Humberside Geologist Online

The Upper Jurassic Kimmeridge Clay

 

Martin Chambers (martinchambers@cottage.karoo.co.uk)

(I can direct you to the references in the text; omitted for the sake of brevity)

 
"Illimitable ocean, without bound,
Without dimension, where length, breadth and highth,
And time and place are lost."
Milton, Paradise Lost
Introduction

Fine-grained mudstones such as the Upper Jurassic UK onshore Kimmeridge Clay Formation (UK onshore KCF) often pose far more problems than can be solved. At the type section located at Kimmeridge Bay in Dorset, these unconformably overlie the coral-rich sandy shallow-water Oxfordian sediments, and grade into the shallow water lacustrine evaporites and limestones of the Portlandian. This succession is of prime interest since it constitutes one of the main North Sea source rock intervals (Baird, 1986). Such rocks are of prime interest to the oil industry since they are, in essence, the beginning of the entire Exploration and Production process, constituting the fundamental part of the source rock-reservoir-seal petroleum play system. Academics have focused on these sediments, arguing the case for productivity versus preservation as a cause of their organic richness (Gallois, 1976; Tyson et al., 1979). Another point of interest is the Bed 44 interval, which has been suggested by Wignall and Ruffell (1990) to mark the northward spread of more arid conditions into northwestern Europe. The UK onshore KCF was deposited in an environment which is not present on the earth today, with much of Western Europe being covered by a global highstand of sea-level (Hallam, 1988), which has been related to the pulsed opening of the Atlantic. The consequence of this is that the UK was covered by a shallow stratified and largely anoxic sea, perhaps less than 100m deep, with occasional landmasses such as the London-Brabant Massif, Cornubian High and Welsh High being exposed. The Kimmeridgian climate was warmer than that of today, with elevated carbon dioxide content. The following is a brief summary of the Kimmeridge Clay Formation, based largely on a study of the Wessex Basin, with further details and references in the PhD thesis of Chambers (2000), which I am currently finishing.

So, why call the succession "the Kimmeridge Clay Formation"? The type section is located in Dorset, UK and encompasses in excess of 200 years' research, initially into the ammonite fauna, but later into many studies related to the problems of source rock formation (see Cox and Gallois, 1981; Gallois, 1998). In addition, this outcrop provides an opportunity for researchers to better understand the sediments, the lateral equivalent of which constitutes one of the principle North Sea source rocks.

Palaeoclimate studies

The Mesozoic Era is widely-believed to have had elevated atmospheric carbon dioxide content and global temperatures (Hallam, 1985; Valdes and Sellwood, 1992; Moore et al., 1992; Valdes et al., 1995; Sellwood and Valdes, 1996; Price et al., 1997; Nunn, 1998), although the link between atmospheric carbon dioxide and global temperatures has been questioned during the Cretaceous (Sellwood et al., 1994; Price et al., 1998), in GCMs (Cess et al., 1993), with some workers reporting a sharp decrease in atmospheric carbon dioxide concentration during the Cretaceous (Kuypers et al, 1999). The questions which have been raised about our understanding of the Cretaceous climate and atmosphere pose a particular problem since many GCM studies assume a warmer Cretaceous due to greater atmospheric carbon dioxide content. In contrast, data for the Kimmeridgian Stage of the Jurassic period suggest that the Kimmeridgian had elevated atmospheric carbon dioxide, was warm and wet with increased cloud cover and a possible monsoon and Mediterranean-type climate over the UK (Valdes and Sellwood, 1992; Price et al., 1997; Sellwood and Valdes, 1997), with the palaeogeography perhaps having some influence on the climate (Moore et al., 1992). GCM (General Circulation Models) models of the Kimmeridgian climate suggest that orbital forcing affected precipitation more than temperature at low and mid-latitudes, whilst orbital forcing affected temperature more than precipitation at high latitudes (Valdes et al., 1995).

