either of these would represent the first manifestation of the Hercynian orogenic cycle in the IPB. Several recent papers (Oliveira 1990; Quesada 1991) concerned with the tectonic evolution of the Hercynian Chain suggest that Hercynian movements began in the Lower Carboniferous. However, previous works by L6colle (1977) and Routhier et al. (1980) highlighted evidence for the occurrence of the Hercynian Bretonic phase (Upper Devonian) in the pre-Dinantian palaeogeography of the IPB, with the formation of regional highs and lows as a consequence of 'long radius epirogenic movements'. Our evidence indicates that the lack of uniformity found in the sediments at the Devonian-Carboniferous boundary in the IPB is evidence of the beginning of the Hercynian Orogeny. As long as the higher zones on the already fractured IPB basin remained linked to the emergent continent, shallow-water sedimentation continued. Locally, this littoral sedimentation was interrupted by episodic, catastrophic influx of sediment and the formation of fandeltas. The deepest areas acted as traps where the accumulation of mass flows derived from the shallow-water horsts occurred. These facies and facies associations are characteristic of
C. M O R E N O ET AL.
160
,',
~
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' '-~::;
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, '..\- ....
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-2m) above which conglomerates become a minor constituent of the succession (Fig. 6). This formation is dominated by coarse sandstones but also contains significant quantities of mudrocks as both relatively tabular bedsets and locally lining erosional hollows. The mudrocks are red, green or grey in colour and sometimes variegated. Most show either desiccation cracks or pedogenic carbonate nodules, thus indicating exposure, often prolonged, of the sediment surface. The sandstones are characterized by numerous erosional surfaces (Figs 7, 8), and internal structures are a mixture of cross-strata and flat or low-angle lamination. Plant debris is common, and Beaconites trace fossils are found on several sandstone surfaces. Only one clear example of inclined heterolithic stratification (IHS; Thomas et al. 1987) has been noted at Shalwy (G646748; Fig. 9). This is interpreted as
N
N
n = 25
n -28
ROELOUGH CONGLOMERATE FORMATION N
n = 19 j
~
~
, 0
i
LARGYSILLAGH SAN D S T O N E FORMATION
. . . . 10
N
SHALWYFORMATION
I,
=
39
SHALWY POINT
CASTLE PORT Fig. 5. Palaeocurrent data from the basal clastics of southwest Donegal. All data are from cross-strata. Arrows indicate vector means.
D I N A N T I A N R I V E R SYSTEMS, N W I R E L A N D
90
-.~
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Fig. 6. (a) Summary stratigraphical log of the main coastal section at Shalwy (see Fig. 2 for location). Scale in metres. RCF, Roelough Conglomerate Formation; LSF, Largysillagh Sandstone Formation; SF, Shalwy Formation; RPF, Rinn Point Formation. (b) Key for (a) and also for Figs 12, 13, 17, 19, 24 and 25.
188
J. R. GRAHAM
hale eptari, an oncretions Mudstone arbonate glaebules Thinly bedded mudrock •& dandstone Inclined Heterolithic Stratification (IHS) J Ripples / Pari~llel lamination / " ~ Cross-bedding e'eollntraclasts
~_......-'~ ~,
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~(
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9
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, --I - '- - IiI Micritic limestone
r~
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,~
Current direction
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~
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--
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limestone
'. i Bioclastic limestone
~_, Erosion suriace
Oolitic limestone
Fig. 6. Continued. the deposit of a point bar. However, the exposure on the tidal platform is limited and does not allow accurate estimation of point-bar width. The preserved bar thickness is 2 m and a width of 15m taken from oblique section is only a crude estimate of the latter. Some constraints on the scale of the river channels are also given by the shape and dimensions of the curved erosion surfaces confining the channel fills. These show sandstone-fill thicknesses of 1-3 m, (occasionally up to 4m), and widths of a few metres up to a few tens of
!
,"~+
"
--
'
metres. Palaeocurrents in the sandstones are again northerly (Fig. 5) although showing more variability than those from the Roelough Conglomerate Formation.
Shalwy Formation (Upper Shalwy Beds). The base of the Shalwy Formation is taken at the incoming of intensely bioturbated mudrocks and sandstones, typically with fossils indicating marine or brackish conditions. The basal contact is a relatively abrupt but conformable passage (Fig. 6 at 97m). The lowest beds
.... "+~i
~
Fig. 7. Sandstones of the Largysillagh Sandstone Formation at Shalwy (G645748) showing complex intersection of erosion surfaces. The mudrock unit right of centre is 2.2 m thick.
DINANTIAN RIVER SYSTEMS, NW IRELAND
189
Fig. 8. Cut bank of small channel to right of person (circled) at Shalwy (G644748).
typically are plant-rich black mudrocks with a gastropod/ostracode fauna and micrites containing a similar fauna. There are some coarsegrained sandstones lining irregular scours and appearing similar to the sandstones in the underlying Largysillagh Sandstone Formation. However, many sandstones have a more tabular geometry and are extensively bioturbated. These sandstones are characterized by ripple-scale bedforms and flat lamination. Some ripples indicate oscillatory currents (using the criteria of De Raaf et al. 1977) and bipolar currents are also common (Figs 5, 6). Several beds of
sandstone are markedly calcareous and contain considerable amounts of broken shell debris, some grading into sandy limestones. In addition to the lithologies above, a most distinctive feature of this formation is the presence of IHS (Figs 6, 9, 10, 11). Some examples of this indicate palaeocurrents, from ripples in the finer-grained strata to small dunes in the sandier varieties, that are strongly oblique to perpendicular to the dip of the IHS. Some of these are bipolar. The dips of the IHS are low, from a few degrees to a maximum of 20 ° . Marine shell debris is common in these units as,
Fig. 9. Inclined heterolithic strata beneath erosive base of coarser grained sandstones at Shalwy (G646748). Cross-beds at lower left (arrowed) indicate currents strongly oblique to the dip of the heterolithic strata. Rucksack (circled) for scale.
190
J. R. GRAHAM
Fig. 10. Sandstone-dominant inclined heterolithic strata (135-138 m on Fig. 6) at Shalwy (G647746). This is also unit SB1 of Nichols & Jones (1992). Person to right for scale.
locally, is fusain, the latter being interpreted as fossil charcoal (Nichols & Jones 1992). The upper part of this formation passes gradationally upwards into alternating beds of bioclastic limestone and calcareous shale known as the Rinn Point Limestone Formation (Rinn Point Beds) (Fig. 6 at 160m). This gradational passage is characterized by the common presence of quartz granules and pebbles and locally by fine-
grained sandstones showing hummocky crossstratification (HCS). The base of the Rinn Point Formation is taken where tabular beds of limestone and shale first become dominant. This coincides with the disappearance of sandstones. This boundary can be seen on the eastern side of Rinn Point (G672755; Fig. 2), near Shalwy Point (G649745), and at Castle Port, south of Dunkineely (G754741; Fig. 1).
Fig. 11. Muddy inclined heterolithic strata rich in fusain at G648745 (Fusain Unit of Nichols & Jones 1992; 141143m on Figt)6). Person in upper part of unit for scale.
DINANTIAN RIVER SYSTEMS, NW IRELAND Some specific bedsets rich in fusain and the sedimentology of adjacent parts of the sequence were investigated by Nichols & Jones (1992) following earlier palaeobotanical work by Scott & Collinson (1978). They interpreted the gently inclined cross-strata that form these bedsets as representing the lower parts of large sandwave slipfaces. By comparison with some laboratory data on the proportion of bedforms preserved due to random topographic variation (Paola & Borgman 1991), and with dimensions of modern sandwaves, they imply large bedforms, several metres and perhaps up to 15m high, and thus significant water depths. However, they also suggested an inshore estuarine setting in which such water depths are spatially very restricted. The interpretation of these bedsets as sandwave deposits is rejected for the following reasons. (a) In many cases, including some considered by Nichols & Jones (1992), the inclined bedsets (cross-strata) consist of alternations of sand and mud, and some are mud- dominant. This is not consistent with an interpretation as sandwaves. Models of sandwaves based on theoretical considerations allied to field examples by Allen (1980) apply only to cohesionless sediments. (b) The angles of inclination reach a maximum of 20 ° but are typically much less and commonly less than 10° (Figs 10, 11). The area is one of negligible tectonic deformation and thus these represent true values. These are inappropriate for the avalanche faces of sandwaves. Where such low values are predicted for sandwaves by Allen (1980), they are accompanied by downslope migrating dune bedforms; mud does not survive except as intraclasts, and Allen noted that such structures had not been fully described either from modern or ancient sediments. (c) In cases where smaller-scale cross-strata are found within the inclined beds they indicate a flow direction strongly oblique to the inclination of the heterolithic bedsets. This was also noted by Nichols & Jones (1992) who described inclined bedsets dipping NW and smaller-scale bipolar beds indicating E-W tidal currents. (d) The bedsets are found in close proximity to indicators of very shallow water or even exposure (e.g. rootlets at 133.8 m in Fig. 6). (e) The bedsets conform to the description of inclined heterolithic stratification (IHS; Thomas et al. 1987), as noted by Nichols & Jones (1992), for which an origin by lateral migration of point bars was proposed. The true profile view of IHS examples described by Thomas et al. (1987) varies from
191
sigmoidal to straight or slightly concave upwards. They suggested that strongly concave-up IHS sets are typically deposited as abandoned channel-filling sequences. All of these profiles are seen within the Shalwy Formation. Thus an interpretation as tidal point bar deposits for the IHS in the Shalwy Formation seems entirely consistent with the gentle dips, heterolithic character, oblique (in places bipolar) palaeocurrents and marine biota, and is the interpretation favoured here. In a novel approach, Nichols & Jones (1992) also attempted to estimate the size of the drainage basin supplying sediment to this coastal zone by estimating the volume of fusain (fossil charcoal) present, from this the biomass needed to produce it, and hence the areal extent of the drainage basin. They concluded that a drainage basin of >95 000 km 2, a little greater than the present land area of Ireland, supplied a 'Shalwy River System' which reached the sea via an estuarine area in south Donegal. Their calculations of the amount of fusain rely on some conservative estimates of the amount of fusain within their 'Fusain Unit' and also on defining a minimum areal extent of the Fusain Unit. The latter relies on correlating fusain-rich beds from three different coastal sections at Shalwy Point, Rinn Point and Muckros Head (Fig. 2). Of the three exposures described by Nichols & Jones (1992), the fusain unit at Shalwy Point is the thickest and richest in fusain. The exposure at Rinn Point is noted to contain much less fusain and its sedimentological character is also slightly different (cf. Nichols & Jones 1992, figs 6b, 9a). Correlation relies primarily on similar stratigraphic position measuring down from the base of the overlying Rinn Point Formation. Unfortunately, the section at Rinn Point contains a fault across which detailed correlation is not possible. The 'distinctive' black mud with septarian nodules used to correlate the beds at Shalwy Point with those at Muckros Head is not present above the IHS bedset designated as the Fusain Unit at Rinn Point. The section at Muckros Head was recognized by Nichols & Jones (1992) as being in a small fault-bounded block, and correlation was based on the fusain unit having a similar overlying bed to that seen above the fusain unit at Shalwy Point. In fact it is possible to correlate the strata at Shalwy Point with this small fault-bounded block, but not in the way suggested by Nichols & Jones (1992). The lower 6.5 m of this block exposed on the tidal rocks belongs to the upper part of the
192
J. R. GRAHAM
Largysillagh Sandstone Formation and contains blocky desiccated mudrocks, pedogenic carbonate glaebules and Beaconites trace fossils, but lack extensive bioturbation (Fig. 12). The basal beds of the Shalwy Formation are similar to those at Shalwy Point but here are succeeded by an IHS unit containing appreciable amounts of fusain and the first signs of a marine fauna. It is this bedset that Nichols & Jones (1992) have correlated with the fusain unit at Shalwy Point, which, it is suggested here, lies some 40 m higher in the succession (Fig. 6 at 141 m). Thus it appears that the bedsets containing the fusain do not all represent the same stratigraphic level. In this case the area over which the high concentration of fusain exists is only that of each individual 'fusain unit'. These can be seen to be lenticular and of limited extent and are interpreted above as representing lateral accretion of tidal point bars. The gently curved, locally sigmoidal shape of the IHS suggests bar heights of up to 4m. The lateral extent of
top of cliff 1^
SHALWY FORMATION ee)
Gc~
individual beds, interpreted as former bar surfaces, is commonly 15-25 m, suggesting bankfull channel widths of 22-38m (Allen 1965; Ethridge & Schumm 1978). Whilst it could be argued that these tidally influenced channels may not be the largest channels in the system, there is no indication either in the coastal sediments (Shalwy Formation) or the fluvial sediments (Roelough Conglomerate & Largysillagh Sandstone Formations) of any larger channels. The general increase in mud and carbonate content at the expense of sand suggests that much of the siliciclastic material was being trapped in the fluvial system. This is to be expected at times of rising base level. The petrography of the sandstones is both compositionally (20% or more feldspar) and texturally immature as noted by Nichols & Jones (1992), who implied that they were first-cycle sediments from a Dalradian source. This petrography is consistent with a relatively local source but, as noted above, the immediately subjacent schists, which extend for 10 km to the north, are scarcely represented. This, together with the subrounded quartzite clasts in the Roelough Conglomerate Formation, suggests that there was little local erosion at the time of deposition of the Roelough Conglomerate Formation, perhaps due to a generally rising base level promoting aggradation within the fluvial environments. Extensive outcrop of psammites and quartzites presently commences some 10 km to the north, with granitic rocks also extensively exposed north of this. This would be a likely source area.
