Orbital Forcing Timescales & Cyclostratigraphy. (Geological Society Special Publication Ser. No 85.)

Orbital Forcing Timescales and Cyclostratigraphy

Geological Society Special Publications Series Editor

A. J. FLEET

GEOLOGICAL SOCIETY SPECIAL PUBLICATION NO. 85

Orbital Forcing Timescales and Cyclostratigraphy EDITED BY

M. R. HOUSE Department of Geology The University, Southampton, UK

A. S. GALE Department of Palaeontology Natural History Museum, London, UK

1995 Published by The Geological Society London

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Contents Preface HOUSE, M. R. Orbital forcing timescales: an introduction

vii 1

KELLY, S. B. & SADLER,S. P. Equilibrium and response to climatic and tectonic forcing: a study of alluvial sequences in the Devonian Munster Basin, Ireland

19

HOUSE, M. R. Devonian precessional and other signatures for establishing a Givetian timescale

37

WEEDON, G. P. & READ,W. A. Orbital-climatic forcing of Namurian cyclic sedimentation from spectral analysis of the Limestone Coal Formation, Central Scotland

51

MOSES, G. P. G. Calibration, analysis and interpretation of depositional cycles in the Early Toarcian of Yorkshire, UK

67

WATERHOUSE,H. K. High-resolution palynofacies investigation of Kimmeridgian sedimentary cycles

75

VALDES, P. J., SELLWOOD,B. W. & PRICE, G. D. Modelling Late Jurassic Milankovitch climate variations

115

COTILLON,P. Constraints for using high-frequency sedimentary cycles in cyclostratigraphy

133

GIRAUD, F., BEAUFORT,L. & COTILLON,P. Periodicities of carbonate cycles in the Valanginian of the Vocontian Trough: a strong obliquity control

143

QUESNE, D. & FERRY, S. Detailed relationships between platform and pelagic carbonates (Barremian, SE France)

165

GALE, A. S. Cyclostratigraphy and correlation of the Cenomanian Stage in Western Europe

177

FISCHER, A. G. Cyclostratigraphy, Quo Vadis?

199

Index

205

Preface The discovery in the 1970s that Pleistocene climates and especially ice-age development were controlled by identifiable orbital parameters in the Milankovitch Band, confirming the views of Milutin Milankovitch in the 1920s, is probably the greatest single advance in palaeoclimatology this century. Changes in temperature recorded by isotopes in calcite of deep-sea foraminifer tests provided a detailed record of ice-sheet advance and decay moderated by precession, obliquity and eccentricity cycles (19-400 ka). As a consequence of this work, the reality of orbital forcing of climate was established as a fact. An added bonus of this discovery is that the identification of individual Milankovitch frequencies allows construction of an orbital forcing timescale graduated by these frequencies. Earlier palaeontological research which gave evidence of the number of days in the year, and days in the lunar month in the past, contributed to calculations on how the precession and obliquity cycles may have differed in geological time. The possibility that Milankovitch cyclicity could form the basis for orbital timescales throughout the geological column was soon recognized, although G. K. Gilbert had seen the same possibilities in the Cretaceous of the Western Interior Basin of North America a century ago. The geological record contains abundant bedding cycles, the duration of many of which fall within the Milankovitch Band. The question as to which of these diverse bedding features record climatic cycles, the processes by which they were generated, and the identification of individual cycle frequencies is a lively and exciting area of research and discussion. The holy grail of this work is the construction of a pre-Pleistocene orbital timescale graduated in Milankovitch units. Bundling of precession cycles within short eccentricity cycles is a readily identifiable feature which is particularly valuable in the development of timescales. This volume is a product of a meeting on orbital timescales and cyclostratigraphy that we organized at the Geological Society of London apartments on 25 and 26 March 1993, and its diverse papers reflect the wide range of sedimentological, palaeontological, geochemical and stratigraphical research presented at that meeting and of the discussions generated also during a field excursion to the Dorset coast which followed that London meeting. M. R. House and A. S. Gale

Orbital forcing timescales: an introduction MICHAEL

R. H O U S E

Department of Geology, The University, Southampton SO17 IBJ, UK Abstract: A brief review is given of orbital patterns affecting the Earth which may be of use in

establishing, for long or short periods, orbital forcing timescales (OFT). The metronomic variations of the Earth-Moon system and of the Earth-Sun orbital patterns produce gravitational and temperature effects which alter the physical environment on the Earth's surface. These give an interpenetrating effect of forcing cycles ranging from twice daily tides, day-night alternations, various tidal patterns and the annual solar pattern. All of these have been used palaeontologically to give precision to short-term age determination in the past. It is cycles of the Milankovitch band which are showing promise of enabling new practical timescales to be established for parts of geological time. These depend on changes in the Earth-Sun distance (perihelion and precession cycles of 19 and 23 ka at the present time), changes in the tilt of the Earth's axis with respect to the Earth's orbit round the Sun (the obliquity cycles of 41 and 54 ka), and changes in the geometry of the Earth's orbit around the Sun (eccentricity cycles of 106 and 414 ka). Since the number of days in the year have changed through time; so have the periods of the perihelion and precession cycles. There is increasing evidence that small-scale sedimentary rhythmic couplets, often grouped into bundles, may represent the effect of some of these; often the precessional couplets are grouped into bundles of five or so within the lower eccentricity period. The disentangling of the interpenetrating cycles to produce an OFT is an exciting problem and challenge for palaeobiology and sedimentology. These should enable numerical dates to be given to biostratigraphic and chronostratigraphic timescales and eventually enable many earth processes to be analysed in real time. 26 Ma oscillations related to the Cosmic Year (c. 260 Ma) have been invoked to explain periodic mass extinctions in the fossil record. But evidence is presented to suggest such extinctions are not, in fact, periodic.

The purpose of this contribution is to provide an introductory review of those orbital patterns which have such an effect on the environment of the Earth's surface that they give potential for the establishment of orbital forcing timescales (OFT) for parts, perhaps eventually much, of Earth history. That the establishment of time in geology, for record of its past events and in the establishment of rates for processes is of major importance is self evident. At present we rely on biostratigraphic scales for the Phanerozoic, but these are relative, not absolute, scales. For the late Mesozoic and Tertiary, in suitable circumstances, the radiometric scales are extremely important, if incomplete; for pre-Cretaceous rocks, however, their increasing sparseness and unreliablity make them of limited practical use. The exciting possibility is that new timescales can be constructed using microrhythmic sequences which may show the effects of particularly precession, obliquity and eccentricity orbital patterns, over frequencies usually referred to as of the Milankovitch band, may provide timescales of considerable refinement. Such cycles affect the solar energy reaching the outer atmosphere because the Earth-Sun distance is changed during them, or seasonal distribution of insolation. The way in which outer atmosphere changes are reflected in local changes on the Earth's surface is undoubtably

complex, and in many ways poorly understood. However, it is thought that resultant sedimentary microrhythms result from changes of sea level, and changes in the pattern of vegetation and erosion on adjacent land areas which are mainly driven by climate. Figure 1 & Table 1 give the range of orbitally forcing frequencies which may contribute to the development of timescales. The recognition of the potential of orbitallyforced microrhythms for the construction of timescales was first most clearly stated by G.K. Gilbert (1895, 1900a, b) and developed further by Barrell (1917). Such ideas followed naturally from the laws of planetary motion established in 1609 and 1618 by Johannes Kepler, and the later recognition, by Newton, of the role of gravitational attraction between planetary bodies. Adhrmar (1842) and Croll (1875) gave an early summary of such views and Charles Lyell considered them in detail in the later editions of his Principles of Geology. However, it was the calculations of Milutin Milankovitch (1920, 1941), using climatic effects of orbital patterns to explain the ice ages, which was the major turning point; but such ideas were not well received at the time. There followed a long period when microrhythmic sequences formed the basis for mathematical studies of series analysis, but with little attempt to invoke the real

From HOUSE,M. R. & GALE,A. S. (eds), 1995, Orbital Forcing Timescales and Cyclostratigraphy, Geological Society Special Publication No. 85, pp. 1-18.

2

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Fig. 1. Logarithmic table of orbital periods which exert gravitational effects on Earth, or which which exert orbitally forced changes in the temporal energy distribution reaching the outer atmosphere of the Earth from the Sun. time dimension, with the exception of some elegant discussions on long records, such as in the late Trias of the Newark Basin (van Houten 1964; Olsen 1984; Anderson et al., 1984) dealing mainly with sub-Milankovitch band effects. The modern phase was undoubtably reached with the calculations of possible orbital forcing to produce documented evidence in ocean cores of temperature changes that really established such theories indisputably (Hays et al. 1976; Imbrie & Imbrie 1979; Covey 1984). The urgency to use such tools to improve the timescale was pressed by House (1985a, 1986a, b) and is part of the theme of this symposium. There have been many symposia and reviews in past years, both of biological rhythmicity (Rosenberg & Runcorn 1975), sedimentological and astronomical aspects (Merriam 1964; Einsele & Seilacher 1982; Berger et al., 1984; Fischer & Bottjer 1991; De Boer & Smith 1994; Smith 1990a, b), and methods of mathematical analysis of microrhythmic sequences (Weedon 1993; Schwarzacher 1964, 1975, 1987). The term cyclostratigraphy has been coined for sedimentary phemomena, but there has been little concentration on techniques to improve the geological timescales. On the scale of daily, monthly and annual effects, the causation of tides essentially followed the recognition of the laws governing planetary motion. A turning point was the classic paper by

Wells (1962), who recognized daily and annual banding in Devonian rugose corals and was able to estimate the number of days in the Devonian year at rather over 400. The recognition of lunar effects followed shortly after (Scrutton 1964), which enabled the periods of the Earth-Moon orbits to be estimated for the Devonian. Since such motion controls the perihelion and precession cycles, it has subsequently been shown by Berger et al. (1989a, b) how these cycles have changed through geological time. The recognition of daily, lunar and annual effects in the shells of bivalves (House & Farrow 1968) was followed by many studies (Scrutton 1978). The annual changes in tree rings have long been known and dendrochronology is now a discipline in its own right extending back over several thousand years. Frequencies of orbital forcing cycles have been divided into the calendar band, solar band, Milankovitch band and galactic band (Fig. 1). Imbrie (1985) used another system, which may be modified here by the inclusion of the highest frequencies as follows: daily band (0-25 h), monthly band ( 25 h-0.5 a), annual band (0.52.5 a), interannual band, (2.5-10 a), decadal band (10-400 a), millenial band (400-10 000 a), Milankovitch band (10 000-400 000 a) and tectonic band 400 000+ a).

Annual and lesser orbital cycles (< 1.0 a) These frequencies have been named the calendar band (Fischer & Bottjer 1991). The principal lowerorder cycles may be separated into the tidal, whose effects result primarily from gravitational changes in the Earth-Moon system and the solar, which result from changes in the energy received from the Sun resultant upon daily to annual changes. The second are well known and merit little attention here, although it should be pointed out that for organisms, as for sedimentation, the interpenetration of these effects can be complex. Emphasis here will be on such factors as contribute to OFT criteria. Cycles at frequencies of < 1 a are recognizable in both the sedimentary record, where they are embraced in the term rhythmites, and in the fossil record, where they show as growth banding where the accretion of tissues reflects environmental rhythms: under suitable circumstances these may be preserved in both plant and animal tissues. It is unlikely that evidence from this source will ever be integrated into a continuous timescale for the past. Nevertheless, for short periods, the documentation of tidal, daily, monthly, equinoxial and annual cycles have already contributed much to short-term environmental analysis, quite apart from the contri-

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bution to factual knowledge on the Earth-Moon orbital parameters in the past.

Tidal effects The orbit of the Moon around the Earth (Fig. 2) attracts a tidal wave around the Earth with a period of approximately once a day, the actual period (24 h 54 min) being termed lunar day, since it is not quite of the same period as the day-night cycle, or solar day. It is fortuitous that, at the present time, the period of the Earth's rotation around it's spin axis is only slightly faster than the orbit of the Moon around the Earth giving such similarity between the solar day and the lunar day: this was not so in the past. However, as Newton demonstrated, because of the centrifugal force resultant upon the rotation of the Earth, not only is the strongest tide developed around the Earth where the plane of the Moon's orbit crosses the Earth (giving the equilibrium tide of Fig. 1), but tides are developed at opposite sides of the Earth in the form of water heaping (tidal nodes) at points closest to and furthest away from the Moon. Thus, given the rotation of the Earth, the typical tidal cycle is formed which gives about two tides a day, a pattern which it is convenient to call semidiurnal (half-day, or half-solar day) but which in reality is half of the lunar day (Fig. 3A). Usually, the two semi-diurnal tides are not equal, that is the alternation of levels reached by successive tides is not the same. Hence, there can be a lower high tide (low high water) followed by a higher high tide (high high water) (Fig. 3A). This diurnal inequality of the tide results from the fact that the orbital plane of the Moon around the Earth

makes a low (and changing) angle with the axis of the Earth's rotation (Fig. 2). This angle, or declination, is not constant, and obviously the inequality is greatest when the declination is greatest. At the time when a tidal node is below the Earth's Equator on one side of the Earth it will be above the Equator on the other side. At a place in mid-latitudes (illustrated in Fig. 2), therefore, the higher high water will be when a point is closest to the Moon. Local geographical effects of coastal shape and seafloor morphology, and of storms, often far away, will modify these simple cycles, often considerably. There is a distinct solar tide which has an interval of 12 h. Although the gravitational attraction due to the Sun is 177 times stronger than that of the Moon, the solar tides are smaller in effect than lunar tides because there is a fundamentally different relationship between the two; the Earth is in orbit around the Sun and hence a state of nonweightlessness occurs because the radial component of force due to the Earth's orbital momentum is almost cancelled out by the gravitational pull of the Sun. It is easy to consider tidal effects solely in terms of the sea, and the effects on the seashore, and limited to the changing distance between high and low tide marks. This would be a mistake. Undoubtedly of greatest importance for tectonics is the continual stresses on the deformable solid earth by tides ranging from the semidiurnal tides to the tidal node cycles of c. 20 a. Rocks may give a less obvious response, but the continual stresses produced in this way will be an important factor in their ultimate relief by fracture, jointing, faulting and folding, and perhaps at much larger scales too. Similarly, the effects caused by cyclical movements

4

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of the atmospheric gases will modify wind movements and pressure systems. But these we have no means, at present, of documenting.

Semidiurnal (0.00135 a) The typical twice-daily rise and fall of sea level can follow several patterns from one in which there is an extreme of semidiurnal change (twice daily) to one which is essentially daily, or circadian, but in reality reflects the period of one lunar day. Both cases have been recorded from sedimentary sequences and fossil shells from the past (see below). This is a means of obtaining precise

information on local tidal regimes and on the timing of astronomical controls. The founder of petrography, H.C. Sorby, over a hundred years ago, appears to have been the first to recognize petrographical differences between the ebb and flood tidal characteristics in the Jurassic Forest Marble of England. He was able to distinguish different directions of depositing flow between the ebb and the flow sedimentary regimes. Examples of such rhythmites from the German coast were reported by Reinecke & Wtinderlich (1967) (Fig. 4B), who found that fine sand was deposited at times of tidal current flow and clays were deposited from suspension during times of

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i~'ig. 4. Semidiurnal signatures in sediments.(A) Semidiurnal inequality and the progressive time progression between high-water slacks and low-water slacks (darker bands): an example from the Upper Carboniferous of Francis Creek, Illinois, USA. Photograph kindly provided by E Broadhurst. (B) A similar example, but from Present day North Sea sediments (modified from Reinecke & Wtinderlich 1967).

minimal current flow at either high or low slack tides. At that locality the ebb tide was much stronger than the flood tide and hence more sand was deposited on the ebb. Note that such records only occur when the environment allows continuous deposition. Modern examples have been discussed by Allen (1981, 1982). An excellent example (Fig. 4A) from the Carboniferous of Illinois has been illustrated by Broadhurst (1988), in which a typical semidiurnal tide pattern is seen where progressive changes in the timing of ebb and flood results in a systematic changes during the neap-spring-neap cycle (Fig. 4A). For fossils, elegant contributions on fine-scale banding in bivalve shells was given by Evans (1972) and Pannella & MacClintock (1968). Evans demonstrated for the Recent cockle, Clinocardium nuttalli from the coast of Oregon, that there was a fine-scale cyclicity in the growth of the ostracum which matched the semidiurnal cycle and which

shows the effects of changing diurnal inequality in the tides (Fig. 5). A rather more systematic study of such changes has recently been published by Ohno (1989). An example in Fig. 6 illustrates an acetate peel made from a polished radial section of a specimen of Cerastoderma edule from mid-tide level of the Bury Inlet, South Wales. This shows a dominant diurnal pattern with only slight evidence of a semidiurnal effect.

Solar day (0.0028 a) Because the lunar day so closely corresponds, at the present time, to the solar day, it is understandable that it is often difficult to disentangle the effects of day-night rhythmicity from daily tide rhythmicity, in the past especially when modulation by a semidiurnal tide effect is not apparent. Thus, the estimation of days in the lunar month, or Earth year, are dependent on whether tidal or insolation effects are dominant. The authors of many such estimates

6

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Fig. 5. (A) Progressive semidiurnal tidal signatures for Empire, Oregon, USA, for a period in June and July 1970. (B) Correlated growth pattern record of Clinocardium nuttalli from the Oregon coast for the same period (modified from Evans 1972).

either do not mention this difficulty or assume that the present day situation of near-identity held in the past. Scrutton (1978) drew attention to this problem. The importance of a circadian biorhythmicity to living things is well known and well documented (Harker 1964; Neville 1967; Clarke II 1974). It operates physiologically, behaviourly and, when approriate, in the deposition of organic tissues. Geological information is only obtainable when fossils record circadian growth increments in preservable hard parts.

Spring tides (0.038 a) Spring tides occur approximately every 14 days when there is an approximate alignment of the Sun, Moon and Earth (Fig. 2). This occurs either at conjunction or opposition of the Moon and Earth with respect to the Sun. Neap tides occur between these when the Sun and Moon are in quadrature with the Earth. Tidal records show this well (Fig. 8A). Sedimentological evidence of this effect in ancient rocks has been recognized and discussed especially by Williams (1989a, b). In the pattern of growth increments in modern

shells such patterns have been well documented (Figs 5 & 6), but there are not many good fossil examples partly because many shells, being made of aragonite, recrystallize after fossilization. In the case of littoral shells, position relative to mean sealevel is important since this controls modulation of shell growth and low tidal forms show the phenomenon least (Farrow 1971, 1972).

Lunar month (0.081 a) At the full Moon alignment of the Earth, Moon and Sun is most perfect and this gives a dominant tidal effect leading to enhanced spring tides. The period of the lunar month at present is 29.53 days, and there are 12.37 lunar months in the year. Tidal gauges show the greater amplitude of spring tides at this time (Fig. 8A). Evidence for the lunar month in the past has been claimed from the Precambrian to the Recent and attempts have been made to show how the number of lunar months per year has increased in the past, reaching in excess of 13 in the Precambrian (Williams 1989a, b). Data has been drawn from corals, bivalves, stromatolites and other groups. The first such study was by Scrutton (1964),

Fig. 6. Semidiurnal, diurnal, lunar and annual (winter) signatures in the growth of the cockle Cerastoaerrnaeaute (Linnaeus) from the mid-shore level in the Bury Inlet, South Wales, UK, based on an acetate peel of a polished radical section, • 80 [by Farrow (1972, pl. 8B)].

ORBITAL FORCING TIMESCALES who claimed lunar month periodicity in Devonian using corals collected by the writer from the Bell Shale of Michigan (Scrutton 1964, Fig. 7). The calculation was based on fine-scale increments thought to be daily. But, as Scrutton (1978), pointed out, whether this referred to the lunar day or solar day is not clear. Thus, the calculation of the lunar month could be in error. Figure 7 shows a well-developed banding of this type. Pompea & Kahn (1979) made similar observations in nautiloids. The cause of the lunar banding is not clear; is this caused by tidal interference at extreme spring tides or is it a lunar-related or biological rhythm connected with spawning? Pannela et al. (1968) produced a graph of days per month showing a rise from the present period of 29.53 to a plateau just below 30 days during the Mesozoic and rising to c. 31.5 in the Cambrian. The more recent review of Williams (1989b) shows greater error bars on the data, but a similar trend. Calculations of the period of the Milankovitch band precession and obliquity in the past depend on accurate knowledge of the period of the lunar month in the past.

7

Equinoxes (0.5 a) These are the two times in the year when the Sun is exactly above the Equator and day and night are of equal duration. The vernal (spring) equinox for the northern hemisphere is about March 21 and the autumnal equinox is about September 23: for the Southern hemisphere it is the reverse. Associated with this are the equinoxial tides which form their annual maxima at this time and storms are often associated with climatic changes.

Annual cycle (1.0 a) This is the frequency which has received most study. There are 365 solar days in the Earth year. Environmentally it represents the complete cycle of the dominant extremes of solar radiation reaching the outer atmosphere of the Earth. There are, however, lag effects in how this operates to affect climate at the surface of the Earth where highest or lowest temperatures are delayed. For sediments the classical work of de Geer (1928), on annual varving was on sediments in

Fig. 7. Diumal rhythms and lunar monthly bands shown on the epitheca of a rugose coral from the Bell Shale, Michigan, USA, collected by the author. (Photograph kindly supplied by Dr. C. T. Scrutton and figured Scrutton 1964).

