Carbonate depositional environments in the late Wenlock of England and Wales

Based on outcrop studies and borehole data, six bedded lithofacies and two reef types are recognized within the Much Wenlock Limestone Formation of the English Midlands and Welsh Borderland. The lithofacies are interpreted to represent a series of carbonate shelf environments extending from below storm wave-base to well above fair weather wave-base. In common with many other shallow marine carbonate depositional systems, the principal controls on lithofacies development were hydrodynamic energy, the supply of fine elastic sediment, and patterns of colonization of the sea floor by organisms. Reef distribution was probably controlled by the nature of the substrate, water circulation, and rate of siliciclastic sedimentation. A depositional model is proposed which incorporates biostratigraphical evidence suggesting that the formation youngs to the west on the northern part of the shelf. Deposition of the Much Wenlock Limestone Formation there began in the West Midlands, where 12 m of microbial limestone were lain down in a mid-shelf setting during a local regression. The remainder of the shelf was dominated by low energy siliciclastic deposition at that time. The West Midlands then returned to somewhat deeper water, lower energy deposition, the resulting impure calcareous muds becoming diagenetically changed into the nodular limestone lithofacies. That lithofacies is commonly overlain successively by the interbedded limestone and silty mudstone lithofacies, and then the crinoidal grainstone lithofacies This vertical lithofacies sequence is uniform over the entire northern part of the shelf, reflecting a gradual decrease in water depth. The crinoidal grainstone lithofacies was deposited as a wave-influenced carbonate sandbody which prograded from east to west. Lithofacies sequences on the southern shelf are laterally impersistent, probably due to greater tectonic instability and topographical variablity.