The Kimmeridge Clay: the most intensively studied formation in Britain

The Jurassic rocks of the World Heritage coast crop out over a distance of about 60 miles between Lyme Regis and Swanage, and represent an unbroken 60 million years of Earth history. Within this succession marine mudstones, principally the Lias Group and the Oxford Clay and Kimmeridge Clay formations, represent about 60% of the time interval. Of these, the Kimmeridge Clay has been the most extensively studied with the result that much is known about its lithostratigraphy, biostratigraphy and geochemistry. Many of the lithological and faunal associations found in the Kimmeridge Clay can be closely matched with those in the Lias and Oxford Clay. All three were deposited in relatively shallow (50m to 200m deep), fully marine environments on continental shelves, and many of the conclusions reached about the depositional history of the Kimmeridge Clay can be equally well applied to the other formations. The Kimmeridge Clay outcrop runs almost continuously from the Dorset coast to the Yorkshire coast, and the formation has an extensive subcrop beneath younger rocks in eastern England and the North Sea (Figure 1). At outcrop the mudstones weather rapidly to clay with the result that there is no natural inland exposure, and at any one time there are rarely more than five man-made sections. The only good exposures are in the cliffs at and adjacent to Kimmeridge and Ringstead bays, and these form the type sections for the Kimmeridge Clay Formation and the Kimmeridgian Stage (Arkell, 1947; Cox & Gallois, 1981).