Stratigraphical relationships and sedimentary environments of the silurian—early Old Red Sandstone of Pembrokeshire

Abstract

In Pembrokeshire, northward Hercynian thrusting has brought together Silurian and Old Red Sandstone rocks of widely different stratigraphical and sedimentological successions. Five structurally separate blocks are centred from south to north on Freshwater, Marloes, Winsle, Rosemarket and Haverfordwest. Facies, current indicators and stratigraphical relationships suggest a southerly sourceland during the Silurian with a northward deepening and thickening of facies, except in the Rosemarket Block, which formed a positive horst structure until Middle Llandovery times. Petrography shows persistent dominance of an acid-intermediate igneous source throughout Silurian in all structural blocks. This points to a south-westerly continental area with rocks of this composition undergoing erosion. Old Red Sandstone floodplains spread into the area from the south in the southern blocks, as the basin filled, while in the northern blocks uplift, tilting and erosion preceded Old Red Sandstone of northerly derivation. Petrography of all Old Red Sandstone deposits indicates a source compositionally identical to the Silurian. Comparison of Pembrokeshire in the Silurian with the present Bering Sea continental margin suggests an analogous structural setting, on the margin of a Japan Sea-type basin. A south to north section in Pembrokeshire represents a section from northward deepening continental shelf deposits of the Freshwater, Marloes and Winsle Blocks, including alkaline volcanics, to shelf edge, continental slope and rise deposits of Rosemarket and Haverfordwest.