8

M.R. HOUSE

lakes near glacial regimes where spring melts and summer organic debris classically comprise a couplet or two of laminae resulting in a single varve. Varves have led to the establishment of a post-Glacial chronology. A more general term for laminated sediments is laminites since their annual period is not often demonstrable. Lacustrine varves through time have been reviewed by Anderson & Dean (1988). There are many examples of ancient varved sediments. For the Tertiary, the Eocene Green River Formation Colorado (Bradley 1929, 1931; Fischer & Bottjer 1991). The Jurassic Todilto Formation (Anderson & Kirkland 1960) has long sequences of varves. Hallam (1960) has claimed occurences in black shales in the Jurassic. For the Trias long sequences have been established in the Lockatong Formation of the Newark Basin (van Houten 1962). For the Carboniferous Kvale et al. (1989) have recognized annual and higher-frequency cycles. Devonian annual varves have been claimed in the Ireton Shale of western Canada (Anderson 1961) and in the Achanarras Limestone of Scotland (Rayner 1963; Trewin 1986). Precambrian varving has been claimed for the Mid-Proterozoic (c. 1.75 Ga) by Jackson (1985). The Elatina laminites (Williams 1989a, b) suggest that there were then c. 400 solar days to the year. Organisms commonly reflect the annual cycle in their deposited tissues. Annual growth rings in plants have established dendrochronology as a discipline (Creber 1977; Fritts 1976a, b) giving a chronology going back over five millenia [Suess (1970), following fundamental contributions of A.E. Douglas in his monumental Climatic Cycles and Tree Growth (1919-1936)]. Studies have been attempted on Mesozoic material (Creber & Chaloner 1985). Similarly for animals, annual bands have been used for dating fish otoliths and bivalves, and such groups, and in addition brachiopods, stromatoporoids and stromatolites, but the time spans involved a very small. An early well-documented study demonstrating annual banding in the cockle Cerastoderma edule was by Orton (1926), in which growth is very restricted during winter-forming winter bands (Fig. 6).

Orbital cycles between annual and the Milankovitch band (1.0 a-10.0 ka) Frequencies within this span have been refered to as the solar frequency band because solar phenomena and atmospheric and magnetospheric reactions to them dominate (Fischer & Bottjer 1991, p. 1065). But several gravitational elements continue to be important. For convenience these are again treated in order of increasing wavelength.

Chandler Wobble (1.17 a) In 1891 S.C. Chandler noted that latitude variations contained two components with periods of 428 days (14 months) and 1 a. Known as the Chandler Wobble, it is considered to be a free wobble, the period being extended from an expected 304 days (on a rigid-body theory) by the response of the deformable Earth to rotational forces (Munk & MacDonald 1960; Smylie & Mansinha 1971; Chinnery 1971). This seems to have no OFT relevance.

El Ni~o or ENSO (1.0-9.9 a) Off the coast of Peru the cold Humboldt Current from the south usually gives way around Christmas (hence E1 Nifio) to tropical and warmer waters from the north. Changes in rainfall, sedimentation and ecology result. Stronger changes occur about every four years and there may be other peak frequencies up to 9.9 a (Quinn & Neal 1987). More extreme E1 Nifio effects than over the previous century occured in 1982 and 1984. It is now recognized that there are global effects climatic of an E1 Nifio type. Whilst these effects may be initiated by orbital factors their operation is complex Sedimentary cycles at about this period are increasingly being interpreted as ENSO effects. Cycles at 4.8-5.6 in the Eocene Green River Formation (Ripepe et al. 1991) may be of this type, confirming their recognition by Bradley (1929), and there may be a weaker periodicity at 33 a.

Lunar perigee (8.85 a) The distance between the Earth and Moon varies and it is closest, at perigee, every 8.85 a. It has been calculated for the Precambrian Elatina Formation and Reynella Siltstone (65 Ma) at 9.7 + 0.1 a (Williams 1998a, b).

Solar year (c. 11.0 a) Sunspots are localized vortices on the surface of the Sun thought to be produced by magnetic activity: spots often appear in pairs with opposite magnetic polarity. The spots reduce luminosity of the Sun and hence the solar energy received by the outer atmosphere of the Earth (Dickey 1979). In 1843 S.H. Schwale discovered that there is a cyclicity when spots reach maximum of every c. 11 a, and this is termed the solar year (a term sometimes used, misleadingly, also for the earth year); spots move latitudinally during the cycle. Higher periods

9

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up to 60-120 a have been thought to be due to similar activity but the 22 year cycle (Hale cycle, see below) is the best attested. These are not, of course, controlled by orbital factors but they have some time relevance. Anderson (1965) reviewed Recent, Pleistocene and pre-Pleistocene records of this cycle. It has been, in particular, long recognized in the Eocene Green River Formation, where Bradley (1929) recognized an 11 year cycle: Crowley et al. (1986) calculated a 10.4-year cycle and Ripepe et aI. (1991) a 10.4--14.7-year cycle. In the Jurassic Todilto Formation, Anderson & Kirkland (1960) recognized this cycle, and also higher wavelength cycles. The cycle has been recognized in the Precambrian (Williams 1981). For the midProterozoic Jackson (1985) considered a 7.0-11.0year cycle to be recognizable.

Lunar nodal (18.61 a) The lunar orbital plane relative to the ecliptic plane (Fig. 2) is not constant but precesses with a present day period of 18.61 a. This is the period of the lunar nodal cycle. There is a wobble on the plane of the Moon's orbit around the Earth such that the direction of tilt changes through 360 ~ in longitude on the celestial sphere over 18.61 a. Although the orientation of the tilt changes, the angle between the plane of the Moon's orbit and the ecliptic is constant at e. 5 ~ (Kaye & Stuckley 1973). The effect of this is to give very enhanced tides at this frequency (Fig. 8B). Williams (1989a, b) has claimed to recognize this cycle in the Proterozoic Elatina Formation and Reynella Siltstone (650 Ga) when, he suggests, the period was 19.5 + 0.5 a.

10

M . R . HOUSE

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Hale cycle (c. 22.0 a) A sunspot cycle twice that of the solar year has been been recognized both in historic time and in geological time. It is named after G.E. Hale who, in 1908, recognized the magnetic character of sunspots (Mitchell et al. 1979). Early records at this frequency were reviewed by Anderson (1965). This is the dominant periodicity in the Miocene Sicilian anhydrites (Fischer 1986) and has a powerful signature in the Devonian Ireton Shale (Anderson 1961).

Cycles of the Milankovitch band (10 ka to 1.0 Ma)

Introduction The complex orbital patterns of the EarthMoon-Sun system presumably result from chance patterns on coalescence during their origin. These orbital effects operate through changing the seasonal distribution of insolation and the distance between the Earth and the Sun from time to time. Such changes alter the amount of solar energy, or insolation reaching the outer atmosphere of the Earth. They are appropriately named after Milutin Milankovitch who used such orbital changes to produce a coherent explanation of the ice ages

(Imbrie & Imbrie 1979; Imbrie 1985). The main cycles of precession, obliquity and eccentricity (Fig. 9), although almost sinusoidal, combine to give quite complex patterns (Fig.10), and changes of insolation flux of c. 5%, and of up to c. 100W m -2. The reflection of these outer atmosphere energy changes at sea level in climatic changes is undoubtedly very complex, not only affecting climate, but by thawing or freezing ice, in changing sea level, and in changing climatic regimes, altering erosional and weathering processes so that the nature and rate of sedimentation will change. These various frequencies vary in relation to the Equator. This geographical control is apparently the reason why at various times in the geological past, when obliquity or precession influences may dominate as the major control of orbitally forced microrhythms, the precessional effects are greater at low latitudes.

Precession cycle (19-23 ka) Precession is usually used for the combination of the precession of the equinoxes and the movement of the perihelion. These cycles (or pseudocycles) refer to the movement of the axial projection of the axis of rotation of the Earth with regard to the stars. It was first recorded in 129 BC by Hipparchus. The

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projected axis today lies close to the Pole Star, Polaris, but through time it precesses through a celestial ellipse with a period of c. 26 ka. This wobble of the Earth's axis is caused by the pull of the Moon and Sun on the Earth's equatorial bulge. Other planets introduce a slight additional precession. The fundamental periodicity varies relative to the ellipse of the orbit, i.e., to the equinoxes or perihelion, with two peaks at the present time of 1900 and 23 000 a. The former, the time off

perihelion, is indicated in Fig. 10, and is an important high-frequency modulator.

Obliquity or tilt (41 ka) The projection on to the sky of the Earth's Equator is the celestial equator. This plane lies at 23.5 ~ from the vertical to the ecliptic, the plane of the Earth's orbit. This angle varies by c. 3.5 ~ fluctuating from 21.5 ~ to 24.4 ~ and back with a period of about

12

M.R. HOUSE

Fig. 11. Photograph of a Jurassic sequence in the Atlas Mountains north of Rich showing the small scale couplets groups as bundles of about five couplets and representing the interpenetration of orbital forcing due to the eccentricity and precession cycles (photograph M.R. House).

41 000 a. This changes insolation energy received by the outer atmosphere in two ways; it changes the intensity of the seasonal cycle and it alters the poleto-equator insolation gradient on which climatic and ocean circulation depend. For a given latitude and season, typical departures from present day values are of the order of • (Imbrie 1985).

Eccentricity (54, 106 and 410 ka) There are several wobbles on the orbit of the Earth-Moon system around the Sun (Fig. 9). Some are true eccentricity factors, but also the orbit itself changes from near-circular to an ellipse. The effect of the latter is considerable, and variations in insolation, which are minimal when the orbit is circular, can reach 30% of the total flux at extremes of the ellipse. The most powerful of these are the 106 and 410 ka cycles. Figure 12, based on the analysis of a 2.5 Ma record of oxygen isotope records from ocean cores, shows the signature (labelled 100 ka).

Interpretation Sedimentary rhythmicity is common in the stratigraphical record (Fig. 11) and is much debated. The only striking suggestion that such patterns are orbitally controlled is when couplets of rhythms are combined into groups of about five couplets as bundles (Fig. 11), since this

suggests the operation of an approximate 5 : 1 ratio in agreement with the approximate ratio between the 23 ka precessional and 106ka eccentricity cycles. Although orbital forcing was invoked as a causation of some sedimentary rhythmicity earlier, there is no doubt that it was only the analysis of the oxygen isotope records in ocean cores, now going back over 2.5 Ma (Fig. 12), which has convinced most sceptics of its importance. Precession and obliquity are dependent upon the character of the Earth-Moon system. Since, as demonstrated earlier, there is good evidence that the lunar day and lunar month have changed with time, then so will both the precessional and obliquity periods. A calculation to correct these periods through geological time has been provided by Berger et al. (1989a, b) (Fig. 13). The eccentricity cycles are not changed by modifications of the Earth-Moon system. Many examples of the operation of Milankovitch frequencies below 106 ka have been documented (see especially Berger et al. 1984; Fischer & Bottjer 1991, De Boer & Smith 1994). Particularly valuable have been the correlations between rhythmic sequences and oxygen isotope measurements (Ditchfield & Marshall 1989) confirming temperature dependence and correlations with carbonate percentages (Weedon & Jenkyns 1990), which might be expected to have a temperaturecontrol effect. The frequency of metre-scale rhythms in the stratigraphic record, in many facies,

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was emphasized by House (1986a, b), who argued that a common interpretation was required. Anderson & Goodwin (1990) considered such an allocycle a fundamental stratigraphic unit, building on their earlier work on Devonian rhythms (Anderson et al. 1984; Goodwin & Anderson 1985). Similar cycles have been recognized in terrestrial sequences (Astin 1990), and in bituminous and evaporitic sequences (BougersmaSanders 1971). For operation of longer-term cycles a fascinating study lies ahead. For example, Heckel (1986) recognized a long series of Pennsylvanian sedimentary cycles which he estimated might be 0.5 Ma each, and related to transgressive pulses due to ice-melting. If these are the 410 ka cycles then the framework for a precise timescale is, established. It is unlikely that a continuous timescale at the Milankovitch band level is possible unless long

term-cycles of the sort recognized by Heckel (1986) can form the basis for a framework, and we are clearly not at that stage yet. Meanwhile, the best approach may be to concentrate on stage duration, and the duration of zones within stages, as has been commenced for the Givetian (House 1992 and this volume) and Cenomanian (Gale 1989 and this volume). For this purpose sequences from basinal or pelagic regions with little asymmetricity in rhythms and with good biostratigraphic control are required, i.e. sequences of the sort illustrated in Fig. 11. Sections such as those of the British Lias or Kimmeridge Clay suffer from too many hiatuses and are from too shallow a facies to be very helpful.

Long period orbital cycles (>1.0 Ma) P e r i o d i c extinctions

There has been much interest in recent years in the possibility that orbital events of very long frequencies may have periodically affected the environment of the Earth. The reappearance of Halley's Comet every 76 a appeared to be a trigger for such thoughts, and especially the return in 1985 and 1986. It was the publication of the claim by Raup & Sepkoski (1982, 1984) that extinction of fossil groups occurred at regular intervals during the post-Palaeozoic that led to the proposal of many hypotheses to explain this. The period suggested by Raup & Sepkoski was c. 26 Ma, but 29 and 31 Ma periods were also proposed. Fischer & Arthur (1977) had made similar suggestions earlier but had

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not assigned a period. Associated in timing with these ideas was the claim by Alvarez e t al. (1980) that the Iridium Clay associated with the Cretaceous/Tertiary (K/T) boundary was caused by cometary dust clouds. On the presumption that Raup & Sepkoski's evidence was correct, many theorists suggested that the Earth periodically passed through regions of cosmic dust clouds. The simplest models invoked the cosmic year, and suggested that the solar system oscillated above and below the galactic plane at the required frequency (Fig. 14), taking it in and out of vulnerable space (Rampino & Stothers 1984). Others invoked a periodically returning Companion Star (Whitmire & Jackson 1984), named the Nemesis Star (Davis e t al. 1984), or the periodic effects of a Planet X (Whitmire & Matese 1985); these speculations have continued (Crawford 1985; Clube & Napier 1982). But from the beginning, there were criticisms of the basic conclusions of Raup & Sepkoski. Some claimed falsity in their statistical tests (Noma & Glass 1987; Stigler & Wagner 1987), or bias in the selection of data (Patterson & Smith 1987), or the timescale definitions (Hoffmann 1985)9 That the K/T boundary was caused by cosmic events was also questioned (Clemens e t al. 1981; Hallam 1987). The crudity of Raup & Sepkoski's data, with only 55 data points used to assess extinctions, and the assumption that the stages they used were of equal duration, raised problems. Accordingly the writer assembled data, at 2 Ma time intervals, for

a period of 320 Ma, with 162 data points, for the history of the marine Ammonoidea families from their origin in the Devonian to their extinction at the KIT boundary (House 1989, 1993). This may well represent the most detailed and closely documented such analysis ever undertaken for the fossil record. A Fourier transform of the data (House 1993) gave no evidence whatever of a 26 Ma, or other, long frequency extinction pattern (Fig. 15). Hence the Raup & Sepkoski proposals must be rejected. Were a regular orbital effect proven then it would be of enormous advantage in calibrating those time scales using the Milankovitch band but regrettably there appears to be no such modulator. Cosmic

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The period for the solar system to move around the Milky Way galaxy (Fig.15) has been variously estimated, but is thought to be c. 220-250 Ma. The suggestion has been made that major glaciation periods in the past might be related to this cycle. The Pleistocene (1.6 Ma), Permian (c. 250Ma), Ordovician (c. 440 Ma) and Vendian (600 Ma) do not appear to be separated by equal amounts on current radiometric data, so this suggestion is highly speculative. This contribution, given here as an introduction to this volume on O r b i t a l F o r c i n g T i m e s c a l e s a n d Cyclostratigraphy, is based essentially on the Special Invitation Lecture given by the author to the Geologists' Association in March 1990.

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Fig.

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sophical Transactions of the Royal Society of London, A306, 291-345. ALVAREZ, L. W., ALVA~Z, W., ASARO, E & MICHEL, H. V. 1980 Extraterrestrial cause for the Cretaceous-Tertiary extinctions. Science, 208, 1095-1108. ANDERSON, E. J. & GOODWIN, R W. 1990. The significance of metre-scale allocycles in the quest for a fundamental stratigraphic unit. Journal of the Geological Society, London, 147, 507-518. 1984. Episodic accumulation and the origin of formation boundaries in the Helderberg Group of New York State. Geology, 12, 120-123. ANDERSON, R. T. 1961. Solar-terrestrial climate patterns in varved sediments. Annals of the New York Academy of Sciences, 95, 424-439. ANDERSON,R. Y. 1965. Solar terrestrial patterns in varved sediments. Annals of the Academy of Sciences, 95, 424-439. - 1984. Orbital forcing of evaporite sedimentation. In: IMBRIE, J., HAYS, J., KUKLA, G & SALTZMAN, B. (eds) Milankovitch and Climate:Understanding the Response to Orbital Forcing. Reidel, Dordrecht, 147-162. -& DEAN, W. E. 1988. Lacustrine varves formed through time. Paleogeography, Paleogeography, Paleoclimatology, 35, 215-235. -& KIRKLAND,D. W. 1960. Origin, varves, and cycles

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HOUSE

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CH1NNERY, M. 1971. The Chandler Wobble. In: GASS, I. G., SMITH, P. J. •

WILSON, R. C. L. (eds) Understanding the Earth. Artemis Press, Sussex, 89-95. CLARKE II, G. R. 1974. Growth lines in Invertebrate skeletons. Annual Review of Earth and Planetary Sciences, 2, 77-99. CLEMENS,W. A., ARCHIBALDP,J. D. & HICKEY,L. J. 1981. Out with a whimper not a bang. Paleobiology, 7, 293-298. CLUBE, S. V. M. & NAPIER, W. M. 1982. Spiral arms, comets and terrestrial catastrophism. Quarterly Journal of the Astronomical Society, 23, 45-66. COVEY, C. 1984. The earth's orbit and the Ice Ages. Scientific American, 250, 421-450. CREBER, G. Z. 1977. Tree-rings - a natural data-storage system. Biological Reviews, 52, 349-383. - & CHALONER, W. G. 1985. Tree growth in the Mesozoic and early Tertiary and the reconstruction of palaeoclimates. Palaeogeography, Palaeoclimatology, Palaeoecology, 52, 35-60. CRAWFORD, A. R. 1985. Spiral arms, comets and terrestrial catastrophism: a discussion. Quarterly Journal of the Astronomical Society, 26, 53-55. CROLL, J. 1875. Climate and Time in their Geological

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Equilibrium and response to climatic and tectonic forcing: A study of alluvial sequences in the Devonian Munster Basin, Ireland S E A N B. K E L L Y 1 & S H A U N

E SADLER 2

Department of Geology, University College Cork, Ireland 1present address: Geochem Group, Chester Street, Sattney, Chester, Cheshire CH4 8RD, UK 2present address: Simon Petroleum Technology, Exploration Services, Llanududno, Gwynedd LL30 1SA, UK Abstract: The Upper Palaeozoic Munster Basin is characterised by thick (> 6 km), Late

Devonian alluvial successions (the Old Red Sandstone) that are well exposed throughout southwest Ireland. These represent the deposits of large-scale, complex terminal fan systems which operated under a semi-arid climate. A coarse biostratigraphic subdivision of these sequences has been combined with established radiometric evidence for Devonian stage durations in order to estimate a sediment accumulation rate for this part of the Munster Basin (0.38-0.46 mm a-l). This rate has then been used to estimate the duration of cyclic variations in a range of sedimentary parameters that have been identified throughout both proximal and distal basin-fill sequences. 'Spot'-data derived from variables, such as coset thickness, maximum bed thickness and sandstone content, were used to generate time series which have been subjected to routine Fourier analysis. The application of the estimated sediment accumulation rate to the three most prominent cycle thicknesses (31-175 m) and the assessment of relative frequencies has been used to establish the approximate cycle periodicities, all of which appear to lie within the Milankovitch Band. It is suggested that these features represent the stratigraphic expression of climatic perturbations related to orbital forcing (particularly eccentricity variation). It is further suggested that the recognition of these cycles throughout the basin-fill may, in future, form the basis of high resolution relative timescale and allow the correlation of proximal and distal sections through the basin-fill at scales comparable to the high frequency cyclicities (c. 40 m). The stratigraphic expression of these cycles and their interpretation as evidence of climatic rather than tectonic variation are considered to be a function of their relationship to the equilibrium time constant ( Teq ) of the Munster Basin 9The establishment of Teq requires an assessment of diffusivity which is largely controlled by the rate of water supply to the depositional systems. The latter is assessed by palaeohydrological reconstructions of the scale and discharge characteristics of the major fluvial distributary systems within the Munster Basin. Two methods have been applied. Method I uses a series of established empirical relationships to estimate palaeohydrological variables and Method II uses the thickness of sets and cosets of cross-strata within alluvial sandbodies to assess discharge and channel depth. The resulting estimates of water flux allow the assessment of diffusivity which suggests a Te_ for the Munster Basin of the order of 2 x 106 a. The Milankovitch Band (105 a) periodicities ~73 of the cycles observed in the basinfill are therefore considered to represent 'rapid' variations ( T < < Teq) in water/sediment flux associated with relatively high frequency climatic perturbations. However, these cycles are superimposed on kilometre-scale, sand-prone 'wedges' that dominate the basin-fill architecture. These are considered to record the basinward progradation and subsequent retreat of the terminal fan systems. The 1.5-2 km thicknesses of these units represent time intervals greater than Teq for the Munster Basin. These features are therefore considered to record 'slow' variations (T > Teq) in base level related to changes in tectonic subsidence rate and/or sediment flux.

