The arenig in South Wales: Sedimentary and volcanic processes during the initiation of a marginal basin

Marine clastic sediments and volcanics of Arenig (Ordovician) age crop out in South Wales. These were deposited after presumed late Tremadoc erosion and subsequent arc volcanicity. Arenig sedimentation was transgressive, and followed significant erosion of the arc volcanics. Arenig conglomerates, sandstones, and mudstones were deposited in deltaic and turbiditic systems. Storm and tidal processes influenced the shallow marine deposits. The minor rhyolitic volcanics extruded during the Arenig reflect the development of Ordovician marginal basin-type volcanics across Wales. Five sandstone petrofacies are defined and reflect differing proportions of these volcanics and of Cambrian and Precambrian basement material. Sedimentation patterns were controlled by intra-Arenig tectonism during an overall rise in sea level. Facies and petrofacies were ponded in small, interconnected, marine sub-basins. Earliest Arenig tectonism and sedimentation, also recognized in North Wales, reflects the initiation of a marginal basin in Wales.