North Mayo Succession
LARGYSILLAGH SANDSTONE FORMATION -V--
mf m c
Fig. 12. Stratigraphic log of a small fault-bounded section on the western side of Muckros Head (G622742 on Fig. 2 ). The Fusain Unit of Nichols & Jones is at 8 m. See Fig. 6 for legend; scale in metres.
A very similar clastic sequence at the base of the Carboniferous succession is exposed in coastal sections in North Mayo (Figs 1, 3, 4). This also displays a lower coarse-grained fluvial unit passing gradationally upwards into a marginal marine unit which is dominantly clastic, and thence into open marine carbonates. Within the limits of current biostratigraphical resolution, it is of comparable age i.e., post-Courceyan to pre-late Arundian as discussed below. A recently published map by (Geological Survey of Ireland 1992) subdivides the succession into units which approximate to these in terms of their position on the ground, although none of their formations are described or defined in text. Their names are used here with formal definitions to avoid unnecessary replication of nomenclature.
DINANTIAN RIVER SYSTEMS, NW IRELAND
193
25
20
X /A t'3
75 6 -
7o_~ -~_~ 100
65 m' ~c
Fig. 13. Stratigraphic log of the upper part of the Minnaun Formation (MF) and the Downpatrick Formation (DF) on the western side of Downpatrick Head (Fig. 4). See Fig. 6 for legend; scale in metres.
Minnaun Formation. This formation is primarily exposed in cliff sections west of Ballycastle which have only limited direct access, but the formation can also be examined in part on the east side of Bunatrahir Bay (Figs 4, 13), although the base is not seen here. The base is an unconformity across which the Carboniferous rocks rest on Dalradian rocks, mostly psammites. One difference from Donegal is that a basal conglomerate unit comparable to the Roelough Conglomerate Formation is absent, and only a metre or so of pebble conglomerate is present at the base near Port (G023419). Despite limited access, the cliff sections west of Ballycastle are easily examined by binoculars and give good lateral and vertical control. The formation is dominated by medium to coarse-grained sandstones displaying numer-
ous erosion surfaces (Fig. 14). Muds are only locally preserved in the lower part of the formation, as in the Largysillagh Formation in Donegal, but their proportion increases upwards. These are accessible at Minnaun (G039418) and in the sections north of BaUycastle. Colours vary from red to green to grey, and desiccation cracks and calcareous glaebules interpreted as palaeosols are very common. Locally these glaebules coalesce to the extent that they form tabular micrites interpreted as calcrete palaeosols. Some intraclastic conglomerates, produced by reworking of these calcareous palaeosols, are present. The sandstones display abundant trough cross-beds with preserved sets mainly 20-60cm thick, but locally up to 1 m. Palaeocurrents from cross-strata consistently show derivation from
194
J. R. GRAHAM
Fig. 14. Typical cliff section in the lower part of the Minnaun Formation at G033419. Note the localized preservation of mudrocks beneath the curved erosion surfaces at centre left of the cliff. Height of cliff is 30 m.
the NW (Fig. 15). However, there are no large bar structures and the cliff sections do not display any large-scale point bar surfaces despite suitable orientations. Thus, as with the sections in Donegal, the river sizes can only be estimated from the shapes and dimensions of the curved erosion surfaces as being a few tens of metres wide and less than 3 m deep.
Downpatrick Formation. A gradational change into the Downpatrick Formation is exposed SW of Downpatrick Head (Gl18416; Fig. 13). The base is defined by a marked increase in bioturbation, by the common presence of IHS, and by the incoming of scattered marine fossil debris. It is thus comparable to the base of the Shalwy Formation. Dimensions of the IHS are similar to those in the Shalwy Formation, indicating channel depths of up to 4 m and point bar widths of 10--25m. Palaeocurrents from ripples indicate flows strongly oblique to the overall dip of the IHS, and a bipolar pattern of large-scale cross-strata is again in evidence (Fig. 15). Overall micritic palaeosols and evidence for exposure are more common than in the Shalwy Formation in Donegal, and horizons rich in marine shells are less common. In places, micritic palaeosols have been produced by the coalescence of carbonate glaebules and are devoid of fauna. However, most micrites in this formation are tabular, bioturbated, and contain some fauna, mainly ostracodes and gastropods. These are interpreted as lagoonal micrites formed in waters of abnormal salinity. The upper part of the formation (Figs 13, 16) shows a gradational passage into the overlying Moyny Limestone Formation which can be
examined at Downpatrick Head (G123428), west of Ballycastle (G063410, G096403) and east of Downpatrick Head at G144417. The base of the Moyny Limestone Formation is taken to
N
T IINNAUN )RMATION
n = 42
0
10
.....
N PATRIC K IMATION
n = 35
Fig. 15. Palaeocurrent data from the Minnaun and Downpatrick formations. All data are from crossstrata. Arrow represents vector mean.
DINANTIAN RIVER SYSTEMS, NW IRELAND
195
Fig. 16. View of the sea stack of Doonbristy, Downpatrick Head. The lower part of the cliff is formed by the uppermost part of the Downpatrick Formation and shows a muddy channel fill overlain by an IHS set. The upper part is formed by tabular limestones and shales forming the base of the Moyny Limestone Formation. Cliff is 25 m high.
be where tabular beds of limestone and shale first become dominant. The Downpatrick Formation appears to thin westwards, as only c. 15m is present in the cliff sections at Benadereen (G057411) whereas c. 90m is present near Downpatrick Head (Fig. 13).
Early Vis~an carbonates and shales In both Donegal (Rinn Point Formation) and North Mayo (Moyny Limestone Formation), tabular bedded carbonates with subordinate shales succeed the predominantly clastic coastal zone sediments. In Donegal e. 120m of these were deposited before the resumption of significant clastic input, whereas in North Mayo only 50-75 m of the carbonate-dominant sequence is present (Figs 3, 17). Like the Downpatrick Formation beneath it, the Moyny Limestone Formation thins westwards, but the difference in thickness may be partly due to non-deposition and erosion (see below). At the present level of biostratigraphical knowledge it is not possible to assess the temporal correlation of these carbonates with any great precision, but they are clearly post-Courceyan and all appear to belong to the Pu biozone (Higgs 1988). In Donegal these carbonates are overlain by a calcareous shale unit (Doorin Shales) at least
120m thick which is quite different in character to anything lower in the succession. It consists of regularly bedded laminated grey mudrocks, variably fossiliferous, with some horizons showing soft-sediment deformation features affecting several metres of succession. Thin lenticular sandstones and limestones are present within these shales. They have been interpreted as prodelta deposits (George & Oswald 1957), although their sedimentology remains to be investigated in detail. Lithologically this unit can be traced south and east where the terms Coolmore and Bundoran Shales have been used (Oswald 1955; George & Oswald 1957). These shales have also yielded Pu biozone microfloras but are thought to be Arundian in age, at least in part (George et al. 1976). In North Mayo, east of Downpatrick Head, the uppermost 25 m of the carbonate-dominant Moyny Limestone Formation contains more calcareous shales and some fine sandstones, and resembles the upper part of the Downpatrick Formation. Numerous examples of IHS are seen and one example (G162411) abnormally rich in fusain is present (Fig. 18). There is also evidence of exposure in the form of desiccation cracks. These rocks can be interpreted in a similar way to the upper parts of the Downpatrick Formation, as a complex of tidal channels and tidal flats formed in a coastal zone which was a little more carbonate-rich. Much work remains to be
196
J. R. G R A H A M 90_
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Fig. 17. Stratigraphic log of the section on the western side of Bunatrahir Bay (Fig. 4) from the upper part of the Downpatrick Formation to the highest exposed beds of the Mullaghmore Sandstone Formation. Subsidiary columns to the right of the main log indicate major bar surfaces laterally equivalent to the channel sandstones discussed in the text. DF, Downpatrick Formation; MLF, Moyny Limestone Formation; MSF, Mullaghmore Sandstone Formation. See Fig. 6 for legend; scale in metres.
done to clarify lateral relationships at this level of the stratigraphy, where there is clearly significant variability.
