The Conditions of Growth of the Wenlock Reefs of Shropshire (England)

Abstract

SCOFFIN, T. P., 1971. The conditions of growth of the Wenlock reefs of Shropshire (England).Sedimentology, 17: 173-219.Distribution and size of the reefs in the Wenlock Limestone (Mid-Silurian, Wenlock area) indicate that the most suitable site for growth was at the seaward fringe of the reef belt. The reef development increased through time as clay-mineral concentration reduced. Many reefs colonized small mounds of coarse crinoidal debris but growth also started on soft muddy substrates. The Wenlock structures have similar characteristics to modern patch reefs: they are small-the average width is 12 m and the thickness 4.5 m-and roughly oval in section. During growth reef surfaces were gently convex and reached heights of between 50 cm and 3 m above the sea bed.The organic framework of all reefs examined consisted of massive and branching tabulate corals, stromatoporoids, branching rugose corals and bryozoans; with stromatolites, laminar corals and stromatoporoids and bryozoans playing the role of reef binders. Most reef builders grew contiguously and are preserved in growth position. Biotic zoning within reefs was not detected. Biomicrites accumulated in interstices near the surface of the reef at the time of, and just shortly after, organic growth. The sea was probably less than 30 m deep and extreme shallowing at the close of the Wenlockian terminated reef development. However, reefs were not broken up by wave or current action. The inter-reef sediments, which were deposited at the same rate as reef growth, are predominantly argillaceous crinoidal biomicrites alternating with thin shale bands. Very few beds of reef-derived talus occur ( Fig.13-15). High influxes of clay mineralsespecially bentonites-brought about local reef death.Diagenetic processes of dissolution, recrystallisation and compaction greatly modified the original fabric of the reefs.

Carbonate depositional environments in the late Wenlock of England and Wales

“…Shergold & Bassett (1970) named eight lithofacies in the formation around Wenlock Edge, using old quarrymen's terms (Table 1), but carried out no detailed petrography and made no environmental interpretations. From the same area, Scoffin (1971) described six lithofacies (Table 1) and produced a model in which the large reefs around Hill Top (Wenlock Edge) formed at the seaward edge of a platform, where conditions were particularly favourable for reef growth. The remainder of the formation in the West Midlands and southern Welsh Borders, with its smaller reefs, was thought to have been deposited coevally in a back-reef setting (Scoffin, 1971).…”

“…From the same area, Scoffin (1971) described six lithofacies (Table 1) and produced a model in which the large reefs around Hill Top (Wenlock Edge) formed at the seaward edge of a platform, where conditions were particularly favourable for reef growth. The remainder of the formation in the West Midlands and southern Welsh Borders, with its smaller reefs, was thought to have been deposited coevally in a back-reef setting (Scoffin, 1971). Biostratigraphical evidence (Bassett, 1974(Bassett, , 1976(Bassett, , 1989 has since suggested that the formation is diachronous, Cocks & Fortey, 1982, with data from Soper &Woodcock, 1990 andBassett et al in Cope, Ingham &Rawson, 1992); the stippled parts indicate land.…”

Modes of origin and genetic pathways of peloids within the Duwi Formation (late Cretaceous) in Eastern and Western deserts of Egypt: A petrologic perspective

“…Similar cases can be found in many Phanerozoic reefs. Encrusting stromatolites with clotted and pelleted fabric are common in Silurian reefs of western England (Scoffin, 1971). Crusts interpreted as in situ microbial precipitates on corals in late Jurassic reefs of southeast England by Sun and Wright (1989), closely resemble the studied peloidal aggregates.…”

Cavities in the reefs of the Wenlock Limestone (Mid-Silurian) of Shropshire, England

“…The structure and conditions of growth of these reefs have been described elsewhere (ScoFFIN, 1971). In this account the term cavity refers to any part of the reef rock which was at some time a fluid-filled void in the solid reef structure.…”