PETROGRAPHIC AND GEOCHEMICAL STUDY OF A KIMMERIDGIAN ORGANIC SEQUENCE - (YORKSHIRE AREA, UK)

A metric organic Sequence from the Kimmeridge Clay Formation of Yorkshire (UK) has been extensively studied with petrographical and geochemical techniques in the aim to define more precisely depositional and preservational conditions of the organic matter. The organic content is ranging from 3 to 9%. As a petrographical tool, the use of scanning electron microscopy with backscattered electron mode is emphasized, because it provides an image of all the constituents of the sediment. The sediment displays a very thin lamination, consists of clays, coccoliths (dispersed or as fecal pellets) and algal organic matter. The presence of organic matter brings about diagenesis and induces carbonate dissolution, pyrite precipitation and silicification. The depositional environment was probably quiet with a fluctuating intensity of anoxia of the sea floor. The beginning of the sequence is characterized by higher sedimentation rate and primary productivity.