Mullaghmore
Sandstone
Formation
Throughout the Donegal Bay area there is a significant coarse clastic intercalation into the otherwise marine, carbonate-dominant Dinantian succession that overlies the basal clastic rocks. This is characterized by coarse-grained, locally pebbly feldspathic sandstones with some indications of exposure (palaeosols, rootlets, etc.). A variety of local names have been u s e d - Mullaghmore Sandstone, Carrowmoran
Sandstone, Donore Sandstone, Kildoney Sandstone, Mountcharles Sandstone - for what appears to be the same unit. The most widely used name, Mullaghmore Sandstone, is preferred here. Biostratigraphical data are still limited and, in particular, correlation between the palynological and foraminiferal biozones requires refinement. TS biozone (Clayton 1985) assemblages have been reported from the Mullaghmore Sandstone at Mullaghmore, Carrowmoran and Mountcharles (Higgs 1988). This biozone appears to be late Arundian to Holkerian in age (Riley 1993). Foraminiferal faunas no older than late Arundian in age have been recovered from the Mullaghmore Sandstone Formation west of Ballycastle (Fig. 17 at
DINANTIAN RIVER SYSTEMS, NW IRELAND
197
Fig. 18. Small-scale inclined heterolithic strata at G 162411 in the upper part of the Moyny Limestone Formation. The set on which the hammer (circled) rests is particularly rich in fusain.
109m) and at Kilcummin Head. Thus the present biostratigraphical data are consistent with the proposed lithological correlation. In the context of this paper it is not possible to describe in detail the large variety of both nonmarine and marine facies in the Mullaghmore Sandstone Formation. Primarily the lower and upper contacts and some of the main fluvial channel sediments will be considered.
tion can be defined by the first major sandstone, there is a significant amount of quartz sand in the upper part of the Moyny Limestone Formation.
'
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9_ MULLAGHMORE SANDSTONE FORMATION
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Base of the Mullaghmore Sandstone Formation The basal contacts of the Mullaghmore Sandstone Formation are well exposed on coastal sections in North Mayo and Sligo, but on the north side of Donegal Bay the base of the (Mountcharles) sandstones is not well seen. The character of this basal contact varies significantly. In the eastern parts of Donegal Bay there is rapid gradation from highly fossiliferous calcareous shales to calcareous sandstones, locally with granules and small pebbles, displaying evidence of wave action. This type of contact is seen at Portmore (G455344), Rochfort Lodge (G800589) (Fig. 19) and Coolmore (G855663; Fig. 1). Further west, between Downpatrick Head and Kilcummin Head (Fig. 4), there is evidence for shallowing in the upper part of the Moyny Limestone Formation, which resembles parts of the Downpatrick Formation in facies. Although the base of the Mullaghmore Sandstone Forma-
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BUNDORAN SHALE FORMATION i
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Fig. 19. Stratigraphic log of the base of the Mullaghmore Sandstone Formation at Rochfo'rt Lodge (G800589) west of Bundoran. See Fig. 6 for legend; scale in metres.
198
J. R. GRAHAM
Fig. 20. Erosional base of the Mullaghmore Sandstone Formation into the Moyny Limestone seen in the upper part of a 30m cliff section at G072412. The sandstone at the top of the cliff is 6.5m thick.
West of Ballycastle, the base of the Mullaghmore Sandstone Formation is spectacularly erosive and cuts down into the tabular carbonates of the Moyny Limestone Formation. On the western limb of an open syncline (G072412) the basal erosion surface has over 4 m of relief (Fig. 20). At G094409 on the eastern limb, only minor relief is visible on the erosion surface (Fig. 17), although fluvial sediments rest directly on shelf carbonates. The basal beds are coarse-grained sandstones with unidirectional cross-strata and are interpreted as fluvial channel deposits. Thus this contact represents a significant local sea-level change, probably at least 8m. In sequence stratigraphy terms this boundary has the characteristics of a Type I unconformity (Van Wagoner et al. 1988). The regional variation in the nature of this boundary suggests that the sea deepened to the south and east. The palaeocurrents from the fluvial facies of the Mullaghmore Sandstone Formation (Fig. 21) indicate transport of coarse sediment from the NNW.
Fluvial channels in the Mullaghmore Sandstone Whilst many of the sandstones in the Mullaghmore Sandstone Formation are clearly marineinfluenced on the basis of bioclastic debris, wave ripples, HCS, and so on, some facies can be interpreted as fluvial. In particular, coarse-grained sandstones with unidirectional cross-strata and
locally associated palaeosols and rootlets appear to be fluvial channel deposits. Coastal exposures in North Mayo and Sligo provide further information on the nature and scale of these channels. Exposures on the western side of Bunatrahir Bay display gently dipping strata cut by an irregular coastline with a few metres of vertical relief. In the lower part of the Mullaghmore Sandstone (Fig. 17 at 70m; G093409), trough cross-bedded sandstones separated by erosion surfaces are well exposed and consistently indicate palaeocurrents from the NNW. These are overlain by a distinctive pale-grey micrite with a buff weathering bioclastic top. This micrite is exposed on both sides of a small headland and then traces around a horseshoeshaped depression in the foreshore where the strata beneath are again exposed. Here they consist of large, gently inclined, heterolithic strata dipping at 10° towards 100 °. Current ripples within the sandy strata indicate palaeocurrents from the N - N W , strongly oblique to these surfaces but roughly parallel to the laterally equivalent trough cross-bedded sandstones. A 30cm Stigmaria is present on one sandstone surface aligned 130-310 ° . These heterolithic strata are interpreted as the products of laterally migrating point bars, and a minimum point bar width perpendicular to the dip of the strata is 60 m. More examples of these structures are exposed further west, particularly at G084409 (Fig. 17,
DINANTIAN RIVER SYSTEMS, NW IRELAND
'CASTLE n = 120
0 I
10 I
I
20 I
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IOWMORAN n = 72
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Fig. 21. Palaeocurrent data from sandstones interpreted as fluvial channel facies from the Mullaghmore Sandstone Formation. All data are from cross-strata; arrows indicate vector means. 102-109m). Here exposures show a continuum from trough cross-beds near the channel centre (Fig. 22) across the gently dipping bar surfaces (Fig. 23). The point bar surfaces dip at 10° towards 180 ° whereas the trough foresets in the channel axis dip at up to 28 ° towards 065 ° . The approximate width of the point bar surface is 75-90 m. Here it is possible to trace the trough cross-bedded sandstones of the axial zone of the
199
channel up to 100 m laterally before encountering a major erosion surface. These field data suggest a channel width of 150-200 m. The top of this sandstone complex is locally draped by thin lenses of limestone containing orthocones and gastropods, and must represent marine flooding of the channel after it was abandoned. A similar style and scale of channel is exposed at Donagh, Co. Sligo (G441353) within the Carrowmoran inlier of the Mullaghmore Sandstone (Fig. 1; Hubbard 1966). Here the point bar surfaces dip at less than 10° to the NNE, whereas well-exposed trough crossbeds in the axial zone of the channel dip consistently towards ESE. Estimated width of the point bar surfaces perpendicular to their dip is 40-50m. Thus the fluvial channel facies of the Mullaghmore Sandstone commonly displays large IHS sets interpreted as former point bar surfaces. The scale of bankfull river width indicated by these structures is a few hundreds of metres rather than the few tens of metres indicated for the channels in the basal clastics. The reasons for this increase in river size are not obvious, and hypotheses are difficult to test. A simple explanation is that river capture increased drainage basin size. Such events may be expected during the evolution of a relatively low relief landscape. However, it is also possible that the increase in drainage basin size during Mullaghmore times was in part due to the exposure of previously shallow-marine areas due to relatively rapid sea-level fall. A full facies analysis of the Mullaghmore Sandstone Formation is currently in progress. Work to date suggests that fluvial channel sands are most common in the most westerly section, west of Ballycastle (Fig. 4), and marine facies are more common further east. However, all sections show limestones with varied marine fauna as well as some unidirectional crossbedded sandstones representing fluvial channels. Thus sedimentation throughout the Mullaghmore Sandstone Formation was in coastal areas, probably in part estuarine. This is consistent with the limestone tops seen on many abandoned fluvial channels (e.g. Fig 17 at 72 m, 100 m, 109 m) and with the presence of bipartite, bipolar sandstones (e.g. Castlenageeha, Figs 4, 24).
Top of the Mullaghmore Sandstone In very general terms the Mullaghmore Sandstone shows an increase in thicker fluvial
200
J. R. GRAHAM
Fig. 22. Large-scale trough cross-strata from the axial channel facies of the sandstone complex at 102-109 m (Fig. 13). Rucksack for scale.
sandstones upsection at Mullaghmore and Carrowmoran (Fig. 1), where most of the formation is seen in one section. In North Mayo only the lower parts of the formation are seen west of Ballycastle, but to the east, at Kilcummin Head (G383209), the upper contact is seen. Here marine sandstones, many with well developed HCS (Figs 25, 26), are abruptly
overlain by bioturbated green mudrocks with common micritic palaeosols (Fig. 27). These palaeosols overlie an erosion surface which has locally cut down into the underlying sandstone (Fig. 28) and which must represent at least a local regressive phase. These micrites and mudrocks are 7 m thick and have at least one horizon of further erosion draped by more
Fig. 23. Inclined heterolithic strata interpreted as point bar surfaces dipping towards the trough cross-bedded sandstones shown in Fig. 22, which lie to the left of this field of view. The map board and rucksack (circled) rest on thin lenses of limestone which represent the flooding of the channel after abandonment.
DINANTIAN RIVER SYSTEMS, NW IRELAND
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micrites and mudrocks. The topmost 70cm of mudrocks are different in character, being extensively bioturbated and displaying trace fossils characteristic of marine horizons seen elsewhere in the formation. Above these rocks is a razor-sharp basal contact of coarse, pebbly bioclastic limestone (Fig. 29) which then passes up into well-bedded limestones and shales with abundant Zoophycos. This contact is taken by the Geological Survey of Ireland (1992) as the base of their Ballina Limestone Formation. Faunas from just above the base of the Ballina Limestone Formation indicate an Asbian age. Some 10km further south, near Killala, this upper contact of the Mullaghmore Sandstone Formation is characterized by extensive development of cross-bedded oolitic limestones, the Killala Oolite Member of the Mullaghmore Sandstone Formation (Geological Survey of Ireland 1992). The upper contact of the Mullaghmore is also seen at Carrowmoran where slightly different aspects are seen on different limbs of an open anticline (Hubbard 1966). To the west
q
0
Ill mfmcCO
Fig. 25. Stratigraphic log of the upper part of the Kilcummin Head section showing the upper part of the Mullaghmore Sandstone Formation and the sharp base of the Ballina Limestone Formation. The small column to the right of the main log represents the erosional downcut and subsequent muddy micritic fill which are partly seen in Fig. 28. See Fig. 6 for legend; scale in metres.
