Palaeozoic Reefs and Bioaccumulations" Climatic and Evolutionary Controls
The Geological Society of L o n d o n
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/~kLVARO,J. J., ARETZ,M., BOULVAIN,F., MUNNECKE,A., VACHARD,D. & VENNIN,E. (eds) 2007. Palaeozoic Reefs and Bioaccumulations: Climatic and Evolutionary Controls. Geological Society, London, Special Publications, 275. KERSHAW, S., LI, Y. & Guo, L. 2007. Micritic fabrics define sharp margins of Wenlock patch reefs (middle Silurian) in Gotland and England. In: ALVARO,J. J., ARETZ, M., BOULVAIN,F., MUNNECKE, A., VACHARD, D. & VENNIN, E. (eds) Palaeozoic" Reefs and Bioaccumulations." Climatic and Evolutionary Controls. Geological Society, London, Special Publications, 275, 87-94.
GEOLOGICAL SOCIETY SPECIAL PUBLICATION NO. 275
Palaeozoic Reefs and Bioaccumulations" Climatic and Evolutionary Controls
EDITED BY J. JAVIER ,/~LVARO University of Zaragoza, Spain and University of Lille I, France MARKUS ARETZ University of Cologne, Germany FRI~DI~RIC BOULVAIN University of Li6ge, Belgium AXEL MUNNECKE University of Erlangen-Niirnberg, Germany DANIEL VACHARD University of Lille I, France and EMMANUELLE VENNIN University of Bourgogne, France
2007 Published by The GeologicalSociety London
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[email protected] CONTENTS Foreword
~kLVARO,J. J., ARETZ,M., BOULVAIN,F., MUNNECKE,A., VACHARD,D. & VENNIN,E. Fabric transitions from shell accumulations to reefs: an introduction with Palaeozoic examples CLAUSEN,S. &/~kLVARO,J. J. Lower Cambrian shelled phosphorites from the northern Montagne Noire, France GANDIN, A., DEBRENNE,F. & DEBRENNE,M. Anatomy of the Early Cambrian 'La Sentinella' reef complex, Serra Scoris, SW Sardinia, Italy /~kLVARO,J. J. & CLAUSEN,S. Botoman (Lower Cambrian) turbid- and clear-water reefs and associated environments from the High Atlas, Morocco HUNTER, A. W., LEFEBVRE,B., RI~GNAULT,S., ROUSSEL,P. & CLAVERIE,R. A mixed ophiuroid-stylophoran assemblage (Echinodermata) from the Middle Ordovician (Llandeilian) of western Brittany, France KERSHAW, S., LI, Y. & Guo, L. Micritic fabrics define sharp margins of Wenlock patch reefs (middle Silurian) in Gotland and England HUBMANN, B. & SUTTNER,T. Siluro-Devonian Alpine reefs and pavements MABILLE, C. & BOULVAIN,F. Sedimentology and magnetic susceptibility of the Upper Eifelian-Lower Givetian (Middle Devonian) in SW Belgium: insights into carbonate platform initiation BOULVAIN, F. Frasnian carbonate mounds from Belgium: sedimentology and palaeoceanography POTY, E. & CHEVALIER,E. Late Frasnian phillipsastreid biostromes in Belgium ARETZ, M. & CHEVALIER,E. After the collapse of stromatoporid-coral reefs - - the Famennian and Dinantian reefs of Belgium: much more than Waulsortian mounds ALMAZAN-VAZQUEZ,E., BUITRON-SANCHEZ,B.E., VACHARD,D., MENDOZA-MADERA,C. & GOMEZ-ESPINOSA, C. The late Atokan (Moscovian, Pennsylvanian) chaetetid accumulations of Sierra Agua Verde, Sonora (NW Mexico): composition, facies and palaeoenvironmental signals BUITRON-SANCHEZ, B.E., GOMEZ-ESPINOSA,C., ALMAZAN-VAZQUEZ,E. & VACHARD,D. A late Atokan regional encrinite (early late Moscovian, Middle Pennsylvanian) in the Sierra Agua Verde, Sonora state, NW Mexico VENNIN, E. Coelobiontic communities in neptunian fissures of synsedimentary tectonic origin in Permian reef, southern Urals, Russia WEmI.ICH, O. Permian reef and shelf carbonates of the Arabian platform and Neo-Tethys as recorders of climatic and oceanographic changes THI~RY, J. M., VACHARD,D. & DRANSART,E. Late Permian limestones and the Permian-Triassic boundary: new biostratigraphic, palaeobiogeographical and geochemical data in Caucasus and eastern Europe ZAPALSKI, M. K., HUBERT, B. & MISTIAEN, B. Estimation of palaeoenvironmental changes: can analysis of distribution of tabulae in tabulates be a tool? Index
vii 1
17 29 51 71
87 95 109
125 143 163 189
201
211 229 255
275 283
Foreword The difficulty of studying reefs and shell accumulations rests primarily on its multidisciplinary position crossing numerous disciplines, such as biostratigraphy, geochemistry, palaeobiology, palaeoecology, petrology, sedimentology and taphonomy. The facies characterization of these bioclastic-bearing strata is, like many other biosedimentary structures, a process that requires the acquisition and integration of a wide and multiscale diversity of observations, which include field (global geometries), sample (fabrics) and thin-section (textures) scales. When we organized an international meeting focused on 'Climatic and Evolutionary Controls on Palaeozoic Reefs and Bioaccumulations' (7-9 September 2005, Paris, France), it was our intention to provide a forum for discussing the evolution of reefs, shell accumulations and their transitional deposits. We invited specialists on a wide range of taxonomic groups, siliciclasticmixed carbonate platforms and Palaeozoic ages to introduce a number of topics that are the focus of current research in the world of Palaeozoic benthic communities. The result of their contributions is presented in this Special Publication, which shows the complexity of intrareef synecological relationships and the diversity of concepts used to characterize both reefs and bioaccumulations. The editors offer, in the introductory paper (,~lvaro et al.), a discussion about concepts and definitions related to reefs and bioaccumulations. The transition between the concepts of reef and shell accumulation is gradual, as illustrated by some Palaeozoic examples, where reworked coquinas were episodically stabilized by encrusting communities and/or early diagenetic cements, in some cases forming the sole for future frame-building fabrics. Three Cambrian works are focused on shell-rich phosphorites from the Montagne Noire, France (Clausen & ,iAvaro), and microbial and archaeocyathanmicrobial reef complexes from Sardinia, Italy (Gandin et al.) and the High Atlas, Morocco (Alvaro & Clausen), the latter directly controlled by volcanogenic turbidity. The increase in biodiversity recorded in Ordovician bioaccumulations is illustrated by the characterization of a distinct echinoderm assemblage rich in ophiuroids and stylophorans (Hunter et al.). Kershaw et al. show that some Silurian reefs from Gotland and the U K have sharp boundaries, with the surrounding sediments terminating abruptly against the reef edge, and the sharp margins made up of automicrites; the sharp reef edges indicate
coherence of the micritic fabric, interpreted as a lithified wall against which bedded limestones were deposited. Hubmann & Suttner provide a review of Alpine Late Silurian-Late Devonian reefs and pavements, spanning a wide range of different autochthonous carbonates (e.g. brachiopod pavements, algal reefs, stromatoporoidcoral patch reefs) as well as allochthonous accumulations (e.g. serpulid accumulations). Two related papers offer an updated synthesis of the establishment of a carbonate platform (Mabille & Boulvain) and the sea-level-controlled evolution of Devonian mounds and atolls in the Dinant Synclinorium from Belgium (Boulvain). Examples for the youngest reefs of the Middle Palaeozoic reef community of stromatoporoids and corals are described from the latest Frasnian of Belgium (Poty & Chevalier). The struggle to establish a successful reef assemblage in the aftermath of the Kellwasser events and the importance of microbial communities in Famennian and Carboniferous reefs is described from Belgium (Aretz & Chevalier). The influence of palaeobathymetry and local synsedimentary tectonics in the establishment of carbonate factories is discussed in two papers focused on the development of Carboniferous chaetetid 'reefs' (Almaz~n-Vazquez et al.) and neighbouring crinoidal thickets (Buitr6n-S~nchez et aL) in Sonora, Mexico, whereas the documentation of sea-floor instability related to the evolution of the Permian foreland basin recorded in the southern Urals is characterized by Vennin. The input of cool waters within tropical PermoCarboniferous seas is analysed in Oman by Weidlich, and opens a large field of future discussions. Th6ry et al. provide new insights into the latest Permian reefs and bioaccumulations from eastern Europe and the Caucasus. And, finally, Zapalski et al. offer an estimation of palaeoenvironmental changes based on the distribution of late Middle Devonian tabulae in tabulate corals from northern France. One of the messages of this collection of papers is the wide diversity of sedimentary geometries and facies displayed by reefs, shell accumulations and transitional composite deposits. Readers will find that the papers in this Special Publication cover specific nomenclatural problems, evidenced by the widespread terminology used to describe skeletal assemblages. Rather than attempt a complete revision of terms, we have touched on some of the major issues at this stage of development in the field: the major
From:/~LVARO,J. J., ARETZ,M., BOULVAIN,F., MUNNECKE,A., VACHARD,D. & VENNIN,E. (eds) 2007. Palaeozoic Reefs and Bioaccumulations: Climatic and Evolutionary Controls. Geological Society, London, Special
Publications, 275, vii-viii. 0305-8719/07/$15.00 9 The Geological Society of London.
