The GEOLOGY OF BRITAIN by Peter Toghill

Author:Peter Toghill
Language: eng
Format: epub
Publisher: Crowood

The south-west province

Carboniferous Limestone

South of St George’s Land the Carboniferous Limestone of Dinantian age was formed in what is called the south-west province. A warm, shallow subtropical sea gradually spread north over the flanks of St George’s Land, laying down tropical carbonates. Around Bristol and the Mendip Hills the Carboniferous Limestone is between 600 and 800 m thick but has a basal shale unit above the Old Red Sandstone before thick limestones appear. The sequence of limestones is well exposed in the Avon Gorge (fig. 90) around the Clifton Suspension Bridge, and it was here in 1920 that Vaughan set up his zonation scheme based on corals and brachiopods. The Carboniferous Limestone is also well exposed in the Mendip Hills where Cheddar Gorge is cut through the sequences, and caves occur in the soluble limestones. The tropical marine limestones contain corals but not reefs, and oolitic limestones also occur. Oolitic limestones are a unique type of limestone better known from the Jurassic rocks of the Cotswolds. They are made of millions of tiny egg-shaped growths (hence the name) of calcium carbonate, usually less than 2 mm in size, which grew in a warm, shallow sea often close to the shoreline. They can be seen growing today on the Bahamas banks of the Caribbean, and their small size is due to the constant abrasion of one oolith against another in shallow, wave-influenced water.

In South Wales the Carboniferous Limestone occurs as a continuous elliptical outcrop around the edges of the South Wales coalfield (chapter 6, fig. 80) where the rocks are part of the South Wales Syncline. This is a large late Carboniferous (Variscan) fold structure with the productive Coal Measures in the central part. The Forest of Dean is a separate syncline to the east of the coalfield, with a thick Carboniferous sequence. In the eastern area of the South Wales syncline the Carboniferous Limestone is around 300 m thick, with the upper stages of the Dinantian missing because of an unconformity at the base of the Millstone Grit. Further west, in the famous area of the Gower Peninsula and west into Pembrokeshire, up to 800 m of massive limestones occur following on from the Upper Old Red Sandstone without a break. The base of the sequence and junction with the Old Red Sandstone is well exposed in southern Pembrokeshire, around Skrinkle Haven west of Tenby. All the Dinantian cycles and mesothems can be identified, and it is apparent that some dolomitisation occurred between cycles. Northwards in Pembrokeshire the Carboniferous shows a thinning of the sequence towards the positive feature of St George’s Land.

The most northerly occurrence of Carboniferous Limestone associated with the south-west province is found in southern Shropshire on Titterstone Clee Hill near Ludlow. Here the basal beds are shales of Courceyan age (the lowest Dinantian) and are followed by 50 m of limestones of the Chadian Stage. No higher Dinantian rocks occur, and the limestones are followed by Millstone Grit-type sandstones and Coal Measures. This locality was said

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