(G441353) oolitic limestones are present in the upper part of the Mullaghmore Sandstone just beneath the channel complex described above. To the east (G467344) the topmost sandstone of
202
J. R. GRAHAM
Fig. 26. Hummocky cross-stratified sandstones from the upper part of the Mullaghmore Sandstone Formation at Kilcummin Head. Hammer for scale.
the Mullaghmore is characterized by a pebbly lag and is then abruptly succeeded by relatively clean biomicrites. West of Mullaghmore headland the continuity of exposure is curtailed by sandy beaches. The uppermost beds exposed at Conor's Island (G657525) also show a prominent sandy oolite. Thus the top of the Mullaghmore Sandstone Formation represents a period of winnowing with oolitic and locally pebble lag facies. This is
i ~
typical of many rapid sea level rises seen in the stratigraphical record. In sequence stratigraphy terms such features are termed transgressive surfaces (Van Wagoner et al. 1988). In the more landward parts of the system these transgressive surfaces might be expected to overlie a nondepositional hiatus, possibly including soil zones (Galloway 1989). The sequence at Kilcummin Head (Fig. 25) conforms to this pattern and suggests that some considerable time may be
!
Fig. 27. Mudrocks with micritic nodules, Kilcummin Head. Note the development of micrites as replacements of burrow fills in the left of the photograph. Lens cap (50 mm) for scale.
DINANTIAN RIVER SYSTEMS, NW IRELAND
203
Fig. 28. Erosional downcut (centre) into the prominent tabular calcareous sandstone filled mainly by mudrocks with micritic nodules, Kilcummin Head. Person is standing on the basal bed of the Ballina Limestone Formation. represented by the uppermost 7 m Mullaghmore Sandstone Formation.
of the
Source of the Mullaghmore Sandstone Whilst much work remains to be done on the Mullaghmore Sandstone it is clear that the overall derivation direction of sediment was from the N N W (Fig. 21). The sandstones are relatively coarse-grained, angular and markedly felds-
pathic, and apparently similar in composition to the sandstones from the basal clastic rocks, despite the inferred increase in drainage basin size. A similar interpretation as first-cycle detritus from the Dalradian basement seems reasonable, since this type of basement geology is likely to have extended some considerable distance to the north and west. Notable exceptions to this general transport direction are found in the Killala area. A well
Fig. 29. Basal contact of the BaUina Limestone on the Mullaghmore Sandstone Formation, Kilcummin Head. Hammer head rests on a micritic palaeosol.
204
J. R. GRAHAM
exposed sandstone body at Castlenageeha (G209348; Figs. 4, 24) displays a lower trough cross-bedded quartzose sandstone with abundant drifted plant stems, and is interpreted as fluvial. This is overlain by cross-bedded oolitic shelly sandstones which are interpreted as marine flooding of an abandoned fluvial channel. The fluvial sandstones are southerly derived and the upper oolitic sandstones are northerly derived. Further north at Kilcummin a well exposed section through most of the Mullaghmore Sandstone Formation shows mainly marine facies. However, there is one major fluvial channel sandstone within the sequence which also shows south to north palaeocurrents. The ultimate source for these sandstones is not clear, but they may be evidence for some exposed basement to the south at this time.
Regional extent o f the Mullaghmore Sandstone The Mullaghmore Sandstone Formation is thickest and best developed north and west of the Ox Mountains basement block in the Donegal Bay area. However, it is also recognized in parts of the Lough Allen Basin between the northern parts of the Ox Mountains and the northern parts of the Curlew Mountains, although it does not appear to be present adjacent to the Curlews Block or in the Ballymote Syncline between the southern parts of the Ox Mountains and the Curlews (Fig. 1; Philcox et al. 1992). Important intercalations of sandstone prograding southwards into the marine sequence are known further east in the northern part of Ireland: e.g. the Clonelly Sandstone of the Omagh Syncline, and the Aughnacloy Sandstone of the Clogher ValleySlieve Beagh area, from which Mitchell & Owens (1990) derived a late Arundian age. The recently published maps of the Derrygonnelly and Kesh areas around Lower Lough Erne (Geological Survey of Northern Ireland (GSNI) Sheets 43, 44, & 56) recognize the Mullaghmore Sandstone Formation and show it to be completely Arundian in age, based on foraminiferal assemblages, with the base of the Holkerian recognized in the overlying Benbulben Shale Formation. Further east in Armagh the Drumman More Sandstone Formation (c. 120m) represents deltaic environments with associated thin coal seams, and is dated as late Arundian or Holkerian in age on the basis of fossiliferous limestones which occur above and below the formation (GSNI Sheet 46). It has also yielded
TS biozone microfloras (Higgs et al. 1988). It thus seems as if this progradational event is recognized throughout an area at least 250 x 70 km.
Controls on Dinantian sedimentation in N W Ireland The onset of sedimentation in NW Ireland in the Dinantian was due to the creation of accommodation space in the crust. Ultimately this implies regional subsidence for which large-scale tectonic processes must have been responsible. Reasonable explanations for an overall extensional regime in the northern parts of the British Isles are provided by Leeder (1988). It is clear from the whole of this large area that there were marked variations in subsidence locally, owing to the subsidence being produced by movements on widely spaced, often basement-inherited faults, giving rise to tilt block and basin provinces (Leeder 1988). Marked local variations of this type are documented in the basal clastic sequences in the northern part of Ireland (Sevastopulo 1981; Mitchell & Owens 1990; Graham & Clayton 1994). The main questions which have been asked of these Carboniferous successions is how much they reflect widespread eustatic events, as suggested by Ramsbottom (1973), rather than relatively local tectonic controls. Notwithstanding the local variations, which reflect variation in amounts of extension, the early Vis6an was clearly a time of widespread transgression (see Cope et al. 1992). However, this transgression was gradual (Sevastopulo 1981) and reflected long-term rise of sea level rather than any rapid, short-term change. At this scale the sea level change may well have been eustatic. The widespread progradation of clastic sediment in late Arundian times, exemplified by the Mullaghmore Sandstone Formation, does suggest a change in sea level which was approximately synchronous throughout NW Ireland. The magnitude of the sea level change can only be assessed crudely at present. The fully fluvial sandstones at the base of the Mullaghmore Sandstone Formation west of Ballycastle cut down over 4m into shallow-marine carbonates which show common wave action but a lack of lagoonal micrites, and rare IHS sets. This suggests a minimum sea-level fall of c. 8 m. Gradational basal contacts of the Mullaghmore Sandstone Formation elsewhere (e.g. Fig. 19) are from sediments interpreted as shallow marine, and certainly within the zone of in situ carbonate
D I N A N T I A N R I V E R SYSTEMS, N W I R E L A N D production. Thus a few tens o f metres w o u l d be a m a x i m u m estimate. The top of the M u l l a g h m o r e Sandstone F o r m a t i o n appears to represent a widespread transgression of similar magnitude. In contrast, the alternations between marine and n o n - m a r i n e conditions within the Mullaghm o r e Sandstone F o r m a t i o n c a n n o t be traced over even a few kilometres, a feature previously reported by H u b b a r d (1966). Thus these alternations can be interpreted as due to lateral facies change in coastal sediments. In order to assess the likelihood of a tectonic or eustatic control for the main M u l l a g h m o r e progradational event, it is necessary to look further afield at temporally equivalent successions. In this respect the lack of precise biostratigraphical correlation remains a limiting factor. Nevertheless, there are clear indications elsewhere in Ireland and in Britain of i m p o r t a n t tectonic activity in A r u n d i a n times. This was a time of d e m o n s t r a b l e fault m o v e m e n t s in the Curlew M o u n t a i n s (Philcox et al. 1989), near N a v a n (Philcox 1989), in the D u b l i n Basin ( N o l a n 1989), and in the Craven Basin of N o r t h e r n E n g l a n d ( G a w t h o r p e 1986). Thus it seems likely that the M u l l a g h m o r e Sandstone p r o g r a d a t i o n m a y have been controlled by regional tectonic causes. To test this hypothesis fully requires further biostratigraphical refinement from both this area and elsewhere to determine whether the sea level fall extended b e y o n d areas where it could be linked to tectonic activity. Discussions on the Carboniferous geology of NW Ireland with C. Ni Bhroin, G. Clayton, G. Nichols and G. Sevastopulo have been much appreciated. Thanks to B. Bluck, P. Shannon and, in particular, P. Strogen for their time and effort which helped to improve this paper.
References ALLEN, J. R. L. 1965. The sedimentology and palaeogeography of the Old Red Sandstone of Anglesey, North Wales. Proceedings of the Yorkshire Geological Society, 35, 139-185. - - 1 9 8 0 . Sand waves: a model of origin and internal structure. Sedimentary Geology, 26, 281-328. CLAYTON, G. 1985. Dinantian miospores and intercontinental correlation. Compte Rendu lOme Congres Advancement Etudes Stratigraphie Geologie Carbonifere, Madrid, 4, 9-23. COPE, J. C. W., GUION, P. D., SEVASTOPULO, G. D. & SWAN, A. R. H. 1992. Carboniferous. In: COPE, J. C. W., INGHAM, J. K. & RAWSON, P. F. (eds) Atlas of Palaeogeography and Lithofacies. Geological Society, London, Memoir 13, 67-86.
