Oncoids as environmental indicators in the Much Wenlock Limestone Formation of the English Midlands

Abstract

The Much Wenlock Limestone Formation of the West Midlands was deposited in a mid-shelf setting and is divisible into three members; the Upper and Lower Quarried Limestone members being separated by the more argillaceous Nodular Limestone Member. Oncoids, composed predominantly of micritic fabrics with Rothpletzella and Girvanella, occur commonly in the Lower Quarried Limestone Member. These oncoids vary from subspherical bodies up to 5 mm in diameter to forms with a highly irregular and branched upper surface which reach 70 mm across. Each form is indicative of a different depositional environment, which is also reflected in the sediment enclosing the oncoid. Equidimensional oncoids in peloidal packstones were formed by continuous rolling, whereas the larger, branched forms enclosed in loosely packed wackestones developed in quieter conditions below wave base. The distribution of oncoid morphotypes in the Lower Quarried Limestone Member shows that small variations in relative sea level were superimposed on the overall middle to late Wenlock regressive episode during which the Much Wenlock Limestone Formation was deposited. The uniformity of the formation throughout the West Midlands indicates that the sea floor was essentially planar over a large area. Vertical variation of oncoid morphology within the Lower Quarried Limestone Member can be traced throughout the area, allowing accurate correlation of relative sea-level variations.

Carbonate depositional environments in the late Wenlock of England and Wales

“…This lithofacies was fully described and interpreted by Ratcliffe (1988). It consists of continuous beds of pale grey limestone separated by thin beds of grey silty mudstone.…”

“…Three environmentally specific oncolite morphotypes occur (Ratcliffe, 1988): Type A oncolites are small and spherical, Type B are larger and more irregular but have a continuous cortex, and Type C oncolites have a coating on one surface only. The base of the Lower Quarried Limestone Member is dominated by Type C oncolites, the middle parts by Type B, and the top by Type A (Ratcliffe, 1988). Microbial reefs are known from this lithofacies (Ratcliffe, 1988, and Section 2.g.1).…”

“…The base of the Lower Quarried Limestone Member is dominated by Type C oncolites, the middle parts by Type B, and the top by Type A (Ratcliffe, 1988). Microbial reefs are known from this lithofacies (Ratcliffe, 1988, and Section 2.g.1). Ratcliffe (1988) demonstrated that this lithofacies was deposited in a relatively low energy environment where water circulation was poor.…”

“…Microbial reefs are known from this lithofacies (Ratcliffe, 1988, and Section 2.g.1). Ratcliffe (1988) demonstrated that this lithofacies was deposited in a relatively low energy environment where water circulation was poor. The Lower Quarried Limestone Member of the West Midlands represents an isolated mid-shelf carbonate bank, which formed when the remainder of the shelf was dominated by fine siliciclastic deposition.…”

Early Silurian sea-level changes

“…Within the Much Wenlock Limestone Formation is a more argillaceous, nodular unit (see, e.g. Ratcliffe, 1988;Jeppsson, Aldridge & Dorning, 1995, fig. 2), perhaps indicative of a slight rise in sea-level.…”