So, what is "orbital forcing"? This is a term used by many palaeoclimate modelers and also researchers of cyclostratigraphy, or the study of cycles of beds. During the last million years, there have been eight glacial periods, each occurring on average every 125,000 years, and each with a warm period that lasted for only 10% of the duration of the glacial. This is clearly a natural phenomenon, occurring at a frequency similar to that of orbital forcing, the concept which links climate change to c. 100,000 year (eccentricity) cycles, 41,000 year (obliquity) cycles and 22,000 year (precession) cycles in the orbit of the Earth around the Sun (Berger, 1979; Dawson, 1992). These variations of orbit affect the amount of solar energy incident on the surface of the Earth, and hence are widely-held to have influenced the climate of the Earth throughout the Quaternary (Shackleton and Opdyke, 1973; Hays et al., 1976). In addition to this natural phenomenon, global temperature records show a significant but irregular temperature rise of between 0.3 and 0.6oC since global records began in 1861 (Barry and Chorley, 1992). This clearly coincides with the onset of industrialisation, and hence industrial emissions generated by large-scale anthropogenic activity may have affected global climate via heating due to their effect on outgoing infra-red radiation (Barry and Chorley, 1992). These industrial emissions are also known as the so-called "greenhouse gases" and comprise water vapour, carbon dioxide, methane and nitrous oxide and ozone (Barry and Chorley, 1992).

The UK Jurassic and Kimmeridgian

Climatically, the Jurassic was a time of elevated temperatures, carbon dioxide levels and sea-levels. These factors elevated marine and terrestrial productivity and hence contributed towards the formation of some of the most carbonaceous deposits in geological history, with source-rocks of the Upper Jurassic containing 25% of global reserves (Klemme and Ulmishek, 1991).

The UK Jurassic has a marked tendency towards a cyclic lithology, usually with a tripartite sequence of clay, sand and limestone (Arkell, 1956). Large internal unconformities are absent in the UK Jurassic, although small ones do exist, for example in the Bajocian of the Cotswolds, and disconformities are frequent, both in the entire Jurassic (Arkell, 1956) and occasionally in the Kimmeridge Clay Formation itself (Birkelund et al., 1983). The base of the UK onshore KCF is marked by an unconformity in Dorset (Cox and Gallois, 1981), as is the Yorkshire section of the Cleveland Basin near Hunmanby which may be related to faulting (Whittaker et al., 1985). Although they worked principally on the North Sea, Rawson and Riley (1982) reviewed the evidence of surrounding areas to identify the effects of the "Late Cimmerian Unconformity", and they suggest that the Baylei Zone "event" was related to this. Unlike other localities, the Dorset area is probably the most complete example of the UK onshore KCF (Gallois, 1998), and thus any cores drilled at Dorset will record most of the sedimentation that occurred during the Kimmeridgian.

The UK onshore Kimmeridge Clay Formation accumulated as a shallow marine deposit (Hallam, 1987) between 154.1 and 145.6 Ma (Gradstein et al., 1994). This was a time of elevated atmospheric carbon dioxide levels and temperatures (Hallam, 1985, 1994; Sellwood and Price, 1993; Valdes, 1993) and a global highstand (Hallam, 1978, 1981, 1988, 1992; Norris and Hallam, 1995). The Kimmeridge Clay Formation at Dorset consists of a series of clays and shales, with some cementstone bands and septarian nodules (Arkell, 1956). This changes further inland, since the Pectinatus Zone of the Upper Kimmeridgian is developed as sands with sandstone doggers at Swindon, Oxford and Aylesbury (Arkell, 1956; Gallois and Cox, 1994). Its maximum thickness is reached around the type section in the Purbeck and Kimmeridge Bay areas, decreasing to half this value twenty miles west around Weymouth, 90-100 m at Swindon and 30-45 m at Oxford (Arkell, 1956; Oates, 1981; Gallois and Cox, 1994). Despite these obvious variations in thickness, a good correlation of the entire UK onshore Kimmeridge Clay Formation is possible (Cox and Gallois, 1981; Wignall, 1993).

