Kimmeridge clay sedimentation — A new cyclic model

Abstract

The Kimmeridgian (Upper Jurassic) fine-clastic sequences of the Boulonnais (northern France) and Dorset (southern England) represent oxygen controlled environments. Five facies types with increasingly deteriorating living conditions, ranging from normal shelf environments to submarine lifeless desert, have been distinguished. The widespread occurrence of this facies in the Late Jurassic North Atlantic Shelf Sea and the palaeogeographic situation require a large-scale model to explain the Kimmeridge Clay sedimentation. Anoxic conditions are generally explained by one of three models, none of which applies directly to the Kimmeridge Clay. A new model, derived from the overall palaeogeographic and palaeoclimatic situation is therefore proposed. In this model, water mass stratification and anoxic bottom conditions refer to a high latitude monsoonal type wind circulation causing a water current and counter current system in the North Atlantic Shelf Sea and the Arctic Ocean. Variations in the amount of oxygen in the bottom waters allow recognition of four different cycles with periods ranging from seasonal to 3 × 10 7 years. They influenced the counter current system and the resulting anoxic conditions.