This paper presents data f r o m the Late Devonian Old R e d Sandstone (ORS) o f southwest Ireland. The ORS has been interpreted as the deposits o f terminal fan complexes which operated under a semi-arid climate and which occupied the western side o f an intracratonic half-graben (the Munster Basin; see Fig. 1). C o m p r e h e n s i v e sedimentological evaluations o f these sequences are available

in previous accounts (e.g. G r a h a m 1983; Williams et al. 1989; M a c C a r t h y 1990; Sadler 1992; G r a h a m et al. 1992; Sadler & Kelly 1993; Kelly & Olsen 1993) and only after a brief s u m m a r y o f the proposed depositional systems is provided here. However, recent analysis o f extensive log sections through this thick (> 6 km), well-exposed alluvial basin-fill succession indicates the presence o f a

From HOUSE,M. R. & GALE, A. S. (eds), 1995, OrbitalForcing Timescalesand Cyclostratigraphy, Geological Society Special Publication No. 85, pp. 19-36.

19

20

S.B. KELLY • S. P. SADLER

Fig. 1. Location and general geological setting of study sections within the northwestern part of the Munster Basin, southwest Ireland.

hierarchy of apparently cyclic fluctuations in a range of sedimentary parameters (Kelly 1992, 1993; Sadler & Kelly 1993). Two of the most important factors influencing aluvial sedimentation over geological timescales are tectonism and climate. Although regular 'cycles', 'megacycles' or 'megarhythms' have been identified in other Devonian alluvial sequences (e.g. Steel 1976), climate is often discounted as a possible driving mechanism. For example, in an assessment of the cyclic sediments of the Hornelen Basin, Bryhni (1978, p. 298) stated that, 'The recurrence of similar megarhythms throughout the entire stratigraphic succession would require a regularity of climate which is rather unlikely'. However, recent work suggests that the influence of regular cyclic fluctuations in climate, which may be related to orbital forcing, is recorded in a number of Devonian continental basins (Hamilton & Trewin 1988; Olsen 1990; Astin 1990; Kelly 1992). Differentiation between the influence of tectonism, climate and other factors, both allocyclic (e.g. eustasy) and autocyclic (e.g. channel switching) in alluvial sequences remains generally difficult. However, the isolation of these separate effects is facilitated if a workable timescale can be applied using, for example, magnetic polarity reversals or the dating of interbedded tuff horizons. An accurate timescale allows the assessment of various rates of processes (e.g. sediment accumulation) and their influence on alluvial stratigraphy or architecture. In this way, Johnson et al. (1986, p. 74), studying fluvial sediments of the Andean foreland basin and the Siwaliks molasse of Pakistan, were able to conclude that, '...the deciding factors in determining exact stratigraphic form are climate and hydrology and not tectonism'. However, in the absence of high resolution

magnetic polarity or radiometric time frameworks for the Munster Basin, previous workers have assessed sediment accumulation rates by reference to a coarse biostratigraphy and accepted Devonian stage lengths (Graham 1983, p. 481). The results of this study indicate that the recognition of Milankovitch Band cyclicity within the Munster Basin allows the development of a high resolution relative timescale. Estimates of sediment accumulation rate allow the application of diffusivity models for alluvial basin-fills (e.g. Paola et al. 1992). These assess the implications of major variables (subsidence rate, sediment flux, gravel fraction, water supply) for the grain-size distribution and architecture of basin-fill successions. One of the main difficulties with diffusivity models is constraining water supply. This paper attempts to resolve the problem by the palaeohydraulic estimation of flow rates in the ancient river systems. Numerous quantitative (empirical) relationships which describe a range of geomorphological and hydrological variables observed within modern fluvial systems are used to evaluate palaeohydrological parameters for the ORS river systems of the Munster Basin. The approach is not a new one and was first applied in detail to the Devonian Wood Bay Group of Spitsbergen by Friend & Moody-Stuart (1972). The objective is to provide estimates of sediment and water flux from the drainage basin (Ad) into the depositional basin (Bb). The basin equilibrium time or 'equilibrium time constant' (Teq) is then calculated using the model developed'by Paola et al. (1992). Teq is the timescale over which a basin naturally responds to cyclic variations in factors such as subsidence and water/sediment flux. In summary, this paper has five main objectives: (1) to provide an estimation of sediment accumulation rates within the Munster Basin; (2) to demonstrate the presence of a hierarchy of cyclicities within the Munster Basin fill and their interpretation as the expression of orbitally-forced climatic variations; (3) to investigate the palaeohydrology of alluvial systems within the Munster Basin; (4) to use palaeohydraulic reconstructions to determine an equilibrium time constant (Te.q) for the Munster Basin; (5) to use T~, to evaluate ttie relative rates of climatically- and "~ tectonically-driven alluvial processes and their expression within the basin-fill.

Location and geological setting The data presented in this paper were recorded from locations in west Co. Cork and south Co. Kerry, Ireland (Fig. 1). Widespread glaciation of this relatively remote upland region has resulted in extensive coastal and inland exposures. These provide thick and locally continuous sections

SEQUENCES IN THE DEVONIAN MUNSTER BASIN through the basin-fill succession which have been logged on a 1 : 5 0 scale. Specific study locations are concentrated in the northeast of the Beara and Iveragh Peninsulas (Fig. 1). Southwest Ireland represents a westerly extension of the Variscan fold belt in Europe. The area is characterized by thick, conformable Late Devonian and Early Carboniferous sedimentary sequences which have been subject to intense deformation and shortening. The structural framework is now dominated by a series of northerly verging, regional anticlines and synclines (Cooper et al. 1986; Price & Todd 1988; Meere 1992). However, shortening has also been accommodated by the development of parasitic, subregional and mesoscale asymmetric fold couplets, a pervasive pressure solution cleavage, thrust propagation on a variety of scales and widespread strike-slip and oblique-slip faulting (Sanderson 1984; Sadler 1992). In addition, subsequent relaxation of deformational stresses and the effects of Jurassic and post-Mesozoic erosional unloading (Keeley et al. 1993) have resulted in the pervasive development of joint sets (Meere 1992). Recent studies have evaluated the distribution and orientation of major compressional faults with respect to regional stratigraphic patterns. Systematic relationships indicate that the current structural configuration of this region is a product of the compressional inversion of a pre-existing

,

..

ll~ Mlddle-Ul~er Devonian

~

21

extensional tectonic framework (Price & Todd 1988; Williams et al. 1989; MacCarthy 1990). The thicknesses of the conformable sedimentary sequences throughout southwest Ireland (locally > 6 km) testify to active sediment accumulation on a subsiding crustal block. The extensional framework is therefore considered to have been related to the development of an intracratonic half-graben the Munster Basin. Subsidence patterns appear to have been largely dictated by extensional offset on a major, southerly dipping, listric fault system that is now traceable in a zone running from Dingle Bay in the west, across to the Galtee Mountains in the east (Williams et al. 1989). However, regional stratigraphical and sedimentological analyses suggest that the extensional framework of the Munster Basin was more complex, with the development of a prominent antithetic structure in the west (Williams et al. 1989) and localized crustal buoyancy associated with proximity to a major granitic pluton (the Leinster Granite) resulting in a complex eastern closure (Price & Todd 1988). Subsidence within the Munster Basin is thought to have been initiated in the late Middle Devonian (Clayton & Graham 1974, 1988) and to have persisted throughout the Late Devonian and into the Early Carboniferous. However, an apparent southerly translation of the locus of crustal extension in the Late Devonian resulted in the superposition of a genetically related sub-basin which

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Fig. 2. Schematic paleogeography illustrating the main depositional systems identified within the Munster Basin (after Graham 1983; Williams et al. 1989). Note the presence of several large distributory systems which are either transverse (from the north) or axial (from the east and west). This study concentrates on the 'Northern System' which deposited the GPE Also indicated are major tectonic features which are thought to have influenced both ORS and subsequent sedimentation.

22

s.B. KELLY t~ S. E SADLER

dominates the south of the region (the South Munster Basin) (Price & Todd 1988; Sadler 1992). Initially, basin subsidence resulted in the deposition of a thick alluvial succession (Graham 1983), with subsequent marine incursion during the Late Devonian restricted to southeastern areas (MacCarthy 1987). However, marine influence persisted and spread throughout the Late Devonian, resulting in the inundation of the whole of the South Munster Basin by Early Carboniferous times. Subsequently, the continuation of this transgressive regime resulted in the drowning of the northern reaches of the Munster Basin and their transformation into a Carboniferous limestone province (Williams et al. 1989). This contrasted with the marine clastic sedimentation characteristic of the South Munster Basin. Palaeontological data constrain the onset of ORS sedimentation in the west of the Munster Basin to the late Middle-early Late Devonian (see below). Conformable relationships with the overlying, fossiliferous marine sequences suggest marine inundation of this area in latest Devonian ('Strunian') times. Lithostratigraphic subdivision of these thick alluvial sequences is commonly based on the regular presence or absence of medium- and coarse-grained sandstone bodies within a pervasive background sequence characterized by the decimetre-scale intercalation of purple very fine/fine grained sandstones and siltstone. This principle has allowed the establishment of a coarse lithostratigraphy which defines the basin-fill architecture. The vertical distribution of mediumand coarse-grained sandstone bodies defines

A

two, kilometre-scale clastic wedges [Chloritic Sandstone/Slehany Formation (CSF) and the Purple Sandstone/Gun Point Formation (GPF)] (Fig. 3) These thicken to the north and appear to amalgamate adjacent to the prominent basin margin fault. A similar coarse-grained alluvial "tongue' has been identified in the south of the region [Sherkin Sandstone Formation (SSF)]. All three project into the finer-grained background sequence (Valentia Slate/Caha Mountain/Castlehaven Formations) (Fig. 3).

Depositional m o d e l Recent sedimentological analysis of the ORS in southwest Ireland has established that the sandprone wedges which extend into the Munster Basin represent the deposits of large-scale terminal fans (Graham 1983; Williams et al. 1989; Graham et al. 1992; Sadler 1992; Sadler & Kelly 1993; Fig. 2). General models of terminal-fan sedimentation, with specific reference to Devonian examples, are provided by Friend (1978) and Kelly & Olsen (1993). The scale and grain-size distribution characteristic of depositional systems within the western part of the Munster Basin suggest that they comprised major distributary networks. These were marked by systematic downstream variations in channel size and sediment grade due to bifurcation, transmission losses and evaporation. Such trends are also characteristic of modern, semi-arid terminal distributary systems, such as those described by Parkash et al. (1983) and Abdullatif (1989). The main lithostratigraphic unit considered in

A~

2km

0

Fig. 3. Schematic north-south cross-section through the ORS in the northwest of the Munster Basin (predeformation), illustrating the main, sand-prone wedges that prograded from the north into finer grained, 'basinal' areas.

SEQUENCES IN THE DEVONIAN MUNSTER BASIN this paper is the GPE A detailed account of the sedimentology of this unit is provided by Sadler & K e l l y (1993). The proximal 'feeder' zone was characterized by large-scale, low sinuosity channels which transported sand and gravel through a low relief, muddy alluvial plain. Further downstream, widespread channel multifurcation in the 'distributary zone' initially resulted in the development of abundant, highly mobile, low sinuosity, sandy bedload streams. Downstream transmission losses due to infiltration and evaporation resulted in the replacement of variable but persistent flows by high energy, unconfined ephemeral floods. These events locally resulted in the scouring and episodic filling of transient, low sinuosity ephemeral channels in the distal reaches of the distributary zone. Eventually, flows reached a 'basinal zone' in which a persistently quiescent regime was episodically punctuated only by low magnitude, shallow sheetfloods. These mainly represented the residual expression of flood pulses originating in the distal distributary zone. T i m e - s e d i m e n t a c c u m u l a t i o n rates In order to assess the rates of various palaeohydrological processes, it is necessary to evaluate time in a stratigraphic context by reference to sediment accumulation rates. Measured or calculated sedimentation rates generally vary in inverse relation to the length of time over which the measurement is made (Sadler 1981). This is a function of the discontinuous nature of sedimentation within the alluvial environment, with accumulation typically punctuated by phases of erosionor non-deposition, and therefore rarely persisting for more than a few days or weeks at any one locality (Sadler & Strauss 1990). The significance of this inherent 'unsteadiness' will be a function of the relevant timescale. At timescales of > 105 a, alluvial units such as the GPF are likely to be stratigraphically 'complete' (Johnson et al. 1988). However, at c. 104 a level of time resolution, the GPF is likely to become 'incomplete', with sedimentary hiatuses introducing significant vertical fluctuations in sediment accumulation (Johnson et al. 1978). Miall (1978) showed that non-marine basins, in various tectonic settings, have sedimentation rates (averaged over time periods of the order of 106 a) of 0.03-1.5 mm a q, and a survey of recent literature suggests that sediment accumulation rates are most frequently in the range of 0.1-0.6 mm a -1 for foreland and successor basins (Miall 1978; Sadler 1981). It has long been recognized that many ORS basins experienced high sediment accumulation rates, primarily as a result of rapid subsidence. Periods of rapid subsidence typically appear to have been 5-10 Ma in duration and rarely persisted

23

for more than c. 20 Ma (Friend 1969), with sediment accumulation rates generally in the range of 0.2-0.6 mm a -1 (Friend 1969, p. 708). Sediment accumulation rates for the Munster Basin may be estimated using radiometric determinations of Devonian stage durations. Sparse fish debris and palynological data indicate that the oldest strata exposed in the Munster Basin are upper Givetian/lower Frasnian in age. Palynological dating of the overlying coastal/ shallow marine sequences indicates that nonmarine sedimentation was largely complete by the end of the Fammenian, although the Tournaisian marine transgression was strongly diachronous (Clayton & Higgs 1979; Sadler 1992). Devonian stages were not equal in length (Friend & House 1964). Although, many earlier workers simply assigned average durations of 7 Ma to each of the Devonian stages (Friend 1969), more recent estimates give durations of 4.6, 10.5 and 3.5 Ma for the Fammenian, Frasnian and Givetian stages, respectively (Harland et al. 1989). It may therefore be estimated that the 6 km alluvial succession observed in the depocentre of the Munster Basin accumulated over a period of 13-16 Ma. This yields a mean sedimentation rate (a) of 0.38-0.46 mm a-1, which is similar to the 0.4 mm a 1 estimated by Graham (1983). These estimates are comparable to sediment accumulation rates derived from other thick GivetianFammenian non-marine successions using a similar approach. For example, the c. 7 km cumulative sediment thickness in the East Greenland Basin indicates sediment accumulation rates of up to 0.5 mm a -1 [Friend 1978 (p. 534), 1981 (p. 153); Friend et al. 1983 (p. 41)]. Cyclicity Well-exposed sections through the basin-fill were routinely logged at a 1 : 5 0 scale. 'Spot'-data recorded include coset size, mean/maximum set size and bed thickness. In order to generate pseudotime series, frequency and percentage data (e.g. calcrete frequency, sandstone percentage) were grouped or averaged into 10m intervals. This allowed data to be analysed using a regular sample spacing. In general, the generation of time series for 'spot' data was based on the maximum value recorded within a 10 m interval. Before proceeding further, it is necessary to consider the significance of the sampling scheme in relation to the statistical characteristics of the palaeohydrological record (cf. Hirschboeck 1988). The preserved sedimentary record represented by the GPF suggests that the recurrence interval (r) of aggradational 'events' related to channel/flood activity and the magnitude-frequency distribution

24

S.B. KELLY & S. P. SADLER

of aggradational 'events' generally have a log normal character, which is typical of both sedimentologic and hydrologic time series (Wolman & Miller 1960) and the rapid response of sedimentary phenomena linked to sediment transport (e.g. bedform development) (Jackson 1975, p. 1528). The selection of a maximum value within a 10 m interval suppresses the filtering effect of stochastic sedimentary processes (Rachoki 1981; Howard 1982). The latter is thought to be minor, given that the sedimentary record is relatively complete (sensu Crowley 1984), considering the character of the depositional system and the timescales involved (see above). Assuming a sediment accumulation rate of approximately 0.4 mm a -1, the sample spacing is equivalent to a time period of c. 25 ka. This is greater than the timescale of any anticipated complex responses (cf. Howard 1982). Although the effective sampling is not uniformly spaced (the original data are discrete), the variation in spot data point spacing is minor in relation to the thickness of the stratigraphic intervals considered. By analogy with modern hydrologic records, we are effectively sampling the maximum recorded flood event in a period of c. 25 ka. The log normal distribution of the data requires the application of a logarithmic transform [x'n = log (Xn)] prior to any time series analysis (cf. Hinnov & Goldhammer 1991).

Individual 'time series' were subject to routine Fourier harmonic analysis and the results are illustrated in Fig. 6 as periodograms. The frequency range extends to 0.05 cycles m -~, which represents the Nyquist frequency for the 10m sample spacing. The majority of the plots illustrate simple analysis of complete data sets, allowing the detection of any regular, long-wave periodicities. Although the periodograms are often 'noisy' they clearly indicate the presence of several pronounced peaks or lines. Three dominant frequencies are definable in nearly all of the spectra from the GPF. The wavelength (m) and estimated duration (based on 'reasonable' values for the sediment accumulation rate; 0.38-0.46 mm a -~) of each of the frequencies are provided in Table 1. Inspection of the various logs (Fig. 4) also reveals the presence of the P1 and P3 cyclicities which correspond to intervals of c. 40 and 150m, respectively. Significantly, all cycle periodicities appear to lie within the Milankovitch Band. This assumes reasonable errors involved in the estimation of sediment accumulation rates using stage durations. However, this method of estimating sediment accumulation rates remains generally uncertain and, in the absence of an absolute timescale. The use of relative frequencies may yield information concerning the possible origins of cyclicity (Fischer

North

South

Derrincullig-Eskna brock

....

~

South Beara

:_----

I

i-,.

0

I

i Im

coset thickness (cm)

1000

1~0

1o0~

maximum sandstone bed thickness (cm)

Fig. 4. Correlation of main study sections within the GPF using variations in sedimentary parameters (all data have been smoothed using a spline function). Note the correlation of small-scale cycles (c. 40 m) and the presence of longer cycles (c. 150 m). Both of these periodicities are thought to reflect climatic changes related to Milankovitch Band orbital parameters, the 100 and 400 ka eccentricity components.

SEQUENCES IN THE DEVONIAN MUNSTER BASIN & Bottjer 1991). The ratio of the two most prominent cyclicities, P1 and P3, is c. 1 : 3.8, which is comparable to the 1:4.1 ratio of the 101 and 414 ka orbital eccentricity cycles (cf. Kelly 1992). If the P1 and P3 cyclicities do represent the expression of these eccentricity components, then the P2 periodicity may correspond to longer elements of the c. 100 ka eccentricity cycle, which are considered to have periodicities of 122-133 ka (Berger 1978; Hinnov & Goldhammer 1991). This conclusion is in accordance with the relative frequencies and apparent duration of the various periodicities based on estimated sediment accumulation rates (Table 1). Longer periodicities have also been detected, the most prominent of which is the c. 700 m P4 periodicity (see Fig. 5), (cf. Sadler & Kelly 1993). If the estimated sedimentation rates are of the correct magnitude then this periodicity may correspond to the 2100ka eccentricity component (Hinnov & Goldhammer 1991). Climatic perturbations, such as those predicted by the Milankovitch Theory, will influence both the sediment and water flux of a closed alluvial system through changes in precipitation and subsequent run-off. Recent studies have confirmed that cyclic changes in insolation are capable of inducing cyclic variations in precipitation (e.g. Rossignol-Strick 1985). The following summary of responses to climatic change is based on numerous studies of Quaternary fluvial systems (e.g. Williams 1970; Dorn et al. 1987; Bull 1988; Maizels 1990) and sedimentological observations of the cyclical deposits within the Munster Basin. Increased aridity is likely to result in a reduction in run-off from the drainage basin and a lowering of water tables within the depositional basin. This is associated with a decrease in streamflow, which may be supplemented by localized fluvial incision on fan surfaces. Conversely, an increase in run-off will promote degradation in the drainage basin and a proportional increase in sediment flux within the depositional basin. Increased run-off will also be reflected by increased discharge or streamflow and an associated increase in the frequency and magnitude of flooding.

Table 1. Cycle lengths (m) derived from power spectra (Fig. 6)

Cycle P1 P2 P3

Wavelength (m) 31-42 55-62 110-175

Estimated periodicity (ka) 78-104 138-155 275-438

The estimated duration of cycles uses a mean (compacted) sediment accumulation rate (a) of 0.4 nun a-1.

25

The rates and processes of calcrete formation are also influenced directly by climate through rainfall and subsequent changes in run-off, which influence the leaching regime, and indirectly by variation in the availability of airborne Ca 2§ (Machette 1985; McFadden 1988). Evidence from Quaternary calcretes suggests that climatic variations may produce systematic changes in profile maturity (Bryan & Albritton 1943). In semi-arid environments, calcrete formation is likely to be initiated when there is a sufficient decrease in effective precipitation (Bull 1991), possibly in association with an increased supply of airborne Ca 2+ (Machette 1985). The maturity of profiles will also increase when the sediment flux is reduced (Wright 1990), as anticipated during relatively arid periods. In spite of dating problems associated with the lack of detailed biostratigraphic and radiometric data within the Munster Basin, it is concluded that long-term sediment accumulation rates were probably between 0.3 and 0.5 mm a 1 and that some of the cyclicities recorded in these successions represent the sedimentary expression of the astronomical forcing of the Earth's climate. The recognition of this 'cyclo-stratigraphy' allows the correlation of the two main study sections within the GPF (Figs 4 & 5). When the Milankovitch Band cyclicity is combined with conventional lithostratigraphic criteria (based on detailed field mapping) the correlation of sediment packages at the same order as the P1 cycles (i.e.c. 40 m) is possible. This is remarkable considering the significant downslope distance between the two sections (30-40km following palinspastic restoration).