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DE RAAF, J. F. M., BOERSMA, R. & VAN GELDER, A. 1977. Wave generated structures from a shallow marine succession, Lower Carboniferous, County Cork. Sedimentology, 24, 451 484. ETHRIDGE, F. G. & SCHUMM, S. A. 1978. Reconstructing paleochannel, morphologic and flow characteristics: methodology, limitations, and assessment. In: M1ALL, A. D. (ed.) Fluvial Sedimentology. Canadian Society of Petroleum Geologists Memoir 5, 703-722. GALLOWAY, W. E. 1989. Genetic stratigraphic sequences in basin analysis I: architecture and genesis of flooding-surface bounded depositional units. American Association of Petroleum Geologists Bulletin, 73, 125 142. GAWTHORPE, R. 1986. Tectono-sedimentary evolution of the Bowland Basin, N. England, during the Dinantian. Journal of the Geological Society, London, 144, 59-71. GEOLOGICAL SURVEY OF IRELAND 1992. North Mayo. Bedrock Geology 1 : 100 000 Series. Sheet 6. GEOLOGICAL SURVEY OF NORTHERN IRELAND 1983. CIogher. 1 : 50 000 Series. Sheet 46. - - 1 9 9 1 . Derrygonnelly and Marble Arch. 1:50000 Series. Sheets 44, 56 and 43. - - 1 9 9 4 . Kesh. 1 : 50000 Series. Sheets 32 and 31. GEORGE, T. N. & OSWALD, D. H. 1957. The Carboniferous rocks of the Donegal Syncline. Quarterly Journal of the Geological Society of London, 113, 137-178. - - , JOHNSON, G. A. L., MITCHELL, M., PRENTICE, J. E., RAMSBOTTOM, W. H. C., SEVASTOPULO, G. D. & WILSON, R. B. 1976. A Correlation of Dinantian Rocks in the British Isles. Geological Society, London, Special Report 7. GRAHAM, J. R. & CLAYTON, G. 1994. Late Tournaisian conglomerates from County Donegal, NW Ireland; fault controlled sedimentation and overstep during basin extension. Irish Journal of Earth Sciences, 13, 95-105. HIGGS, K. 1988. Stratigraphic palynology of the Carboniferous rocks in Northwest Ireland. Geological Survey of Ireland Bulletin, 3, 171-201. , MCPHILEMY, B., KEEGAN, J. B. & CLAYTON,G. 1988. New data on palynological boundaries within the Irish Dinantian. Review of Palaeobotany and Palynology, 58, 61-68. HOLLAND, C. H., AUDLEY-CHARLES, M. G., BASSETT, M. G., ET aL. 1978. A Guide to Stratigraphical Procedure. Geological Society, London, Special Report 10. HUBBARD, J. A. E. B. 1966. Facies paterns in the Carrowmoran Sandstone (Vis6an) of western Co. Sligo, Ireland. Proceedings of the Geologists Association, 77, 233-254. LEEDER, M. R. 1988. Recent developments in Carboniferous geology: a critical review with implications for the British Isles and NW Europe. Proceedings of the Geologists Association, 99, 73-100.
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MITCHELL, W. I. & OWENS, B. 1990. The geology of the western part of the Fintona Block, Northern Ireland: evolution of Carboniferous basins. Geological Magazine, 127, 407-426. NICHOLS, G. & JONES, T. M. 1992. Fusain in Carboniferous shallow marine sediments, Donegal, Ireland: the sedimentological effects of wildfire. Sedimentology, 39, 487-502. NOLAN, S. C. 1989. The styling and timing of Dinantian syn-sedimentary tectonics in the eastern part of the Dublin Basin, Ireland. In: ARTHURTON, R. S., GUTTERIDGE, P. & NOLAN, S. C. (eds) The Role of Tectonics in Devonian and Carboniferous Sedimentation in the British Isles. Yorkshire Geological Society Occasional Publication, 6, 83-97. OSWALD, D. H. 1955. The Carboniferous rocks between the Ox Mountains and Donegal Bay. Quarterly Journal of the Geological Society of London, 111, 167-186. PAOLA, C. & BORGMAN, L. 1991. Reconstructing random topography from preserved stratification. Sedimentology, 38, 553-565. PHILCOX, M. E. 1989. The mid-Dinantian unconformity at Navan, Ireland. In: ARTHURTON, R. S., GUTTERIDGE, P. & NOLAN, S. C. (eds) The Role of Tectonics in Devonian and Carboniferous Sedimentation in the British Isles. Yorkshire Geological Society Occasional Publication, 6, 67-81. , BAILY, H., CLAYTON, G. & SEVASTOPULO, G. D. 1992. Evolution of the Carboniferous Lough Allen Basin, Northwest Ireland. In: PARNELL, J. (ed.) Basins on the Atlantic Seaboard." Petroleum Geology, Sedimentology and Basin Evolution. Geological Society, London, Special Publication, 62, 203-215.
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SEVASTOPULO, G. D. & MCDERMOTT, C. V. 1989. Intra-Dinantian tectonic activity on the Curlew Fault, north-west Ireland. In: ARTHURTON, R. S., GUTTERIDGE, P. & NOLAN, S. C. (eds) The Role of Tectonics in Devonian and Carboniferous Sedimentation in the British Isles. Yorkshire Geological Society Occasional Publication, 6, 55 66. RAMSBOTTOM, W. H. C. 1973. Transgressions and regressions in the Dinantian: a new synthesis of British Dinantian stratigraphy. Proceedings of the Yorkshire Geological Society, 39, 567-607. RILEY, N. 1993. Dinantian (Lower Carboniferous) biostratigraphy and chronostratigraphy in the British Isles. Journal of the Geological Society of London, 150, 427-446. SCOTT, A. C. & COLLINSON, M. E. 1978. Organic sedimentary particles. In: W. B. WHALLEY (ed.) Scanning Electron Microscopy in the Study of Sediments. Geo Abstracts, Norwich, 137-167. SEVASTOPULO, G. D. 1981. Lower Carboniferous. In: HOLLAND, C. H (ed.) A Geology of Ireland. Scottish Academic Press, Edinburgh, 147-171. THOMAS, R. G., SMITH, D. G., WOOD, J. M., VISSER, J., CALVERLY-RANGE, E. A. & KOSTER, E. H. 1987. Inclined heterolithic stratification- terminology, description, interpretation, and significance. Sedimentary Geology, 53, 123-179. VAN WAGONER, J. C., POSAMENTIER, H. W., MITCHUM, R. M., VAIL, P. R., SARG, J. F., LOUTIT, T. S. & HARDENBOL, J. 1988. An overview of the fundamentals of sequence stratigraphy and key definitions. In: WILGUS, K. C., HASTINGS, B. S., KENDALL, C. G. ST. C., POSAMENTIER, H. W., ROSS, C. A., & VAN WAGONER, J. C. (eds) Sea Level Changes- an Integrated Approach. SEPM Special Publication, 42, 39-45.
Cyclic emersion surfaces and channels within Dinantian limestones hosting the giant Navan Zn-Pb deposit, Ireland GIANCARLO
RIZZI ! & COLIN
J. R. B R A I T H W A I T E
Department of Geology and Applied Geology, University o f Glasgow, Lilybank Gardens, Glasgow G12 8QQ, UK 1 Present Address." Blackbourn Geoconsulting, 30 Coltbridge Terrace, Edinburgh EHl2 6AE
Abstract: The Carboniferous succession in the Navan Mine area, which lies near the northern margin of the Dublin Basin, rests unconformably on folded Lower Palaeozoic rocks. The informal stratigraphic nomenclature of the mine is used throughout. Sedimentation began in the late Devonian-early Courceyan with Red Beds reflecting deposition in braided streams and alluvial fans. The overlying Laminated Beds consist of shallow-marine barrier sandstones, tidal-flat/lagoonal mudstones, and sabkha evaporites. The Muddy Limestone is a mud-dominated carbonate sequence reflecting deposition in an open elastic-influenced lagoon. The Pale Beds that follow, and on which attention is focused here, contain at least 44 peritidal and shallow-shelf depositional cycles. The overlying Shaley Pale and Argillaceous Bioclastic Limestones reflect deeper water open-sea conditions. This sequence is truncated by a conspicuous erosion surface overlain by Chadian submarine debris-flows and limestone turbidites. The succession reflects a progressive deepening of the waters as subsidence outpaced sediment accumulation, but emersion surfaces capping cycles indicate that neither deposition nor relative sea-level rise were continuous. Cyclicity and emergence are believed to reflect the interaction of regional subsidence, glacio-eustatic sea-level oscillation, and sediment supply. Emersion surfaces occur in the Laminated Beds and throughout the Pale Beds, and include palaeosols, in situ breccias, pinnacled and hummocky surfaces, and karst-modified topography. Deeply incised channels at four intervals point to larger-scale incision. The lithoclast-bearing conglomerates which they contain indicate extensive emergence nearby. The distribution of these features may be related to the margins of outcrops of Old Red Sandstone and Lower Palaeozoic rocks. The Dinantian limestones in this region host several Zn-Pb ore deposits including Europe's largest at Navan. The results of this study suggest a relationship to the margins of former emergent carbonate terrains.
Intraformational emersion surfaces marked by palaeokarst and palaeosols have been widely reported from Dinantian platform limestones in the US (Briskey et al. 1986), Britain (Walkden 1987) and mainland Europe. By contrast, few examples have been described from generally similar limestones in Ireland, perhaps reflecting the paucity of inland surface exposures. However, palaeosols have been reported at Moyvoughly, County Westmeath (Harwood & Sullivan 1991), Andrew & Poustie (1986) described meteoric cements from Tatestown, County Meath, and Pickard et al. (1992) noted both meniscus cements and rhizoliths at Kentstown and Walterstown, in County Meath. Closely spaced drilling around the Navan Z n - P b deposit has generated more than 1000 cores. Over 70 of these have been examined and logged, together with underground exposures. As a result, numerous previously undescribed
emersion surfaces have been identified within the Lower Dinantian Limestones. The aims of this paper are: (1) to present an inventory of these surfaces; (2) to provide an interpretation of them, and finally (3) to consider the implications of this interpretation for the evolution of the Lower Dinantian (Courceyan) ramp on the northern margin of the Dublin Basin.
Summary lithostratigraphy and sedimentology Navan Mine lies about 1 km west of Navan close to the northern margin of the Dublin basin (Fig. I). Detailed descriptions of the lithostratigraphy of the Dinantian succession are available in Andrew & Ashton (1985), Ashton et al. (1986,
From STROGEN, P., SOMERVILLE,I. D. & JONES, G. LL. (eds), 1996, Recent Advances in Lower Carboniferous Geology, Geological Society Special Publication No. 107, pp. 207-219.
208
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Fig. 1. Solid geology of the Navan area with the location of Navan mine. Modified from Ashton et al. (1992).