viii
FOREWORD
climatic, environmental and evolutionary factors that controlled the Palaeozoic development of shell accumulations and reefs. We hope that this volume attracts the attention of everyone interested in the fascinating diversity of Palaeozoic reefs and shell accumulation. It will be useful to senior undergraduate and postgraduate students of Earth Sciences and engineering. We also hope that it may prove useful to professionals
who explore and economically exploit Palaeozoic skeletal-rich strata. The editors would like to thank all contributors and referees for their rapid and stimulating collaboration. Thanks also for their suggestions, discussions, editorial work and constructive reviews. J. J. _~lvaro, M. Aretz, F. Boulvain, A. Munnecke, D. Vachard & E. Vennin
CONTENTS Foreword
~kLVARO,J. J., ARETZ,M., BOULVAIN,F., MUNNECKE,A., VACHARD,D. & VENNIN,E. Fabric transitions from shell accumulations to reefs: an introduction with Palaeozoic examples CLAUSEN,S. &/~kLVARO,J. J. Lower Cambrian shelled phosphorites from the northern Montagne Noire, France GANDIN, A., DEBRENNE,F. & DEBRENNE,M. Anatomy of the Early Cambrian 'La Sentinella' reef complex, Serra Scoris, SW Sardinia, Italy /~kLVARO,J. J. & CLAUSEN,S. Botoman (Lower Cambrian) turbid- and clear-water reefs and associated environments from the High Atlas, Morocco HUNTER, A. W., LEFEBVRE,B., RI~GNAULT,S., ROUSSEL,P. & CLAVERIE,R. A mixed ophiuroid-stylophoran assemblage (Echinodermata) from the Middle Ordovician (Llandeilian) of western Brittany, France KERSHAW, S., LI, Y. & Guo, L. Micritic fabrics define sharp margins of Wenlock patch reefs (middle Silurian) in Gotland and England HUBMANN, B. & SUTTNER,T. Siluro-Devonian Alpine reefs and pavements MABILLE, C. & BOULVAIN,F. Sedimentology and magnetic susceptibility of the Upper Eifelian-Lower Givetian (Middle Devonian) in SW Belgium: insights into carbonate platform initiation BOULVAIN, F. Frasnian carbonate mounds from Belgium: sedimentology and palaeoceanography POTY, E. & CHEVALIER,E. Late Frasnian phillipsastreid biostromes in Belgium ARETZ, M. & CHEVALIER,E. After the collapse of stromatoporid-coral reefs - - the Famennian and Dinantian reefs of Belgium: much more than Waulsortian mounds ALMAZAN-VAZQUEZ,E., BUITRON-SANCHEZ,B.E., VACHARD,D., MENDOZA-MADERA,C. & GOMEZ-ESPINOSA, C. The late Atokan (Moscovian, Pennsylvanian) chaetetid accumulations of Sierra Agua Verde, Sonora (NW Mexico): composition, facies and palaeoenvironmental signals BUITRON-SANCHEZ, B.E., GOMEZ-ESPINOSA,C., ALMAZAN-VAZQUEZ,E. & VACHARD,D. A late Atokan regional encrinite (early late Moscovian, Middle Pennsylvanian) in the Sierra Agua Verde, Sonora state, NW Mexico VENNIN, E. Coelobiontic communities in neptunian fissures of synsedimentary tectonic origin in Permian reef, southern Urals, Russia WEmI.ICH, O. Permian reef and shelf carbonates of the Arabian platform and Neo-Tethys as recorders of climatic and oceanographic changes THI~RY, J. M., VACHARD,D. & DRANSART,E. Late Permian limestones and the Permian-Triassic boundary: new biostratigraphic, palaeobiogeographical and geochemical data in Caucasus and eastern Europe ZAPALSKI, M. K., HUBERT, B. & MISTIAEN, B. Estimation of palaeoenvironmental changes: can analysis of distribution of tabulae in tabulates be a tool? Index
vii 1
17 29 51 71
87 95 109
125 143 163 189
201
211 229 255
275 283
Fabric transitions from shell accumulations to reefs: an introduction with Palaeozoic examples J. J A V I E R / i ~ L V A R O 1,2, M A R K U S A R E T Z 3, F R t ~ D I ~ R I C B O U L V A I N 4, A X E L M U N N E C K E 5, D A N I E L V A C H A R D 2 & E M M A N U E L L E V E N N I N 6
1Departamento Ciencias de la Tierra, Universidad de Zaragoza, 50009 Zaragoza, Spain 2Laboratoire LP3, UMR 8014 du CNRS, UniversitO de Lille I, 59655 Villeneuve d'Ascq, France (e-mail:
[email protected]) 3Institut fiir Geologie und Mineralogie, Universitiit zu K61n, Ziilpicher Strasse 49a, 50674 Kdln, Germany 4pktrologie skdimentaire, B20, Sart Tilman, Universitd de Likge, 4000 Likge, Belgium 5Institut fiir Paliiontologie, Universitiit Erlangen-Niirnberg, Loewenichstrasse 28, 91054 Erlangen, Germany 6UMR 5561 CNRS, Biogkosciences, Universitk de Bourgogne, 6 bd. Gabriel, 21000 Dijon, France Abstract: One unresolved conceptual problem in some Palaeozoic sedimentary strata is the boundary between the concepts of 'shell concentration' and 'reef'. In fact, numerous bioclastic strata are transitional coquina-reef deposits, because either distinct frame-building skeletons are not commonly preserved in growth position, or skeletal remains are episodically encrusted by 'stabilizer' (reef-like) organisms, such as calcareous and problematic algae, encrusting microbes, bryozoans, foraminifers and sponges. The term 'parabiostrome', coined by Kershaw, can be used to describe some stratiform bioclastic deposits formed through the growth and destruction, by fair-weather wave and storm wave action, of meadows and carpets bearing frame-building (archaeocyaths, bryozoans, corals, stromatoporoids, etc.) and/or epibenthic, non-frame-building (e.g. pelmatozoan echinoderms, spiculate sponges and many brachiopods) organisms. This paper documents six Palaeozoic examples of stabilized coquinas leading to (pseudo)reef frameworks. Some of them formed by storm processes (generating reef soles, aborted reefs or being part of mounds) on ramps and shelves and were consolidated by either encrusting organisms or early diagenesic processes, whereas others, bioclastic-dominated shoals in barrier shelves, were episodically stabilized by encrusting organisms, indicating distinct episodes in which shoals ceased their lateral migration.
A paramount quantity of information characterizes the formation and distribution of shell concentrations and reefs in modern and Cenozoic strata, but the knowledge of Phanerozoic shell concentrations and reefs is decreasing when increasing in age. The Palaeozoic is a key time span in the history of life owing to the wide occurrence of skeletonized metazoans (the so-called 'Cambrian explosion': Zhuravlev & Riding 2000) and the successive biodiversifications related to: (i) major extinctions, such as the end-Ordovician (Cooper 2004), the Frasnian-Famenian boundary (Buggisch 1991; Copper 2002; Racki & House 2002) and the endPermian extinction (Wignall & Hallam 1992); and (ii) major community replacements, such as the Lower-Middle Cambrian transition
(Debrenne 1991; Zhuravlev 1995), the Cambrian-Ordovician transition (Barnes et al. 1996) and the so-called 'Mid-Carboniferous extinction' (Tappan & Loeblich 1988; R a y m o n d et al. 1990; Vachard & Maslo 1996). Shell concentrations (also named bioaccumulations, coquinas or lumachelles) are defined as 'relative dense accumulations of biomineralized animal remains with various amounts of sedimentary matrix and cement, irrespective of taxa composition and degree of post-mortem modification' (Kidwell et al. 1986). By contrast, the concept of reef is more complex: reefs can form as a result of microbial growth, mixed microbial-skeletal growth, complex microbially devoid biomineralized metazoan assemblages, or the accumulation of reef-builder
From:/~LVARO,J. J., ARETZ,M., BOULVAIN,F., MUNNECKE,A., VACHARD,D. & VENNIN,E. (eds) 2007. PalaeozoicReefs and Bioaccumulations: Climaticand Evolutionary Controls'.Geological Society, London, Special Publications, 275, 1-16. 0305-8719107l$15.009 The Geological Society of London.
2
J.J. ALVARO E T AL.
remains (Webb 1996). Following Wood's (1998) definition, reefs 'develop due to the aggregation of sessile epibenthic marine organisms, with the resultant higher rate of in-situ carbonate production than in surrounding sites'. Although the relative capacity of some gregarious epibenthic organisms bearing mineralized skeletons to be preserved in living position is a common characteristic of reef organisms, this character is also shared with other non-reef metazoans adapted to soft substrates and forming bundles-like clusters. One group of these organisms, named 'secondary soft-bottom dwellers' by Seilacher (1984), differs from mud stickers (e.g. non-encrusting pelmatozoans and sponges) in the heavy weight of their shells. These metazoans, mainly brachiopods and molluscs, develop common cup-, boulder- and fan-shaped convergent morphologies favouring recliner strategies (Seilacher 1984; Savazzi 1999). Skeletal concentrations can be subdivided, according to their biostratinomic features, into biogenic, sedimentary, diagenetic and mixed concentrations (Kidwell et al. 1986). Biogenic concentrations can be intrinsic or extrinsic in character, the former generated by the organisms that produce the hard parts resulting from intrinsic gregarious behaviours of autochthonous and parautochthonous skeletal organisms (e.g. preferential colonization by larvae, and single colonization events of opportunistic species), and the latter produced by other organisms that interact with skeletonized organisms on their discarded hard parts leading to the formation of parautochthonous and allochthonous concentrations (e.g. hard-part-rich fecal masses and shell-filled pits). Sedimentary concentrations result from physical (usually hydraulic) processes of concentration, in which hard parts behave as sediment particles and non-bioclastic matrix is either reworked or fails to accumulate (e.g. shelly storm lags, aeolian beach pavements, channel lags in fluvial, intertidal and subtidal environments or shell-paved turbidities). Diagenetic concentrations form or are significantly enhanced by processes acting after burial, such as compaction or selective dissolution of matrix. And, finally, mixed concentrations form by the interplay of two or more kinds of the aforementioned processes, and can display both autochthonous, parautochthonous and allochthonous shell assemblages. One unresolved conceptual problem in some of these mixed shell concentrations is the boundary between the concepts of 'shell accumulation' and 'reef'. Some of them are transitional coquina-reef deposits, because either distinct
flame-building skeletons are not commonly preserved in growth position, or skeletal remains are episodically encrusted by 'stabilizer' (reef-like) organisms, such as calcareous and problematic algae, encrusting microbes, bryozoans, foraminifers and sponges. Some of these centimetre- to metre-thick, mixed shell assemblages are named biostromes, and are described as tabular or sheetlike, bioclastic beds. Biostromes are built up by alternating gregarious settlement and hydraulic reworking leading to multiple events of hard-part concentrations. Originally, the term biostrome was coined by Cumings (1932), who defined it as a stratiform bed composed 'mainly or exclusively of shell remains', so that it included the modern concepts of shell accumulation and reef. This was subsequently constrained by Kershaw's (1994) concept of 'parabiostrome', where the constructing organisms are less than 20% in place. Therefore, the (para)biostromes formed through the growth and destruction, by fair-weather wave and storm wave action, of meadows and carpets bearing flame-building (archaeocyaths, bryozoans, corals, stromatoporoids, etc.) and/or epibenthic, non-flame-building (e.g. pelmatozoan echinoderms, spiculate sponges, and many brachiopods) organisms. Another mixed shell accumulation results from the colonization of shell accumulations by bottom-dwelling, substrate-dependent organisms. This process, named 'taphonomic feedback' by Kidwell & Jablonski (1983), allows a better understanding of the influence of hard parts on the ecological success of living benthos. The reworking of skeletal material by waves and storms commonly provides scattered hard shells in otherwise soft-bottom habitats. This can facilitate colonization and reproductive success by species that require or prefer hard substrates (mainly epibenthic suspension feeders), and at the same time inhibit earlier species that can tolerate only the initial soft-bottom conditions (Kidwell 1991; Kidwell & Bosence 1991). In some cases, preferential colonization by shelled benthos of isolated shells, on otherwise disadvantageously soft sea floor, is the first stage of development in many reefs: these mixed accumulations are usually interpreted as reef soles and reef pioneer communities (Lecompte 1954). The aim of this paper is to outline the fabrics and geometries of Palaeozoic bioaccumulations and reefs, and to illustrate some transitions between both biosedimentary geometries. The role played by distinct encrusting organisms and early diagenetic cements, stabilizing the sea floor as a response to hydrodynamic fluctuations and palaeoecological relationships, is documented in detail.