The Dorset type area of the UK onshore Kimmeridge Clay Formation (UK onshore KCF) is divided into 13 ammonite zones, which run from Pictonia baylei Zone at the base to the Virgatopavlovia fittoni Zone at the top (Cox and Gallois, 1981; Gallois, 1998). The boundaries of several of these Zones have been extended beyond the range of a pure biozone (ie the range of an index taxon) to lithological markers or "events" (Cox, 1990; Cox et al., 1994). This approach facilitates fieldwork and intra- or inter-basinal well correlation (eg Penn et al., 1986). However, this scheme relates to the sensu anglico definition of the Kimmeridge Clay Formation, which allows subdivision of the Kimmeridgian into the Lower and Upper Kimmeridgian Substages. This division occurs at the top of Bed 35/ base of Bed 36, which marks the disappearance of the ammonite species Aulacostephanus and is the equivalent of the base Portlandien (sensu gallico) and the presumed correlative of the base of the Tithonian and Volgian Stages (Gallois, 1998; see also Cope, 1995b). This biostratigraphic zonation is widely-accepted following studies of perisphinctid ammonites (Callomon and Cope, 1971; Cope, 1967, 1974, 1978, 1980) based on earlier palaeontologicalwork (see details in Cope 1980 and Cox and Gallois, 1981). The Formation is also divided into 63 beds on palaeontological and lithological criteria (Gallois, 1998). In Dorset, the lower Kimmeridge Clay Formation runs from baylei (at the base) to autissiodorensis, and the upper Kimmeridge Clay Formation runs between elegans to fittoni (at the top). The Kimmeridge Clay Formation at Dorset is underlain unconformably by Oxfordian sands and overlain by the Portland Sands. The dominant lithology is that of an organic-rich mudstone, which varies between organic shale, coccolith limestone, bituminous coccolith limestone, dolomite and a siltstone. Calcite-rich stone bands occur throughout the section, and occasional concretions are also found. Several marker beds can be traced across the full width of the outcrop, a distance of about 400 km (Cox and Gallois, 1981).

The UK onshore Kimmeridge Clay Formation lithology is arranged into a succession of sedimentary rhythms, which differ between the lower and upper parts of the Kimmeridge Clay Formation (Whittaker et al., 1985). The UK onshore Kimmeridge Clay Formation comprises cycles of silty mudstones and siltstones which rest on an erosion surface, and these are overlain by medium or dark grey shelly and fissile mudstones which become lighter, more calcareous and more fissile upwards, running between Autissiodorensis and Baylei ammonite zones. In the Upper Kimmeridge Clay Formation, these rhythms consist of fissile, brown to black and shelly bituminous shales, which grade upwards by increasing carbonate content through medium and dark grey mudstones into pale grey calcareous mudstones and argillaceous limestones. Superimposed onto these small-scale rhythms are larger-scale fluctuations of calcite and bitumen content.

The large-scale variations in calcite and kerogen content combine to give the Kimmeridge Clay Formation a very distinctive geophysical log pattern, which is typified by highly serrated formation density log and sonic log signatures. Bituminous beds have a high gamma, low sonic and low density values. Gamma decreases and sonic increases upwards through each rhythm, until the calcareous beds generate the lowest gamma and highest sonic values at the top of each rhythm. Other cycles include those detected in the geochemistry, which occur at the meter-scale (Dunn, 1974).

 

Development of the Wessex Basin and the effects of faulting on the Kimmeridge Clay Formation

The UK onshore Kimmeridge Clay Formation accumulated in the Wessex Basin, which is actually composed of four sub-basins, namely the Portland-Wight Basin, the Dorset Basin, the Vale of Pewsey Basin and the Weald Basin. Thus, the Wessex Basin may be defined as a series of post-Variscan sedimentary depocentres and intra-basinal highs which developed across southern England and its adjacent offshore areas (Underhill and Stoneley, 1998). The basin covers parts of Dorset, Hampshire, East Devon, Somerset and Wiltshire, and is bounded to the southwest and west by the Amorican and Cornubian Massifs, to the north by the London Platform (or London-Brabant Massif) and to the south by the Central Channel High, although its northeastern and northwestern boundaries are less well defined (Underhill and Stoneley, 1998). The northwestern limit is taken to be a poorly-defined boundary running from the Quantock Hills across the Central Somerset Trough to the western extension of the London Platform (Underhill and Stoneley, 1998).

