The influence of a sudden climatic change on marine deposition in the Kimmeridgian of northwest Europe

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Abstract
A sudden change from humid style to semi-arid style deposition markedly affected the accumulation of the Upper Kimmeridge Clay in southern England. Many of the changes appear to be related to a change in sedimentation rate at this time. Thus softground faunas are replaced by firmground faunas; diagenetic dolostones formed in the methanogenic zone are replaced by sulphate reduction zone carbonate nodules; and depositional gradients, recorded by lateral biofacies changes, becomes steeper. The evidence available is in accord with a decline in offshore sedimentation rates during this interval. Other changes, such as a decline in kaolinite abundance, were more directly controlled by the ‘drying-out’ of the hinterland. Similar changes, elsewhere in the marine geological record, could be used as climatic indicators. The climatic change is part of a wider, northern hemisphere dry event which affect a broad area in the late Jurassic. The Kimmeridge Clay of southern England was one of the last depositional environments to be influenced by the climatic change at this latitude.