PALAEOZOIC COQUINA-REEF TRANSITIONS
Palaeozoic shell concentrations Overlying the earliest occurrence of skeletonized microfossils in the Ediacaran and earliest Cambrian, shell concentrations are found in most Palaeozoic lithofacies, except in high-porosity sedimentary rocks (such as conglomerates and breccias) because of shell dissolution. Thickness, abundance, packing, internal fabric and fossil preservation are all affected by changes in sediment accumulation rate and vary locally (Kidwell 1991). The physical scale, frequency of occurrence and taphonomic attributes of shell beds in different sedimentary environments are controlled by intensity and frequency of storms, background current and wave agitation, and diagenetic processes (Aigner 1985; Brett & Baird 1986; Speyer & Brett 1988; Kidwell 1991; Speyer 1991) Patterns of shell accumulation varied over Palaeozoic time because of changes in the diversity and environmental distribution of skeletonized organisms and those that interact with skeletal parts. Kidwell (1990) recognized two different modes of shell concentrations: archaic and modern modes. The archaic mode of shell concentration is represented in Palaeozoic and Triassic strata, and is characterized by relatively thin concentrations dominated by brachiopods and other epifaunal and semi-infaunal organisms (except for crinoidal calcarenites). In contrast, the modern mode is primarily CretaceousQuaternary in age, and contains thin pavements and full three-dimensional bioclastic concentrations dominated by molluscs and other epifaunal and fully infaunal organisms. Li & Droser (1992, 1997) further recognized distinct differences across the Cambrian shell beds, and between Cambrian and Ordovician shell beds. Cambrian shell beds are dominated by trilobites and occur as thin pavements, whereas Kidwell's (1990) archaic mode first really occurred during the Early Ordovician with the onset of the Ordovician biodiversification. After the Ordovician, the shell beds became more common, thicker and taxonomically diverse (Kidwell & Brenchely 1994). As a result, Li & Droser (1997) proposed to distinguish three types of shell concentrations, named Cambrian, post-Cambrian (but still Palaeozoic) and modern modes. In addition, although fluctuations in biodiversity of shell concentrations are primarily related to the appearance of new biomineralized clades, they are also controlled by the ecospace occupation: Cambrian faunas utilized relatively little ecospace, occupying mostly epifaunal guilds, and subsequently diversified during the rest of the
3
Palaeozoic in association with a distinct diversification in infaunal and pelagic guilds (Bottjer et al. 1996).
Palaeozoic reef geometries and their nomenclatural problem The variety and complexity that make reefs and mounds so interesting is also responsible for long-lasting problems with concepts and definitions. It is not the aim of this introduction to review reef classifications, but to give some working definitions of the most frequently used terms. Recent and extensive reviews by James & Bourque (1992), Bosence & Bridges (1995), Wood (1998, 2001), Kiessling et al. (2002), Riding (2002), and Schlager (2003), and give full access to nature and characteristics of reefs and mounds. A major distinction between reefs (sensu stricto) and other kinds of buildups was highlighted by Lowenstam (1950): 'reefs and 'ecological reefs' of Dunham (1970) are organically produced wave-resistant topographic structures'. This concept was further developed by Heckel (1974), in which he proposed that reefs are buildups which display evidence of potential wave resistance or growth in turbulent water and of control over the surrounding environment. Although these characteristics may be relative and difficult to be established in ancient buildups, this definition satisfied a need to give a special status to reefs. Other buildups, usually lime-dominated, were grouped under the term 'mound' or 'mud mound'. This term was popularized by Wilson (1975), but attributed to so many different examples of buildups that it lost parts of its meaning. This probably led James (1978) to propose the term 'reef mound' for quiet-water buildups growing below the fairweather wave base, rich in poorly sorted lime mud. Later, James & Bourque (1992) simplified matter by referring to 'mounds'. They also suggested that there are three end members of mounds that would grade into reefs in a tetragonal diagram (Fig. 1). These three mound types are microbial or cryptalgal mounds, skeletal mounds and mud mounds. Microbial or cryptalgal mounds are formed by stromatolites and/or thrombolites; skeletal mounds (such also correspond to the 'biodetrital mounds' of Bosence & Bridges 1995) are those with organisms baffling, trapping or stabilizing mud; and mud mounds are those resulting from piling of lime mud with minor amount of benthic biota. Several case studies show that overlapping of this mound type is frequent as is vertical
4
J.J. J~LVARO E T AL. (1988) distinguished a kind of incomplete ecological succession, the so-called 'arrested reef successions', in which full climax stages are not formally attained. These pioneering stages are typical in environments in which strong external control limits the potential of reefs to reach the climax stage. Palaeozoic examples of coquina-to-reef transitions
Fig. 1. Schematic classification of reef and mounds, together with variation of some basic parameters. The dark arrow represents an evolution from mud mound to reef via skeletal mound and, possibly, cryptalgal mound due to bathymetric decrease.
evolution from one type to another, or from mound to reef (Textoris 1966; Bourque & Gignac 1983; Boulvain 2001). This mound-reef transition is usually related to shallowing (Hoffman & Narkiewicz 1977; Boulvain 2001) rather than to ecological maturation (Walker & Alberstadt 1975). It is accompanied by textural evolution, community replacements (Fagerstrom 1991) and change in the dominant early diagenetic process, from organomineralization in mounds to cementation in reefs (Neuweiler et al. 1999; Schlager 2003). The wide diversity of patterns preserved in reefs allow the application of Root's (1967) concept of community guilds, each consisting of several species competing for the same 'class of environmental resources'. These are called constructor, baffler, binder, destroyer (borers, raspers, etc.) and dweller (passive members). The three first guilds build the rigid framework of reefs at a rate of upwards accretion that exceeds the rate of deposition of the adjacent levelbottom sediments to give the reef positive topographic relief (Fagerstrom 1988). If the vertical succession of different kinds of shell concentrations is commonly related to community replacements, which are directly controlled by changing external factors (Copper 1988), reef systems can display radial facies changes associated with intrinsic ecological successions. An ecological succession is 'an orderly, directional, and predictable, pioneer-to-climax process of community and species development' (Copper 1988). Ecological successions take place in environments where external physicochemical constraints (responsible for the aforementioned community replacements) are not undergoing major changes. In addition, Copper
This section documents six examples of stabilized coquinas. Some of them formed by storm processes (generating reef soles, aborted reefs or being part of mounds) on ramps and shelves and were consolidated by either encrusting organisms or early diagenesic processes, whereas others bioclastic-dominated shoals in barrier shelves - were episodically stabilized by encrusting organisms, marking distinct episodes in which shoals ceased their lateral migration. The presence of microbial mats (commonly called stromatolites and thrombolites) is conspicuous, coating numerous erosive discontinuities where the crusts commonly fossilize substrate irregularities enhanced by skeletal fabrics. Where these erosive discontinuities are repeated vertically the episodic growth of microbial mats can be used as a time record of when they interrupted background-sedimentation patterns, which are characterized by the amalgamation of high-energy events that involved repeated shell accumulations. The biological response of microbial communities to coat stratigraphic discontinuities can be considered as an integral part of dynamic stratigraphy, as they enhance the preservation and identification potential of interruptions in the background sedimentation on substrates devoid of burrowing activity. A semantic problem is related to the use of apparent synonym words related to microbial fabrics, such as 'thrombolite', 'thromboid' or 'clotted fabric'. Kennard (1994) followed the general terminology proposed by Kennard & James (1986), except for the replacement of 'mesoclots' with 'thromboids' (Shapiro 2000, p. 168): 'I recommend that the term 'thromboid' should not be used because 'mesoclot' is a suitable word for meaning the mesostructural elements, and the similarity of 'thromboid' with 'stromatoid' (macrostructural form of a stromatolite in the revision of Grey, 1989) will be misleading' (Shapiro 2000, p. 169). As explained in this work and those included within this Special Publication, this nomenclatural problem is still open to discussion according to the experience of each research worker.
PALAEOZOIC COQUINA-REEF TRANSITIONS
Cambrian shell concentrations stabilized by microbial mats One example of successive shell accumulationmicrobial crust alternations is illustrated by the amalgamation of: (i) millimetre- to centimetrescale, high-energy sedimentary events, characterized by deposition of broken to disarticulated shells; (ii) interrupted by episodes with extremely low sedimentation rates that led to development of centimetre- to decimetre-thick, microbial reefs (Fig. 2a). The case study reported here is taken from the Micmacca Breccia, a member of the lowermost Middle Cambrian Jbel Wawrmast Formation from the Moroccan Atlas that deposited on temperate-water substrates. The member consists of reddish-brownish, volcanoclasticbearing, bioclastic limestones that alternate with shale or sandstone strata. Each limestone, up to 1.2 m thick, is composed of amalgamated units (up to 0.4 m thick), separated by erosive surfaces, which show a vertical modification of textures:
5
the lenses that directly cover the scour surfaces show chaotic lithoclast orientations, passing upwards into packstones and local grainstones and brecciated volcanoclast-dominated levels (Fig. 2b), in some cases forming low-angle laminae. The shell accumulations display a wide diversity of trilobite debris, echinoderm ossicles, and subordinate heteractinid and hexactinellid sponge spicules, chancelloriid sclerites, brachiopods and helcionellids (fidvaro & Clausen 2006). The aforementioned limestone units are separated by stromatolitic crusts, which are up to 8 cm thick, and vary from wrinkled to domal and columnar in shape (Fig. 2c). The stromatolites grew perpendicularly to substrate surfaces, and generally followed their irregularities. Although the stromatolitic microfabric was largely the consequence of in situ calcite precipitation, there is evidence of litho- and bioclastic incorporation into the structures, in a manner analogous to many modern microbial mats generating agglutinated stromatolites (Riding 1999).
Fig. 2. (a) Field aspect of the Micmacca Breccia limestones with marked intrabed erosive discontinuities covered by packstone-dominated shell concentrations capped by microbial mats, Tarhoucht, Anti-Atlas. (b) Photomicrograph of a packstone with polyphase bioclasts replaced by iron oxides: e, echinoderm ossicle; ch, chancelloriid sclerite; t, trilobite sclerite, Lemdad valley, High Atlas; scale = 250 ~tm. (e) Wrinkled (wm) to domal (dm) stromatolites encrusting a shell concentration and topped by a wacke-packstone; Tarhoucht, Anti-Atlas; scale = 1 mm. (d) Trapping of bioclasts and volcanoclasts within a microbial mat, subsequently covered by a packstone, Lemdad valley, High Atlas; scales = 1 mm.
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J.J./~LVARO ET AL.
Sediment consisting of quartz, feldspar, corroded micas, volcanoclastic sand- and silt-sized grains, and skeletons was episodically trapped and bound into the surface, increasing upwards in some crusts from 10 to 60% in volume (Fig. 2d). No evidence of grazing and boring organisms was found. The limestones of the Micmacca Breccia represent low-relief shoals that recorded abrupt changes in hydrodynamic patterns. These are indicated by the presence of centimetre- to decimetre-thick, microbial (stromatolitic) reefs that reflect successive interruptions of the background sedimentation, represented by the aforementioned high-energy coquinas.
Ordovician shell concentrations stabilized by encrusting bryozoans Late Caradocian-early Ashgillian echinodermbryozoan thickets and bundles characterize temperate-water substrates located on the northern margin of Gondwana. The Ashgillian, highlatitude, mixed substrates of the Lower-Ktaoua
Formation, in the Alnif area (eastern Anti-Atlas, Morocco), have recorded three distinct carbonate factories, displaying some examples of coquina-reef transition, primarily controlled by accommodation space, distance from source areas and subsequent siliciclastic input, and benthic-community replacements (Alvaro et al. 2006). The first benthic community is preserved on the foresets of siliciclastic shoal complexes. It is volumetrically dominated by brachiopods and non-pelmatozoan echinoderms, whereas robust bryozoans and phosphate-shelled brachiopods are secondary (Fig. 3a, b). The brachiopods are dominated by heterorthids and drabovids, which are epibenthic, fixosessile and plenipedunculate, and rafinesquinids interpreted as ambitopic, liberosessile juvenile brachiopods capable to change into quasi-endobenthic adult stages. This brachiopod assemblage was apparently adapted to sandy substrates, where heterorthids and drabovids with highly developed diductor muscles were able to prevent the sharp closure of their valves in turbulent waters, and
Fig. 3. (a) Carbonaceous siltstone of the Lower-Ktaoua Formation rich in calcite-walledbrachiopods and fragile-ramified bryozoans paralleling the inclined foreset of a shoal; scale= 2 mm. (b) A robust-ramified bryozoan inclined to the foreset plane; scale = 2 mm. (c) Non-pelmatozoan echinoderm with flat base parallel to the foreset plane; scale = 2 cm. (d) Distal phosphatic limestone (top arrowed) with alternation of encrusting bryozoans (eb) and brachiopod-dominated tempestites (b); scale= 5 cm.