Palaeohydrology The interpretation of these cyclicities as the product of orbitally forced climatic changes rather than tectonic pulses may be clarified by estimating the rate at which the depositional basin (Munster Basin) is likely to have responded to imposed fluctuations in autocyclic variables. This involves an assessment of diffusivity (Paola et al. 1992) which is largely controlled by water supply. This parameter may be constrained by the palaeohydrological reconstruction of the Devonian fluvial systems. The estimation of palaeohydrological parameters from alluvial sandbodies is possible using a number of empirical relationships (e.g. Williams 1988), although many workers insist on the use of confidence intervals for statistical significance (Leeder 1973; Ethridge & Schumm 1978) and errors may be introduced by the improper algebraic manipulation of empirical equations (Williams &Troutman 1987; Williams 1988). In addition, such power functions do not

26

s.B. KELLY • S. E SADLER N

~

( ;lenlle.~kEsknabrock

~

South Reara

~

~

.....~z~ ~~~

34

~

S

",

~.~

/;ili

Itane % oleann

sandstone %

sandslone %

readily account for the large and generally unpredictable natural variability in relations between one site and another (Knighton 1975; Richards 1977; Rhodes 1977; Park 1977). Although it is generally accepted that the sedimentary basins of the ORS continent probably experienced warm to hot climates, this does not necessarily imply that discharge patterns in rivers which fed the depositional systems were strongly ephemeral or 'flashy'. A significantly different ('wetter') climate may have prevailed in the upland source areas where discharge is likely to have originated (Allen 1986). Consequently, data from drainage basins in a wide variety of climatic, topographic and tectonic settings have been used to develop empirical relationships from which the palaeohydrology of the Munster Basin may be estimated. Palinspastic facies maps provided by Williams et al. (1989) suggest that the terminal fan systems in the 'northern' part of the Munster Basin (CSF and GPF systems) had a maximum downstream extent of 90-110 km (Fig. 2). This assumes a N-S layer parallel shortening strain of c. 2 : 1 (e.g. O'Sullivan et al. 1986). The easterly and westerly limits of the fans are less well constrained but

Fig. 5. Correlation of basin-fill sequences as recorded in the two main study sections, illustrated by logged sandstone content (smoothed using a spline function). Note that the CSF variations in sandstone content are more 'rapid' fluctuations (P4 cycles) with a periodicity of c 700 m (in addition to the relatively short-term P1, P2 and P3 cycles illustrated in Fig. 4). These pulses are thought to reflect long-term climatic change due to cyclical variations in the eccentricity component with a periodicity of 2100 ka.

recent mapping suggests that the systems were c. 120 km wide. These dimensions are consistent with the data plotted by Heward (1978, p. 677, Fig. 3) which indicate that alluvial fans with areas ranging from 103 to 104 km 2 are characterized by fan lengths generally equal to, or frequently greater than, fan widths. The depositional area (Bb) covered by the large-scale 'northern' terminal fan system in the western Munster Basin was therefore c. 13 2 0 0 k m 2. In order to estimate the sediment/water flux of this system two approaches have been used. The first method uses quantitative relationships between factors such as drainage basin size, sediment yield and discharge observed in modern drainage basins. The second method uses direct estimates of channel depth and discharge based on sedimentary structures within alluvial sandbodies. The results of these palaeohydraulic reconstructions will be used to establish an equilibrium time constant for the Munster Basin. Method 1

Rates of denudation for particular present-day drainage basins have been calculated using the

SEQUENCES IN THE DEVONIANMUNSTER BASIN sediment transport rate in rivers or the rates of sediment accumulation in reservoirs (Langbein & Schumm 1958; Schumm 1963). As the Munster Basin was effectively closed, the volume of sediment within the individual depositional systems provides a direct measure of the amount of denudation that took place in the drainage basin (cf. Friend & Moody-Stuart 1972). The known area of the depositional system (Bb) and the estimated rate of sediment accumulation (a) are used to establish the volume of compacted sediment deposited annually

(v~):

Vc = aB b

(1)

where a is in m a l and Bb is in m 2. The value of Vc is then used to calculate the uncompacted volume of sediment deposited annually (Vu) using the relationship: Vu = Vc pc/Pu

(2)

where Pc and Pu are specific weights of compacted and uncompacted sediment with assumed values of 2700 and 1380 km m -3, respectively. The latter is an average value derived from 'open air' density estimates of 1490, 1314 and 1 2 5 0 k g m -3 for alluvial sand, silt and clay, respectively (FAO 1981, p. 173; see also Ingles & Grant 1975, p. 305). It should be noted that values of 9u will vary according to grain size and clay content. A value for a of 0.4 m m a -1 provides an estimate of c. 107 m 3 a -1 for Vu. An alternative method would be to use porosity evaluations to decompact the vertical sequence, although this is largely precluded by the virtual absence of primary porosity within these sediments as a result of their low grade metamorphic character (lower greenschist facies). However, in spite of the lack of porosity estimates and complexities relating to non-quantifiable volume loss during deformation, decompaction factors of 2 for sandstone and 3 for siltstone/ claystone are considered reasonable. In order to establish a crude mean decompaction factor for the basin-fill succession, the distribution of sandstone and siltstone must be accounted for. Sandstonedominated proximal and medial sequences are effectively "balanced' by more silt-prone distal and basinal deposits. The vertical alternation of proximal and distal sequences within the basin-fill therefore suggests that a crude mean decompaction factor of 2.5 is likely to be most applicable. This value compares well with that derived from Equation 1. The value of Vu derived in Equation 2 may be used to estimate the size of the drainage basin (Ad) through the relationship: Ad = 0.0168 VO'92

(3)

27

(R 2 = 0.82: standard errors; intercept (i) = 0.34, coefficient (c) = 0.05 log units). This relationship was derived from regression analysis of data from 89 drainage basins with values of Vu in the range 103-101~ 3 a -1 and Ad varying in the range 10~ 2. For a = 0.4 mm a t , Ad is estimated to be c. 50 000 km 2. The empirical relationships (2) and (3) suggest a direct link between drainage area and sediment yield. Both run-off and sediment yield from drainage basins are influenced by a number of additional, dependent and independent variables, which include bed-rock properties, drainage basin elevation/relief, main channel slope, basin length, rainfall intensity (duration and frequency) and vegetation (Langbein & Schumm 1958; Jansen & Painter 1975; Walling & Webb 1983). However, for a wide range of climatic regions, sediment yield (m 3 km -2 a -1) consistently decreases as a function of drainage area (Branson et al. 1981). This trend appears to be related to storage of eroded sediment within slope and channel systems (Trimble 1975). Climatic change and sediment source also have implications for sediment yield and the dimensions of the resulting alluvial fan (Bull 1991, p. 276). Empirical relationships between alluvial fan area, Af, and drainage basin are Ad, are expressed in the form Af = c A ~ (Bull 1964; Hooke 1968). The constants vary according to the lithology of the drainage basin. For a drainage basin with an area of 50 000 k m 2 comprising 60-70% sandstone, a fan area of c. 18 000 k m 2 is anticipated. For a drainage basin of the same size, but schistose in composition, a fan area of 43 000 krn 2 is predicted. The volumes of Holocene fan deposits correspond to estimated bedload sediment yields that range from 50 to 200 m 3 km 2 a ~ for watersheds underlain by granitic and metamorphic rocks. Significantly, estimates for the drainage basin that supplied the GPF depositional system suggest a sediment yield of c. 2 0 0 m 3 km -2 a -1 which is consistent with a proposed derivation from predominantly metamorphic rocks and older sediments within the Caledonides. The estimated drainage basin area (Ad) is used to establish mean annual discharge (Qd) through the relationship: Qd = 0.019A ~

(4)

(R 2 = 0.81: standard errors; intercept (i) = 0.11, coefficient (c) = 0.03 log units). Equation (4) is based on data from 259 drainage basins worldwide, with Ad in the range 10110 7 k m 2 and Qd in the range 10-1-105 m 3 s-1. This relationship is similar to that of Leopold et al. (1964), who further state that the relationship between discharge and drainage area can be

28

S.B. KELLY & S. P. SADLER

Table 2. Estimates of drainage basin area (Ad), length (Ld) , mean discharge (Qd) using Method I Sediment accumulation r a t e (mm a-1) a 0.1 0.2 0.3 0.4 0.5 0.6

Drainagebasin area (km2) Ad

Drainage basin length (km) Ld

Drainage basin mean annual discharge (m3 s-1) Qd

13 000 25 000 36 000 47 000 58 000 68 000

330 450 540 620 690 760

80 130 180 230 280 320

Various sediment accumulation rates (a) have been used, although the most likely rate is thought to be approximately c. 0.4 mm a-l.

expressed in the form Qf = aA b, where Qf is bankfull flood discharge and Ad is drainage area. The values of a and b depend on numerous factors including relief and climate. These relationships suggest estimated palaeodischarge volumes for the GPF drainage basin of c. 200 m 3 s-1 (Table 2). The length of the drainage basin (Ld) may be estimated using the relationship: L~ = 2.5A ~

(5)

(R2 = 0.83: standard errors; intercept (i) = 0.19, coefficient (c) = 0.03 log units). This relationship is based on data from 52 drainage basins with values of Aa in the range 104-107 km 2 and Ld in the range 102-104kin. Equation 5 is similar to that given by Hack (1957) and indicates that the relationship is fractalic (Leeder 1993). The estimates for Ad, Ld and Qa are given in Table 2 for varying sediment accumulation rates. The values for Ad and Ld are similar to those derived for Lower Old Red Sandstone drainage in the British Isles by Simon & Bluck (1982), who estimated a drainage basin of at least 5.2 x 104 km 2, with a 'valley distance' of c. 530 km. Method II

The second method of palaeohydraulic reconstruction uses the thickness of sets and cosets of cross-strata within sandbodies to assess discharge and channel depth. Allen (1968, Fig. 6.4) has shown that the mean height of dunes is proportional to flow depth. This has been supported by a number of field studies (Williams 1971; Wijbenga & Klassa 1983). Echo sounder profiles illustrated by Cant (1982, Fig. 31, p. 127) and Collinson (1986, Fig. 3.11, p. 28) indicate that bedform size will respond rapidly to discharge variations. A recent detailed field study by Gabel (1993) confirms the rapid response of dune height, length and migration rate to changes in discharge.

The size of the original bedforms is assessed by reference to preserved set height. The relationship between these parameters has been considered by several workers (Nummedal 1973; Paola & Borgman 1991). Harms &Fahnestock (1965) found that approximately half the dune height is preserved. Relative dune height [ratio of dune height (h) to water depth (d)] can vary from c. I (Simons 1971) to a very small value, with an average between 1/6 (Yalin 1964) and 1/3 (Nordin & Algert 1965). Allen (1968) derived the relationship h = 0.086d 1"19, where h is the mean bedform height (m) and d the mean water depth over the bedform (m). Although Allen's data indicate a _+50% variation in water depth for a given bedform height the relationship has been used by previous workers (e.g. Miall 1976). An alternative method of estimating channel depth is to assume that the bedforms achieved 'optimal' steepness values such that h/I= 0.6, where h is dune height and I is bedform wavelength (Haque & Mahmood 1986), and that the depth of the flow is I/7 (Jackson 1976). In order to reconstruct the dune height a preservation ratio of 0.24 is used (Paola & Borgman 1991). Combining these relationships yields d = 9.9Sp, where d is flow depth and Sp is preserved cross-strata set thickness. In order to estimate mean annual basin discharge (Qb) the following relationship is used: Qb = 32d18

(6)

(R2 = 0.68: standard errors; intercept (i) - 0.04, coefficient (c) = 0.01 log units). This relationship was derived from regression analysis of 150 rivers with Qb in the range of 0.5-18 000 m 3 s-1 and depths (d) in the range of 0.15-8.0 m. The power function given in Equation 6 can be compared to those of other workers (e.g. Knighton 1987) which indicate that the water surface width, w, and mean flow depth, d, of a stream change in proportion to some power of

SEQUENCES IN THE DEVONIAN MUNSTER BASIN water discharge, Q, such that w = aQ b and d = cQ a. For sand bed rivers, w - - a Q ~ and d = cQ ~ where likely values of a and c are 4.3 and 0.56, respectively. These relationships have been used to estimate the discharge of the rivers within the depositional basin. The minimum, mean and maximum set sizes observed in the basin-fill sequences indicate the range of discharges within the channels. The results are given in Table 3. Significantly, estimates of discharge in the depositional basin (Qb) derived from Method II are comparable to those for the drainage basin (Qa) provided by Method I though it is emphasized that Method II is independent of estimated sediment accumulation rate. Both methods suggest that the GPF depositional system could have been fed by a single river which had a mean annual discharge of c. 200 m 3 s-1. It is also possible to estimate gradient (slope) within the depositional basin, as various studies have demonstrated an empirical relationship between gradient and water discharge for different channel patterns (e.g. Leopold & Wolman 1957). Power functions derived by a number of workers indicate that gradient (G) and mean discharge (Q) are related by a power function of the form G = aQ c, in which the exponent c is of a constant (fixed) value and all variation, caused largely by the effects of sediment, is expressed in the coefficient a. For braided streams with discharges in the range 1.25-200 m 3 s-1 and a mean slope of 0.00096, Osterkamp (1978) established that a = -0.0019 and c = -0.31. These values are similar to those given by Leopold & Wolman (1957)(a= 0.0125 and c = - 0 . 4 4 ) and Lane (1957) ( a = 0 . 0 0 4 1 and c = -0.25). The differences between these relation-

Table 3. Channel depth (d) and discharge (Qb) estimates based on cross-strata thickness (Method II) Section within GPF

Depth (m) d

Discharge (m3 s-l) Qb

KE (mean) KE (min) KE (max) KF (mean) KF (min) KF (max)

2.8 1.9 4.2 2.6 1.9 3.6

210 100 420 180 100 330

Within each coset, the maximum set thickness observed has been used. The depth is then estimated using the two methods described in the text, the value d being the average of these two estimates. (Note: the difference between the two values for depth was never more than 17% and was, on average, only 5%). Section KE and KF refer to the lower and upper logged sections from Iveragh (see Fig. 4).

29

ships are due mainly to variations in the grain size of the sediment transported and the use of bankfull discharge rather than mean discharge in some of the earlier data sets. The relationship given by Osterkamp (1978) is for streams dominated by fine- to medium-grained sand, whereas the other relationships are based on data sets derived from both sand and gravel bed channels. Typical gradients on large fans dominated by braided streams are typically in the range 0.001-0.0003 (Stanistreet & McCarthy 1993). Using the relationship G = 0.0041Q -~ and assuming a mean Q of 200 m 3 s-1, a slope estimate of 0.001 is obtained for the GPF depositional system.

Basin equilibrium and response Many recent basin modelling exercises use a concept of diffusion which assumes that sediment influx is proportional to slope and transport in the downslope direction (Colman &Watson 1983; Kenyon & Turcotte 1985; Waltham 1992). However, it is noted that diffusive relationships between deposition and streamflows (e.g. Flemmings & Jordan 1989) imply a linear relationship between transport and slope and it has been suggested that this may represent too simplistic an approach (Leeder 1993). The following discussion is based on the model of basin filling developed by Paola et al. (1992). This model combines a linear diffusion equation for sediment dispersal with a simple mass balance model based on selective deposition for grain size partitioning and downstream fining. Paola et al. (1992) demonstrated from first principles that diffusivity is mainly controlled by the average rate of water supply and the type of streams in the system. The model allows the assessment of four independent governing variables whose influence on basin sedimentation is significant: input sediment flux, subsidence rate, gravel fraction in the sediment supply and diffusivity (which is mainly controlled by water supply). The stratigraphic response of the basin to all of these factors, except gravel fraction, depends on the ratio between the timescale over which the changes take place and a time constant that is unique to each basin or depositional system. The models of Paola et al. (1992) make use of the fact that natural systems take a finite length of time in order to respond fully to any imposed change and that the period of time necessary for recovery will reflect the magnitude and frequency of the imposed change and the magnitude of the geomorphic system (Howard 1982). Based on the scaling of the equations, Paola et al. (1992) defined the characteristic time constant for basin response, the equilibrium time

30

s.B. KELLY & S. P. SADLER

constant (T..), as To. = L2/v, where L is the basin length in th~ directl~)n of transport and v is the diffusivity. In physical terms, if a basin is allowed to evolve under constant rate and distribution of sediment input, the surface topography (stream profile) will reach steady state in a time approximating to Teq. For basin modelling purposes, the real importance of the equilibrium time constant is that the form of the basin response is determined by whether the timescale of the superimposed variations, T (e.g. variations in basin subsidence or variations in water/sediment flux due to climatic change), is substantially greater or less than Teq. Paola et al. (1992) state that 'slow' processes operate at timescales such that T >> Teq' whereas 'rapid' variations are those operating at timescales of T

I _1I1I

l lIli',

!

0

//i

~.:

r.i ~ / . I i 7 , / / .." . - / .,11_-/

/

I 0

I

I

~ o

I

~.,7':~.:

~

~.J

"~

:~..,

85

86

H. K. WATERHOUSE

therefore demands that great caution be exercised when using time series techniques to analyse geological sequences, as thickness measurements may not necessarily be equated with time. Power Spectral Analysis can be used to detect regular cycles within a time series. It can detect and separate a number of different frequencies, or regular cycles, in a time series consisting of several cycles with different frequencies, amplitudes and starting points superimposed upon one another, as orbitally forced cycles often are. The results of the power spectral analyses of the palynofacies results are displayed in Figs 5 & 6.

Results The raw palynofacies count data is given in Table 3. Absolute palynofacies particle abundances are displayed, in Fig. 2, as bar charts of the three main particle categories (marine palynomorphs, terrestrial palynomorphs and terrestrial debris) for the complete sampled section at Kimmeridge Bay. In addition, the individual particle categories are displayed as barcharts, in Fig. 3, for the top three sedimentary cycles only. The ratios are not

chorate

proximate prasino saccate pollen

spore5

dinocysts dinocysts phy'tes

brown wood

cuticle

displayed in full - only those which were considered to contain useful palaeoenvironmental information are shown. The results of time series analyses are presented as periodograms in Figs 5 & 6. It should be noted that none of the time series analyses were carded out on smoothed data sets. Smoothing the time series would eliminate high frequency cycles, or 'noise', and perhaps produce clearer periodograms, but precision may be lost. It was possible that, in this high-resolution study, organic particle cycles of quite short duration may have been present and these would be lost if the data were smoothed. The bar charts show a cyclic pattern of variation in the absolute abundances of all the organic particle groups. In general, there is one peak, sometimes a double peak, and one trough in the abundance graphs per lithological cycle, the peak occurring within the bituminous-rich shale bed and the trough spanning the remainder of the cycle. The cycles in absolute organic particle abundance therefore appear to have the same duration as the lithological cycles, and to be more or less in phase with them. The periodograms support this observation. Towards the base of the lithological section the

eq. r'ded eq. shp black wd black wd

hii

.=

i

N 9

iN Im

m mm I

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,

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L particles per mg

% of rock wt adiacenl samples not significantly separable due to Lycopodium variation p

adjacent samples not significantly separable due to low count values of these categories in the total count (see text)

Fig. 3. Variation in absolute palynofacies particle abundances in individual palynofacies categories, and of mineral component percentages of the rock, through the topmost three cycles of the Kimmeridge Bay section. Bed numbers follow those of Fig. 1. Where the statistical significance of variations between adjacent palynofacies samples is not sufficient to separate them this is indicated on the graphs.

87

PALYNOFACIES OF KIMMERIDGIAN CYCLES cyclic pattern becomes less distinct, with increased variation between peaks. In addition, there is a trend towards higher absolute particle abundances in the terrestrially derived groups in the lower part of the section. In Figs 3 & 4 the variations within individual cycles can be seen more clearly. The chorate and proximate dinoflagellate cysts display very similar patterns; a mostly smooth cyclicity corresponding to the lithological cyclicity, with occasional unusually high abundances in places. Terrestrial palynomorphs tend to show more abrupt changes. The bisaccate pollen in particular show this very clearly, with a strong peak on the periodogram indicating a wavelength of 122 cm for the cyclicity. The spores have a very similar distribution to that

marine p'morphs :

terrestrial debris

dinocysts :

prasinophytes

of the bisaccate pollen. Both pollen and spores display very similar abundance patterns and display a cyclicity more or less in phase with that of the dinocysts, but perhaps with the peak in abundance occurring slightly higher in the bituminous shale. Cuticle shows a rather different cyclicity to that of the marine and terrestrial palynomorphs. In contrast to other groups it has low, or decreasing, abundances during the bituminous-rich shale beds. In addition, it appears to display two peaks per lithological cycle. The first peak occurs immediately before, or at, the base of the bituminous-rich shale, and the second about half way through the lithological cycle, immediately above the bituminous-rich shale. The peaks vary slightly in size but there does not appear to be one

.

f

spores and r'ded:sharp black laths : pollen: equid, equid, terrest, debris black wood black wood

spores :

saccate pollen

.

.

.

.

.

.

.

.

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~.-.