The creation of the Wessex Basin post-dates the development and closure of the Devano-Carboniferous proto Tethys or Rheic Ocean (Underhill and Stoneley, 1998). Permian Variscan tectonics led to basin extension and its associated sedimentary fill, and sedimentation persisted until the Late Cretaceous, when the Wessex Basin was subjected to basin inversion (Underhill and Stoneley, 1998). Subsequent development of the overlying Hampshire Basin allowed accumulation of Tertiary sediments, with sedimentation terminating in the Oligocene (Underhill and Stoneley, 1998). Thus, according to the above tectonic history, the Permian-Oligocene succession can be divided into three megasequences. These megasequences are the Permian to Lower Cretaceous (extensional), Upper Cretaceous (inversion) and Tertiary (compressional) megasequences (Hawkes et al., 1998).

Liassic rifting was accompanied by a marine transgression, probably in response to the westerly propagation of oceanic spreading in the western Tethys (Hawkes et al., 1998). This transgression may be divided into six shallowing-upwards sequences, which were probably related to pulsed propagation of Atlantic rifting. The Lower Kimmeridge Clay Formation is the fifth of these six sequences. It is separated from the Upper Kimmeridge Clay Formation by a distinctive carbonate unit and an unconformity. The Upper Kimmeridge Clay forms the last of these six sequences, and its deposition occurred in response to renewed rifting and regional subsidence. Facies display an overall shallowing-upwards profile, from the marine mudstones and shallow marine sands of the Kimmeridge Clay and Portland Sandstone through to the brackish to marginal marine Purbeck Beds.

Differences between onshore and offshore Kimmeridge Clay

There is a strong contrast in the tectonic styles of the UK onshore and offshore Kimmeridge Clay Formation at this time. The offshore Kimmeridge Clay Formation accumulated in a series of grabens and basins formed during a Kimmeridgian phase of rifting. The Jurassic tectonic history of the North Sea is very complicated at this point in time. The Triassic graben system of North-western Europe was polarised to a few major rifts such as the North Sea Rift, Polish-Danish Troughs, the Porcupine Trough and the Celtic Sea-Bristol Channel and Western Approaches Basins (Ziegler, 1978). Unlike its North Sea equivalent, the effects of tectonics on the deposition of the UK onshore Kimmeridge Clay Formation were minimal (Kent, 1975). These effects were restricted to periodic lithospheric upwelling driven by halokinetic diapirism of the underlying Zachstein salts, thereby forming a series of swells and basins (Hallam and Sellwood, 1975). Later workers have suggested that the diapirism was related to the Triassic Mercia Mudstone Group rather than the Permian evaporites (Smith and Hatton, 1998).

Early rifting of the Central and North Atlantic did cause some slight displacement along faults bounding the Wessex Basin during the Hettangian-Bajocian and latest Oxfordian onwards respectively (Jenkyns and Senior, 1991). Major basin inversion occurred during the Late Cretaceous to Tertiary (eg Ruffell and Wignall, 1990; Hillis, 1993; Beeley and Norton, 1998) as extensional faults were reversed during Tertiary contraction (Harvey and Stewart, 1998). This basin inversion led to the formation of a periclinal fold at Kimmeridge (Underhill and Stoneley, 1998). Hydrocarbons are currently extracted from the core of this fold, although they are sourced in the Lias rather than the Kimmeridge Clay Formation (Underhill and Stoneley, 1998). This is the only oilfield of the three producing oilfields in the Wessex Basin (Wytch Farm, Wareham and Kimmeridge) which has shown any hydrocarbon remigration into younger structural inversion structures (Underhill and Stoneley, 1998).

