Comparison of major sequences of organic-rich mud deposition in the British Jurassic

Abstract

The British Jurassic contains 3 major sequences of organic-rich shales: the Jet Rock Formation (Lower Toarcian), the Lower Oxford Clay (Callovian), and the Kimmeridge Clay (Kimmeridgian). In addition minor sequences are found in the Blue Lias Formation and the Shales-with-Beef Formation (Hettangian-Sinemurian). The Lower Oxford Clay has the lowest average organic carbon content and was deposited in mildly oxygenated bottom waters. The sediment profile consisted of a thin upper oxidizing layer, with reducing conditions occurring a few cm below the sediment surface. The Jet Rock Shales were laid down in very poorly oxygenated bottom waters. Here reducing conditions extended up to the sediment surface and this resulted in the preservation of a higher organic carbon content. The Kimmeridge Clay shows a much wider variation in organic carbon content than the other examples, indicating accumulation in an environment which periodically fluctuated between mildly oxygenated and totally anoxic.