--ii..........

m

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I

I

.... i::

I

h=e I

.

-

sample in which the total value of all the constituents of the ratio is less than 50. the ratio thereby possibly lacking significance at that point

Fig. 4. Variation in the ratios between selected palynofacies particle groups through the topmost three cycles of the sampled section at Kimmeridge Bay. Bed numbers follow those of Fig. 1. The ratios are plotted on a logarithmic scale and adjacent samples which were not found to be statistically separable are indicated.

~

H. K. WATERHOUSE

dominant peak within each cycle. This is confirmed by examination of the periodogram which has its dominant peaks at 76 and 60 cm, which represent a cycle, or cycles, of approximately half the duration of the 120 cm cycle of other particle groups. The equidimensional sharp and the lath-shaped black wood, the brown wood, the degraded palynomorphs and the total particle abundances all display a clear cyclicity in phase with the lithological cyclicity and with that of the terrestrial palynomorphs. The periodograms record strong cycles at c. 119 cm. The equidimensional rounded black wood, like the cuticle, has two peaks in abundance per lithological cycle. The periodogram records a cycle at 64 cm but nothing near 120 cm, indicating that both of the two peaks within each cycle are equally important. Many of the ratios in Fig. 4 display cyclic variation with the most prominent cyclicity being at c. 120 cm. These groups do vary from one another, however, in their types of distribution. Several groups also have a clear cyclicity but are not in phase with the lithological cyclicity or the main palynofacies cyclicity. Figure 3 contains the results of the elemental analysis evaluations for the WL3 and WL4 cycles. It can be seen that the % TOC displays a cyclic distribution, with the pattern being very similar in both cycles. It is low in the mudstone, increasing gradually through the bituminous shale above to a definite peak at the top of it. It then decreases rapidly at the base of the mudstone above. The percentage of clay, quartz and other minerals displays a variation similar to that of the % CaCO 3, both being almost exactly the opposite to that of the % TOC.

Discussion Explanations for the deposition of such large amounts of organic matter as those found in the Kimmeridge Clay Formation have been advanced by a number of workers, often using analogies with Recent environments such as the Black Sea. There have been two main schools of thought, one favouring a mainly preservational control on organic matter accumulation and the other favouring productivity. Calvert & Pedersen (1990) have suggested that the settling flux of organic carbon, closely linked to the rate of primary production, was the main control on accumulation of organic matter irrespective of bottom-water oxygen values. Pedersen & Calvert (1990) noted that sediments accumulating in the modem Black Sea are not particularly enriched in organic matter despite the presence of an anoxic water column. They suggested that oceanic anoxic events, such as in the Cretaceous Atlantic for example, were brought about by sluggish circulation and that increases

in primary production, reflecting changes in the ocean-atmosphere system, constitute a more tenable explanation for the occurrence of modem and Quaternary carbon-rich sediments and Cretaceous black shales. Lallier-Verg~s et al. (1993) proposed that tile organic sediments of the Kimmeridge Clay Formation were deposited under an oxygenated water column with a redox boundary that oscillated above and below the water-sediment interface, mainly due to variations in the organic flux or productivity. Huc et al. (1992), in a study of the Dorset Kimmeridge Clay based on petrography and geochemistry of organic matter, proposed that variations in the amount and nature of organic matter were mainly controlled by changes in biomass. They also showed that changes in depositional conditions occurred together with changing productivity. These changes were mainly reflected in the modification of burrowing features. Tribovillard et al. (1994) favour phytoplankton productivity as the main driving force of variations in organic matter concentration, with redox conditions of the depositional environment acting as a positive feedback effect. Gallois (1976) invoked a delicate balance between palaeogeography, a series of subsiding basins linked by relatively narrow straits between land areas of low relief, and nutrient supply as particularly favourable to the formation of algal blooms. Another theory suggests that phytoplankton blooms were a symptom, rather than a cause, of widespread anaerobic bottom conditions and that preservational factors rather than productivity were the major control (Tyson et al. 1979). Tyson (1987) proposed a widespread depletion of oxygen with deposition of organic-matter-rich mudstone cycles due to limited, probably silled, connections to the open ocean, and high biological productivity. Wignall (1989) and Myers &Wignall (1987) suggested, in this context, that a climate-induced temperature stratification (thermocline) was the predominant control on the different lithologies. This situation may have been storm-limited, with water depth being the principal control on the distribution of bituminous shales (Wignall 1989). Many other authors invoke water stratification as a characteristic parameter of the depositional conditions of the Kimmeridge Clay Formation. Miller (1988) suggested that stratification was due to salinity variations. Pratt (1984), studying the Cretaceous Greenhom Formation of Colorado, found that sedimentary and organic geochemical data indicated a close association between palaeoclimate, salinity of surface water, strength of bottom-water currents, and amount and composition of organic-matter preservation in the sediment. She suggested that

PALYNOFACIES OF KIMMERIDGIANCYCLES palaeoclimatic and palaeoceanographic factors that influenced mixing and current strength in the water column profoundly affected the amount and type of organic matter preserved. She proposed that dry periods were characterized by a well-mixed water column and low preservation of organic matter, and wet periods with high river discharge led to salinity stratification of the water column and quiescent oxygen depleted bottom water and high preservation of organic matter. Bottom-water oxygenation was therefore primarily controlled by the strength and frequency of benthic currents rather than by the rate of oxygen consumption in the benthic environment. Oschmann (1988) proposed the North Atlantic Water Passage model, which is a seasonallycontrolled modified upwelling model with temperature stratification. His interpretation was not related to any cycles of possible orbital origin but referred to shorter-scale variations and the overall palaeoecological situation. Cyclic variation in the lithology of the Kimmeridge Clay Formation has often been suggested as being due to vertical movement of the oxic-anoxic interface (Irwin 1979; Tyson et al. 1979; Myers & Wignall 1987). Scotchman (1991), using Tyson's (1989) method of calculating kerogen abundances, found that the second highest phyTOC values (an estimate of the absolute abundance of terrestrially derived structured organic debris) from a number of Kimmeridge Clay locations were recorded at Kimmeridge Bay in Dorset. This suggested to him that increased terrigenous kerogen input was associated with high sedimentation rates. He also found that proximal shelf samples had the lowest phyTOC values, and suggested a sedimentation rate control on kerogen facies in the Kimmeridge Clay. In addition, he suggested for the Kimmeridge Bay location that dilution of a relatively constant flux of amorphous marine organic matter by variable amounts of structured terrestrial-derived kerogen occurred towards basin centres, where higher sedimentation rates increased the relative proportions of the terrestrial components. He also demonstrated that both sedimentation rate and differential organic-matter preservation were major controls on the kerogen facies of the Kimmeridge Clay Formation. Myers &Wignall (1987) have suggested that the sedimentation rate exerts a control on the organic matter and carbonate content of the mudstones of the Kimmeridge Clay Formation. Downie (1957), studying the Kimmeridge Clay of Dorset, found a close correlation between the quantity of kerogen and the proportion of terrestrial palynomorphs. This suggested that bulk kerogen is also terrigenous and he proposed that the Kimmeridge Oil Shale was formed by an increase

89

in swamp vegetation and therefore an increase in transported swamp-generated organic matter. Tribovillard et al. (1994) have stated that dilution effects by inorganic components (detrital clay and quartz and biogenic coccolith carbonate) of the sediment cannot account for the TOC cyclicity in the Kimmeridge Clay in Yorkshire. They studied the clay minerals in two cycles and obtained results which indicated a homogeneous terrigenous detrital supply. No qualitative variation was detected in the clay assemblage and minor quantitative variations were seen within and between the cycles. Muller (1959) noted that there was apparently no direct relationship between the rate of sedimentation and the total pollen content of the modern sediments of the Orinoco Delta, which was not surprising as the sources and supply conditions were different. He stated that climatic changes may be expressed by the bulk changes in composition of vegetation and the amount of river discharge. Both Tribovillard et al. (1994) and Herbin et al. (1991) report that the main source of carbonates in the cycles of the Yorkshire Kimmeridge Clay was coccoliths. NChr-Hansen (1989) used visual and chemical kerogen analysis to investigate the Lower Kimmeridge Clay. He recognized three distinct kerogen facies (palynofacies); one dominated by AOM, a second consisting mainly of dinocysts, acritarchs, spores, pollen, cuticles and foraminiferal test linings, and a third facies containing predominantly woody particles. He interpreted these associations to be the result of vertical fluctuations in the oxic-anoxic boundary of the sediment-water interface and only to a lesser extent of variations in the organic input. This was in agreement with earlier models. Williams & Douglas (1983) conducted an examination of sequences of shale, clay and limestone from the Kimmeridge Clay of Dorset. They sampled from three points within the section used in the present study and, using a number of techniques, found no major qualitative differences in the organic-matter content of the sedimentary units. However, major variations in the quantity of organic matter were observed, shales containing greater proportions than either the clays or limestones. They suggested a combination of mineralogical dilution of kerogen and sedimentary preservation effects as the causes. Ebukanson & Kinghorn (1985) investigated the influence of environmental factors on the distribution of kerogen types contained in the different lithologies of the Kimmeridge Clay of southern England. They found no simple linear relationship between kerogen types and the associated lithologies. However, there was a relationship between kerogen type and organic richness of rock samples. The laminated mudstones were found to

90

I-I. K. WATERHOUSE

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flV;I! 2000 off the coast to < 50 in deep ocean basins. Distances of transport may have exceeded 5000 km. Groot (1966) noted that there appeared to be a relationship between the number of pollen grains and the quantity of mineral matter in suspension in the estuary of the Delaware river. He suggested that the two types of particles are transported together and in a similar fashion, i.e. primarily by water currents. Pollen grains per g of suspended sediment (clayey silts and silty clays) varied between 16 000 and 25 000. Traverse & Ginsburg (1966), studying the surface sediments of the Great Bahama Bank, stated that pine pollen (bisaccate) is large and buoyant and, once delivered to the water, remains suspended for long periods of time and is deposited as a rather sensitive indicator to hydrographic features, especially turbulence. On swamps and tidal fiats they found nearly 5000 pine pollen g-l, and in all other samples 0-558 per g. Hoffmeister (1954) reported 7500 grains of pollen per g in a sample and a ratio of 1 : 4 of large to small pollen, indicating proximity to a shoreline, in ancient basins of deposition, with regularly decreasing amounts of pollen per g and a decreasing ratio of large to small grains moving away from the shore. But, Traverse & Ginsburg (1966) noted an inverse relationship between pine pollen in the water and that in the sediment. This they explained as being due to sediment patterns not source. Pollen highs were regularly found where water turbulence was minimal. Cross et al. (1966), studying the source and distribution of palynomorphs in the gulf of California, found a few thousand to 80 000 spores

93

and pollen grains per g of dried bottom sediment; a few hundred to over 20000 pine pollen; 2000-25 000 cuticle fragments; up to 3 000 000 tracheids (5-50 lam) and up to 17 000 dinocysts per g of sediment Low concentrations of palynomorphs were found to occur along some shores, probably due to dilution by terrigenous sediments, but they built up offshore. Some palynomorphs decreased in number offshore while others (e.g. pine) became selectively increased in relative frequency. In this study no taxonomic determination of the spores and pollen was undertaken and, as a result, it would not be feasible to relate their abundances in the samples to changes in the source vegetation. This would be difficult anyway in such a distal setting due to differential sorting during transportation. Ratios such as the bisaccate pollen:spore ratio, the l i g h t : h e a v y spore ratio, and the terrestrial palynomorphs : wood ratio, in which one particle type is less buoyant than the other whilst having similar provenances, may all record variations in energy conditions of the transporting medium and, thus, variations in marine current circulation or perhaps run-off. The terrestrial debris found in palynological preparations is composed of broken-down parts of terrestrial plants. It may therefore provide a record of several aspects of the terrestrial environment and of the conditions of transportation of particles from there to the environment of deposition. Two characteristics of these particles were noted during analysis - the absolute abundances, and the condition of the particles as classified by size and shape. As the particles have different morphologies, they are likely to have different hydrodynamic properties and, therefore, different distributions. Various measures of environmental variation which can be inferred from these particles are listed in Table 4. The black laths are likely to have a greater buoyancy than other particles as they would have a lower mass to surface area ratio, and therefore be hydrodynamically equivalent to smaller solid particles. These particles would remain in suspension for relatively longer periods of time than other particles and therefore be an indicator of low energy conditions or a distal setting. This is the case as indicated in the literature (e.g. Tyson 1989; Cope 1981), In addition, most black laths were noted to be angular rather than rounded which is also suggestive of low energy conditions with very little abrasion occurring and/or that they consisted of a more resistant material (Cope 1981). Van Buchem et al. (1994) noted that opaque particles found in their samples can be rounded, blocky or splinter-like. SEM examination revealed to them that most of these particles showed anatomical details of the kind found in carbonized wood fragments of terrestrial plants (Cope 1981).

94

H. K. WATERHOUSE

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H.K. WATERHOUSE

pronounced at low latitudes. Perhaps, therefore, the position of southern England during the Kimmeridgian was such that it was within an overlap of the latitudes where the controls of both the precessional and obliquity cycles had effect. This would explain the presence of the secondary (precessional) cycles within each major (obliquity) cycle. One of the precessional peaks in palynofacies abundance is situated (segment la) close to, but slightly below, the major peak in palynofacies abundance (the obliquity 'high' of segment lb) of the longer obliquity cycle, and is therefore partly disguised by it. It may be noted, when the complete sampled section is studied (Fig. 2), that the major peaks in abundance often appear as double peaks. When studied in more detail these can be seen as the peaks in absolute abundance of almost all palynofacies categories in segment lb and of marine palynomorphs, equidimensional rounded black wood and cuticle in segment la of the typical cycle. It is thought that the distance between these two 'peaklets' varies through the longer complete sampled section (see Fig. 2). This would be expected to occur if precessional variation were superimposed on obliquity variation whose duration is not exactly twice that of the precessional variation. For example, if the terrestrial palynomorph abundances in the top part of the complete Kimmeridge Bay section (Fig. 2) are studied, it can be seen that the distance between the two 'peaklets' of each major peak seems to decrease moving down-section to a point within the WL3 shale bed, where there appear to be two major peaks situated more closely together than the rest, within one bituminous shale bed with no mudstone bed between them (note that the WL3 cycle analysed in detail in this work does not include the lower part of the WL3 bed which was considered to possibly be a separate cycle). Each peak in the WL3 bed also possibly has two 'peaklets' within it. It is suggested that this pattern represents the gradual shift of a superimposed precessional cycle (the lower 'peaklets' in these particular cycles, plus a second peak in equidimensional rounded wood which cannot be seen clearly in Fig. 2) along the wavelength of an obliquity cycle. At times the obliquity and one of the precessional peaks in palynofacies abundance would coincide (as they nearly do in the three cycles studied here in detail) and at others they would be completely separate, as has possibly occurred in the lower half of the sampled section at Kimmeridge Bay (Fig. 2). It was for this reason that the study of the Kimmeridge Bay section was continued downwards - to investigate whether it was possible to recognize this expected shift. However, as the patterns lower down the section became more complicated it was

rather difficult to interpret them in any meaningful way using the current sampling interval, although it certainly does show more than one major peak in palynofacies particle abundance per lithological cycle. Van Buchem et al. (1992), studying chemical cyclicity of Lower Lias mudstones of Yorkshire, UK, found that at some times the eccentricity signal was best preserved, while at others the obliquity and/or precessional signal was dominant. Van Buchem et al. (1994) further suggested that, in the Lower Lias mudstones of Yorkshire, orbitally forced climatic changes influenced the depositional system at two levels: (1) short-term variations affecting the storm frequency and perhaps magnitude; and (2) longer-term variations affecting weathering and clay production in the source area. It is proposed here that two orbital cycles produced different palaeoenvironmental effects in the Kimmeridge Clay of Dorset. The precessional cycle is suggested to have had its greatest effects on the terrestrial environment and terrestrially-derived variations in palynofacies (the second peak in abundance in each cycle occurred only in terrestrially derived particle groups), while the obliquity cycle had its greatest effect on the marine environment (the current intensity, plankton productivity and preservational conditions). A discussion of the possible nature of these effects now follows. A discussion of the variation in dinocyst abundance is first required. It may be noted that the first peak in dinocyst abundance, in segment l a, is not accompanied by peaks in the other particle groups (except equidimensional rounded black woods and cuticle). It is therefore likely that this peak is productivity, rather than preservation, controlled. This distinction could not be made without the use of absolute abundance techniques. Indeed, Berlin & Brosse (1992), in a high-resolution petrographical and geochemical study on cycles from the Exodus Zone of the Yorkshire Kimmeridge Clay Formation, proposed a higher primary productivity and higher sedimentation rate at the base of their cycle which, according to the patterns of their results, would probably correspond to the segments 3 and la of the present work. It is also possible that the peak in dinocyst abundance could be due to good benthic preservational conditions in the marine environment but low terrestrial output into the marine basin. It should be noted, however, that the dinocysts display two peaks in absolute abundance situated closely together, one at the same time as the first equidimensional rounded black wood precessional peak (la) as well as one during the obliquity controlled (lb) peak, while other particle groups display one peak. Perhaps this indicates both an obliquity and a

PALYNOFACIES OF KIMMERIDGIANCYCLES (different) precessional control on the dinocysts. However, they do not display the expected second precessional peak in segment 2a. There are several possible explanations for this pattern. Dinocyst production could be precessional controlled, with the peak in segment l a being a productivity peak and the (obliquity) peak in segment lb being preservation controlled, as the high absolute abundances of other particle groups would suggest. The lack of a productivity peak in segment 2a could therefore be due, in some way, to the probably rather stronger influence of the obliquity cycle producing a breakdown of stratification and benthic anoxia by increasing the intensity of ocean-current circulation. This would have prevented the otherwise expected (productivity generated) peak in dinocyst abundance from being preserved due to the low preservation potential, with the higher preservation potential in segment la (due to a slowing of ocean circulation and build-up of stratification and anoxia) allowing the first productivity generated peak to be preserved. It is proposed that this is the most likely explanation for the patterns. There is still, however, a peak in equidimensional rounded black wood recorded in segment 2a despite the apparently low preservation potential. If this explanation for the dinocyst peaks in abundance were correct, it would suggest that the abundance of equidimensional rounded black wood was controlled by either the energy of the transporting medium, which is thought to be greatest in segment 2a, or some factor acting on the terrestrial environment which affects neither the other terrestrially derived particles nor the dinocysts, A further discussion of the equidimensional rounded black wood variation patterns is given below. Alternatively, the dinocyst peak in segment la could be part of the obliquity cycle to which most other palynofacies particle groups responded slightly later. It would still, however, be productivity controlled. In this case, the earlier peak in dinocyst abundance than in other particles could be due to the obliquity generated conditions in that part of the cycle particularly favouring dinocyst production. For example, the conditions prevalent during the build-up to the low-energy, low benthic oxygen probably stratified water-column conditions of segment lb could be ideal for high dinocyst production, possibly including blooms. This production would then decrease as the water column became increasingly stratified, with maximum stratification (and minimum oxygen and minimum current intensity) in segment lb. The second dinocyst abundance peak (in lb) would be preservation controlled, as are most other particle groups, despite possibly much lower plankton

107

productivity. This would explain the lack of a second (precessional) peak in dinocyst abundance in segment 2a. A third explanation could be that two preservation-controlled dinocyst peaks were preserved within the bituminous shale, but that the supply of terrestrial debris was much lower during the first peak, therefore resulting in the lack of a peak in terrestrial particles in la due to low supply, despite high preservation potential. However, there is little conclusive evidence from other palynofacies indicators (e.g. the ratios), or from previous workers' observations, to support such a large variation in terrestrial supply in such a small segment of the cycle and in such a distal setting. It is suggested, therefore, that the first dinocyst peak in abundance was productivity controlled. Tribovillard et al. (1994) state that TOC acts as an indicator of productivity of organic-walled plankton. Perhaps this would explain the fairly high TOC values which correspond to the first dinocyst abundance peak, with the main peak in TOC values being an indicator of preservational conditions, as the other high particle abundances are. However, this still requires an explanation for the presence of a productivity peak at this part of the cycle and it needs to be determined why this peak occurs during the build-up of stratification rather than during its breakdown. Breakdown of anoxia is likely to be a more common trigger of high productivity due to remixing of stored nutrients back into the water column, although it does not necessarily require large amounts of stored nutrients or widespread anoxia. Irwin (1979) has suggested vertical mixing of nutrients as a method for coccolith propagation. Calvert & Pedersen (1990) suggested primary production as a main control on accumulation of organic matter irrespective of bottomwater oxygen values. Whichever interpretation is preferred, it is still likely that the two peaks in dinocyst abundance per cycle have different origins, the first being productivity controlled and the second preservational. At the beginning of their cycle in TOC variation, which showed similar patterns to the TOC variations of the cycles of this work, Bertrand & LallierVerges (1993) noted high SRI values where the TOC values were lowest. This difference in the two measures, which was in contrast with the rest of the cycle where they showed similar patterns, they interpreted as most probably being due to early sulphate reduction in anaerobic microenvironments, such as fecal pellets, aggregates, or living or dead organisms, close to the photic zone. This occurred in the same part of the cycle as the first dinocyst abundance peak of this work, for which an explanation of high dinocyst productivity is difficult to determine.

