The Kimmeridgian of England was characterised by the deposition of a widespread and frequently organic-rich mudrock formation, the Kimmeridge Clay. Significant lateral thickness variations are attributable to active faulting which is occasionally recorded within the sediments as a number--of earthquake-generated structures. The slowest subsidence rates occurred on the London platform and depositional depths increased away from this positive feature both to the north and the south. Organic-rich shales preferentially accumulated in the deeper basinal areas where the thickest sequences accumulated. Even in the deepest locations storms were able to influence the depositional environment. This is reflected by a range of tempestite beds and by the highly opportunistic character of the benthic fauna which responded to the brief storm-induced oxygenation events. Palaeocommunity analysis indicates that a number of variables were controlling the faunal distribution. Substrate consistency, environmental stability and oxygen levels all interacted to produce a diverse range of community types composed of a few euryoxic, opportunistic molluscs. A major change occurs in the middle of the Upper Kimmeridge Clay when softground faunas are replaced by firmground faunas. This appears to reflect a marked reduction in sedimentation rate related to a sudden increase in aridity at this time. Sandy marginal marine facies were developed on the London Platform throughout much of the Kimmeridgian; they are best seen at outcrop in the cliff sections of the Boulonnais. Facies range from shallow marine glauconitic sandstones with intensive bioturbation and a fauna of large bivalves, to shell-poor shoreface sandstones with well preserved sedimentary structures. Analysis of this latter facies indicates that storms and waves were the two principal sources of hydrodynamic energy. Tides were probably of negligible importance in the Kimmeridgian epeiric seas. Local cyclicity in the Boulonnais sediments appears to be due to small-scale fluctuations of narrow, nearshore facies belts; they are not related to the more ubiquitous shale: mudstone rhythms which characterise the offshore mudrocks. Investigation of eustatic sea level changes indicates a Jurassic highstand of sea level in the Lower Kimmeridgian followed by a slow progressive fall through the Upper Kimmeridgian. The highstand appears to have produced relatively deep water conditions in many epeiric settings throughout the world which led to organic-rich shale formation. Finer scale eustatic sea level changes are more equivocal although a highstand may have occurred in the Eudoxus zone. No substantiating evidence was found for the fine scale eustatic fluctuations recently proposed by the Vail team.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:536551 |
| Date | January 1988 |
| Creators | Wignall, Paul Barry |
| Publisher | University of Birmingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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