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 th e English Midlands and Welsh Borderland. The lithofacies are interpreted to represent a series of carbonate shelf environments extending from below st orm wave-base to well above fair weather wave-base. In common with many oth er shallow marine carbonate depositional systems, the principal controls on lithofacies development were hydrodynamic energy, the supply of fine elast ic sediment, and patterns of colonization of the sea floor by organisms. Re ef distribution was probably controlled by the nature of the substrate, wat er circulation, and rate of siliciclastic sedimentation. A depositional mod el is proposed which incorporates biostratigraphical evidence suggesting th at the formation youngs to the west on the northern part of the shelf. Depo sition of the Much Wenlock Limestone Formation there began in the West Midl ands, where 12 m of microbial limestone were lain down in a mid-shelf setti ng during a local regression. The remainder of the shelf was dominated by l ow energy siliciclastic deposition at that time. The West Midlands then ret urned to somewhat deeper water, lower energy deposition, the resulting impu re calcareous muds becoming diagenetically changed into the nodular limesto ne lithofacies. That lithofacies is commonly overlain successively by the i nterbedded 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 wat er depth. The crinoidal grainstone lithofacies was deposited as a wave-infl uenced carbonate sandbody which prograded from east to west. Lithofacies se quences on the southern shelf are laterally impersistent, probably due to g reater tectonic instability and topographical variablity.