Abstract

Lithofacies analysis has been undertaken on a suite of apparently monotonous organic-rich mudstones from the Upper Jurassic Kimmeridge Clay Formation (KCF) in the Wessex Basin, southern England. Using a combination of hand-specimen description, whole-rock geochemistry, optical petrography, and electron optical petrography, five lithofacies have been identified. These are: clay-rich mudstones, silt-rich mudstones, nannoplankton-rich mudstones, laminated mudstones, and concretionary carbonates. These facies can be distinguished from one another on the basis of physical sedimentary structures and the relative proportions of allochthonous, autochthonous, and diagenetic components in each. All five of the KCF lithofacies studied were deposited in a marine shelf environment below fair-weather wave base but in relatively shallow water close to storm wave base. Deposition occurred in regions where sand and other coarse clastic allochthonous debris were not being supplied and the bottom waters varied from oxic to anoxic. The main factors influencing the development of the five facies were: distance from source area, primary productivity, bottom-water anoxia, absolute bathymetry, clastic dilution, and local sediment accumulation rates. Specifically, the clay-rich mudstones were deposited in regions farthest down the sediment-transport paths, commonly in areas by-passed by the main sediment supply and beneath surface waters of relatively low primary productivity. In contrast, the silt-rich mudstones were deposited in more proximal areas, closer to the source of the sediment, in regions where surface productivity and sediment accumulation rates were much higher. The nannoplankton-rich mudstones accumulated beneath regions where productivity in the surface water layers was fairly high (dominated by coccolithophoroid production) and the supply of clastic sediment was insufficient to significantly dilute the autochthonous fraction of the sediment. Unlike the other facies, laminated mudstones were deposited in areas where the bottom waters were anoxic, surface productivity was enhanced, and local sediment accumulation rates were high. The concretionary carbonates were precipitated by diagenetic processes in areas where sediment accumulation rates were very low. These facies descriptions have enabled a detailed sedimentological study of the Kimmeridge Clay Formation to relate depositional and diagenetic processes directly to the facies present, thereby allowing classical facies analyses to be undertaken on apparently homogeneous mudstone successions.

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