PALAEOZOIC COQUINA-REEF TRANSITIONS rafinesquinids would have been able to change their position on unstable substrates. Sphaeronitid and aristocystitid echinoderms are also abundant (Fig. 3c). Despite their lack of encrusting strategies, they display their flat bases parallel to the foreset laminae, indicating episodic quiescent episodes in the shoal migration. As a result, this brachiopod-echinoderm benthic community marks episodes of decreased energy in prograding sandy shoals. The second benthic community, preserved in distal phosphatic limestones, is dominated by disarticulated to fragmented bryozoans and brachiopods (Fig. 3d). Common growth forms of bryozoans are robust and delicate branching, and multilaminar and unilaminar encrusting (nomenclature after Nelson et al. 1988). Carbonate- and phosphate-shelled brachiopods, trilobites, bivalves, ostracodes, ortoconic nautiloids and conulariids are locally abundant. The widespread variety of bryozoans indicates the establishment of bryozoan thickets that helped to stabilize episodically the substrate but without trapping significant amounts of mud. Finally, the top of the Lower-Ktaoua Formation is recognizable as a phosphatized coquina, which contains the peak in biodiversity patterns of the whole studied succession. It includes trilobites, brachiopods, bivalves, conulariids, orthoconic nautiloids, disarticulated pelmatozoan stems and scattered encrusting bryozoans. Brachiopods consist of fixosessile, plenipedunculate heterorthids and rhynchonellids, as well as the liberosessile ambitopic, quasi-infaunal xenambonitid Aegiromena aquila aquila. The condensed level also contains abundant trilobite debris derived from calymenids, dalmanitids, cheirurids and illaenids. Other taxa are sinuitid, eotomariid and holopeid gastropods, and hyolithids. This phosphatized coquina indicates development of condensation associated with marine depositional hiatuses, which allowed repeated early diagenetic phosphate cementation and encrustation made up by bryozoans.
Silurian shell concentrations stabilized dur&g early diagenesis The strata of Gotland (Sweden) represents an extraordinary well-preserved example of tropical carbonate platform development during Silurian times (late Llandovery-late Ludlow), and reflects a series of stacked carbonate platforms (Calner et al. 2004). On Gotland, a huge variety of carbonate facies is developed, from extremely shallow-water deposits mostly on the eastern part of the island to open-marine shelf deposits dominating the western part (Samtleben et al.
7
1996, 2000). Accordingly, the variety of different shell accumulations is very remarkable. However, a common characteristic of most of these deposits is that cementation occurred early in diagenetic history. Except for some crinoidal limestones, most coarse-grained limestones on Gotland do not show any fitted fabrics (Fig. 4b, c). This indicates that the original pore space was filled by calcite spar prior to mechanical compaction (cf. Bathurst 1995). Stylolites, which according to Bathurst (1991) are defined as 'serrated interface between two masses of rock', are common, and represent structures developed during late diagenesis. However, stylolites form only on sediment that has been lithified before. An early diagenetic lithification is also typically observed in fine-grained limestones from Gotland, where undeformed trace fossils and organic-walled microfossils document an early stabilization (= cementation) of the original carbonate mud (Munnecke & Samtleben 1996; Westphal & Munnecke 1997). These observations point to one of the fundamental questions in carbonate petrology (Bathurst 1970): how to cement a carbonate sediment while it is still largely uncompacted. Where was the source of such an enormous quantity of CaCO3? On Gotland, most of the section is developed as alternation of limestones and marls, which exhibit a wide morphological variety from nodular marl-dominated alternations to well-bedded limestone-dominated alternations. According to Munnecke & Samtleben (1996), lithification of the limestones in these alternations took place just below the sea floor in the shallow-marine burial realm. The source of the carbonate cement lithifying the micritic limestones is a selective dissolution of aragonitic constituents in the intercalated marl layers. Consequently, the marls are not cemented and represent a residual sediment depleted in aragonite. Owing to ongoing sedimentation, the marl layers were more and more compacted. But, what about coarse-grained shell accumulations? Where is the source for their carbonate cement? Bathurst (1995) pointed out that 'the style of pressure dissolution in pure limestones, fitted-fabric or stylolite, depends strongly on the availability of aragonite in the original sediment'. This, however, was questioned by Railsback (1996). But how can we prove the former existence of aragonite as source for the carbonate cement when it is dissolved during diagenesis? And when - that is, in which diagenetic environment - does aragonite dissolution occur? Cherns & Wright (2000), who compared early silicified faunas from Gotland with non-silicified faunas, show that aragonite dissolution must
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J.J./kLVARO E T AL.
Fig. 4. All thin sections come from the Silurian of Gotland, Sweden. (a) Irregular hard ground with thin pyrite-enriched layer, and reworked intraclast in the overlying sediment (Slite Group, Wenlock, proximal shelf facies, locality Hagan~s 1) ; scale = 1 cm. (b) Reworked intraclast (arrow) with sharply eroded bryozoan (left) and crinoid (right) debris. The fact that matrix as well as biogenic components are sharply truncated indicate that the reworked layer was already completely lithified prior to erosion (?H6gklint Formation, locality Gutev~igen); scale = 1 mm. (c) Shell accumulation dominated by bivalve shells, partly with geopetal fillings. All components show micritic envelopes, which is a common feature in shallow, warm water. The bivalve shells are completely recrystallized to clear, blocky calcite spar (Halla Formation, marginal marine to lagoonal facies, locality near Gothemhammar 1); scale = 2 mm. (d) Vertically stacked bivalve and brachiopod shells embedded in a fine peloidal-grainstone matrix. The sharp boundaries of the bivalve shells indicate that dissolution of the aragonitic shells occurred after cementation of the matrix. Interestingly, the bivalve shells are partly filled with fine-grained sediment. This indicates that after the aragonite dissolution fine-grained material was flushed into the pore network filling primary (left arrow) and secondary pores (the dissolved shells). Sometimes the shells are filled geopetally (right arrow) (Halla Formation, marginal marine-lagoonal facies, locality near Gothemshammar 1); scale = 2 mm.
have happened very early in diagenetic history. Their finding is confirmed by our results: Figure 4d illustrates that aragonite dissolution in the limestones of Gotland must have occurred very early, but after cementation. The sharp boundaries of the former aragonitic bivalve shells indicate that the first step in diagenesis was the lithification of the matrix. Later, the shells were dissolved and partly refilled with fine-grained sediment. This must have taken place where soft mud was still available, i.e. close to the former seafloor. The bivalve shells themselves, however, cannot be the source for the cementation of the matrix because: (a) the limestone was cemented
prior to dissolution of the aragonite shells; and (b) the amount of CaCO 3 provided by this process would be far too low. So, a source outside the limestone bed has to be considered (see above). An early cementation of the Silurian rocks on Gotland is also documented by the occurrence of abundant hard grounds and reworked intraclasts, mainly in proximal environments (Fig. 4a, b) (Samtleben et al. 2000). Hard grounds are sedimentary surfaces that have existed as hardened sea floor prior to the overlying sediment (Flfigel 2004). In general, hard-ground formation can be explained in two different ways:
PALAEOZOIC COQUINA-REEF TRANSITIONS (i) by an interruption in sedimentation resulting in cement precipitation directly from sea water; and (ii) by an exhumation of a limestone bed that was lithified close to the surface, i.e. a few decimetres or metres below the sea floor. Such sedimentary depth is assumed for the cementation of the limestones in limestone-marl alternations (Munnecke & Samtleben 1996). Changes in the local hydrodynamic regime can easily result in erosion of the topmost part of the sediment, which consists of soft, unlithified mud, and thus resulting in an excavation of the early cemented limestone layer. As long as the water energy is high enough to prevent accumulation of new carbonate sediment this layer represents a hardened sea floor, and might be either encrusted by hard-bottom communities, or corroded or bored by chemical processes or biological activities (bioerosion). As the internal character of hardground beds is indistinguishable from 'normal' limestone layers on Gotland, and no traces of early marine cements have been found (Fig. 4a), this second process is assumed to be the common process in hard-ground formation (Munnecke 1997). A similar process is proposed for the formation of so-called 'hiatus concretions' (Voigt 1968). It is more and more accepted that aragonite dissolution and early cementation is a very important process in early marine carbonate diagenesis, even in non-tropical carbonates (Munnecke & Samtleben 1996; Cherns & Wright 2000; Melim et al. 2002; James et al. 2005; Munnecke & Westphal 2005; Dix & Nelson 2006). However, there is an ongoing discussion as to whether the bulk of the carbonate cement is provided by dissolution of mud-sized aragonite derived from shallow carbonate platforms or by dissolution of benthic molluscs (Cherns & Wright 2000; Wright & Cherns 2004; see discussion in Munnecke & Westphal 2005).
Nebuloids: gel-stabilized shell concentrations & Devonian carbonate mounds Late Frasnian Petit-Mont Member mounds occur in the southern part of the Dinant Synclinorium and in the Philippeville Anticline (SW Belgium). These mounds are 30-80 m thick and 100-250 m in diameter. They are embedded in shale, nodular shale and argillaceous limestone. Mound growth typically initiated from below the photic and storm wave base zones and continued into shallow-water environments. Above an argillaceous limestone substrate, the
9
first mound facies consists of spiculitic wackestone with stromatactis, which becomes progressively enriched in crinoids and corals, then in peloids, stromatoporoids and cyanobacteria (named Pm3 in Boulvain 2001). The shallower facies consists of algal-coral-peloid wackestone and packstone with green algae and thick algal coatings. A core of algal and microbial bindstone sporadically occurs within large mounds (Boulvain 2001). Of particular interest is the pink limestone with corals, crinoids, stromatactis, fenestrae and lamellar stromatoporoids (Pm3): this facies marks the entrance of the mounds into the storm wave zone. Corals are generally tabular
(Alveolites,
Phillipsastrea,
Thecostegites),
branching (Thamnopora, Senceliaepora) or fasciculate (Thamnophyllum); solitary rugose corals are also present. Receptaculites is locally abundant. Cyanobacteria form partial coatings around particles, and peloids are common and irregular. This muddy facies includes enigmatic structures consisting of decimetre-scale pockets or decimetre-thick beds of dark grey fibrous cement containing sorted brachiopods and crinoids (Fig. 5b, c). These particular structures (called 'nebuloids' by Boulvain 2001) may pass laterally, by reduction in the proportion of spar, into a network of centimetre-scale stromatactis or fenestrae. In some mounds, the nebuloids are laterally uninterrupted over tens of metres and show a rhythmic pattern consisting of decimetrethick spar networks separated by layers of pink limestone (Fig. 5a). In these large-scale structures the fossil content changes laterally: brachiopods are prominent in the central part of the mounds, although crinoids are more abundant close to the flanks. This reflects the usual lateral zonation of the mounds at this growth stage. In thin section, the fibrous cement corresponds to isopachous crusts of radiaxial calcite, with micron-sized inclusions of dolomite, and are very rich in organic matter (Fig. 5d). Internal sediment is very rare, although it is common in other types of fenestrae. Sorting of bioclasts, rhythmic pattern, local hummocky cross-stratification and position of this facies in the shallowing-upwards mounddevelopment sequence indicate that these nebuloid structures are formed by storms. Periodic increasing of water agitation was responsible for lime mud erosion and the concentration of bioclasts in beds or pockets. After deposition, fossils were cemented by early fibrous cement. It is, however, difficult to date the precipitation of the fibrous spar. Was it truly synsedimentary, before settling of the muddy interlayers, or did it postdate mud accumulation? Several lines of
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J.J. J~LVARO ETAL.