1992), Philcox (1989), Anderson (1990), and McNestry & Rees (1992). Rizzi (1992) described the sedimentology summarized below. The Courceyan succession (Fig. 2) rests unconformably on deformed Lower Palaeozoic black shales, turbidites and volcaniclastic rocks and is divided into two groups, a lower Navan Group and an upper Argillaceous Bioclastic Limestone (ABL) Group (cf. Ashton et al. 1992). A formal stratigraphy has been erected for the Dinantian of the region (Rees 1987; Strogen e t al. 1990), but since this paper deals exclusively with the Navan Mine area, the informal mine nomenclature is retained here. The Navan Group is divided into five units: Red Beds, Laminated Beds, Muddy Limestones, Pale Beds and Shaley Pale Limestones. The ABL Group consists of both argillaceous bioclastic limestones s e n s u s t r i c t o and Waulsortian facies limestones. Both groups are truncated by a Chadian 'erosion' surface, and are overlain by the Fingal Group of Nolan (1989), consisting, at the mine, of the Boulder Conglomerate and Upper Dark Limestones. The Red Beds, up to 50m thick, consist of conglomerates, cross-bedded and rippled sandstones, siltstones and mudstones arranged in fining-upwards cycles, some of which are capped by calcretes. They are interpreted as fluvial or alluvial fan deposits (Strogen e t al. 1990; Rizzi 1992).
The Laminated Beds record the initial transgression of the Lower Carboniferous sea (Philcox 1984; Andrew & Ashton 1985) and reflect deposition in a variety of environments (McNestry & Rees 1992; Rizzi 1992). The lower part of the succession is characterized by a transition from silty shales to siliciclastic sandstones, recording the progradation of a shallow-marine barrier-beach complex. Above these, algal-laminated limestones, calcisiltites, fenestral calcite mudstones and black shales are associated with oncolites and a nodular anhydrite (now silicified) and resemble peritidal and lagoonal deposits of the present Trucial coast (cf. Kendall & Skipwith 1968). The succession is capped by thin bioclastic grainstones. The Laminated Beds are cut by a channel sequence about 20 m deep and 100-500 m wide, which isopach data show extends roughly NW-SE (Fig. 3). This contains about 15m of sand to small-pebble grade, burrowed and crossbedded grainstones, forming the Limestone Conglomerate. These are arranged in fining-up cycles about 1.5 m thick with sharp erosive bases. Grains include bioclasts and siliciclastic grains as well as lithoclasts of palaeosols. Cross-sections correlating cycles indicate that there are at least five minor channels stacked vertically within the sequence. The Muddy Limestone consists of about 20 m of shaley, bioclast-rich calcite mudstones and
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A rich assemblage was recorded in the Polygnathus communis carina Zone (Fig. 3). The hybodont teeth of Mesodmodus (Figs 7e, f) and tooth plates of bradyodonts (e.g. Fig. 7g) resembling Psephodus and Psammodus appear first in this zone. Mesodmodus occurs in the Kinderhookian (=early Tournaisian) of Iowa (St John & Worthen 1875). Tooth plates of Psephodus and Psammodus are very common in the Early-Middle Carboniferous of the East European Platform, the Kuznetsk Basin and the British Isles (Obruchev 1964). A more extensive radiation of the Early Carboniferous chondrichthyans took place in the Vis6an G. texanus-M, beckmanni Zone. The ichthyoassemblages are considerably increased by new chondrichthyan groups with various types of dentition: Xenacanthus ?nebraskensis Johnson (Figs 4f-h), Lissodus (Fig. 7a-d), Orodus, and petalodontids were also recorded there (Fig. 3). Teeth of Xenacanthus ?nebraskensis occur in the late Pennsylvanian of North America (Johnson 1984), the late Vis6an of the Polar Urals (Kozhim River) and in the Serpukhovian (early Namurian) of the Moscow region. The remains of Lissodus resemble some varieties of L. wirksworthensis Duffin from the late Vis+an of England (Duffin 1985). Teeth of Denaea similar to those of D. meccaensis Williams from the late Pennsylva-
420
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Gnathodus bilineatus bollandensis-Adetognathus unicornis conodont Zones of the South Urals (Figs 3, 6h, i). These teeth are very similar to the ones of the recent frilled shark Chlamydoselachus anguineus Garman. Apart from the chondrichthyans, the vertebrate assemblages include remains of palaeoniscids and acanthodians.
Systematic
description
Protacrodus aequalis, sp. nov. Fig. 6a-g.
Protacrodus sp. 'C': Turner, 1982. 125-126, Fig. 7.
Etymology. Latin aequalis, equal. Holotype. LP 11-18, Laboratory of Palaeontology, St. Petersburg University. South Urals, Sikaza; Early Tournaisian, Siphonodella sulcata conodont Zone. Other material. 12 complete teeth: LP 11-19LP 11-23 and LP 11-33-LP 11-38. Occurrence. South Urals, Sikaza, Ryauzyak; Late Famennian-early Tournaisian, early S. praesulcata-S, sulcata conodont Zones. Diagnosis. The teeth have three separate cusps in the crown. These cusps are almost equal in size, short and wide, straight or laterally curved, labio-lingually flattened. The cusp striation
consists of clear ridges diverging from the cusp top (3-6 ridges on the labial or lingual surfaces of the cusps). The ridges are sigmoidal on the lateral cusps. The tooth base is directed lingually at nearly a right angle to the crown. There is a lamina on the labial border of the base. The base is triangular or round in form. One or two canals open on the apical border. The basal surface of the tooth is weakly concave. Remarks. This species was found in the Famennian of the Kuznetsk Basin, Tom' and Yaya Rivers and Australia, Queensland (Turner 1982). The striation of the new species is similar to that of some teeth of Xenacanthus ?nebraskensis Johnson. The teeth of Protacrodus vetustus Jaekel differ from the described species by the pyramidal crown with fused basal parts of the cusps and a short and wide base. Also the cusps of P. vetustus are rounded in cross-section.
Symmorium sp. Fig. 5a-1.
Material.
218 teeth: L P l l - 8 - L P l l - 1 7 and LP 11-39-LP 11-246. Occurrence. South Urals, Sikaza, Ryauzyak, Popovskiy; early Tournaisian, S. sulcata-S. duplicata conodont Zones. Description. The specimens vary from teeth with five cusps in the crown (Figs 5c, d) to teeth with 15 cusps (Fig. 51).
SOUTH U R A L S C A R B O N I F E R O U S C H O N D R I C H T H Y A N S
421
Fig. 4. (a-e) Thrinacodus sp., teeth, South Urals, Sikaza; (a, b, d, e) early Tournaisian, (e) late Famennian. (a) LP11-1, occlusal view, S. duplicata Zone. (b) LP 8-2, lateral view, S. sulcata Zone. (c) LP 11-2, occlusal view, Late Pa. expansa Zone. (d) LP 11-3, occlusal view, S. duplicata Zone. (e) LP 11-4, occlusal view, P. communis carina Zone. ([)-(h) Xenacanthus ?nebraskensis Johnson, teeth South Urals, Sikaza, Vis6an, G. texanus-M. beckmanni Zone: (f) LP 11-5, (g) LP 11-6, (h) LP 11-7, occlusal views. Scale bars are 0.25 mm. All the teeth have a strong concavity in the central part of the labial surface (Fig. 5c, f,j,1). There are two processes on each side of the concavity (Fig. 5a, c,e,g). All the cusps are striated with 3-5 straight ridges on the labial and lingual surfaces. The largest lateral cusps are approximately half or a third the height of the main cusp. The teeth with a crown of five cusps have a semicircular base with two indistinct buttons
on the apical surface (Fig. 5c, d). The base width is about l mm. There is no clear b o u n d a r y between the base and the crown on the labial and lingual surfaces. The main cusp bears a lateral carina separating the labial and lingual surfaces (Fig. 5c, d). The outermost lateral cusp pair is higher than the inner ones. The teeth with numerous lateral cusps have a wide crescent base (Fig. 5i,j, 1). The base width is up to 5 mm. There are no indistinct buttons on
422
A. I V A N O V
Fig. 5. Symmorium sp., teeth, South Urals, Sikaza, early Tournaisian. (a), (b) S. sulcata Zone; (c)-(l) S. duplicata Zone. (a) LP 11-8, labial view. (b) LP 11-9, basal view. (c) (d) LP 11-10: (c) occlusal and (d) lingual views. (e) LP 11-11, labial view. (f) LP 11-12, occlusal view. (g) LP 11-13, basal view. (h) LP 11-14, labial view; (i) LP 11-15, basal view; (j), (k) LPll-16. (!) LPII-17, occlusal views. Scale bars are 0.25 ram.
SOUTH URALS CARBONIFEROUS CHONDRICHTHYANS
423
Fig. 6. (a)-(g) Protacrodus aequalis sp. nov., teeth, South Urals, Sikaza, early Tournaisian, S. sulcata Zone. (a), (b) LP 11-18, holotype: (a) labial and (b) occlusal views. (c) LP 11-19, lingual view. (d) LP 11-20, labial view. (e) LP 11-21, occlusal view. (f) LP 11-22, labial view. (g) LP 11-23, labial view. (h)-(i) Denaea sp., teeth, South Urals, Sholokh-Sai, Serpukhovian, G. bilineatus bollandensis-A, unicornis Zone, occlusal views: (h) LP 11-24, (i) LP 11-25. Scale bars are 0.25 mm.
the apical surface. The rows of canal openings are located on the apical and basal surfaces of the base (Fig. 5b, g,i,j,1). The crown is clearly separated from the base, especially in the teeth bearing 15 cusps. Remarks. Wang (1989) described various teeth designated as 'Cladodus' spp. from the Devonian/Carboniferous boundary beds (S. praesulcata-S, duplicata Zones) of the Dapoushang Section, China. One specimen (Wang 1989, pl. 28, Fig. 3) is very similar to the species described here. Teeth of Symmorium occidentalis (Leidy), S. reniforme Cope and other Carboniferous species differ from specimens described here in having a higher and thicker central cusp, and clear and large paired buttons on the lingual
and basal surfaces of the base (Zidek 1973; Williams 1985). Teeth of specimens from the South Urals differ from the Famennian teeth of Symmorium (Long 1990) in possessing fused bases of cusps in the crown and a less concave labial surface.
Discussion and conclusions Chondrichthyan remains have great significance as indicators of facies, changes in environment, sea depth and relationship between basins. In the localities of the South Urals described above, chondrichthyans are recorded in increasing numbers and diversity from the Early
424
A. IVANOV
Fig. 7. Lissodus sp., teeth, South Urals, Sikaza, Vis+an, G. texanus-G, beckmanni Zone: (a)-(b) LP 11-26: (a) lingual and (b) occlusal views. (e) LP 11-27, labial view; (d) LP 11-28, occlusal views; (e)-(f) Mesodmodus sp., tooth, South Urals, Sikaza, Late Tournaisian, Pol. communis carina Zone, LP 11-29: (e) occlusal and (f) labial views. (g) Bradyodont tooth plate, South Urals, Sikaza, Vis+an, G. texanus-M, beckmanni Zone, LP 11-30, occlusal views. (h) Ctenacanthid scale, Sikaza, Early Tournaisian, S. sulcata Zone, LP 11-31. (i) Orodontid scale, Sikaza, Vis+an, G. texanus-M, beckmanni Zone, LP 11-32. Scale bars are 0.25 mm.