108

H.K. WATERHOUSE

If the TOC peak of Bertrand & Lallier-Verg~s (1993) is taken to represent the same point within a cycle as the TOC peaks of the present work (lb), then both peaks (according to Bertrand & LallierVerges 1993) would be the result of increased planktonic productivity, with the lower part of the cycle (below the peak in la) having the lowest plankton productivity but a high degree of sulphate reduction in microenvironments in the upper part of the water column. The results of the present study, however, suggest the opposite pattern for the bituminous shale part of the cycle. The other parameters requiring an explanation are the distributions of equidimensional rounded black wood and cuticle, which are very different from that of other palynofacies data. Their main features are a low absolute abundance during segment lb, where all other particles peak, a peak in segment 2a of equidimensional rounded black wood when all other particles have a low abundance, and a peak in segments 3/la where all other particle groups, except dinocysts, have low abundances. The low equidimensional rounded black wood and cuticle abundances in segment lb occurs during the time of lowest marine energy conditions, lowest oxygen and probably stratification of the water column. The low abundances of these groups must be due to a low supply to the site of deposition, as all other organic particles (both terrestrial and marine) are preserved in high absolute abundances. It must, therefore, be determined why these components have a lower supply than other terrestrially derived debris. The low abundance of cuticle can be explained by the low marine energy conditions. Cuticle is the strongest indicator of proximity of all the terrestrial particles analysed. In such a distal setting it would be low in abundance anyway, but where the energy of the transporting medium is especially low cuticle would be expected to be absent. An explanation for a low supply of equidimensional rounded black wood may be obtained from the paragraphs below where possible reasons for its peaks in abundance are discussed. One explanation of why there is no peak in dinocyst abundance in segment 2a when equidimensional rounded black wood peaks (if both were precession controlled) could be that the two particle types have very different origins, the dinocysts settling through the water column and probably being degraded on their way through it, while the equidimensional rounded black wood has a more horizontal supply, perhaps carried mainly by bottom marine currents once in the marine basin. It is also possible that the equidimensional rounded black wood is more resistant to degradation than

the dinocysts and is therefore not degraded in the well oxygenated conditions of segment 2a. There are very few clues in the data obtained to provide assistance with explaining the presence of the two peaks in equidimensional rounded black wood in segments 2a and 3/la. However, a number of speculative explanations can be suggested. Berlin & Brosse (1992) suggested that terrestrial organic debris particles became more rounded when energy was lower (and TOC therefore greater). An increase in equidimensional rounded black wood with respect to other, angular wood particles would be expected to represent a decrease in energy of the transporting medium allowing more time to be spent in transit for abrasion of black wood particles (from sharp to more rounded) and oxidation from brown to black wood to occur. The peaks in absolute abundance of equidimensional rounded black wood could not be due to any transportational effects as they show the opposite pattems to those expected, such as high abundance during segment 2a where energy is thought to be much higher than in lb where their abundance is low. It is possible that the equidimensional sharp wood and black laths consist entirely of charcoal and the equidimensional rounded black wood represents the only true (unburnt) wood, which was subsequently oxidized in transit. The equidimensional rounded black wood would therefore have a different origin, within the terrestrial environment, from the other types of black wood and therefore possibly be a reflection of vegetation change (Cope 1981). However, the brown wood displays a similar pattern of variation to the equidimensional sharp black wood and black laths (charcoal), and not the equidimensional rounded black wood as would be expected in this case. Therefore, an explanation for the similar abundance variations but different origins of those particles would be needed which could also explain the similar origins but different abundance variations of brown and equidimensional rounded black wood. An explanation which does not require different origins for the different equidimensional black wood types and similar origins for the equidimensional rounded black wood and brown wood is that the equidimensional rounded black wood has undergone a different method of transportation (including storage - see below) on land, or intertidally, before reaching the marine basin, from the other types of wood, whose abundances could then be a reflection purely of transportational processes or preservation in the marine basin. There is no indication from the other recorded data that the difference in abundance patterns of the woods

PALYNOFACIES OF KIMMERIDGIANCYCLES could be due to differences in marine transportation processes. Equidimensional rounded black wood could therefore be brown wood that has undergone oxidation or equidimensional sharp black wood that has undergone abrasion during transit (mostly whilst still on land). It would therefore have had to have spent a longer time in transit than the fresh brown wood or the sharp black wood and its shape would certainly indicate this. This could be due to extended periods of storage on land or intertidally, and not necessarily slower transit times (for which there is no evidence). This scenario, and those described below, could apply equally if the equidimensional rounded black wood was derived from oxidation of brown wood or physical abrasion of sharp equidimensional black wood particles (perhaps charcoal), or both. Scotchman (1991) noted that in almost all Kimmeridge Clay samples that he studied, vitrinite (wood) particles were generally small, not very abundant and occasionally showed signs of oxidation. Their reflectances suggested that many were reworked from older sediments. This he found to be more common in basin margin areas rather than in basinal areas, such as Kimmeridge Bay. Nevertheless, it is possible that, during certain parts of the cycle (3/la and 2a), a situation occurred where older sediments were able to be reworked, or the storage areas (on land or intertidally) of the oxidizing brown wood or abrading equidimensional sharp black wood to be plundered, and their particulate organic contents incorporated into the resulting marine deposits. A possible climatic explanation of this could be that the shift of climatic belts, which is a known effect of precessional forcing, could result in reworking of previously little eroded areas. Kutzbach & Otto-Bliesner (1982) stated that this shift would alter the boundaries between wet and dry zones, and the balance between monsoonal and zonal wind circulation. This could result in reworking of sediments in a number of ways. A change in direction of the prevailing winds could expose previously sheltered coastal areas, possibly storing equidimensional rounded black wood, to wave erosion. An increased frequency of storms would have a similar effect. Increased run-off would also expose terrestrial sediments to renewed erosion. A difference in type of vegetation may alter the supply of wood. For example, a forest-type vegetation would increase the supply of wood on land (although not necessarily its output to the marine basin). Another type of vegetation, such as desert or scrub, could induce greater terrestrial erosion (by rain and wind) and perhaps, therefore, remove previously stored equidimensional rounded black wood to the marine basin. An established vege-

109

tation of this type would also then keep it low in the marine basin by preventing its accumulation by oxidation and abrasion on land. A taxonomic study of the pollen grains may provide additional information, as would a study of the reflectivities of the different types of wood. If the difference in origin between equidimensional rounded black wood and other types of black wood were vegetational (or proxy-vegetational due to vegetation affecting erosion; Herring 1977), this could also fit into a precessional-controlled model for the patterns of variation seen. For example, a slight shift in climate belts could alter the type of vegetation. In the climatically sensitive 'zones', between 20 and 40 ~ latitude, even small changes in climatic conditions would be likely to change to position of the climate belts and, as such, the depositional environment (Van Buchem et al. 1994). The high sensitivity of the climate system has been illustrated by Kutzbach & Guetter (1984) and Kutzbach &Otto-Bliesner (1982), who demonstrated the importance of low latitude solar insolation changes as a modulator of monsoon circulation during the last 9 ka. It is proposed, therefore, that the most likely explanation for the apparently anomalous abundance patterns of the equidimensional rounded black wood is not any marine process or a direct result of vegetation change but that in some way a precessional-controlled shift in climate belts, and therefore vegetation, alters the supply of equidimensional rounded black wood (whether derived from equidimensional sharp black wood by abrasion or brown wood by oxidation) by alternatively allowing storage of the particles somewhere outside the marine basin and plundering these storage areas, therefore increasing the supply to the marine site of deposition. Any of the possibilities discussed above could also alter the horizontal supply of sediments and nutrients to the site of deposition, as well as possibly affecting the vertical mixing in the marine basin (e.g. salinity stratification due to increased run-off). This would, in turn, affect the plankton productivity which is also thought to respond to precessional orbital forcing. A variation in marine current circulation accompanying the shift in climate belts (but less important than the more dominant obliquity marine current control) could also explain the presence of a plankton peak here. Quantitative results were obtained from the reflected light work (on five samples - C9, C l l , C13, D2 and D20) and used to try to determine whether there was a difference between equidimensional rounded black wood and equidimensional sharp black wood, apart from their shape. Initial results indicated that equidimensional sharp black

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wood consisted of c. 15% vitrinite ('fresh wood'), 70% fusinite/semifusinite ('charcoal') and 12% inertinite ('oxidized wood') (Stach et al. 1982), while equidimensional rounded black wood consisted of c. 25% vitrinite, 65% fusinite/semifusinite and 7% inertinite. In other words, they both consisted of similar percentages of fusinite/semifusinite but equidimensional rounded black wood contained a greater proportion of vitrinite and equidimensional sharp black wood a greater proportion of inertinite. If equidimensional sharp black wood had spent less time in transit than equidimensional rounded black wood, it would be expected to contain a higher proportion of vitrinite ('fresh wood') than the equidimensional rounded black wood. That it does not could suggest several things. It could contain a similar or greater absolute amount of vitrinite but the larger amount of inertinite lowers the relative amount of vitrinite. Altematively, to fit into the storage theory, the two types could have had similar origins but have taken different journeys to the site of deposition, with equidimensional sharp black wood undergoing more chemical degradation during transit and the equidimensional rounded black wood more physical abrasion. Or, perhaps, vitrinite from different plants had different resistances to degradation and the proportions of wood types within each group reflects vegetation changes. A final explanation could be that the equidimensional sharp black wood contains less vitrinite because the vitrinite is still brown in colour, and therefore classified in a different group, because it has been little oxidized due to less time spent in transit. The question of the different compositions of the individual wood groups warrants further investigation. It should be remembered that the reflectance results discussed here were obtained from a small number of particles in a small number of samples. It is not unusual, when using percentage data, for one component to vary at the expense of another, as do the TOC and the clay, quartz and other mineral components of the samples of this work. What is interesting in the data of this work is that the CaCO 3 displays a similar variation to that of the clay and other minerals which comprise the major (80-98%) portion of the rock. When one component comprises such a large proportion of the total it would seem more likely for smaller components, such as the TOC and CaCO 3 here, to vary in the opposite way to the major one and in a similar manner to each other. The patterns seen here suggest that the conditions which favour an increased percentage of clay and other minerals to occur in a sample also favour the deposition of CaCO 3. This is reasonable, as CaCO 3 is more readily precipitated in oxygen-rich (and therefore more energetic) waters, and clay and other

minerals, most of which would be terrestrially derived in these samples, are likely to be more abundant when terrestrial run-off is higher or marine currents more energetic and therefore more able to transport particles. It has been shown that the percentage of terrestrially derived inorganic particles (clay, quartz, etc.) varies through a cycle, decreasing during the top of the bituminous shale. This could be a reflection of two things. It could represent an actual decrease in terrestrially derived mineral matter, which may give a false high reading of absolute organic particle abundances due to less dilution by mineral matter. This would only be true if the relative decrease in terrestrially-derived mineral matter was also an absolute decrease, which cannot be determined from the existing data. It could also be due to an increase in TOC in the marine basin during the deposition of the bituminous shales, due to either an increase in organic matter preservation (lower marine energy) and/or an increase in plankton productivity. Despite the fact that the TOC percentage is much lower overall than that of the clay and other minerals, it is suggested that during the bituminous shale deposition it had the same major control as the factors affecting the mineral matter percentages. In this case, a decrease in marine-bottom current energy would be the cause of increased preservation and/or productivity of organic matter and therefore TOC and, hence, relatively less clay and other terrestrial minerals. There are two factors which support this theory. This was a relatively distal setting so variations in terrestrial run-off, and therefore dilution by terrestrial particles, would have less effect than marine currents would have. Van Buchem et al. (1994), studying Lower Lias mudstones, proposed that terrestrial run-off could be excluded as a cause for TOC variation as the clay mineral distribution showed no correlation with TOC variations on a layer scale. In addition, the terrestrial organic particles (except the equidimensional rounded black wood and cuticle) show the opposite distribution to the clay and other minerals through the whole cycle. This suggests that their abundance is preservation-controlled because both of these components would otherwise be expected to vary in a similar manner since they are transported by the same agents. This also lends weight to the argument that a high absolute abundance of marine palynomorphs in the lower part of the bituminous shale was productivity controlled. The terrestrial organic particles and TOC have lower values in the lower (la) than in the upper (lb) part of the bituminous shale but the per cent of clay and other minerals is higher. If this was due to higher terrestrial output it should both dilute the marine component and coincide with an increased terrestrial organic debris component, neither of

PALYNOFACIES OF KIMMERIDGIAN CYCLES which occur. Therefore it is probably only an effect of a decreased TOC component. A summary of the interpreted palaeoenvironmental changes through a typical Kimmeridge Bay palynofacies cycle is as follows. Precessional (positive ?) orbital forcing shifts climatic and vegetational zones, and allows areas on land or intertidally storing wood, which is being altered (from equidimensional sharp black wood or brown wood) to equidimensional rounded black wood, to be eroded and their contents carried into the marine basin. This is accompanied by a peak in dinocyst productivity in the marine basin (1 a). The high point of an obliquity cycle causes marine currents to slow sufficiently to allow a build-up of anoxia in the particular palaeogeographic position of the Kimmeridge Bay area at that time. This allows all organic particles to be preserved in high abundances. There is a low supply of equidimensional rounded black wood and cuticle at this time due to the (negative ?) precessional forcing (lb). Precessional (positive ?) forcing again shifts climate belts and allows storage areas of equidimensional rounded black wood to be plundered and the particles to be transported to the marine basin. Decreasing obliquity values keep ocean currents circulating normally and keep preservation potential of organic matter low, except where there is a particularly high supply of resistant material (equidimensional rounded black wood) (2a, b). The cycle then begins again (3). It must be remembered, however, that if the proposals advanced here that the palynofacies particle abundances show the effects of the precessional cycle 'moving along' an underlying obliquity cycle, the effects of the two cycles will not always coincide in the pattern as those described for the 'typical' palynofacies cycle here, as this is based mainly on the top three cycles of the Kimmeridge Bay section studied. In conclusion, it is proposed that the 'major' palynofacies cycles in the Kimmeridge Bay section represent the climatic effects of orbital forcing by the obliquity cycle and the 'secondary' palynofacies cycles, those of forcing by the precessional

11 |

cycle, and that the position of southern England during Kimmeridgian times was such that the Kimmeridge Bay area was under the influence of both the obliquity and precessional orbital variations, but perhaps with the orbital influence being slightly stronger. It is also proposed that the obliquity cycle exerted a stronger control on the marine realm by influencing the ocean-current circulation and, therefore, plankton productivity and preservation conditions, and that the precession cycle affected both the marine and terrestrial realms but exerted its control mainly on the terrestrial environment. By using quantitative palynofacies analysis as a tool and employing high-resolution sampling techniques, detailed palaeoenvironmental variations, more detailed than those recognizable in the sedimentological characteristics, can be identified through a lithological section. The most important of these were the precessional-forced peaks in abundance and the variation within the apparently uniform bituminous shale beds. In addition, the use of a method of estimating absolute palynofacies particle abundances has allowed particles with different provenances, but subsequently found superimposed upon one another in the distal setting of the Kimmeridge Bay section, to be studied separately. This provides a possible means of identifying separate variations, that may have different palaeoenvironmental causes, within and between the marine and terrestrial environments. Quantitative palynofacies analysis, in conjunction with high-resolution sampling techniques, has therefore proved to be a powerful tool and a sensitive indicator in the palaeoenvironmental and palaeoclimatic investigation of the cycles in this study, and future work on the detailed effects of orbital forcing of climate would benefit from the use of such techniques. This work was supported by a NERC research studentship. Thanks are also due to the Smedmore estate for permission to collect samples, C. Mar-Molinero for advice on time series analysis, to J. E. A. Marshall and M. R. House, and to R. V. Tyson and another, anonymous, referee for many useful comments.

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, WILSON, R. C. L. & DOWNIE, C. 1979. A stratified water column environmental model for the type Kimmeridge Clay. Nature, 277, 377-380. VAN BUCHEM, E S. P., MELNYK, D. H. & MCCAVE, I. N. 1992. Chemical cyclicity and correlation of Lower Lias mudstones using gamma ray logs, Yorkshire, UK. Journal of the Geological Society, London, 149, 991-1002. , MCCAVE, I. N. & WEEDON, G. E 1994. Orbitally induced small-scale cyclicity in a siliclastic epicontinental setting (Lower Lias, Yorkshire, UK). In: DE BOER, E L. & SMITH,D. G. (eds) Orbital Forcing and Cyclic Sequences. Blackwell Scientific Publications, Oxford, 345-366. VAN DER ZWAN, C. J. 1990. Palynostratigraphy and palynofacies reconstruction of the upper Jurassic to Lowermost Cretaceous of the Draugen Field, Offshore Mid Norway. Review of Palaeobotany and Palynology, 62, 157-186. VENKATACHALA,B. S. 1981. Differentiationof amorphous organic matter types in sediments. In: BROOKS, J. (ed.), Organic Maturation Studies and Fossil Fuel Exploration. Academic Press, New York, 177-200. WALL, n. 1965. Microplankton, pollen and spores from the Lower Jurassic of Britain. Micropaleontology, 11, 151-190. --, LOHMANN, G. P. & SMITH, W. K. 1977. The environmental and climatic distribution of dinoflagellate cysts in modem marine sediments from regions in the North and South Atlantic Oceans and

adjacent seas. Marine Micropalaeontology, 2, 121-200. WATE~HOUSE, H. K. 1992. Quantitative Palynofacies Analysis of Jurassic Climatic Cycles. PhD thesis, University of Southampton. WEEDON, G. P. 1986. Hemipelagic shelf sedimentation and climatic cycles: the basal Jurassic (Blue Lias) of southern Britain. Earth and Planetary Science Letters, 76, 321-335. The detection and illustration of regular sedimentary cycles using Walsh Power Spectra and filtering, with examples from the Lias of Switzerland. Journal of the Geological Society, London, 146, 133-144. WIGNALL, E B. 1989. Sedimentary dynamics of the Kimmeridge Clay: tempests and earthquakes. Journal of the Geological Society, London, 146, 273-284. 1991. Test of the concepts of sequence stratigraphy in the Kimmeridgian (Late Jurassic) of England and northern France. Marine and Petroleum Geology, 8, 430-441. WILLIAMS, D. B. & DOUGLAS, J. G. 1983. The effects of lithologic variation on organic geochemistry in the Kimmeridge Clay of Britain. In: BJOROY, M. et al. (eds) Advances in Organic Geochemistry, 1981. Wiley, Chichester, 16-27. & SA~GEANT,W. A. S. 1967. Organic-walled microfossils as depth and shoreline indicators. Marine Geology, 5, 389-412.

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Modelling Late Jurassic Milankovitch climate variations E J. V A L D E S , B. W. S E L L W O O D 1 & G. D. P R I C E 1

Department of Meteorology, University of Reading, 2 Earley Gate, Whiteknights, Reading RG6 2A U, UK. 1Postgraduate Research Institute for Sedimentology, The University, Whiteknights, Reading RG6 2AB, UK. Abstract: Although largely circumstantial in character, evidence for orbitally-forced

(Milankovitch) climate changes in Jurassic microrhythmic successions, such as those of the Lias and Kimmeridgian, is becoming more persuasive. We present here the results of experiments, using a general circulation model, testing ways in which orbitally-induced variations in solar energy might be translated into a Jurassic climate response. In particular, we address the problem of the 100 ka (eccentricity-forced) cycle. This is generally considered to have only a small direct effect on solar input. It would be expected to have little impact on an ice-free Earth (commonly assumed for the Jurassic). Nonetheless, this weak signal is claimed to have been recognized in many Jurassic successions. Our results simulate, for the Late Jurassic, the possible effects at the 'minimum' and 'maximum' extremes of seasonal forcing (i.e. comparable with those affecting the Earth at 115 and 9 ka BP, respectively). Model predictions are critically evaluated against the geological database. At times of 'minimum seasonal forcing' there is a significant expansion in the area of the Northern hemisphere monsoon. In the tropics, changes in precipitation predominate over changes in temperature, whereas at high southern latitudes there are very large seasonal variations in temperature, and heavy winter snows. During these times the model is close to predicting a modest, but significant, Jurassic ice-cap in the Antarctic. Ice build-up is particularly likely over uplands. Such an ice-cap disappears at times of 'maximum seasonal forcing'. Waxing and waning of ice may thus provide the elusive mechanism for metre-scale sea-level changes. It is argued that apparently similar microrhythms (e.g. limestone-shale) might be the sedimentary response to different climatic signals in different climate zones, cause and effect being exactly the opposite in some circumstances.

Successions rhythmically bedded on a decimetre to metre scale (often interpreted to be cyclic) are a common feature of Jurassic successions worldwide, 'pervading every Jurassic facies' according to House (1986). Since the pioneering days of Kltipfel (1917) and Brinkmann (1929) a vast literature has blossomed, concerned with both the description and interpretation of rhythms, microrhythms and cycles, interpretations often reflecting the fashion of the time. In the early parts of this century epeirogenetic interpretations were popular. Subsequently diagenetic and autocyclic mechanisms (reviewed in Hallam 1964, 1986) were favoured, followed in the last couple of decades by eustatic and generalized climatic controls (Sellwood 1972; Anderton et al. 1979). At the moment interpretations involving refined climatic controls (orbitally-forced Milankovitch models) are in vogue (e.g. House 1985, 1986; Weedon 1985, 1993; Weedon & Jenkyns 1990), even though the evidence in support of such interpretations are often no better than circumstantial [see discussion in Hallam (1986) and Wignall (1989)]. Evidence for eccentricity,

obliquity and precession cycles has also been claimed on the basis of natural gamma-ray logs used at outcrop and in boreholes (e.g. Van Buchem et al. 1992). The objective of this paper is not to re-review the nature and origin of rhythmic cyclic successions in the Jurassic but to shed light on the way in which Milankovitch climatic signals might effect preservable responses in the sedimentary record of the Jurassic Earth. Because of growing interest in the mechanisms of climate change, controls of eustasy and the potential use of Milankovitch cyclicity in high resolution stratigraphy, it is important to understand the ways in which Milankovitch signals might effect changes in climatic patterns on the Earth. The approach adopted here is the employment of a version of the general circulation model (GCM) of the UK Universities Global Atmospheric Modelling Programme (UGAMP). As will be seen, some of the climatic responses predicted are not, perhaps, those that might intuitively be expected. The results of some of modelling experiments underline the principle that apparently similar

From HOUSE,M. R. & GALE,A. S. (eds), 1995, OrbitalForcing Timescalesand Cyclostratigraphy, Geological Society Special Publication No. 85, pp. 115-132.