Fig. 5. All pictures come from the Late Frasnian Petit-Mont Member mounds, Philippeville Anticlinorium, Belgium. (a) Rhythmic succession of nebuloids in the middle part of the Tapoumont mound; So = bedding. (b) Nebuloid structures (grey) in reddish limestone; pencil (arrowed) for scale; Les Bulants mound. (e) Close-up of a nebuloid, cemented by dark organic matter-rich radiaxial spar; a Receptaculitesneptuniis included in the spar to the right of the picture; coin (50 eurocents) for scale. (fl) Thin section in a nebuloid, showing dark organic matter inclusions (arrowed) and crinoid ossicles; crossed nicols, Tapoumont quarry; scale = 2 mm.
evidence suggest that cement was precipitated into a gel-like microbial mat stabilizing the storm layer: first, the unusual richness of the radiaxial cement in organic matter; and, second, the lateral transition of the nebuloids into mud with stromatactoid fenestrae. This could be explained by progressive lateral reduction of the mat, allowing mud to settle between the bioclasts. Unusual absence of internal sediment could also be explained by the presence of an organic gel.
Carboniferous shell concentrations stabilized by microb&l communities Shell accumulations are common features of many Carboniferous carbonate platforms. Some of these concentrations display important lateral extension, and thus are of stratigraphic importance; for example, the Upper Vis6an Orionastrea bed of England (Cossey et al. 2003). The composition of shell beds varies significantly in time and space. 'Encrinites', which are limestones of
various textures dominated by crinoid debris, are ubiquitously distributed (e.g. in the so-called 'Petit-Granit' in Belgium). Many Mississippian shell beds are macroscopically composed of brachiopods and/or corals. The compositional complexity of Pennsylvanian shell beds is similar to that of the older counterparts, but chaetetid sponges and calcareous algae play a more prominent role. The case studies described herein are Mississippian in age, and it is our aim to provide examples of the involvement of microbial communities into the hydrodynamic context of these shell beds. The first example is taken from the Middle Vis~an Lives Formation of the Namur Syncline (Belgium). The formation consists of a set of shallowing-upwards parasequences (Chevalier & Aretz 2005). An ideal parasequence starts with subtidal marine bioclastic packstones-grainstones, passing upwards into stromatolitic boundstones or peloidal mudstones of intertidal-supratidal environments, finally
PALAEOZOIC COQUINA-REEF TRANSITIONS capped by palaeosols. In one of the parasequences, a decimetre-thick bed rich in brachiopods, conventionally named the Composita bed, marks the transition from bioclastic limestone to lime mudstone. Without record of major macroscopical changes, the bed can easily be laterally traced over several kilometres. However, a microfacies analysis reveals a very heterogeneous composition of the bed: texture and allochem composition differ over short distances, with most of the bed grading between bioclastic grainstone (Fig. 6a) and oncoidal packstone (Fig. 6b). Microbial coating, resulting in advanced stages in oncolite formation, is ubiquitous in the bed, although increasing vertically in abundance and thickness. However, although reworking seems to be an important process in the shell bed formation, this trend may be partly obscured. The microbial coatings are important for the stabilization of the mobile substratum in which the shell bed formed. The development of proper boundstone fabrics is not observed. The transitional character of the shell bed fabric is
11
further expressed by the formation of small patch reefs, which initiated upon this bed (Chevalier & Aretz 2005). However, the reef fabrics show fundamental compositional and structural differences compared to the shell bed facies. The second example is taken from the Upper Visdan of the Montagne Noire (southern France). In the quarry of Castelsec, a small coral patch reef is topped by three well-bedded units, which are composed of partly argillaceous bioclastic limestones (Aretz & Herbig 2003). The lowest of these units is 2.8 m thick and comprises abundant small clisiophyllid and axophyllid rugose corals. Associated with the small corals are productid brachiopods, and larger and/or colonial rugose corals. All small rugose corals are coated with microbial crusts, which are differentiated into two crust morphotypes'. (i) crusts forming through incrustation of coral skeletons, thus resulting in a more circular geometry; and (ii) crusts of more aligned habits, which do not necessarily encircle corals, but seem to bound a number of previously encrusted (?) corals
Fig. 6. (a) Bioclastic-dominated part of the Compositabed; larger bioclasts are mainly brachiopods and gastropods; note the presence of thin microbial coatings surrounding some clasts; Compositabed (Middle Visdan), Engihoul Quarry ; scale = 2 mm. (b) Oncoidal grainstone; note the high diversity of encrusted bioclasts; Compositabed (Middle Visdan), Engihoul Quarry; scale = 2 mm. (c) Inner circular crust (morphotype 1) around a small solitary rugose surrounded by more aligned crusts (morphotype 2); outcrop picture, Castelsec Quarry, Montagne Noire; scale = 5 mm. (d) Complex microbial-dominated crust on coral skeleton; note the incorporation of grains; thin section from below the shell bed, Castelsec Quarry, Montagne Noire; scale = 1 mm.
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J.J./i.LVARO E T A L .
together. Both morphotypes are well preserved on weathered surfaces (Fig. 6c), but crusts of the first morphotype seem more abundant, as parts of the unit tend to develop a nodular appearance. The composition of these microbial crusts is complex, and involves dark micritic layers, calcimicrobes, trapped carbonate particles and sparry cement layers. The importance of the microbial crusts for shell bed formation is their bounding activity, which results in a wellpreserved coral rubble, and the homogenous appearance of the shell bed. However, the relative smallness of the corals indicates an originally somewhat unfavourable environment for corals. The abundance and composition of microbial crusts in this shell bed is very similar to other deposits in the Montagne Noire (Fig. 6d), but the failure to develop a substantial reef development, as seen in other outcrops (Aretz & Herbig 2003), remains enigmatic. It is important to remark that microbial communities played a key role in the formation and preservation of many Mississippian shell beds, although they seem to be absent in others. Therefore, the functional role of microbial communities, which is mainly stabilization through binding, either remains open or is fulfilled by other encrusting organisms, such as bryozoans and chaetetid sponges. However, shell beds with an important microbial community commonly tend to be more compact and massive than other shell beds.
Permian shell concentrations stabilized by richthofenid brachiopds, calcisponges and bryozoans In the Mixteco Terrane (SW Mexico), diverse Permian strata onlap the Acatlfi.n metamorphic substrate as a result of a Devonian (probably Ligerian) suture. The Middle-Upper Permian Olinal~i Formation (Guerrero state, Mexico) consists of three (lithostratigraphically unnamed) successions of, in ascending order, siliciclastic, carbonate and siliciclastic character (Flores de Dios & Buitr6n 1982; Vachard et al. 2004). The lower siliciclastic succession is Wordian in age, and is composed of a lower massive conglomerate, a middle black shale bearing middle-Wordian goniatites (Vachard et al. 2004), and an upper palaeodelta complex with terrestrial plants and brachiopods (Buitr6n et al. 2005). The overlying carbonate-dominated succession can also be subdivided into three levels: a lower stromatolitic level (Flores de Dios et al. 2000), probably early Capitanian in age, a middle part rich in channelized accumulations of giant
fusulinids (e.g. Polydiexodina capitanensis) and an upper part containing mud mounds, dated as late Guadalupian based on the presence of the fusulinid Codonofusiella extensa (Vachard et al. 1993); its genus is commonly Lopingian (=Late Permian) in the other parts of the world. The third siliciclastic succession consists of shales with scattered indeterminate, but apparently Permian, ammonoids (Flores de Dios & Buitr6n 1982). If the mud mounds of the Olinalfi. Formation are late Capitanian in age, and grew up into the Lopingian, they would represent the youngest Permian evidences of North American reefs. As in the Lopingian reefs of South China (e.g. Rigby et al. 1989; Weidlich 2002), the Olinal~ mud mounds are built in particular by calcisponges and richthofeniid brachiopods. These deposits are first composed of crinoid rudstones with rare Polydiexodina fusulinids, the whole fabric recording current structures and channel fillings. The richthofeniid and encrusting productid brachiopds occur directly attached to the surface of these encrinites (Fig. 7a, b). The space between the richthofenids is filled with micrite and/or crinoidal wackestone with thin carbonate cements (Fig. 7a, b). This benthic assemblage is subsequently replaced by sphinctozoan calcisponges (Fig. 7c) and encrusting bryozoans, in association with the aforementioned richthofeniids. At the top of the mud mounds, some stromatolites cap the buildups (Fig. 7d), which were finally drowned and buried by ammonoidbearing shales. In summary, the Olinal/t mud mounds display a vertical evolution that started by an accumulation of crinoidal ossicles, subsequently stabilized by richthofeniid brachiopods, secondarily replaced by sphinctozoan calcisponges and bryozoans, and finally capped by microbial mats.
Concluding remarks The occurrence of new biomineralized metazoans during the Palaeozoic, and their synecological relationships with microbial communities, have greatly increased the concepts and nomenclature necessary to understand the evolution of benthic communities through this key time span. Although the concepts of reef and shell accumulation are clear, numerous transitional coquina-reef deposits reflect fluctuations in hydrodynamic conditions and palaeoecological relations leading, in some cases, to the episodic stabilization of epibenthic non-flamebuilding meadows and carpets, and reef communities. Some of these skeletal-rich geometries
PALAEOZOIC COQUINA-REEF TRANSITIONS
13
Fig. 7. (a) Two ventral richthofeniid valves, probably Cyclacantharia sp., directly attached to an encrinitic or bioclastic, slightly sandy, wackestone; note the numerous vesicles; Late Capitanian. (b) Another contact, although slightly modified by a small stylolite, of an encrinitic substrate of well-sorted ossicles and richthofenids; Late Capitanian. (c) Sphinctozoan calcisponges and encrusting bryozoa, associated with richthofeniids. (d) Stromatolites (top right) participating in the constructions with calcisponges (bottom). All samples from the Olinalfi section, Guerrero, Mexico; Late Capitanian.
can be described using Kershaw's concept of 'parabiostrome', which takes into account the percentage of constructing organisms preserved in living position.
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Lower Cambrian shelled phosphorites from the northern Montagne Noire, France SI~BASTIEN C L A U S E N 1 & J. J A V I E R .A,L V A R O 1'2
~Laboratoire de Palkontologie et Palkogkographie du Paldozofque, U M R 8014 CNRS, Universitk des Sciences et Technologies de Lille, 59655-Villeneuve d'Ascq, France (e-mail." Sebastien.
