The Silurian Transgression of a Palaeoshoreline: The Area between Old Radnor and Presteigne, Welsh Borderlands

David C Ray,Oskar Bremer,Anna L Claussen,John Moseley,Helen E Hughes,Arthur C Tingley,Emilia Jarochowska,Jean Borgomano

doi:10.2113/2021/7866176

Abstract

AbstractQuarries between Old Radnor and Presteigne, Welsh Borderlands, expose a Silurian nearshore succession, which onlaps a rocky palaeotopography of the Neoproterozoic basement that had been uplifted along the Church Stretton Fault Zone. The succession documents the Aeronian to Sheinwoodian transgression of an island or islands, with the following sequence of events: deposition of shallow marine sandstones (Folly Sandstone Formation), regional uplift, preservation of a rocky shoreline and associated deposits (Dolyhir Rudite Member), deposition of limestones characterized by a profusion of coralline algae and the abundant remains of reefs (Dolyhir and Nash Scar Limestone Formation), and finally deposition of trilobitic silty mudstones (basal Coalbrookdale Formation). Facies analysis, carbon isotope (δ13Ccarb) values, sequence stratigraphy, and collections of bryozoans, conodonts, thelodonts, and trilobites have been used here as a means of refining our stratigraphic understanding of this unique succession. The revised stratigraphy demonstrates many similarities with the adjoining Midland Platform and the wider Silurian world. Notable features include the globally recognized early Sheinwoodian carbon isotope excursion and sea-level changes of regional and global extent. As one of the best examples of its kind, the palaeoshoreline and nearshore succession of Old Radnor and Presteigne acts as a depositional model for ancient rocky shores worldwide.

Highlights

The area that encompasses the settlements of Old Radnor and Presteigne straddles the border between England and Wales (Herefordshire-Powys border) and is notable for its inclusion in a region for which there is no modern published geological map and accompanying text describing the geology [1]
This is despite the classic geology of the area, which has been important in advancing the development of science through the 19th and early 20th centuries [2]
There is a rare Silurian succession that documents the flooding of the Neoproterozoic basement, which had been uplifted along the Church Stretton Fault Zone (Figure 1)

Summary

Introduction

The area that encompasses the settlements of Old Radnor and Presteigne straddles the border between England and Wales (Herefordshire-Powys border) and is notable for its inclusion in a region for which there is no modern published geological map and accompanying text describing the geology [1]. The aims of this study are to better understand the lithostratigraphy, distribution, age, and depositional environments of the Llandovery-Wenlock succession between Old Radnor and Presteigne; the units investigated are the Folly Sandstone Formation, the Dolyhir Rudite Member, the Dolyhir and Nash Scar Limestone Formation, and the basal part of the Coalbrookdale Formation. These aims have been achieved by geological mapping, the logging of sections which are stratigraphically extensive and contain important lithological boundaries, the investigation of borehole records provided by Tarmac (Dolyhir and Strinds quarries) and the British. As a means of communicating age information, the Aeronian, Telychian, and Sheinwoodian stages have been described and subdivided using internationally recognized stage slices [19, 20] and informally subdivided into early, middle, and late substages (Figure 1)

Folly Sandstone Formation

Dolyhir Rudite Member of the Dolyhir and Nash Scar Limestone Formation

Findings

Lithostratigraphy and Distribution of the Dolyhir and