|Peripatus Home Page Paleontology >> Silurian Period >> Wenlock Limestone||Updated: 18-May-2017|
AbstractThe Much Wenlock Limestone Formation of Wales and the Welsh Borderland preserves one of the most diverse, and well-preserved fossil assemblages known. More than 600 species of invertebrates have been recorded, including crinoids, corals, brachiopods, trilobites, algae and bryozoans. The formation was deposited during the early Silurian when this area of Britain was covered by a relatively warm, shallow shelf sea.
The Much Wenlock Limestone is exposed in Shropshire and the Welsh borderlands, cropping out continuously from Ironbridge in the northeast to Ludlow in the southwest, forming the conspicuous escarpment of Wenlock Edge, Shropshire. Visitors can access it easily from the Wren’s Nest Nature Reserve in Dudley, Shropshire, and several public walks (in the order of 3 km) are maintained by the National Trust.
It mostly comprises alternating grey nodular limestones and softer shales. The nodular limestones often pass laterally into more tabular limestones made up of shell debris. (After Toghill 2006, p. 132.) The limestone has been quarried at least since medieval times, and some have been restored and can be seen today (e.g., on the ‘Lime Kiln Walk’ from Presthope).
“As well as reef-building animals, brachiopods, gastropods and trilobites lived in this reef environment … and are found fossilized today within the reefs, and also in the material which broke off during growth and rolled down the sides of the reef to form apron deposits” (Toghill 2006, p. 133).
The Caledonian Orogeny and associated closure of the Iapetus Ocean during the Silurian resulted in a steady shallowing of the seas overlying Shropshire. At the same time, the tectonic unit which carried southern Britain was drifting north into the southern subtropics. Consequently, the Much Wenlock Limestone was laid down on a continetal shelf, the Midland Platform, in shallow, warm water, similar to the present-day Bahamas.
“Within the nodular and shell limestones occur the famous reef limestones formed … in a shallow (probably less than 10 m deep), warm, clear subtropical sea, similar to parts of the Carribean today. By the middle Silurian, Wenlock Epoch … southern Britain had moved to 20 degrees south of the equator, a similar latitude to present-day Tahiti in the south Pacific. The reefs are referred to as patch reefs that grew as individual patches on the shallow seabed. They are not part of a barrier reef … nor are they fringing coral atoll reefs” (Toghill 2006, p. 132-133).
Toghill, P. 2006: Geology of Shropshire (second edition). Crowood Press: 1-256.
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