CLASSIFICATION OF JURASSIC MARINE SHALE SEQUENCES - EXAMPLE FROM THE TOARCIAN (LOWER JURASSIC) OF GREAT-BRITAIN

A classification of Jurassic marine shales was proposed based on an analysis of the Toarcian (Upper Lias) sediments of the Lower Jurassic of Yorkshire, Great Britain [U.K.]. Using sedimentological, body-fossil and trace-fossil features, this sequence of apparently monotonous shales was subdivided into 3 facies: Normal shale: a homogeneous bioturbated sediment often containing sideritic nodules or horizons. Trace-fossils were dominated by Chondrites; benthic body-fossils were abundant and diverse. Restricted shale: consisting of laminated sediment with scattered calcareous concentrations. Bioturbation was generally sparse, with thin discrete pyritic burrows, while the benthic fauna was dominated by deposit feeding protobranch bivalves. Bituminous shale: a finely laminated sediment with pyritic calcareous concretions, little or no bioturbation, and a benthic fauna which was sparse and entirely epifaunal. Each facies was interpreted in terms of modern-day shelf mud environments and the balance between dissolved O2 and H2S in the sediment column.