So how does this relate to Yorkshire? Rare outcrops of the UK onshore KCF are found outside Dorset as far north as Filey Bay and the Vale of Pickering area, although it was not until a program of borehole correlation was undertaken that the geological community could be certain that these could be correlated. Mainly undertaken by the BGS (lead by Dr. Ramues Gallois and Dr. Beris Cox), this programme showed how the Dorset section could be correlated to the East England Shelf (Penn et al., 1986), with a borehole at Winestead being one of the furthest north in this region. The East England shelf had a different type of sedimentation to the Wessex Basin, since it is condensed, probably as a consequence of tectonics related to the development of the North Sea (eg Penn et al., 1986). Cox and Gallois (1981) highlight that certain ammonite species may be found in Lincolnshire and Yorkshire, although a number of misidentifications have been made, for example confusion of the ammonite Pictonia with large raseniids and aulacostephanids with smooth body chambers. Wider European correlation is difficult, with separate definitions of the Kimmeridge Clay Formation being applied in the English (sensu anglico), French (sensu gallico), North Sea (Kimmeridgian/Tithonian) or Russian (Kimmeridgian/Volgian) terms (see Cope, 1995 for further details). These problems of correlation largely relate to the provinciality (or spatial restriction) of certain ammonites. Perhaps the most useful paper for this society is that of Wignall (1993) (detailed in the Yorkshire Geological Society) dealing with the Golden Hill SSSI site. This site contains a 25m section, and spans the mid-Hudlestoni to mid-Pectinatus ammonite Zones of the Upper Kimmeridge Clay Formation. However, as is often noted by many workers in the field (including on occasion, me!), identification of Kimmeridgian ammonites is often difficult since rib densities and style (prosiradiate etc.) are often the only distinguishing characteristic; perhaps a better solution is to look at the collection provided by Wignall for public display at the Yorkshire Museum. Ammonites of the Golden Vale site are usually less mature than those in Dorset and are encrusted with the oyster Liostrea multiformis which attached themselves as pseudoplanktonic parasites to living ammonites (Wignall, 1993). On occasion, these outlived the host ammonite, sinking to the soft sediment of the seabed, where they overgrew their host. Additionally, these organic-rich sediments are devoid of bioturbation, unlike those of Dorset. The lack of bioturbation and highly organic nature of these sediments has been taken by Wignall (1993) as an indicator of an anoxic water column.

Conclusions

I hope to have communicated as briefly as possible some of the many theories as to the nature of deposition of this cryptic fine-grained mudstone. All too often mudrocks are dismissed, yet of all the sedimentary rocks in the record, these pose perhaps some of the most fascinating questions to us all. Where did the clays come from? What is the actual cause of organic richness in the rock record (I must admit that I'm pretty much sold to the productivity school)? Why are the sedimentation rates so high (see Chambers et al. in press)? Can we learn anything about future climate change from these successions (I am of the opinion that they are best used to test GCMs for prediction of future climate change and also in the use of these GCMs to predict economically viable deposits of minerals and oil)? I am in the process of finishing my PhD and writing several papers; if anyone knows of someone who needs a sedimentologist.........

Key References

As this is a website for Hull Geological Society members, the following references are given to members for figures and other useful data (ammonite hunters pay attention to the founding work of Cope and Ramues Gallois' and Paul Wignall's excellent work), and will prove instructive (I'm a little worried about copyright laws, academic hit squads and annoying close friends, else I would include some!).

CALLOMON, J.H. and COPE, J.C.W. (1971) The stratigraphy and ammonite succession of the Oxford and Kimmeridge Clays in the Warlingham borehole. Bulletin of the Geological Society of Great Britain, 36, 147-176

CALLOMON, J.H. and COPE, J.C.W. (1995) The Jurassic Geology of Dorset. In: Taylor, P.D. (Ed.) Field Geology of the British Jurassic. Geological Society of London, London. pp 51-103

CHAMBERS, M.H. (in draft) A high-resolution mineralogical study and chemostratigraphy of the UK onshore Kimmeridge Clay Formation. Unpublished PhD thesis, University of Reading.

CHAMBERS, M.H., LAWRENCE, D.S., SELLWOOD, B.W. and PARKER, A. (in press) Annual layering in the Upper Jurassic Kimmeridge Clay Formation, UK, quantified using an ultra-high resolution SEM-EDX investigation. Sedimentary Geology

COPE, J.C.W. (1967) The palaeontology and stratigraphy of the lower part of the Upper Kimmeridge Clay of Dorset. The Bulletin of the British Museum (Natural History), 15, 1-79

COPE, J.C.W. (1974) Upper Kimmeridgian ammonite faunas of the Wash area and a subzonal scheme for the lower part of the Upper Kimmeridgian. Bulletin of the Geological Survey of Great Britain, 47, 29-37

COPE, J.C.W. (1978) The ammonite faunas and stratigraphy of the upper part of the Upper Kimmeridge Clay of Dorset. Palaeontology, 21, 45-56