[email protected]) 2Departamento Ciencias de la Tierra, Universidad de Zaragoza, Ciudad Universitaria, 50009-Zaragoza, Spain
Abstract: Shelledphosphorites of Early Cambrian age are common in the Av6ne-Mendic autochthonous unit (Marcory Formation) and the M61agnes nappe ('Heraultia beds' of the Lastours Formation), northern Montagne Noire (France). Palaeogeographically, the concentration of phosphate took place along the shelf edge between a stable inner platform (southern Montagne Noire) and an unstable slope-to-basin sea floor preserved in the northern Montagne Noire. Petrography, back-scattered SEM (scanning election microscopy) and elemental mapping by EDS (energy dispersive system) show that the phosphorites were generated by repeated alternations of low sedimentation rates and condensation forming hardgrounds, in situ early diagenetic phosphogenesis, winnowing and polyphase reworking of previously phosphatized skeletons and hardground-derived clasts. The successionof repeated cycles of sedimentation, phosphate concentration and reworking led to multi event phosphate deposits rich in allochthonous particles. Associated accumulations of exhumed and reworked pyrite clasts reflect final deposition in a mainly dysaerobic substrate.
A critical aspect of the NeoproterozoicCambrian transition is the occurrence of phosphogenic events contemporaneous with the 'Cambrian explosion' of metazoans, some of which are characterized by phosphate-shelled skeletons. The origin of phosphate shells (original biomineralization v. secondary epigenesis) is key to a better understanding the palaeoceanographical and palaeogeographical factors that controlled Neoproterozoic and Lower Cambrian phosphorites (Brasier 1990). In the present day, phosphorites form in two distinct environments: (i) along west-coast margins, where dynamic upwelling of nutrientrich water enhances primary productivity and development of oxygen minimum zones, thereby favouring accumulation of organic-rich sediments and subsequent phosphogenesis; and (ii) along current-dominated margins, where low sediment accumulation rates allow intense biogenic mixing of surficial sediments, giving rise to supersaturation of pore waters with respect to apatite (F611mi et al. 1991). In addition, porewater concentrations of dissolved apatite may be enhanced by microbial activity, which can mediate apatite precipitation (Wilby & Briggs 1997; Xiao & Knoll 1999). The purpose of this paper is to analyse the occurrence of Lower Cambrian phosphogenesis
in the northern Montagne Noire, France. This study takes into account the geochemical, petrographical, sedimentary and palaeogeographical aspects of the phosphorites, which offer key data to understand the geodynamic patterns of this margin of Gondwana after the Cadomian orogeny.
Geological setting and stratigraphy The Montagne Noire (southern France) consists of a complex framework of tectonostratigraphic units (Fig. 1a), which are grouped into three main structural domains (G~ze 1949): (i) a metamorphic axial zone, made of complex domes of gneiss and migmatites surrounded by micaschists, which has yielded Precambrian acritarchs in the 'Schistes X' unit (Fournier-Vinas & D6bat 1970); (ii) the southern flank composed of large nappes involving Lower Cambrian-Carboniferous strata; and (iii) the northern flank made of imbricated tectonic nappes composed of Lower Cambrian-Silurian rocks. Our study is focused on two stratigraphic units located on the eastern part of the Lacaune Mountains, northern Montagne Noire: the lower part of the Marcory Formation from the Av~ne-Mendic autochthonous unit, and the 'Heraultia beds' (uppermost part of the Lastours Formation) from the
tkLVARO,J. J., AREXZ,M., BOULVAIN,F., MUNNECKE,A., VACHARD,D. & VENNIN,E. (eds) 2007. Palaeozoic Reefs and Bioaccumulations: Climaticand Evolutionary Controls. Geological Society, London, Special From:
Publications, 275, 17-28.0305-8719/07/$15.00 9 The Geological Society of London.
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S. CLAUSEN & J. J./i~LVARO
Fig. 1. (a) Pre-Hercynian rocks of France and neighbouring areas with location of the Montagne Noire. (b) Map of the tectonostratigraphic units that form the Montagne Noire. (e) Geological map of the M61agues nappe and the Av6ne-Mendic unit; modified from Gu6rang6-Lozes & Burg (1990).
Fig. 2. (Neoproterozoic?)-Lower Cambrian stratigraphic framework of the M61agues and Brusque nappes, and the Av6ne-Mendic unit; summarized from Cobbold (1935), Thoral (1935), Roques & Vachette (1970), Courtessole (1973), Hamet & All+gre (1973), Rolet (1973), Boyer-Guilhaumaud (1974), Alsac & Donnot (1978), Donnot & Gu6rang6 (1978), Gachet (1983), Courjault-Rad6 (1985), Debrenne & Courjault-Rad6 (1986), Kerber (1988) and Gu6rang6-Lozes & Burg (1990).
M61agues nappe (Fig. 1b). The Lower Cambrian of the M61agues nappe offers close lithostratigraphic similarities with the fossiliferous Minervois and Pardailhan nappes (southern Montagne Noire) where the Lower Cambrian litho- and,biostratigraphic units were formally defined (Alvaro et al. 1998a). By contrast, the Lower Cambrian(?) lithostratigraphy of the Av6ne-Mendic units is complicated by the presence of volcano-sedimentary complexes,
which are absent in the southern Montagne Noire. The lower part of the Marcory Formation in the M61agues nappe (Fig. 2), with a total thickness of approximately 1000 m, consists of alternations of sandstones and green and black shales that contain phosphatic limestone nodules and beds. These occur interbedded within slope-related deposits, and grade upwards into the shallower marine carbonate strata of the
CAMBRIAN PHOSPHORITES FROM FRANCE Pardailhan Formation (Rolet 1973). The presence of the earliest Cambrian ichnogenus Taphrelminthopsis at the uppermost part of the Marcory Formation, considered by Seilacher (1997) as a senior synonym of Psammichnites, allows a broad correlation of these strata with the Tommotian-Atdabanian transition (Dor6 1994; Alvaro & Vizcaino 1999). The archaeocyathanbearing limestones and dolostones of the overlying Pardailhan Formation in the Brusque nappe (northern Montagne Noire: Debrenne & Courjault-Rad6 1986), and the Minervois and Pardailhan nappes (southern Montagne Noire: last revision in Alvaro et al. 1998a) are Botoman in age. The Av6ne-Mendic unit contains an incomplete Lower Cambrian volcano-sedimentary succession, underlain by the Mendic granite (broadly dated as 600-507Ma: Roques & Vachette 1970; Hamet & All6gre 1973; Gu6rang6-Lozes & Burg 1990) and overlain by the tectonic contact with the M61agues nappe. In ascending order the stratigraphic units comprise the so-called 'blavi6rites' (rhyolitic tufts and breccias) and the Layrac volcano-sedimentary complex (both units with an undetermined Neoproterozoic-Early Cambrian age), and the Pardailhan and Lastours formations (Fig. 2). The upper part of the latter contains some bedded phosphatic limestones (named 'Heraultia beds'), which have yielded a phosphatized skeletal and shelly microfauna composed of annelid and chancelloriid sclerites, brachiopods, crustaceans, gastropods, halkieriids, trilobites and problematica (Cobbold 1935; Kerber 1988). The presence of the mollusk Heraultia, characterized by a pandemic distribution in the Early Cambrian (Miiller 1975; Runnegar 1981), was taken by Kerber (1988) as indicative of a Botoman age. This proposal was revised by Gubanov (2002), who dated them as early Tommotian. However, this is incompatible with the age of the underlying archaeocyathanbearing Pardailhan Formation. More Lower Cambrian phosphorites are reported from other nappes of the northern Montagne Noire and the neighbouring C6vennes (e.g. Orgeval & Capus 1978; Notholt et al. 1979; Prian 1979, 1980; Notholt & Brasier 1986; Southgate 1986), but they are not documented here.
Palaeogeographical context The tectonostratigraphic patterns displayed by the Cambrian sedimentary rocks of the southern and northern Montagne Noire are still a matter of debate. The Lower Cambrian of the southern Montagne Noire represents what was a
19
homogeneously subsiding, stable platform where volcanic activity was absent, except for giving rise to: (i) the increased feldspar content in arkoses of the Marcory Formation (CourjaultRad6 1985), interpreted as the volcanoclastic influence of a magmatic activity recorded in the northern Montagne Noire; and (ii) the presence of volcanic and hydrothermal deposits associated with the massive sulphide-ore stratabound of the Salsigne district (Minervois nappe: L6pine et al. 1984, 1988). The southern Montagne Noire lacks Cambrian phosphorites, black shales, reworked pyrite clasts and slope-related strata (Alvaro et al. 1998b; Debrenne et al. 2002). Conversely, the Lower Cambrian of the northern Montagne Noire is characterized by an episodic tectonic instability of the platform with development of slopes and localized dysoxic substrates, phosphogenic episodes and volcanogenic influence, the last related to oceanic extension and rifting recorded in the northernmost part of the northern Montagne Noire (Alsac et al. 1987). Therefore, the northern Montagne Noire represents a heterogeneously subsiding, unstable margin bordering the relatively stable inner platform preserved in the southern Montagne Noire. Relics of primary to early diagenetic evaporites, reported from Lower Cambrian sedimentary rocks of SW Europe, are also present in the Lastours Formation of the southern Montagne Noire (Alvaro et al. 2000, 2003). This suggests development of an arid subtropical belt in the Early Cambrian southern Hemisphere (Alvaro et al. 2000), which affected the margin of Gondwana in the Montagne Noire region coevally with the phosphogenesis that took place on this shelf margin.
Methods Both originally phosphatic and secondarily phosphatized shelly and skeletal microfossils were extracted from the limestone matrix by dissolving the rock in a dilute (10% by volume) acetic acid. Diagenetic processes were investigated by a combination of petrographic observations: replacements, overgrowths and cross-cutting relationships of cements were clarified via optical and cathodoluminescence microscopy of ultra-thin sections, and backscattered scanning election microscopy (SEM) on polished and etched surfaces. Geochemical analyses were made by an energy dispersive system (EDS) of elemental mapping attached to the SEM (see Martill et al. 1992). This approach allows differentiation of polyphase phosphatic encrustation and impregnation: owing to slight
20
S. CLAUSEN & J. J. ALVARO
Fig. 3. Phosphatic heterogeneous hardgrounds of the M61aguesnappe. (a) Crenulation schistosity. (b) Detail of boxed area with lineation of blavieritic and pyrite grains cemented by phosphate. (c) & (d) Cemented blavieritic clasts, pyrite and bioclasts; all scales= 2 mm. p, pyrite grain; b, blavieritic (rhyolitic) grain; sf, bioclast.
modifications in the geochemical composition of apatite cements.