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cyclic responses may be produced for different reasons in different climate regimes. Although there has been much work on Jurassic cyclicity, and much recent speculation about possible climatic controls, there has been much less effort in trying to understand the nature of climate change itself, yet this is of fundamental importance. The effects of Milankovitch forcing are well recognized in Quaternary and some Pliocene successions, particularly in the deeper marine record (e.g. Shackleton 1993; Thunell et al. 1991). Climate change is forced by variations in the radiative energy arriving at the top of the atmosphere. This is controlled by three orbital parameters, namely the eccentricity of the Earth's orbit (which has periods of c. 100 and 400 ka), the obliquity of the rotation axis (period of c. 40 ka) and the precession of the equinoxes (period of c. 20 ka). They change the seasonality of incoming solar radiation. The annual mean solar radiation changes only by very small amounts, yet the climate itself changes both seasonally and over an annual average. Further, the 100 ka eccentricity forcing has a small direct effect on the solar input. It's effect mainly controls the size of the precession signal. Thus, it is surprising that the 100 ka signal is so large in the Pleistocene, and is frequently claimed to have been recognized in the prePliocene (e.g reviews in Crowley & North 1991). There are now many explanations for the dominance of the 100 ka signal (for review see Imbrie et al. 1992). All of them imply some feedbacks between various components of the climate system, of which the most important, given late Pleistocene boundary conditions, is the growth of the major ice sheets. The standard theory is that the initial glacial response to orbital forcing will occur at high Northern hemisphere latitudes. If the orbital parameters are such that Northern summers receive less radiation than normal, then summers will be relatively cool and winter snows will not be entirely melted. Ice albedo feedback will then result in further cooling and, hence, further growth of ice. Such an orbital configuration occurred at 115 ka BP. A similar argument, but in reverse, explains deglaciation, for which the orbital configuration at 9 ka Be is a prime example. If an expansion in the area of ice-fields is the main feedback process which amplifies the 100 ka Milankovitch period, then we would expect to see a much weaker eccentricity climate signal during times when the Earth was generally warmer ('equable'), such as the Mesozoic (e.g. Crowley & North 1991; Hallam 1993). Alternatively, ice may not be the major feedback process during such periods. Oceans, continental configuration and changes in the carbon dioxide content of the

atmosphere (e.g. Berner 1992) may also be of crucial importance. Further, Saltzman & Maasch (1988) and Maasch & Saltzman (1990) have suggested that the 100 ka period signal is the result of a self-sustaining oscillation of the climate system. They propose that this signal would exist even if there were no external forcing at the eccentricity period. If this is true, then it introduces some serious problems for interpretation of pre-Pliocene sedimentary rhythms because such a self-sustaining oscillator could possibly change frequency in a climate regime very different from that of the Peleistocene.

Numerical climate modelling of Milankovitch cycles In order to understand the quantitative links that exist between changes in the incoming solar radiation and climatic parameters, such as temperature and rainfall, it is necessary to develop numerical models of the climate system. There are currently two main types of models, both of which have their strengths and weaknesses. The first type of model, generally referred to as energy balance models (EBM), can be used to simulate the climatic change over a full glacial/interglacial cycle. The most sophisticated versions include sub-models for the atmosphere, ocean, ice sheets, and lithosphere (e.g. Berger et al. 1993). However, even with the most powerful computers, such complicated models can only be run for Milankovitch time-scale periods by making a number of severe approximations. The most common of these is to calculate the latitudinal/ height variations only. No longitudinal variations are included. Further, some potentially important processes, such as clouds and monsoons, cannot be properly represented. Berger et al. (1993) have used such a model to simulate the last glacial/interglacial cycle with considerable success. For the last glaciation the evolution of predicted temperatures and ice volumes are remarkably similar to the observed variations. In particular, it correctly reproduces the longer timescale (100 000 a) climate variation. Further diagnosis of their model shows that the amplification of the longer timescale variations are the result of feedbacks related to ice cover and carbon dioxide. The clear implication is that the 100 000 a response would be considerably weaker on an Earth without significant ice. The strength of this type of model is that it calculates the transient response. The weakness is that the model has many simplifications and assumptions. It would be potentially difficult to

LATE JURASSIC CLIMATE VARIATIONS modify such a model for a different climate regime, and a different palaeogeography. Further, the assumption of a zonally symmetrical climate may be particularly unreliable for the Triassic and Jurassic, when the supercontinent of Pangaea, and its related mega-monsoons, significantly disrupted zonal circulation patterns (Kutzbach & Gallimore 1989). The alternative type of numerical climate model is the GCM. This model can be used to simulate the full regional and global climate, but cannot be run for 100 000 a. Instead, the model can only be run for a 'snapshot' at particular times during a Milankovitch cycle. It predicts all climate variables. In particular, it is possible to examine the relative changes of temperature and the hydrological cycle. This information can be used to aid the interpretation of sedimentary deposits. The GCM approach has had considerable success in explaining the evolution of climate over the last 20 000 a (e.g. COHMAP 1988), as well as increasing our understanding of regional climate change for specific periods (e.g. Kutzbach & Wright 1985; Kutzbach & Guetter 1986). However, there are also a number of problems. Rind & Peteet (1985) showed that GCM simulations, using CLIMAP sea-surface temperature reconstructions for the last glacial maximum, were in contradiction with estimates of land temperatures at high altitudes. There is currently much debate as to whether it is the data, or the models, that are incorrect. There is also a problem in understanding the onset of glaciation. Rind et al. (1989) showed that the GCM of the Goddard Institute for Space Studies (GISS) would not start to grow an ice sheet if the orbital parameters alone were modified to those for 115 ka BP The cool Northern hemisphere summers still melted all of the winter snows. Winter snow lasted through the summer only if sea-surface temperatures were changed to those at the last glacial maximum (probably an unreasonable assumption). Such a serious disagreement, between models and data, may just be an artifact of the particular model used. Oglesby (1990) showed that the NCAR CCM1 model had no problems at maintaining winter snow, and he speculated that the GISS model may be a 'hot' model that has difficulty maintaining snow cover, whereas the NCAR model may be a 'cold' model that has difficulty removing snow cover. The UGAMP model, employed by us, appears to lie in between these two extremes (D. Buwen, pers. comm.). In the northeast of Canada, Alaska and Siberia, winter snow cover does last through the summer. GCMs and EBMs have also been used to

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examine 'snapshots' of climate of the more distant past. Oglesby & Park (1989) and Park & Oglesby (1990, 1991, 1992) have examined the role of orbital changes on Cretaceous sedimentary rhythms. They show that the precession effects dominate their model, especially the hydrological balance in the tropics. They suggest that orbitally induced changes of the 'precipitation minus evaporation' budget can result in cyclic variations of anoxic waters in shallow basins and in fluctuations in terrigenous input to shelf environments. Obliquity changes seemed to be of less importance, even at high latitudes. Crowley et al. (1992a, b) showed that the 100 ka period could be amplified at times of extreme continentality, as in the Triassic and Early Jurassic. They showed that the temperature maximum of low-latitude land exhibited a significant 100 ka oscillation as a result of the interaction between the twice yearly passage of the Sun across the Equator, and the seasonal timing of perihelion. The magnitude of the temperature response implies significant changes in the hydrological cycle of the monsoon and are of sufficient magnitude to leave an imprint on the geological record. Their model suggested that high latitudes did not experience the same amplification, although feedbacks within the climate system could influence high latitudes. Crowley et al. (1993) have also investigated Milankovitch variations for Carboniferous climates. Using a GCM they showed that there were large changes in climate between cold and warm summer orbits. In particular, snow cover extended over large parts of Gondwanaland during cool summer orbits, but almost disappeared during warm summer orbits. The extent of the ice was in reasonable agreement with the geological data concerning ice cover. Further, they suggested that the interpretation of warm winters in Gondwanaland (Yemane 1993) could be the result of preferential preservation of the deposits representing the warm Milankovitch phases. They suggested that GCMs should be run for a number of orbital parameters in order to fully document climate for the past. These studies, therefore, suggest that the 100 ka variations should be strong during periods of glaciation (especially at high latitudes), or during periods when the Earth has supercontinents (a strong climate signal occurring at low latitudes only). For other periods the 100 ka period is less well understood and should provide a weaker, and less well-represented, signal. In this paper results are presented from a GCM investigating the sensitivity to Milankovitch for the late Jurassic (Kimmeridgian Stage). This is a time period in which the supercontinent of Pangaea is breaking

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up but, as in the Cretaceous, there are generally 'equable' conditions.

Model description The model presented here is similar to that used in Valdes (1993) and Valdes & Sellwood (1992). It is a version of the UGAMP model. This model is spectral, using a triangular truncation at total wave number 31 (a horizontal grid of 3.75 ~ x 3.75~ and 19 levels in the vertical. The parameterization schemes are the same as those used in Valdes (1993). They include the Betts-Miller convective adjustment scheme (Betts & Miller 1986; Slingo et al. 1994), and Morcrette radiation (Morcrette 1990). The former scheme resulted in significantly moister tropics than was found in Valdes & Sellwood (1992), which used Kuo convection (Kuo 1974). The model is adapted to Kimmeridgian boundary conditions by changing the coastlines (from Alan Smith 1991, pers. comm.), mountains (both the mean height and the variance), surface roughness and albedo, sea-surface temperature (SST), and carbon dioxide concentration. All other parameters, including the Earth's orbital characteristics, were kept at their present-day values. The details of these changes are explained in Valdes & Sellwood (1992). In Valdes (1993) and Valdes & Sellwood (1992), we prescribed a zonally uniform SST of 2 7 ~ cos (latitude) and it did not change with the seasons. The temperatures were consistent with the energy balance of the model, provided that carbon dioxide concentrations were 3-4-times present day values. Such figures are consistent with those calculated by Berner (1992). By fixing these temperatures, we effectively implied a change of poleward transport of oceanic heat flux. It was found that the annual mean oceanic heat transport was significantly weaker than that of the present day. In order to examine the model's response to Milankovitch variations it is inappropriate to keep the SST constant because this will artificially damper the climate changes. The best approach would be to use a fully dynamic, coupled ocean-atmosphere model. However, such models are still being developed for present-day simulations and are also extremely expensive computationally. Instead, a similar approach to that used for many future climate change scenarios was adopted (Gates 1990), namely a mixed-layer ocean. The ocean is modelled as a single thermodynamic slab, 50 m thick, with a prescribed oceanic heat flux. Sea ice is modelled using the scheme of Slingo (1985). Thus, the SST can change due to changes in the energy entering the slab

(a sum of latent, sensible and radiative fluxes), but not due to changes in the oceanic circulation. The oceanic heat flux is the annual mean value from the simulation in Valdes (1993). Thus, it has both longitudinal and latitudinal structure, but no seasonal variations. This approach is similar to that used by Barron et al. (1993) for the Cretaceous, except that they used an oceanic heat transport which was a simple multiple of the present day value. Since there is currently no reliable way of determining palaeoocean heat flux, both approaches represent a gross simplification of the true climate system. In the following sections the results of three simulations, representing different extremes, are presented. The first experiment will be referred to as the control. It uses current orbital parameters (obliquity -- 23.44 ~ eccentricity -- 0.0167, longitude for perihelion relative to vernal equinox -282.04~ The only difference between this simulation and that employed in Valdes (1993) is the use of a mixed layer ocean. The other two experiments were chosen so as to demonstrate the possible effects, on the Jurassic Earth, of extremes in the radiative forcing corresponding with the changes that have affected the Earth's orbital parameters over the last 120 000 a. At 115 ka bp the orbital configuration (obliquity = 22.41 ~ eccentricity = 0.04142, longitude of perihelion- 291.02 ~ corresponded to a cold Northern-hemisphere summer. This allowed winter snow to last through the summer, resulting in the growth of the major ice sheets (a time of 'minimal seasonal forcing'). The opposite occurred 9 k a ago (obliquity=24.238 ~ eccentricity= 0.01928, longitude of perihelion = 131.26~ Warm summers throughout the mid-Holocene resulted in the melting of the huge Devensian ice sheets that had extended over North America and Europe (time of 'maximum seasonal forcing'). Both of these configurations primarily altered the seasonality of incoming solar radiation. At most latitudes, the annual mean changes were relatively small. Figure 1 shows the changes in incoming solar radiation for the two periods. For many months and latitudes, the changes exceed 40 W -2. By comparison, a doubling of carbon dioxide, or a 1% change in the solar constant produce a change in the net radiative forcing of c. 4 W m -2. However, both of these processes operate throughout the year. The control was run for a simulated-time 'snapshot' of 10 a, whereas the sensitivity experiments were run for 5-6 a, starting from the end of the 10 a control integration. We also performed a further experiment using the control simulation but with initial land ice covering all land polewards of 45 ~ latitude. All results presented here are averages over the last 3-4 a (simulated) years of the

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Results of the control simulation The results for the control simulation (using present-day orbital parameters) are similar to, but not identical with, those shown in Valdes (1993). The inclusion of a seasonally and longitudinally varying SST results in important changes in the tropical circulation. The seasonal mean surface temperatures for December-February average (DJF) and June-August average (JJA) shows the development of a warm pool of water in the Western Tethys. During the Northern-hemisphere summer this region is particularly notable, there being an extensive region of water in excess of 28~ just North of the equator. The temperatures are 1-2~ warmer than those noted in the simulation of Valdes (1993), which used prescribed SSTs. These relatively warm ocean temperatures have an important effect on the general circulation, and especially the hydrological cycle. The amount of water vapour in the atmosphere is a non-linear function of temperature. Thus, an increase of temperature from 27~ to 28~ is more important than a similar change between 10 and ll~ for example. The total precipitation for DJF (Fig. 3a) and JJA (Fig. 3b) closely follows the maximum in the SST. There is a band of heavy precipitation over the

Northern Tethyan ocean in JJA that only impinges on land in a small belt over Northern Africa and the Eurasian Peninsula. This predicted area of precipitation over Africa is in better agreement with geological observations of bauxite, coal and evaporite distributions (Fig. 4a and b) than were the original simulations in Valdes (1993) and Valdes & Sellwood (1992). The formation of evaporites and other hydrologically sensitive deposits is probably more clearly shown by examining the surface-soil moisture for DJF (Fig. 5a) and JJA (Fig. 5b). This is a more straightforward diagnostic than the 'precipitation minus evaporation' field since, in the latter, the evaporation should be a measure of the potential evaporation, not the actual value. Soil moisture takes this into account. Regions which are seasonally dry are regions where, potentially, either laterites or evaporites may form (Sellwood & Price 1993). Such regions occur over many of the areas bordering the tropical Tethys sea. This situation was not observed when the model was run with prescribed annual mean SSTs (Valdes 1993).

Modelling the Kimmeridgian simulating 'minimum seasonal forcing' This orbital configuration produces less summer and more winter solar radiation. It produces relatively cool summers and warm winters, as shown in Fig. 6a and b. These show the difference in seasonal mean surface temperature between the 'minimum seasonal forcing' simulation (based on

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Signification o f bundles The previous features lead to the consideration again of the signification of cycle bundles. When these bundles succeed one another regularly, with a constant average number of beds between 4 and 5 [4.03 in the Majolica Formation; 4.88 in the Scaglia Bianca (Schwarzacher & Fischer 1982), 5 in the Middle Triassic of Italian Dolomites (Goldhammer et al. 1987)], this signifies that two superposed periodicities occur with a ratio of c. 5, like those of precession and eccentricity. But, in other cases, the signification of bundles is different. The average number of cycles per bundle may exceed 5 [5.9 and 7.5 in the Irish Carboniferous (Schwarzacher 1964, 1975); 7 in the Scaglia Cinerea (Lower Eocene at Gubbio, Italy)]. These features are close to that of the Lower Barremian of Mont Ventoux where the bundles are composed of 15-25 unit-cycles. When bundles of 3-6 cycles appear, either isolated, or grouped by two, three or four, within regular alternating successions it can be assumed these bundles correspond to temporary accelerations of sedimentation rate. They increase abnormally the number of cycles in a given series. From this follows this question: what are the most appropriate sedimentation rates in a series which allows the use of cycles as chronologic tools ?

Ideal rate o f sedimentation

~ioturbated

depQsit

l

1

Fig. 2. Effect of sedimentation rate variation on cyclicity. It is assumed that, < 5 cm thick, a cycle is destroyed by bioturbation. (a) In the unbioturbated succession, all the cyclicity orders are individualized from the marl-limestone couplet to the lamination (not represented here); one cycle > 5 cm thick is taken into account; with an increase of sedimentation rate all the cycles grow thicker so that five of them outpace 5 cm. (b) In the bioturbated succession only one order of cyclicity (the marl-limestone couplet) is represented; the others, < 5 cm, have been destroyed by bioturbation; with an increase of sedimentation rate, the five potential cycles reaching, then outpacing 5 cm are individualized and may be taken into account.

The first example is taken from the Vocontian Basin (SE France), thought to represent sediment at c. 1000 m depth on average and corresponding to open and well-oxygenated environments. In the Lower Cretaceous stratotypic succession of Angles, marl-limestone decimetric couplets have been counted from the Berriasian to the Barremian. The total number of cycles is 1080 (Rio et al. 1989) for a mean total duration of 22.8 Ma (average value from five different geochronological scales). Consequently, the mean duration of a cycle is close to 21 000 a. The succession being 635 m thick, the average sedimentation rate is 28.8 m Ma -1. The Angles Succession shows a general composition balanced between marls and limestones but with a prominent carbonate fraction at the two extremities: the Berriasian and the BarremianBedoulian (Cotillon 1971). Bed bundles are relatively few, this fact agreeing with a rather low

CONSTRAINTS IN CYCLOSTRATIGRAPHY sedimentation rate. Besides, important sections including the Upper Berriasian, the Lower Valanginian and the Middle Hauterivian, exhibit a very regular pattern with nearly equal proportions between beds and interbeds. A second example is from the Appennines of Umbria (Central Italy) where the Middle Cretaceous testifies to a deeper and less oxidized environment than in the Vocontian Basin. A cored succession at Piobbico is characterized by a weak sedimentation rate of 5 m Ma -1 (Herbert & Fischer

Transmission .Orn

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1986; Premoli Silva et al. 1989); it shows 87 m of a very homogeneous series made of couplets, 10 cm thick on an average, grouped in bundles and super-bundles. These three orders of cycles are assumed to correspond to the precession and eccentricity cycles (Fig. 3). The small thickness of cycles and their good preservation are explained by dysaerobic to anaerobic depositional periods illustrated by layers of laminated black shales enriched with organic carbon. These periods of poor bottom circulation have prevented a major bioturbation. The examples of Angles and the Appennine Successions could be regarded as limit possibilities of cyclostratigraphy based on the precession cycles. The upper limit is located at Angles, where some tendancies to bundle individualization occurs, signifying episodic periods of sedimentation rate acceleration. The lower one, in Umbria, shows that in spite of a lithological contraction of cycle orders and of the thickness of marl-limestone couplets, the record of precession cycles is preserved thanks to a relatively closed environment having prevented major bioturbation. So, between 5 and 30 m Ma -1, the sedimentation rate may allow the individualization of 21 000 a cycles and their use for geochronological objectives. However, in the Piobicco series, cyclicity displays the same signification, from a periodicity point a view, whatever the level, owing to the homogeneous facies and regular cycles. On the contrary, the vertical facies succession at Angles is unhomogeneous, with carbonate formation in the Lower Berriasian, the Lower and Upper Hauterivian, the Upper Barremian and marly intervals with sparse carbonate beds in the Middle and Upper Valanginian, the Middle Hauterivian. From this follows this question: are the orbital signals similarly recorded in the two kinds of facies?

Constraints of facies

A

B Fig. 3. Part of the cored Upper Albian Piobbico series (Umbria, Central Italy). Analytical process, based on the brightness (A) and CaCO3 content (B) of deposits, clearly displays three superposed cyclicities corresponding to" fundamental marl-limestone couplets; bundles of couplets (lettered); and super-bundles (dashed lines). After Herbert & Fischer 1986.

A first answer comes from the investigations of Clerc-Renaud (1988) on the Upper Jurassic-Lower Cretaceous alternating series at Fontcalent (Eastern Spain). The spectral analysis of this succession has pointed out frequent shifts occurring particularly at the transition between lithological units. A second answer lies in the Angles series (Fig. 4). Harmonic analysis of cycles based on their constant duration, i.e. 21 000 a (Rio et al. 1989) has shown that most carbonate intervals of the series (Berriasian, Lower Valanginian and Barremian) record nearly identically all the cycle periods except the shortest; besides, the longest of them (up to 1 Ma or more) are well recorded. Conversely,

138

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the most marly intervals, for instance, the Upper Valanginian, have recorded the shortest periods, essentially from 20 to 100 Ka. Very similar features have been observed by Clerc-Renaud (1988) in Upper Jurassic - L o w e r Cretaceous succession at Fontcalent. Two possible explanations are suggested. A theory based on the modality o f sedimentation.