Tournaisian to the Vis6an. The environment of the deep part of the shelf with abundant benthic and planktonic invertebrates is believed to be typical for the Vis6an in the South Urals, especially in the G. texanus-M, beckmanni Zone where chondrichthyan remains are the most abundant (Smirnov & Plyusnin 1975; Kochetkova et al. 1981). Xenacanthid taxa such as Xenacanthus ? nebraskensis and X. luedersensis Berman were found in the marine sequences together with hybodontids and petalodontids
(Schultze 1985). However, other xenacanthid species occur in freshwater assemblages. Thrinacodus as well as other phoebodonts are common in the deposits with abundant conodont and ammonoid assemblages. Neither of these groups occur in the Tournaisian and Vis6an shallow water vertebrate assemblages of the Moscow Synclise, but there are numerous sclerophagous chondrichthyans such as Helodus, 'Orodus', Psephodus, Deltodus and others, as well as sarcopterygians. In the South Urals assemblages
SOUTH U R A L S C A R B O N I F E R O U S C H O N D R I C H T H Y A N S
predator sharks with high migration potentials predominated, e.g. Symmorium, Denaea, stethacanthids and ctenacanthids. The sclerophagous chondrichthyans are rare in the Early Carboniferous of the South Urals. I gratefully acknowledge the financial help of the project IGCP 328 and the coordination committee of EDE '94 Symposium to attend the symposium in Dublin and present a paper. I am greatly indebted to V. Pazukhin (Ufa) for the vertebrate materials and stratigraphical information.
References BARSKOV, I. S., ALEKSEEV, A. S., GOREVA, N. V., KONONOVA, L. I. & MIGDISOVA,A. V. 1984. [The Carboniferous conodont zonation of the East European Platform]. In: MENNER, V. V. (ed.) [Palaeontological Characteristics of the Carboniferous Stratotype and Supporting Sections of the Moscow Syneclise (Conodonts, Cephalopods)]. Moscow University Press, 143-150 [in Russian]. DUFFIN, C. J. 1985. Revision of the hybodont selachian genus Lissodus Brough (1935). Palaeontographica Abteilung, 188A, 105-152. GINTER, M. 1990. Late Famennian shark teeth from the Holy Cross Mts, Central Poland. Acta Geologica Polonica, 40, 69-81. - - & IVANOV, A. 1992. Devonian phoebodont shark teeth. Acta Palaeontologica Polonica, 37, 55-75. JOHNSON, G. D. 1984. A new species of Xenacanthodii (Chondriehthyes, Elasmobranehii) from the Late Pennsylvanian of Nebraska. In: MENGEL, R. M. (ed.) Papers in Vertebrate Paleontology" Honoring Robert Warren Wilson. Carnegie Museum of Natural History, Special Publication 9, 178-186. KOCHETKOVA, N. M., LUTFULLIN, YA. L. & PAZUKHIN, V. N. 1981. Scheme of stratigraphy and correlation of the Early Carboniferous on the South Urals. Ufa [in P.ussian].
425
KULAGINA, E. I., RUMYANTSEVA,Z. S., PAZUKHIN, V. N. & KOTCHETOVA, N. N. 1992. [Lower/ Middle Carboniferous Boundary in the South Urals and Central Tien Shan]. Science, Moscow [in Russian]. LONG, J. A. 1990. Late Devonian chondrichthyans and other microvertebrate remains from Northern Thailand. Journal of Vertebrate Paleontology, 10, 59-71. NEWBERRY, J. m. & WORTHEN, A. H. 1866. Descriptions of New Species of Vertebrates, Mainly from the Sub-Carboniferous Limestone and Coal Measures of Illinois. Geological Survey of Illinois Report, 2, 9 141. OBRUCHEV, D. V. 1964. Subclass Holocephali. In: OBRUCHEV, D. V. (ed.) Agnathans, fishes. Fundamentals of Paleontology, 11. Science, Moscow, 238-266 [in Russian]. ST JOHN, O. H. 8~: WORTHEN, A. H. 1875. Descriptions of fossil fishes. Geological Survey of Illinois, Paleontology, 6, 245-488. SCHULTZE, H.-P. 1985. Marine to onshore vertebrates in the Lower Permian of Kansas and their paleoenvironmental implications. University of Kansas Paleontological Contributions, 113, 1-17. SMIRNOV, G. A. & PLYUSNIN, K. P. 1975. History of the geological development of the Urals during the Carboniferous time. In: Carboniferous of the Urals. Sverdlovsk, 3 14 [in Russian]. TURNER, S. 1982. Middle Palaeozoic elasmobranch remains from Australia. Journal of Vertebrate Paleontology, 2, 117-131. WANG, S.-T. 1989. Vertebrate microfossils. In: JI QIANG (ed.) The Dapoushang Section. Science Press, Beijing, 103-108. WILLIAMS, M. E. 1985. The 'cladodont level' sharks of the Pennsylvanian black shales of Central North America. Palaeontographica Abteilung, 190A, 83-158. ZIDEK, J. 1973. Oklahoma paleoichthyology, Part 2: Elasmobranchii (Cladodus, minute elements of cladoselachian derivation, Dittodus, and Petrodus). Oklahoma Geology Notes, 33, 87 103.
Mid-Dinantian brachiopod biofacies from western Ireland D A V I D A. T. H A R P E R
& ANNA
L. J E F F R E Y
Department o f Geology, University College, Galway, Ireland
Abstract: New silicified brachiopod faunas from lower and middle Vis~an horizons have been retrieved from the vicinity of Loughs Carra, Corrib and Mask in west Connaught. In addition, carbonate shell beds dominated by Linoprotonia are recorded from mid County Galway. At present over 40 species of brachiopod are known, from current sampling programmes and the literature, variably associated with bryozoan, coral, echinoderm, mollusc and trilobite faunas; microvertebrates such as sharks teeth are locally common. Four main assemblages are recognized; in ascending order, Arundian strata at Ardnasillagh, Lough Carra, Kilbeg Wood and Ballintober in a northern belt are variably dominated by Rhipidomelta, Schizophoria, Leptagonia, Rugosochonetes, Krotovia, Echinoconchus, Tylothyris, Punctospirifer and Spiriferellina. In addition to genera recorded from Arundian strata elsewhere, Streptorhynchus, Plicochonetes, Dictyoclostus, Pleuropugnoides and 'Spirifer' also occur in the diverse Kiltiernan fauna assembled last century. In a southern belt, possible Holkerian horizons at Dunsandle are less diverse, with faunas dominated by Composita, whereas limestones of Holkerian-early Asbian age at Kiltullagh Bridge have quite different faunas with Schizophoria, Brochocarina, Minythyra and Cleiothyridina; these faunas are more similar to the diverse Asbian assemblages described from County Fermanagh in the north of Ireland. In a central belt, in the Bunoghanaun area, Holkerian to early Asbian horizons are dominated by relatively thick shell beds in pure limestones with mainly opportunist large linoproductids; this fauna is not silicified.
Mid-Dinantian carbonate facies developed adjacent to Loughs Carra, Corrib and Mask in counties Galway and Mayo have yielded locally abundant brachiopod faunas. Deposition occurred on the western part of an exterisive carbonate platform adjacent to an older Precambrian-Lower Palaeozoic massif to the west (Cope et al. 1992). Three main belts are defined in terms of both bio- and lithofacies (Fig. 1). A northern belt of impure, commonly cherty limestones of mainly Arundian age overlies basal Carboniferous sandstones around parts of Loughs Carra, Mask and Corrib. A central belt is dominated by relatively pure limestones, probably ranging in age from Holkerian to Asbian, whereas a southern belt developed east of Oranmore is characterized by a younger, Holkerian-Asbian development of impure muddy and commonly cherty limestones. The brachiopod fauna from the central belt is of low diversity, with shell beds dominated by large linoproductids, whereas the impure limestone facies contains diverse silicified assemblages with a range of bionomic shell types reflecting a spectrum of habitats in shallowwater, nearshore environments.
Stratigraphical setting Despite adequate exposure and many fossiliferous localities, the Lower Carboniferous rocks immediately east of Loughs Corrib and Mask
have been largely neglected in comparison with Dinantian successions elsewhere in Ireland (Sevastopulo 1981). In broad terms, basal clastic facies of Chadian age pass upwards through a variety of nearshore impure carbonate facies, probably ranging in age from late Chadian to Holkerian and succeeded by the purer carbonate facies of the Burren-type limestones. As a whole the succession youngs to the south although there is some local tectonism manifest in a number of open folds with shallowdipping limbs and some faulting. The initial mapping of the region by the Geological Survey (Kinahan 1865, 1869; Kinahan et al. 1867; Kinahan & Nolan 1870; Kinahan & Symes 1871) last century covered the areas around the three loughs in terms of four main units: in ascending order, the Carboniferous Sandstone and the Lower, Middle and Upper limestones. The upper boundary of the Lower Limestone was drawn north of Oranmore. The Middle or Upper Limestone developed in a 'black earthy facies with shales', exposed northwest of Athenry, was differentiated from the Upper Limestones in a pale grey, crinoidal facies. It is probably the 'black earthy facies with shales' that corresponds to the intercalation of muddy limestone noted here at Kiltullagh Bridge and Dunsandle Station. MacDermot & Sevastopulo (1972) presented a more detailed up-to-date analysis of the shelf
From STROGEN,P., SOMERVILLE,I. D. & JONES, G. LL. (eds), 1996, Recent Advances in Lower Carboniferous Geology, Geological Society Special Publication No. 107, pp. 427-436.