COPE, J.C.W. (1980) Kimmeridgian correlation chart. In: COPE, J.C.W., DUFF, K.L., PARSONS, C.F., TORENS, H.S., WIMBLEDON, W.A. and WRIGHT, J.K. (Eds.) A Correlation of Jurassic Rocks in the British Isles. Part 2: Middle and Upper Jurassic. Geological Society of London, Special Report, 15, 76-85

COPE, J.C.W. (1994) The nature and resolution of Jurassic ammonite biozones. Geobios, 17, 127-132

COPE, J.C.W. (1995a) Introduction to the British Jurassic. In: Taylor, P.D. (Ed.) Field Geology of the British Jurassic, Geological Society of London. pp 1-7

COPE, J.C.W. (1995b) Towards a unified Kimmeridgian Stage. Petroleum Geoscience, 1, 351-354

COPE, J.C.W. and SOLE, D.T.C. (2000) Ammonite jaw apparatus from the Sinemurian (Lower Jurassic) of Dorset and their taphonomic relevance. Journal of the Geological Society of London, 157, 201-206

COX, B.M. and GALLOIS, R.W. (1981) The stratigraphy of the Kimmeridge Clay of the Dorset type area and its correlation with some other Kimmeridgian sequences. Report of the Institute of Geological Sciences No. 80/4, HMSO, London. pp 1-45.

COX, B.M., GALLOIS, R.W. and SUMBLER, M.G. (1994) The stratigraphy of the BGS Hartwell Borehole, near Aylesbury, Buckinghamshire. Proceedings of the Geologists' Association, 105, 209-224

GALLOIS, R.W. (1976) Coccolith blooms in the Kimmeridge Clay and origin of North Sea Oil. Nature, 259, 473-475

GALLOIS, R.W. (1998) The stratigraphy of and well-completion reports for the Swanworth Quarry No. 1 and No. 2 and Metherhills No. 1 boreholes (RGGE Project), Dorset, Technical Report WA/97/91. British Geological Survey, Exeter. 102 pp

GALLOIS, R.W. and COX, B.M. (1994) The Kimmeridge Clay and underlying strata (Upper Jurassic) at Swindon, Wiltshire. Proceedings of the Geologists' Association, 105, 99-110

GALLOIS, R.W. and MEDD, A.W. (1979) Coccolith-rich marker bands in the English Kimmeridge Clay. Geological Magazine, 116, 247-262

HALLAM, A. (1988) A reevaluation of Jurassic eustasy in the light of new data and the revised Exxon curve. Sea-Level Changes - An Integrated Approach. SEPM Special Publication No. 42, 261-273

HOUSE, M.R. (1995) Orbital forcing timescales: an introduction. In: HOUSE, M.R. and GALE, A.S. (Eds.) Orbital Forcing Timescales and Sequence Stratigraphy. Geological Society Special Publication No. 85, the Geological Society, London. 1-18

MILLER, R.G. (1990) A paleoceanographic approach to the Kimmeridge Clay Formation. In: HUC, A.Y. (Ed.) Deposition of Organic Facies, AAPG Studies in Geology No. 30, AAPG, Tulsa. 13-26

OSCHMANN, W. (1988) Kimmeridge Clay sedimentation - a new cyclic model. Palaeogeography, Palaeoclimatology, Palaeoecology, 65, 217-251

OSCHMANN, W. (1990) Environmental cycles in the late Jurassic northwest European epeiric basin: interaction with atmospheric and hydrospheric circulations. Sedimentary Geology, 69, 313-332

OSCHMANN, W. (1991a) Anaerobic-poikilaerobic-aerobic: a new facies zonation of modern and ancient neritic redox facies. In: EINSELE, G., RICKEN, W. and SEILACHER, A., (Eds.) Cycles and Events in Stratigraphy. Springer-Verlag, Berlin. 565-571

OSCHMANN, W. (1991b) Distribution, dynamics and palaeoecology of Kimmeridgian (Upper Jurassic) shelf anoxia in western Europe. In: TYSON, R.V. and PEARSON, T.H. (Eds.) Modern and Ancient Continental Shelf Anoxia. Geological Society Special Publication No. 58, Geological Society, London. 381-395

TYSON, R.V. (1996) Sequence stratigraphical interpretation of organic facies variations in marine siliciclastic systems: general principles and application to the onshore Kimmeridge Clay Formation, UK. In: HESSELBO, S.P. and PARKINSON, D.N. (Eds.) Sequence Stratigraphy in British Geology. Geological Society Special Publication No. 103, the Geological Society, London. 75-96