The phosphorites of the M~lagues nappe The phosphatic limestones of the Marcory Formation are interbedded with grey-black, well-laminated-massive shales (Fig. 2). They occur in one of the thrust sheets that characterize the southernmost edge of the M61agues nappe. Their outcrop has a lenticular shape, up to 100 m long, and is laterally bounded by intra-Marcory Hercynian faults. Influenced by the tectonic deformation, the fabric of the limestones is dramatically affected by an asymmetric crenulation schistosity (Fig. 3a). Other deformation processes are illustrated by the wealth of solution seams and stylolites, concentrated along cleavage surfaces. The presence of millimetre- to centimetre-thick, phosphate-rich laminae, easily identifiable by their grey-black colour, is directly controlled by this schistosity: the phosphatic bands display microfolding lineation formed by stretching and necking (boudins) following
the crenulation (Fig. 3a). We consider these laminae 'hardgrounds' following Southgate's (1986) concept of 'phosphatic hardground', i.e. a surface of synsedimentary lithification including grainy pavements of submergent or semi-emergent origin. Two kinds of hardgrounds can be distinguished in the Marcory Formation: (i) thinner (less than 3mm), homogeneous crusts made up of amorphous collophane; and (ii) thicker (up to 2 cm thick), heterogeneous (polymictic) lag deposits where a complex mixture of allochems is cemented with collophane (Fig. 3b). Hardgrounds are laterally discontinuous, and show a sharp upper contact and a lower gradational phosphatic concentration. Hardgrounds are interbedded with limestones (up to 80cm thick) that commonly exhibit normal grading from packstones to mudstones rich in skeletons and extraclasts (up to 2 cm in size) embedded in a sparry calcite mosaic. Although their allochems commonly display a randomly oriented fabric, long axes of sparry calcite crystals and elongated clasts are in some cases aligned subparallel to the crenulation
CAMBRIAN PHOSPHORITES FROM FRANCE axial planes displayed by the hardgrounds, resembling flow structures. Cathodoluminescence shows that identification of the original fabric of the sparry cement was a clast-supported breccia, dominated by subangular (secondarily subrounded in volume) calcite clasts, skeletons, shells and extraclasts, with dull red luminescence. This breccia is cemented by blocky calcite cements up to 100 gm across, which display an orange-bright yellow luminescence. The heterogeneous hardgrounds contain silty to medium sand-sized particles, composed of quartz, feldspar, rhyolitic clasts ('bliov6rites'), mica flakes, pyrite and other opaques, rip-up mud clasts, skeletons, shells and reworked phosphatic intraclasts derived from both homogeneous and heterogeneous crusts. These components are also dispersed in the sparry mosaics of calcite (Fig. 3c). Intraclastic phosphatic clasts are subrounded-subangular, and contain variable amounts of inclusions such as silt-sized quartz, rhyolitic clasts and mica flakes. Skeletal and shell material is largely composed of abraded and fragmentary, calcitic conical microfossils, up to 8 mm long, of uncertain affinity and commonly telescoped. Skeletons and shells occur as both isolated debris and embedded in multiphase composite clasts. The latter contain first-generation bioclasts, accretionary mud-sized sediment and calcite and phosphate cements truncated at borders, indicating that at least parts of the sea floor were lithified. Lithic extraclasts consist of reworked pyrite and other opaques, quartz, feldspar, chert and rhyolitic ('blavieritic') fragments and mica flakes, all silt to medium sand sized (Fig. 3d). Reworked concentrations of pyrite consist of single and composite grains. Although they are concentrated in the phosphatic hardgrounds, they also occur dispersed throughout the mosaics of sparry calcite. Reworking of pyrite is shown by mechanical breakage of pyrite grains, and the polyphase nature of some compound pyrite agglomerates. In addition, the aforementioned skeletons, shells and extraclasts (blavierites) can be coated by single or multiple laminae of collophane. The polymictic character and diversity of the extra and brecciated intraclasts reflect a complex provenance from different sources. The clasts were eroded from an upslope composed of: (i) siliciclastic muddy substrates (reworked as consolidated rip-up mud clasts) and characterized by anaerobic or minimally aerobic conditions (due to reworking of pyrite grains); (ii) an inherited palaeotopography composed of 'blavieritic' rocks, which crops out in the neighbouring Av6ne-Mendic unit; and (iii) carbonate substrates.
21
The limestones display lenticular concentrations of intra- and extraclasts, overlying distinct erosive surfaces, alternating with phosphatic hardgrounds. The episodic development of hardgrounds is evidence for two kinds of sedimentation rates: (i) normal, or background, conditions represented by upslope carbonate productivity and reworking of carbonate substrates; punctuated by (ii) condensation episodes represented by fall in terrigenous input and cementation of phosphatic sediment. There are neither foreset nor laminated structures to document a current or wave mode of transport, whereas evidence of erosive surfaces and polyphase transport of hard parts is abundant. This is indicated by local size sorting of shells and allochems, high-energy textures (lags related to underlying erosive bases) and preservation of shells with exotic matrix. The telescoped nature of the conical calcite-shelled skeletons also shows the influence of current activity. The abundance of multiphase compound clasts suggests multiple depositional, cementation and erosive events. The mixture of rhyolitic ('blavieritic') extraclasts, breccia, phosphatic and skeletal intraclasts indicates reworking processes on a complex submarine slope, which favoured exhumation and reworking of different lithotypes. The first erosion and transport of the allochthonous clasts are believed to have been variably aerobic, whereas their final sedimentation took place under dysaerobic conditions. Exhumation and concentration of pyritic debris require a sediment-starvation in a normally dysaerobic environment. Completely anaerobic conditions are not necessary for preservation of reworked pyrite grains because, once formed, pyrite may remain stable in a minimally oxygenated environment for a considerable time, for example in distal turbidites and contourites (Baird & Brett 1986).
The phosphorites of the Av~ne-Mendic unit In the vicinity of Saint-Geni6s de Varensal and Marcou (Fig. 1), the 'Heraultia beds', up to 20 m thick, are located at the upper part of the Lastours Formation. They consist of mottled, irregularly laminated, dolomitic limestones (Fig. 4a). The mottling aspect of these phosphoritic limestones is the result of the abundance of millimetre- to centimetre-thick, nodules of dolomite embedded in grey-white limestone. The patches of orange-coloured ferroan dolomite are subelliptical sucrosic mosaics of euhedral dolomite rhombs, up to 300 gm across. The mottling fabric displays a disorganized arrangement of centimetre-thick, sheet-like, contorted beds locally interrupted by breccia channels.
22
S. CLAUSEN & J. J. ALVARO
Fig. 4. Phosphorites of the Av6ne-Mendic unit. (a) Mottled aspect of the 'Heraultia beds' with squeezed and folded structures (dolostone laminae in light grey colours). (b) Intraclasts filling a channel; scale = 5 cm. (c) Cross-laminated structure by arrangement of bioclasts and extraclasts; scale = 1 cm. (d) Polyphase clasts with attached cements (c), see longitudinal section of an hyolith (hy) and oblique sections of helcionellids (he); scale = 2 mm.
Truncation surfaces, intraformational slumps and synsedimentary folding are common. Slumping and sliding has resulted in deformation of entrained material, lithologically similar to the underlying sediments, which displays a range of brittle 'firm' and soft-sediment deformation, contorted and folded. On a smaller scale, sedimentary boudinage (Fig. 4a) resulted from extensional stresses within a poorly lithified to unlithified sediment mass. Interbedded clastsupported, intraformational breccia is largely oligomictic and channelized (up to 1.6 m thick). The elongated character of its angular clasts ('chips' of Fig. 4b), up to 5 cm long, suggests provenance from the sheet-like contorted beds cut by the channels. Both the contorted tabular beds and breccias are wackestones-packstones rich in polyphase clasts (Fig. 4d), trilobites (Micmacca? albesensis Cobbold 1935) and other arthropods, brachiopods, halkieriids, helcionellids, hyoliths, and other skeletonized microfossils (see systematics in Kerber 1988). Skeletons are disarticulated and commonly broken, and locally display low-angle laminae (Fig. 4c). Both the microsparitic matrix
and the intraparticle porosity of the skeletons contain a (calcarenitic) mixture of fragmented skeletons, opaque minerals and phosphate silt-sized clasts. The aforementioned structures document different types of downslope movements by basal sliding of plastic to semi-rigid sediment masses. The reworked material was developed on slope(s) close to the sedimentary environments from which they were derived. Slope deposition from gravity flows occurred when substrate was both cohesive (brecciation and development of minor channels) and unconsolidated (disturbed and slumped substrate rich in disarticulated and broken skeletons). The lack of rhythmic turbidites suggests low-angle slopes, in contrast with the conventional steepened submarine fan models, which imply base-of-slope to basin deposition dominated by turbidites.
Diagenetic cements The porosity-occluding phases recorded in the shelly and skeletal phosphorites of the Marcory and the 'Heraultia beds' are similar (Table 1).
C A M B R I A N PHOSPHORITES F R O M F R A N C E
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LATE PERMIAN EASTERN EUROPE AND CAUCASUS
Robuloides, Lingulina, Pseudolangella, 'Frondina" and advanced Colaniella; and the fusulinids Palaeofusulina nana Likharew, P. wangi Sheng, and P. sinensis Sheng with also species of Reiehelina, Codonofusiella and Parananlingella (Kotlyar et al. 1999b; Pronina-Nestell & Nestell 2001). Our samples contained bituminous bioclastic packstones with Diplosphaerina ex gr. inaequalis (Derville), Paraglobivalvulina sp., Pseudovermiporella nipponica (Endo), Midiella zaninettiae (Altiner), Reichelina media Miklukho-Maklay, Palaeofusulina wangi and Nodosinelloides sp., with more than 60% of the gymnocodiacean red algae (predominant Permocalculus sp., at the stage Dzhulfanella, and subordinate Gymnocodium exile Mu). The Nikitian facies contains also some sponge mounds (Gaetani et al. 2005). The Urushtenian facies consists of bioclastic limestones with abundant algae Permocaleulus and brachiopods. The microfauna is rich and diversified, and, compared with the Nikitian limestone, is characterized by Lasiodiscus,
Pseudomidiella, Graecodiscus, Neodiscus, Multidiscus, Hubeirobuloides, Nankinella and Parareiehelina (Pronina-Nestell & Nestell 2001). One of our samples at the top of the Urushtenian facies ('sample n ~ 2' in Fig. la) contains
Gymnocodium
exile, Shamovella obscura Permocaleulus fragilis (Pia), Palaeofusulina wangi, Pseudovermiporella nipponica, Hemigordiellina sp., Midiella zanitettiae and Nodosinelloides sp. This sample demon(Maslov)
and
strates definitively the contemporeanous character of the Nikitian and Urushtenian facies (Palaeofusulina wangi has not been discovered in the Urushtenian facies by Pronina-Nestell & Nestel! 2001, table 1 p. 211), and indicates, at least locally, stable palaeoecological and, consequently, geochemical conditions up to the PTB. The brachiopod assemblage of this facies is composed of bioherm-building genera, especially Leptodus, Richthofenia and Permianella (Kotlyar et al. 1999b). The colonial rugose coral Waagenophyllum is also mentioned (Kotlyar et al. 1999b). The top of Urushtenian facies consists of burrowed mudstones. Our samples show local pyritizations and only one undetermined
Nodosinelloides. The Changhsingian Lagoon The facies are generally rich in organic matter and were preserved in a restricted environment. They could correspond with a bay or lagoon, on a rimmed carbonate ramp (Burchette & Wright 1992). In particular, the boreholes associated
259
with the field sections revealed a lagoon protected in the south (Fig. la, b). This lagoon stretches for at least 50 km towards the NW from the Belaya to Raskol-Cliff (Fig. lb). It generally exhibits an assemblage of algae, bryozoans, brachiopods, sponges, Shamovella obscura, Gymnocodium exile, Reichelina media and Geinitzina spp. Pronina-Nestell & Nestell (2001) described similar assemblages of foraminifers dominated by Palaeofusulina nana at Gefo, west of Nikitin (Fig. lb). The lagoon is limited towards the north by an emerged zone that was made clearly visible by the drilling at Bagovskaya (Figs lb & 2b). According to Pinchuk (pers. comm.), in this borehole the Jurassic deposits (1044m thick) rest on Permian deposits biostratigraphically characterized up to the Murgabian (middle Middle Permian) crinoidal sandstone by Platycrinites ex gr. laevis Millot. They were deposited on a sandy shoal where phyllitic components from the Precambrian metamorphic substratum and conglomeratic sandstones are also present. These metamorphic Proterozoic green slates (Fig. 1) were penetrated at the base of the drill hole up to -3500m (Fig. 2b). Nevertheless, the presence of the palynomorph Protohaploxypinus sp. and the smaller foraminifer Geinitzina cf. ovata Lange (Pinchuk & Mikerina pets. comm.), just below the Jurassic, would indicate some Late Permian deposits before the emergence and erosion (Fig. 2b). We agree with this hypothesis as we consider that 'Geinitzina' ovata is not a true Geinitzina but most probably a Pachyphloides or a very primitive Colaniella, both taxa whose FAD (first appearance datum) is Late Permian in age (Gaillot & Vachard unpublished data). Consequently, although limited by a fault (Fig. lb), the northern extension of the lagoon is probably entirely preserved. As another piece of evidence of the coastal line, we observed a reddish sandy molasse with granite and Palaeozoic components in the Rufabgo ravine, tributary of the Belaya River (Fig. l b). The horizon is dated as Early Triassic in the transgressive Yatyrgvart Formation. Pinchuk & Kotlyar (pets. comm.) have observed Ammodiseus minutus Dunn here (Fig. l a) and a perhaps locally characteristic Nodosaria, as in the neighbouring section at Balka Svinachay near the Sakhray River (Fig. la, b). We suppose that these two foraminifers might be, respectively: (a) some primitive, planispirally, undivided, tubular cornuspirid miliolids, which are the first to reappear after the PTB, probably Rectocornuspira kahlori Br6nnimann, Zaninetti & Bozorgnia, mentioned as occurring immediately on the PTB in several localities of southern Turkey and
260
J.M. THI~RY ETAL.
southern Iran; and (b) a nodosarioid, relatively common in the same levels, the genus Polarisella (see Vachard et al. 2005; Gaillot 2006).
