Carboniferous Main Limestone of the East Crop in South Wales

Publisher Website
Google Scholar
Abstract
Summary: The East Crop in South Wales, between Blorenge and Risca, displays an incomplete Avonian sequence, Main Limestone in the northern outcrops suffering extinction by Namurian overstep. The Main Limestone, where it occurs, consists almost unrelievedly of secondary dolomites, but in the Afon-Lwyd valley near Blaenafon the strata emerging from beneath the Millstone Grit are in places relatively unaltered and are composed of primary rock types&mainly shelly oolites&closely comparable with those of the Oolite Group of Breconshire. Thicknesses rarely exceed 150 feet, and all the beds appear to belong to lower Tournaisian Main Limestone. A comparable development is found for several miles south of Pontypool. In the southernmost outcrops at Risca a greater thickness (about 360 feet) is preserved. Analogy with the sequence to the south-west suggests it may include upper Tournaisian beds, but the dolomitized rocks have most of their fossils obliterated, so that correlation is uncertain. The Visean stage is nowhere represented. Isopachytes reveal an eastward thinning of the Main Limestone beneath the Millstone Grit at a rate of about 1 in 60 in the north, of about 1 in 12 near Risca. The trend of the isopachytes follows fairly closely the present strike (which between Blaenafon and Pontypool may also have been a depositional strike), and clearly displays the influence of a contemporary prototypic Usk anticline on Namurian overstep.

This publication has 8 references indexed in Scilit: