Sedimentary dynamics of the Kimmeridge Clay: tempests and earthquakes

Storms are suggested to be an important influence upon the formation of epicontinental black shales, controlling both their distribution and facies types. A variety of storm-produced event beds are described from the Kimmeridge Clay of Dorset. These include horizons of graded rip-up clasts, silt laminae, thin graded mud horizons and shell pavements. Biostratinomy and population dynamics indicate that the pavements represent brief benthic colonization events in a predominantly anaerobic-dysaerobic environment. Differential near-surface lithification has also affected the sediments, leading to the formation of flow phenomena. These include loop structures, microfolding and brecciation. Earthquake-induced fissuring also occurs at a number of horizons. The Kimmeridge Clay shows well-developed cyclically alternating organic-rich shales and mudstones on a decimetre scale. A feedback mechanism of storm-induced benthic oxygenation and temperature-stratified inhibition of storm mixing may account for the abrupt nature of the cyclicity. Subsidence and sedimentation rate were probably important factors controlling the thickness and organic-richness of the sediments whilst the influence of storms was important in controlling the distribution and facies types of these important source rocks. The model proposed for the Kimmeridge Clay may be of general application to many epicontinental black shales in the geological record.