428
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by deeper-water basinal facies (MacDermot & Sevastopulo 1972, fig. 5). In his lithological review of the Lower Carboniferous stratigraphy of the Irish Midlands, Philcox (1984) included the areas east of the loughs on the fringe of his Dunmore Province• This province is characterized by a thick basal sandstone overlain by condensed argillaceous,
MID-DINANTIAN BRACHIOPODS A p p r o x i m a t e range of brachiopod faunas NW SE
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Fig. 2. Schematic stratigraphical section with locations of main brachiopod faunas. bioclastic limestones; mud-mounds are rare and isolated. More recently, Drew & Daly (1993), in the course of investigations of karstification and groundwater in the region, published a generalized bedrock map based on a variety of unpublished sources. Much of the region is mapped as 'Pure Limestone', although three main tracts are dominated by 'Muddy Limestone'; the latter occur west of Ballinrobe, County Mayo, and in two larger areas south of Kilmaine and east of Oranmore, County Galway. O'Raghallaigh et al. (1995) have described, in general terms, the Carboniferous succession in north Clare and east Galway within the context of the mineral deposits of the region. Throughout most of the area studied, Dinantian limestones appear faulted against the Dalradian and Lower Palaeozoic basement. Around Loughs Corrib and Mask the Dinantian is developed as massive to thick-bedded limestones generally lacking shale, but with minor chert horizons. The limestone succession ranges in age from Chadian to Asbian. Both east and south of Galway, particularly on the Burren and the Aran islands, similar lithological developments have been termed, informally, the Burren Limestone (Gallagher 1994). However, the overlying Brigantian Slievenaglasha Formation includes
over 100 m of coarse bioclastic limestones, dominated by crinoid debris and extensive chert horizons (Drew 1989; Gallagher 1994). Data for the younger Carboniferous strata in northwest Clare are noted in Fitzgerald et al. (1994). With the exception of the northern development of the Holkerian-Asbian pure limestone facies of the Burren Formation, no formal stratigraphical units have been defined or recog-nized in the area studied. The stratigraphical terminology defined here (Fig. 2) is preliminary, and provides the necessary framework to insert the new brachiopod data. Few conodonts have been extracted from this succession; age constraints have been provided by foraminiferans and algae (G. D. Sevastopulo & I. D. Somerville pers. comm.) together with some data from coral assemblages (I. D. Somerville pers. comm.). Detailed locality information together with taxonomic descriptions of the silicified brachiopod faunas are documented in Jeffrey (1992).
Brachiopod assemblages With the notable exception of Brunton's studies (1966, 1968, 1984, 1987) of the Asbian faunas of
430
D. A. T. HARPER & A. L. JEFFREY
the Glencar Limestone, County Fermanagh, little research has been focused on Irish Carboniferous brachiopods since M'Coy's (1844) extensive monograph on the Carboniferous fossils of Ireland. Silicified faunas are documented and illustrated for the first time from this part of Ireland. In broad terms, two main types of brachiopod assemblage are apparent: firstly, relatively diverse assemblages with a wide range of taxa and shell types, usually silicified and often associated with impure, cherty limestones; and secondly, lower diversity unsilicified faunas in purer limestones lacking chert, dominated by large linoproductids and, higher in the succession, gigantoproductids. These faunas, with the exception of the younger gigantoproductids, are discussed below in terms of bionomic shell types (see also Brunton 1987).
The Ardnasillagh, Ballintober, Kilbeg Wood and Lough Carra faunas-northern belt (Arundian) These silicified faunas (Figs 3 & 4) are associated with a variety of nearshore, impure, cherty limestone facies. Many of the limestones are typically skeletal grainstones, less commonly packstones, with low proportions of quartz and feldspar minerals. In general terms the faunas are dominated by the following genera (Table 1): Rhipidomella, Schizophoria, Leptago-
nia, Rugosochonetes, Krotovia, Echinoconchus, Tylothyris, Punctospirifer and Spiriferellina. A range of shell types is represented. Fixed pedunculate orthides such as Rhipidomella and Schizophoria together with the younger shells of the spiriferides Tylothyris, Punctospirifer and Spiriferellina required patches of hard substrate such as other shells or rocks during most of their life cycles. Both Echinoconchus and Krotovia had pedicle valves coated with spines providing some anchorage and stabilization within the sediment. The Rugosochonetes lay recumbent within the soft substrate, together with the adult shells of the spiriferides. For example, adult Tylothyris possessed imbricate stegidial plates with an atrophied pedicle; the shells rested with their posterior surfaces partly within the substrate (Brunton 1984, p. 80). This ambitopic strategy was probably followed by a variety of spiriferides (Legrand Blain 1986). The strophomenide Leptagonia, however, was ambitopic, initially fixed by a pedicle but in later life the stalk atrophied and the brachiopod pursued a recumbent life strategy, quasi-infaunally within a soft substrate (Brunton 1987).
The Kiltiernan fauna, Lough Mask-northern belt (Arundian) A large fauna assembled last century by William King from the area of Kiltiernan adjacent to Lough Mask (Northern Belt) has yet to be precisely localized (Harper 1988). This fauna is silicified and contains most of the brachiopods found in adjacent Arundian horizons noted above, but in addition Streptorhynchus, Plicochonetes, Dictyoclostus, Pleuropugnoides and 'Spirifer' occur (Table 1). The assemblage is diverse, with bryozoans, echinoderms and molluscs together with corals including Siphonophyllia garwoodi Ramsbottom & Mitchell. In addition to the ambitopic, pedunculate, recumbent and spinose forms noted in Arundian strata elsewhere, this Kiltiernan fauna contained further ambitopic taxa such as the rhynchonellide Pleuropugnoides, the recumbent Plicochonetes, and the spinose Dictyoclostus. The orthotetidine Streptorhynchus may have embedded the apex of its conical pedicle valve in the sediment for support.
The Dunsandle Station fauna-southern belt (probably Holkerian) A low diversity brachiopod fauna dominated by Composita? was etched from fine-grained limestones in the railway cutting adjacent to Dunsandle Station. The facies is mainly lime mudstone with occasional patches of skeletal wackestone.
The Kiltullagh Bridge fauna-southern belt ( Holkerian-early Asbian ) This silicified fauna was retrieved from impure, cherty limestones adjacent to Kiltullagh Bridge in the southern belt. The limestones are finegrained skeletal wackestones. The fauna is similar in diversity to the older Arundian assemblages of the northern belt; it is dominated by the genera Schizophoria, Brochocarina, Rugosochonetes, Minythyra and Cleiothyridina together with some indeterminate productids (Table 1). Pedunculate forms such as Schizophoria together with Cleiothyridina and Minythyra required a firm substrate for attachment, although the small valves of Minythyra may have been fixed to other shells (Brunton 1987). Rugosochonetes was a recumbent form, and the orthotetidine Brochocarina may have been cemented to rocks or other shells during at least some of its ontogeny.
MID-DINANTIAN BRACHIOPODS
431
Table 1. Overview of the main stratigraphically-located brachiopodfaunas discussed in this study. Age
Location
Holkerian-early Dunsandle Station Asbian Kiltullagh Bridge
Brachiopod species
Abundance Size range (mm)
Composita sp. 1 Schizophoria resupinata (Martin) cf. lata Demanet Brochocarina sp. cf. B. wexfordensis
Rare (5) Rare (3)
2-16 20-52
Rare (5)
5-9.5
(Smyth) Productide superfam, gen. et. sp. indet. Minythyra sp. nov.
Rare (1) Common
40 1.5-2.5
(>30) Bunoghanaun
Cleiothyridina sp. cf. C. fimbriata (Phillips) Rugosochonetes sp. Linoprotonia ashfellensis Ferguson
Common
,-,80
(>30) Arundian
Lough Carra
Craniscus? sp. Rhipidomella michelini (L6veill+) Leptagon& sp. cf. L. analoga (Phillips)
Rare (1) Common (~60) common
~12 3-15 1.5-11
(>40) Rugosochonetes sp. cf. R. celticus Muir-Wood Krotovia sp. (?spinulosa)
Rare (--4) Common (>10) Rare (~5) Eomarginifera sp. Echinoconchus sp. cf. E. punctatus (J. Sowerby) Common (~15) Common Spiriferellina insculpta subsp, nov.
2-7.5 1-6 12.5 2.5-6.5 2-7.5
(>20) Beecheria sp. 2 Kiltiernan Lough Mask
Ballynalacka Kilbeg Wood
1-13
Common (>15) Common (>2O) Common (>25) Common (>11) Common (-,~40) Common
30-50
Rhipidomella michelini (L6veill+) Plicochonetes buchiana (de Koninck) Echinoconchus punctatus (J. Sowerby) Dictyoclostus semireticulatus (Martin) Schellwienella crenistria (Phillips) Pleuropugnoides pleurodon (Phillips) Actinoconchus planosulcata (Phillips) Composita ambigua (J. Sowerby) Syringothyris cuspidatus (J. Sowerby) Spirifer bisulcatus (J. Sowerby) Tylothyris laminosa (M'Coy) Balanoconcha saccula (J. de C. Sowerby) No brachiopoda data Schizophoria resupinata (Martin) gigantea (Demanet) Cleiothyridina sp. cf. C. fimbriata (Phillips)
Composita sp. 2 Tylothyris laminosa (M'Coy) subsp, nov. Ardnasillagh
Common (13)
Cleiothyridina sp. Tylothyris sp.
17-24
7-18 3-13.5 2-15
(>40) Punctospirifer sp. Chadian
Lemonfield
Common (~30)
9-20
No brachiopoda data
Data for the Kiltiernan locality are based on collections and notes by William King in the James Mitchell Museum. University College Galwav (see Harrier 19gg~.
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MID-DINANTIAN BRACHIOPODS
433
Table 2. Data from the Mid-Dinantian locality in the Kiltiernan Townland
Age
Location
Brachiopod species
Abundance
Size range (mm)
Mid-Dinantian
Kiltiernan Townland
Orbiculoidea sp. Rhipidomella michelini (L6veill~) Globosochonetes sp.
Rare (2)
3
Common (>20) Rare (1) Rare (5)
2-4
Common (>30) Rare (~10) Common (~20) Common (~15) Rare (~7)
1.5-5.5
Pleuropugnoides pleurodon (Phillips) Hustedia radialis (Phillips) Cleiothyridinafimbriata (Phillips) Crurithyris urei (Fleming) Crurithyris sp. cf. C. urei (Fleming) Spiriferid gen. et. sp. indet. Unispirifer? sp. Beecheria sp. 1
The Bunoghanaun fauna-central belt ( Holkerian-early Asbian ) This brachiopod fauna was collected from a series of exposed limestone pavements in the townland of Bunoghanaun and adjacent areas, Corrandulla, County Galway. The limestones are relatively pure, typical of the Burren Formation, lacking terrigenous material, cherts and apparently silicified fossils. The limestones are mainly pure skeletal packstones. The fauna is of relatively low diversity, dominated by shell beds of the large linoproductid brachiopod Linoprotonia ashfellensis Ferguson, together with clumps of the massive cerioid coral Lithostrotion vorticale (Parkinson). These large productoids were clearly recumbent, but like many other Palaeozoic brachiopod shell beds such as those formed by Ordovician and Silurian trimerellides and Silurian pentamerides, the large shells probably formed cosupportive shell banks.
The Kiltiernan Townland fauna-southern belt ( Mid-Dinantian) A second Kiltiernan locality, in south County Galway (southern belt), yielded a remarkable
15 1.5-4
1.5-5 1.2-5