The reefal phenomenon An important reefal barrier stretched for a length of at least 50-70 km south of the lagoonal area. It settled there with up to 70-100 m of buildup, between the Belaya, Bolshaya and Malaya Laba river basins. The reefal sequence spread out into the Urushtenian facies with bioclastic, biogenic limestones with algae, foraminifers, sponges, bryozoa and reef-building brachiopods. Two field sections of late Changhsingian correspond to the back-reef area at Nikitin ravine and at Raskol-Cliff, west of the Belaya River at the presumed end of the lagoon. Some field sections have been studied at Khuko in the Main Range, 36 km N N W of Soci; we found there in an offreef area olistolites of late Changhsingian sediments. Near the Nikitin section, Severnaya ravine already corresponds to an off-reef field section only 2 km SSE from Nikitin (Fig. lb). The Urushtenian facies provides the best classification concerning the reefal phenomenon as a result of our field sampling at Nikitin ravine. There, beds of bioclastic Urushtenian limestones are massive, layered and about 5 m thick. The limestones have inclusions of marl limestone that contain accumulations of small foraminifers. Associated brachiopods bioherms of Richthofenia, often with Crurithyris or Leptodus, are repeated in association with sponges, and bryozoa in most of the beds. Some accumulations of algae Gymnocodiaceae (Permocalculus, Gymnocodium) and Dasycladales Mizzia form another characteristic biota.
Palaeobiogeographic compar&on with Transcaucasia and southern Crimea The best known Middle-Late Permian Transcaucasian sections are those at Arpa (southern Armenia), Sovetashen (Armenia) and Djulfa (Adzerbadjan) (Figs 3 & 4). The top of the Dorashamian-Changhsingian contains cherts and silicifications at Arpa and Khashik, and red marls at Sovetashen (Rostovtsev & Azaryan 1973; Alekseev et al. 1983; Zakharov et al. 1996). The different taxa of foraminifers clearly suggest the different palaeobiogeographical assignments of NW Caucasus and Transcaucasia during the whole of the Permian. In particular, the fusulinids Orientoschwagerina abiehi MiklukhoMaklay and Eopolydiexodina persica (Kahler) mentioned by Akopian (1974) and Kotlyar et al.
(1989) in Armenia also exist in Iran (Abadeh: see Kobayashi & Ishii 2003a, b), many species of Codonofusiella are common between Armenia (Kotlyar et al. 1984) and Abadeh (IranianJapanese Research Group 1981), and the genus Pseudodunbarula is known both in Transcaucasia (Kotlyar et al. 1984) and in the Abadeh area (Mohtat-Aghai & Vachard 2005). The smaller foraminifers identified by Pronina (1988) and Mohtat-Aghai & Vachard (2005) are also similar. Contrary to NW Caucasus, Changhsingian reef mounds are apparently lacking in Transcaucasia. Stromatolites are known in Armenia at the base of the Triassic, in all probability correlatable with those of Abadeh recently re-described by Heydari et al. (2000, 2003). During the Triassic a phase of rifting is particularly well known from the Carnian (Trachyceras Zone) during which, according to Vuks (2000), the subsidence is at a maximum, as previously supposed by Nikishin et al. (1998a, b). This pre-Norian phase precedes the collision of the Transcaucasian plate with the northern boundary of the Caucasian chain of the Cimmerian orogenesis. Subsequent to an emergence, creating an unconformity, a Jurassic suite of volcaniclastic sediments was deposited. The emergence is even more pronounced south of the Caspian, in the Alborz Mountains, by a layer of laterite (Stampfli 1978). Another important field section was studied by the first author at Dizi (Fig. 4, see later), in the region of Svenetis Kedi in Georgia, NE Transcaucasia. It is located at an altitude of about 3000 m on the banks of the high Inguri River, 80 km east of Suchimi. Located south of the Main Range of the Greater Caucasus, it appears to belong to the northern boundary of the Trancaucasian Terrane (Georgian Block: Khain 1982). The section shows a continuous sequence from the Eifelian to the Triassic epochs and is separated from the Jurassic by an unconformity. According to the work of the Oceanological Institute of Moscow (Kazmin pers. comm.), these series are folded and were overturned to the north, as at Nikitin, during the pre-Jurassic Cimmerian orogeneses, and are here metamorphosed to the greenschist facies. In the lower series, the jaspers and marbles are dated by conodonts as Devonian and Early Carboniferous. These horizons correlate with the levels known in southern Armenia at Danzik near Arpa (Fig. 3) and dated by Bonnet (1947). This upper part of the sequence also contains sandstones, often coarse grained, undoubtedly belonging to the Triassic. They cover a lower complex of Palaeozoic, which presents a chaotic facies of
LATE P E R M I A N EASTERN EUROPE AND CAUCASUS
261
Fig. 3. (a) Recent geological structures with an attempt at a palaeogeographical extension of Tethys Ocean during Dorashamian time in South Armenia area. (b) Sovetashen Permo-Triassic section according to Aleskeev et al. (1983).
262
J.M. THI~RY ET AL.
slides and relics of mudflows formed at the continental slope (Kazmin & Sborshchikov 1989). Their Permian olistolites are similar to those of the Taurida Formation of the Crimea, recently re-examined by Kotlyar et al. (1999a). Moreover, Taurida of Crimea and Transcaucasia have many palaeobiological points in common during the Middle-Late Permian owing to: (a) the diversity of the Parafusulina sensu lato (i.e. Skinnerella, Paraskinnerella and rare true Parafusulina) (compare Kotlyar et al. 1999a and Akopian 1974); (b) the presence of the neoschwagerinids genera: Cancellina, Armenina, Neoschwagerina ex gr. simplex and Praesumatrina (compare Kotlyar et al. 1999a and Leven 1998); (c) the diversity of Eopolydiexodina (compilation in Vachard & Bouyx 2002); and (d) the absence of Changhsingian reef mounds, although richthofenids and calcisponges have been present in Crimea since the Midian (Kotlyar et al. 1999a). The Dizi zone may mark the margin of the Georgian block and the proximity of the Hercynian suture of the Caucasus with the Russian platform (Fig. 4). Consequently, the Hercynian sector of NW Caucasus appears biogeographically separated from the microplate Crimea Taurida-Georgian Block-Lesser Caucasus-Armenia-Djulfa-Alborz. This microplate was initially named 'Extragondwan Realm' by Vachard (1980), but is most generally described in the literature under the name of 'Cimmerian Terranes'. The microplate Crimea-TranscaucasiaAlborz is also characterized by PalaeozoicTriassic olistolites present in Armenia at Sevan Lake, where a Permo-Triassic rift appears to exist (Bonnet 1947; Stampfli & Pillevuit 1993). Here a thin crust (Ershov et al. 1998) was formed, which was uplifted in front of a Cimmerian magmatic belt. A back-arc Permian-Triassic basin extended south of Sevan Lake, at the northern margin of the Neo-Tethys Ocean. This area showed a great seismic and volcanic instability, which even expressed itself in Permian-Triassic sedimentation. This instability was again perceptible in Holocene times by intense volcanism (Karakhanian et al. 2002, fig. 3). The Armenian and Georgian blocks have aggraded against one another since the Bajocian (Khain 1982) and, in turn, against the Russian platform, north of the Dizi area. The Cimmerian Orogeny is thus recognized more or less at the base of the Norian in the three regions studied above: Kuban, Georgia and Armenia. It corresponds to an extension of the Neo-Tethys before its major expansion during the Bajocian-Callovian (second phase of the Cimmerian Orogeny). We suggest that the
Palaeotethys probably closed partially at the base of the Late Permian (Saalian Orogeny), as indicated by the pre-Murgabian conglomerates observed in the Nikitin section, in Bulgaria (Yanev 2000), south of the Caucasus, and in the Alborz Mountains (Stampfli 1978; Stampfli et al. 1991). In the Nessen Formation (Latest Permian) in this region (Lys et al. 1978), the microfauna and algae appear to be very similar to those in certain southern Armenian sections (Khashik: Pronina 1988).
Biikk Mountains (Hungary) and neighbouring countries In Hungary, the PTB is known from the Biikk Mountains and the Transdanubian Range (e.g. Haas et al. 2006 with bibliography) (Figs 5 & 6a). The Bfikk Mountains are located in NE Hungary, near the border with Slovakia and Ukraine (Fig. 6). There, Balogh (1980) divided the Late Permian-Early Triassic series, respectively, into Nagyvisny6 and Gerennav~r limestones. The Nagyvisny6 Limestone (250-280 m thick) consists of black and dark grey, thinbedded limestones with shaly intercalations. The Gennevfir Limestone (110-140 m thick) contains, at its base, successively 50 cm of mudstones or Earlandia wackestones (with the conodont Hindeodus parvus recognized as characteristic of the PTB) and 8.5 m-thick stromatolites (Hips & Haas 2006). We have studied several field sections in the Btikk Mountains (Fig. 6a). Other good exposures of the PTB are located in Serbia, west of Belgrade, as well in a bore core at Gartnerkofel 1 (Austria) (Fig. 6a). The latter corresponds to the Bellerophon Formation, which appears in the Latest Permian as a significant bioclastic limestone with gymnocodiacean algae. In the Btikk Mountains (Nagysvisny6), the microfauna were accurately described from black bituminous limestone by B6rczi-Makk (1992) and B~rczi-Makk et al. (1995). We found in our samples accumulations of Permocalculus with rarer Gymnocodium, and foraminifers Earlandia, Paraglobivalvulina, Septoglobivalvulina? ex gr. decrouezae K6yluoglu & Altiner, Dagmarita, Paradagmarita, Shindella?, Pseudovermiporella, Hemigordius, Multidiscus, Geinitzina, Pachyphloia and Robuloides. Finally, we found only Changhsingian assemblages in our samples from the Btikk Mountains, contrary to B~rcziMakk (1992), which emphasized a Midian age for the series, or a very reduced DorashamianChanghsingian part (B~rczi-Makk et al. 1995). This Changhsingian age is indicated by: (a) the
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