Upper Old Red Sandstone (Farlovian) Paleogeography in South Wales and the Welsh Borderland

Abstract

ABSTRACT The Upper Old Red Sandstone, reaching a maximum thickness of about 1200 feet, rests with a marked depositional break on gently folded Lower Old Red Sandstone and Silurian rocks. Two major sedimentary cycles are represented in the sequence of the Upper Old Red Sandstone through to the Tournasian (Carboniferous) Lower Limestone Shales. In each cycle alluvial deposits graduate up into marginal-marine strata. The alluvial deposits, arranged locally in fining-upwards cycles, are chiefly cross-stratified or flat-bedded sandstones and conglomerates, and subordinate siltstones with proofs of exposure. Compared with modern sediments these lithologies appear to represent stream channel and floodplain sedimentary environments. The marine strata following in each major cycle are fossiliferous san stones, siltstones, and shales comparing with the deposits of modern coastal barrier complexes. Post-dating the Middle Devonian phase of the Caledonian Orogeny, the Upper Old Red Sandstone accumulated on a coastal plain bearing comparison with that of the Gulf of Mexico, which lay between uplands (St. George's Land) in the north and a newly created deep-water marine trough in the south. Cross-stratification and regional sandstone petrology suggest that several rivers, each with a distinctive drainage basin geology, constructed the plain and that these rivers flowed from north to south, off the uplands. The plain experienced two marine transgressions from the south during Upper Devonian and early Carboniferous times, each relative rise of the sea leading first to alluviation.