Shallow subtidal sediments of Soudleyan (Caradoc) age in the Berwyn Hills, North Wales, and their palaeogeographic context

Abstract

Soudleyan rocks in the Berwyn Hills, North Wales, consist essentially of marine mudstones and thin siltstones and fine sandstones, but also include interbedded pyroclastic formations which are at least partly subaerial in origin. In the marine sediments the association of current and wave ripples, small steep sided channels, rare desiccation cracks plus a Planolites , Skolithos, Teichichnus and Cruziana -dominated ichnofauna associated with shallow marine benthic brachiopod-dominated shelly assemblages suggests a shallow subtidal origin with water depths of less than 25 m. Four lithofacies (regular layered, irregular layered, mottled and homogeneous) are differentiated on the basis of their geometry, lithology, sedimentary and biogenic structures and associated faunas, and are described in detail. Comparable subtidal facies extend northwestwards across North Wales into the Arenig-Bala and Snowdon-Tremadoc districts and southeastwards into South Salop, with slightly deeper water argillaceous facies in the intervening Shelve and Breidden Hills areas. Southwest of the Berwyn Hills offshore mudstones are present in the Corris-Cader Idris and Builth-Abbey Cwmhir areas and clastic turbidites in the Cardigan area. A regional palaeogeography for the Soudleyan of the Welsh Basin is proposed.