JU

by thin-shelled pectinids, oysters Plagiostoma, and the byssally attached Inoceramus and Oxytoma. Inoceramus had a thin prismatic shell and a broad flattened shape and these features were probably adaptations to prevent it sinking into the muddy sea floor. Brachiopods were generally rare but became abundant at some horizons where the substrate was more stable. The commonest brachiopods were rhynchonellids (Tropiorhynchia, Calcir-hynchia) and terebratulides.

The sediments were intensively bioturbated, so that much of the substrate was probably 'soupy', except when slower sedimentation rates allowed consolidation or partial cementation of the sea floor. Trace fossils included burrows of suspension feeders and deposit feeders. The 'U' burrows (Diplocraterion, Rhizo cor allium) often penetrated the tops of the limestone beds, while more complex burrow systems (like Chondrites) descended from the bases of many calcilutite beds. Both types of burrows were filled with sediment from the beds above (the 'U' burrows in the limestones were filled with clay, and the complex burrows in the clay are filled with lime mud) showing that the changes in sediment types were original and not just the result of subsequent movement of calcium carbonate in the rock. The normal 'background' sediment was probably coccolithic lime mud, its slow rate of accumulation being reflected in the abundance of unsorted ammonites in some limestones. Periodic influxes of terrigenous material, including comminuted plant debris, produced the interbedded mudstones. Such phases of more rapid deposition upset the oxygen budget of the basin and caused bituminous shales to form. With waning clay supply, there was a subsequent reversion to undiluted carbonate accumulation. This facies occurs as the Blue Lias in Dorset and in central England, and as the Belemnite Marls, again in Dorset.

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