In the Angles Series, carbonate-rich formations, such as the Hauterivian and Berriasian, exhibit a rather high rate of sedimentation (30.6 and 30 m Ma -], respectively, against 28.8 m Ma -1 for the total Berriasian-Barremian interval). This feature agrees with previous results (Cotillon 1985), showing an increase of sedimentation rate from marls to limestones within basic couplets of alternating deposits. But the Berriasian at Angles is partially composed of thick carbonate units formed by amalgamated cycles separated only by millimetric clayey seams, or dry diastems, or even without physical boundary. These features imply an irregular and discontinuous sedimentation suited for the unusual recording of long cycles. This last trend is more developed in the Barremian succession (Fig. 4) which is also carbonate-rich but where the sedimentation rate is reduced (15 m Ma-1); so, a more important effect of hiatus between

cycles, i.e. on the bed contacts, may be suspected as well as an important loss of material during the burial diagenesis. Consequently, a bed-scale rapid deposition may be included in a slowly deposited formation. This might explain the seemingly inconsistent results obtained with the same section, through a harmonic analysis differing from that used by Rio et al. because the latter is based on the variation in CaCO 3 content (Giraud et al. this volume). Conversely, the sedimentation is slower, more regular and continuous in marly intervals such as the Valanginian. This fact is illustrated by a transitional contact between carbonate beds and marly interbeds. This character, and the less active bioturbation in marls favour the recording of short cycles (see also Arthur et al. 1984). On the contrary, long cycles often remain undetected in marls where beds are sparse and variations of CaCO 3 content are not always lithologically expressed. This explanation must be considered cautiously given the results obtained using a different method (Giraud et al. this volume). A theory based on outer controls o f sedimentation.

Terrigeneous sources would react more quickly than carbonate facies (namely platforms and plankton) to the climatic oscillations related to orbital cycles. Clerc-Renaud (1988) reached the same conclusion but also took into account the distance separating sources from depositional areas. In summary, cyclostratigraphy must be employed essentially in series where carbonate and marls are nearly equally represented and form regular altemations. It must be avoided for facies which are too restricted, either in marls or limestones.

Constraint

of sedimentary

fluxes

The characteristics of material fluxes controlling the deposition of alternating successions may influence the recording of orbital cycles. A previous study based on image analysis of oceanic cores (Cotillon 1992) has shown that, during low sea level, deposits with abundant nutrients of continental and marine origin result in especially high carbonate fluxes, marked by a great variability (Fig. 5). These features could contribute to explaining the weak record of short harmonic periods as observed in the Berriasian and Barremian carbonate intervals of Angles Succession, which essentially correspond to low-stand deposits. Conversely, during high sea-level periods, because of the withdrawal and interference of sources, and of the screen effect caused by drowned

CONSTRAINTS IN CYCLOSTRATIGRAPHY

.

.

139

B

Fig. 5. Variability of material fluxes held to characterize the response of sources forced by external influences: climate, tectonics sea-level variations. (A) Low sea-level. Continent and ocean are close. From the sources there is a good transmission (by amplitude and frequency) of signals to the oceanic domain via the material fluxes. The vicinity of sources makes them not very interferent causing a high flux variability. Abundant fluxes and important sedimentation rates can be expected. 1, Silicate terrigenous, carbonate (clastic) and organic (land-derived) fluxes; 2, planktonic flux; 3, detrital flux; 4, activated up-wellings. (B) High sea-level. Continent and ocean are distant. Poor transmission of signals to the ocean because they are partly trapped by epicontinental domain. The remoteness of sources makes them interfere, lowering the flux variability. Reduced fluxes and sedimentation rates can be expected. 1, Silicate terrigenous and organic (land-derived) fluxes; 2, partial flux diversion; 3, weakened residual flux; 4, planktonic flux; 5, detrital flux.

continental margins, fluxes decrease and become regular while sedimentation moves towards a more marly character.

Constraint of the age of deposits Lastly, cyclostratigraphy must take into account the variation of precession and obliquity periods with time, owing to the continuous evolution of the Earth-Moon system. So, at the beginning of the Phanerozoic, these periods were 17 000 and 28 000 a long, respectively, while they are 21 000 and 41 000 a today (Algeo & Wilkinson 1988; Berger et al. 1989). So, the precession cycle, which is the more convenient cyclostratigraphic unit because it is the more clearly recorded in deposits, must not be quantified as 21 000 a by referring to the Present. De Boer & Wonders (1984) have suggested this unit be named 'the Gilbert', which is not an intangible quantified value.

Conclusion Given the outer limits of forcing of orbital origin undergone by sedimentation, cyclostratigraphy, based on high-frequency cycles, seems to be the ideal way for accurate datings whatever the age, environment and diagenetic transformation of deposits. But this method must be used cautiously in successions showing some peculiar characteristics. (1) Successions must be as regular and continuous as possible, then the following depositional environments have to be excluded: those of too strongly dynamic waters which generate erosional features; those of coastal areas where sea-level variations induce sedimentary cycles but also emersions and their associated gaps; and localities where significant detrital or reefal sedimentation disturbs the record of cycles. Conversely, sheltered and relatively deep environnements, down the continental slope, are privileged. In them, planktonic productivity and

140

P. COTILLON

detrital fluxes have the best possibility to express orbital messages with maximum faithfullness. (2) Successions should show very weak diagenetic alterations. (3) In open and moderatly deep basins, where deposits are bioturbated, the sedimentation rate must fall within the 20-30 m Ma -1 range. In deeper and more restricted basins, with or without a weak bioturbation, cycles are preserved in spite of a sedimentation rate as weak as 5 m Ma -1 or less. Accurate correlation through high-frequency cycles requires close sedimentation rates in the compared series. (4) The most favourable facies for using cyclo-

stratigraphy is a balanced alternation between marl and limestone, deposited during high sea level periods related to the maximum regularity of material fluxes and lithological cycles. Such are the conditions for the best record of an orbital periodicity spectrum. Formations displaying major carbonate enrichment should be avoided because their deposition is too irregular and mineral transfer too important during the diagenesis. (5) Lastly, the duration of reference orbital cycles must be defined according to the age of deposits, taking into account their changes through geological time.

References ALGEO, T. J. & WILKINSON, B. H. 1988. Periodicity of mesoscale phanerozoic sedimentary cycles and the role of Milankovitch orbital modulation. Journal of Geology, 96, 313-322. ARTHUR, M. A., DEAN, W. E., BOTrJER, D. & SCHOLLE, P. A. 1984. Rhythmic bedding in Mesozoic Cenozoic pelagic carbonate sequences : the primary and diagenetic origin of Milankovitch like cycles. In : BERGER,A. L., IMBRIE,J., HAYS,J., KUKLA,G. (~ SALZMAN,B. (eds), Milankovitch and climate, 1. Reidel, Dordrecht, 191-222. BERGER,A., LOUTRE,i . F. & DEHANT,V. 1989. Influence of the changing linear orbit on the astronomical frequencies of pre-Quatemary insolation patterns. Paleoceanography, 4, 555-564. CLERC-RENAUD, T. 1988. Recherche de pdriodicitds relatives & la variation des param~tres orbitaux dans la sddimentation alternante du Jurassique supdrieur - Crdtacd infdrieur. Th~se de Doctorat de l'Universit6 Paris VI. COTILLON,e., 1971. Le crdtacd infdrieur de l'arc subalpin de Castellane. Stratigraphie et sddimentologie. Mrmoires du Bureau de Recherche Gdologique et Mini~re, 68. 1985. Les variations h diffrrentes 6chelles du taux d'accumulation srdimentaire dans les srries prlagiques alternantes du Crrtac6 infrrieur, consrquence de phrnom~nes globaux. Essai d'rvaluation. Bulletin Socidtd Gdologique de France, Srr. 8, I, 1, 59-68. 1987. Bed scale cyclicity of pelagic Cretaceous successions as a result of world-wide control. Marine Geology, 78, 108-123. 1991. Varves, bed and bundles in pelagic sequences and their correlation (Mesozoic of SE France and Atlantic). In: EINSELE,G., RICKEN,W. & SEILACHER, A. (eds) Cycles and Events in Stratigraphy, Springer, Berlin, 820-839. 1992. Search for eustacy record in deep tethyan deposits through the study of sedimentary flux variations. Application to the Upper TithonianLower Aptian series at DSDP Sites 534 (Central Atlantic). Palaeogeography, Palaeoclimatology, Palaeoecology, 91, 263-275. ~, FERRY, S., GAILLLARD,C., JAUTEE, E., LATREILLE, G. & RIO, M. 1980. Fluctuations des param~tres du

milieu marin dans le domaine vocontien (France SE) au Crrtac6 infrrieur : mise en 6vidence par l'6tude des formations marno-calcaires alternantes. Bulletin Socidtd Gdologique de France, SEt. 7, XXII, 1,733-742. & Rio, M. 1984. Cyclic sedimentation in the Cretaceous of D.S.D.P. sites 535 and 540 (Gulf of Mexico), 534 (Central Atlantic) and in the Vocontian basin (France). Initial Reports of the Deep Sea Drilling Programme, 77, 339-378. CRUEL, J. 1875. Climate and Time in their Geological Relations: A Theory of Secular Change in the Earth's Climate. Appleton, New York. DE BOER, P. L. 1983. Aspects of Middle Cretaceous pelagic sedimentation in Southern Europe. Geologica Ultraiectina, 3, 112 pp. & WONDERS, A. A. H. 1984. Astronomically induced rhythmic bedding in cretaceous pelagic sediments near Moria (Italy). In: BERGER, A. L., IMBRIE, J., HAYS, J., KUKLA, G. & SALZMAN,B. (eds) Milankovitch and climate, I, Reidel, Dordrecht, 177-190. DE MASTER, D. J., MCKEE, B. A., NITIROUER, C. A., BREWSTER, D. C. & BISCAYE,P. E., 1985. Rate of sediment reworking at the HEBBLE Site based on measurements of Th-234, CS-137 and Pb-210. Marine Geology, 66, 1/4, 133-148. FERRY, S. • MONIER, P. 1987. Correspondances entre alternances marno-calcaires de bassin et de plateforme (Crrtac6 du Sud-Est de la France). Bulletin Socidtd Gdologique de France, Serie 8, III, 5, 961-964. GALE, A. S. 1989. A Milankovitch scale for Cenomanian time. Terra Nova, 1,420--425. GILBERT, G. K. 1895. Sedimentary measurement of geologic time. Journal of Geology, 3, 121-127. GOLDHAMMER,R. K., DUNN,P. A. & HARDIE,L.A. 1987. High-frequency glacio-eustatic sea level oscillations with Milankovitch characteristics recorded in middle Triassic platform carbonates in Northern Italy. American Journal of Science, 287, 853-892. GUtNASSO, N. L. & SHINK, D. R. 1975. Quantitative estimate of biological mixing rates in abyssal sediments. Journal of Geophysical Research, 80, 21, 3032-3043. HART, M. B. 1987. Orbitally induced cycles in the

CONSTRAINTS IN CYCLOSTRATIGRAPHY chalk facies of the United Kingdom. Cretaceous

Research, 8, 335-348. HATTIN, D. E. 1971. Widespread, synchronously deposited burrow-mottled limestone beds in Greenhorn limestone (Upper Cretaceous) of Kansas and Central Colorado. Bulletin of the

American Association of Petroleum Geologists,

55, 412-431. HERBERT, T. D. & FISCHER, A. G. 1986. Milankovitch climatic origin of Mid-Cretaceous black-shale rhythms in Central Italy. Nature, 321, 739-743. --, STALeARD, R. E & FISCHER, A. G. 1986. Anoxic events, productivity rhythms and the orbital signature in a mid-cretaceous deep-sea sequence from central Italy. Paleoceanography, 1, 495-506. HIeGEN, F. J. 1991. Astronomical forcing and geochronological application of sedimentary cycles in the Mediterranean Pliocene/Pleistocene. Geologica Ultraiectina, 93. MILANKOVITCH, M. 1941. Kanon der Erdbestrahlung

and seine Anwendung auf des Eiszeitenproglen. Academic Royale Serbe, Belgrade, special edition, 133. NIT~OUSER, C. A. DE MASTER, D. J., MCKEE, B. A., CUTSHALe, N. H. & LARSEN, I. L., 1984. The effect of sediment mixing on Pb-210 accumulation rates for the Washington continental shelf. Marine Geology, 54, 201-222. PENG, T. H., BROECKLER,W. S. & BERGER, W. H., 1979. Rate of benthic mixing in deep-sea sediments as determined by radioactive tracers. Quaternary. Research, 11, 141-149. PESTIAUX, P. & BERGER, A. 1984. Impacts of deep-sea processes on paleoclimatic spectra. In: BERGER, A. L., IMBRIE, J., HAYS, J., KUKLA, G. & SALZMAN, B. (eds), Milankovitch and Climate,l, Reidel, Dordrecht, 493-510.

141

--,

VANDER MERSCH, I., BERGER, A. & DUPLESSY,J. C. 1988. Paleoclimatic variability at frequencies ranging from l cycle per 10,000 years to 1 cycle per 1,000 years: evidence for nonlinear behaviour of the climate system. Climatic Change, 12, 9-37. PREMOLI SILVA, I., RIPEPE, M. & TORNAGHI, E 1989. Planktonic foraminiferal distribution record productivity cycles : evidence from the AptianAlbian Piobbico core (Central Italy). Terra Nova, 1,443-448. RIO, M., FERRY, S. & COT~eeoN,E 1989. Periodicit6s dans les s6ries p61agiques alternantes et variations de l'orbite terrestre. Exemple du Cr6tac6 inf6rieur dans le Sud-Est de la France. Comptes Rendus Acad(mie des Sciences, Paris, 3119, (II), 73-79. SCHWARZACHER,W. 1964. An application of statistical time series analysis of a limestone-shale sequence. Journal of Geology, 72, 195-213. 1975. Sedimentation models and quantitative stratigraphy. Developments in Sedimentology, 19, Elsevier, Amsterdam. -& FISCHER, A. G. 1982. Limestone-shale bedding and pertubations of the Earth's orbit. In: EINSELE,G. & SEILACHER, A. (eds), Cyclic and Event Stratification, Springer, Berlin, 720-95. TEN KATE, W. G. & SPRENGER, A. 1989. On the periodicity in a calcilutite-marl succession (SE Spain). Cretaceous Research, 10, 1-31. VAN WOERKOM,A. J. J. 1953. The astronomical theory of climatic changes. In: SHAPLEY,H. (ed.), Climatic

Change. Harvard University Press, Cambridge, MA, 147-157. WEEDON, G. P. 1989. The detection and illustration of regular sedimentary cycles using Walsh power spectra and filtering, with examples from the Lias of Switzerland. Journal of the Geological Society, London, 146, 133-144.

Periodicities of carbonate cycles in the Valanginian of the Vocontian Trough: a strong obliquity control E G I R A U D 1, L. B E A U F O R T 2 & E C O T I L L O N 1

1Centre des Sciences de la Terre, Universit( de Lyon, 27-43 Boulevard du 11 Novembre 69622 Villeurbanne Cedex, France 2Laboratoire de G(ologie du Quaternaire, CNRS Luminy case 907, 13288 Marseille Cedex 09, France Abstract: A high-resolution study of variations in carbonate productivity, quantified by CaCO 3

content and by colour intensity (grey level) for a Valanginian rhythmic succession is presented in the Vocontian Trough. Spectral analysis techniques reveal the presence of strong CaCO3 cycles linked to cyclic variations in the Earth's orbit. The results of the analysis suggest that obliquity cycle is the most clearly defined signal whereas the presence of CaCO3 cycles related to the precession signal is less distinct. The transition from limestone-dominant alternations in the lower Valanginian to marl-dominant alternations in the upper Valanginian is characterized both by an increase of sedimentation rate and a change from the precession as the dominant forcing in the lower, calcareous part of the Valanginian, to obliquity as the dominant forcing for the upper marly part of the Valanginian. An orbitally calibrated chronology is presented for the Valanginian based on the identification of the carbonate cycles. The application of band-pass filterings to the original carbonate record allows the extraction of 91 carbonate cycles related to precession and 137 cycles related to obliquity. The proposed duration for the Valanginian stage is 7.04 Ma.

In pre-Quaternary alternating pelagic successions, the thickness of a marl-limestone couplet is often the main lithological signal used for the search for orbital cycles in the sedimentary record (De Boer 1982; Dean et al. 1981; Fischer & Schwarzacher 1984; Schwarzacher & Fischer 1982; Rio et al. 1989; Weedon 1989). However, the use of CaCO 3 content rather than bed-interbed thicknesses provides some advantages. The data do not suffer from the ambiguity of the qualitative designation of a couplet. In a more continuous record the calcium carbonate content in pelagic sediments is more valuable for studies of orbital forcing (Mayer 1991); information, i.e. higher resolution, of variations concerning periods are accurate and smaller than the duration of a bed-interbed couplet. A convincing demonstration of the predominant astronomical forcing in the development of the carbonate system is furnished by the works of Hilgen (1991) in the Mediterranean Pliocene and Pleistocene, and Mayer (1991) in the Central Pacific Plio-Pleistocene. The Vocontian lower Cretaceous alternations representing variations in carbonate productivity are chosen as the framework for a study of carbonate content fluctuations. The rhythmic bedding (from bed-interbed thickness) has been previously linked with cyclic variations in the Earth's orbit (Rio et al. 1989), but the large dis-

persion of frequencies, with only a narrow part in the Milankovitch frequency band, was not conclusive. A new quantitative study on those cyclic sequence (from bed-interbed thickness), through application of the Walsh spectral method (Huang et al. 1993), reveals for the ValanginianHauterivian interval the importance of obliquity in controlling the rhythmic sedimentation, whereas the precession cycle is less important. These results are interesting, especially as they are not in agreement with other examples of astronomical forcing in pelagic successions. On the one hand, De Boer (1991) concluded, from a compilation of literature examples, that sedimentary systems located at c. 30 ~ (the paleolatitude of the Vocontian Basin for the studied time period) are particularly sensitive to the precession of the Earth's axis. On the other hand, Fischer (1986), in a review on pelagichemipelagic series, remarks that many long sequences (such as the Vocontian alternations) are compound with segments reflecting both the precession and eccentricity cycles with, in other parts, the obliquity cycle. Therefore, the main goal of this present study is: (1) To demonstrate the connection between cyclic variations in CaCO 3 content and astronomical parameters. (2) To determine accurately the periodicities of carbonate cycles and then to compare them with the results of Huang et al.

From HOUSE,M. R. & GALE,A. S. (eds), 1995, Orbital Forcing Timescalesand Cyclostratigraphy, Geological Society Special Publication No. 85, pp. 143-164.

143

F. GIRAUD ET AL.

144

(1993). This is performed by spectral analysis of the resulting carbonate record, at two different scales but focuses on the Valanginian stage using a high resolution analysis from a drilling core of an upper Valanginian set of beds with determination of variations in colour intensity concurrently with the quantification of carbonate content; and also a study of the carbonate content record through the whole Valanginian. On this an estimate of the duration of the Valanginian on the basis of the carbonate cyclicity is proposed. G e n e r a l i t i e s on s t u d i e d a r e a

Geological setting The Angles Section (southern Subalpine Ranges) is chosen to illustrate the Vocontian alternations (Fig. 1). This selection is guided by the following features: (1) Angles is the hypostratotype of the Valanginian stage (Busnardo & Thieuloy 1979); (2) it has a typical sequence of pelagic limestonemarl couplets; (3) the access and exposure are good; (4) tectonic disturbances are minor; and (5) no biostratigraphic hiatuses are observed. The Vocontian Trough is defined sensu stricto (Cotillon 1984) as a relatively deep pelagic sedimentation area with cyclic deposits where sedimentation is mainly characterized by vertical fluxes of fine pelagic biocarbonate and clay particles. From the late Jurassic, and throughout the early Cretaceous, this basin was open towards the Tethys

and surrounded by slopes and platforms with hemipelagic facies and temporarily by shallow carbonate banks. At that time, the tectonic regime was extensional. Platform margins were split into tilted blocks by NW-SE and WSW-ENE faults generating high subsidence contrasts (Cotillon 1985a). The sedimentation was influenced by this tectonic regime as demonstrated by the occurrence of slumps into the basin and by rapid lateral changes of thickness and facies. From the end of the Jurassic until the Aptian, the Vocontian Trough was located at a palaeolatitude of 25-30 ~ (Savostin et al. 1986). In terms of basin paleogeography the Valanginian stage is divided in two periods. The first begins with a transgressive episode emphasizing the transition from a system of carbonate platforms initiated during the Beniasian towards a system of deep-water carbonates with deposition of alternating cycles in the basin where fine terrigenous materials are mixed with biogenic carbonate in various proportions. The second episode is marked by a deepening trend. The basin is largely open to the Tethys and there are also communications with the Boreal basins by way of the Jura, Franche-Comt6 and Alsace platforms (Cotillon 1984).

Lithostratigraphy & biostratigraphy (Fig. 2) Lithology. At Angles the Valanginian is 240 m thick. The alternations consist of a repetition of

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Fig. 1. Paleogeography of the Subalpine Basin from the Late Jurassic to the Early Cretaceous.

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