TYSON, R.V., WILSON, R.C.L. and DOWNIE, C. (1979) A stratified water column environmental model for the type Kimmeridge Clay. Nature, 277, 377-380

UNDERHILL, J.R. and STONELEY, R. (1998) Introduction to the development, evolution and petroleum geology of the Wessex Basin. In: UNDERHILL, J.R. (ed.) Development, Evolution and Petroleum Geology of the Wessex Basin. Geological Society Special Publication No. 133, Geological Society, London. 1-18

WIGNALL, P.B. (1989) Sedimentary dynamics of the Kimmeridge Clay: tempests and earthquakes. Journal of the Geological Society of London, 146, 273-284

WIGNALL, P.B. (1990) Ostracod and foraminifera micropaleoecology and its bearing on biostratigraphy: a case study from the Kimmeridgian (Late Jurassic) of North West Europe. Palaios, 5, 219-226

WIGNALL, P.B. (1991a) Test of the concepts of sequence stratigraphy in the Kimmeridgian (Late Jurassic) of England and northern France. Marine and Petroleum Geology, 8, 430-441

WIGNALL, P.B. (1991b) Dysaerobic trace fossils and ichnofabrics in the Upper Jurassic Kimmeridge Clay of Southern England. Palaios, 6, 264-270

WIGNALL, P.B. (1991c) Model for transgressive black shales? Geology, 19, 167-170

WIGNALL, P.B. (1993a) The stratigraphy of the Upper Kimmeridge Clay (late Jurassic) of Golden Hill, Vale of Pickering, North Yorkshire. Proceedings of the Yorkshire Geological Society, 49, 207-214

WIGNALL, P.B. (1993b) Distinguishing between oxygen and substrate control in fossil benthic assemblages. Journal of the Geological Society of London, 150, 193-196

WIGNALL, P.B. (1994a) Mudstone lithofacies in the Kimmeridge Clay Formation, Wessex Basin, Southern England: implications for the origins and controls of the distribution of mudstones - discussion. Journal of Sedimentary Research, A64, 927-929

WIGNALL, P.B. (1994b) Black Shales. Clarendon Press, Oxford. 124pp

WIGNALL, P.B. and HALLAM, A. (1991) Biofacies, stratigraphic distribution and depositional models of British onshore Jurassic black shales. In: TYSON, R.V. and PEARSON, T.H. (Eds.) Modern and Ancient Continental Shelf Anoxia. Geological Society Special Publication No. 58, Geological Society, London. 291-309

WIGNALL, P.B. and MAYNARD, J.R. (1993) The sequence stratigraphy of transgressive black shales. In: KATZ, B.J. and PRATT, L.M. (Eds.) Source Rocks in a Sequence Stratigraphic Framework, AAPG Studies in Geology No. 37, AAPG, Tulsa. 35-48

WIGNALL, P.B. and MYERS, K.J. (1988) Interpreting benthic oxygen levels in mudrocks: a new approach. Geology, 16, 452-455

WIGNALL, P.B. and PICKERING, K.T. (1993) Palaeoecology and sedimentology across a Jurassic fault scarp, N.E. Scotland. Journal of the Geological Society of London, 150, 323-340

WIGNALL, P.B. and RUFFELL, A.H. (1990) The influence of a sudden climatic change on marine deposition in the Kimmeridgian of northwest Europe. Journal of the Geological Society of London, 147, 365-371

ZIEGLER, P.A. (1990) Geological Atlas of Western and Central Europe. Shell International Petroleum, Maatschappij.13-16, 17-28, 29-67, 68-90 and 91-122

Acknowledgements

I would like to extend my thanks to Mike Horne for presenting me with this opportunity to enlighten the Society with brief details on the Kimmeridge Clay Formation. As always, I am indebted to Martyn Pedley (Hull University) for getting me into geology, and my current supervisors, Bruce Sellwood and Andrew Parker (PRIS, Reading). Finally, Paul Wright (Cardiff; ex-PRIS) needs a mention after his inspiring "grow up, you don't really want to be an accountant because they're......" speech. Cheers, Paul!

 

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