Intraformational conglomerates and scoured surfaces in the lower old red sandstone of the Anglo‐Welsh cuvette

Publisher Website
Google Scholar
Abstract
Scoured surfaces occur in the upper part of the Lower Old Red Sandstone of the Anglo‐Welsh cuvette between cyclothems consisting of intraformational conglomerate→sandstone→siltstone. The conglomerates are formed chiefly of rocks which can be matched in the deposits of the cuvette, and contain little or no gravel from more distant sources. The scoured surfaces persist laterally and are preserved typically as casts. They discordantly cut earlier beds, and show a variety of discordantly‐filled minor structures (sole marks) due to current scour: flute‐like and lobate forms, current crescents, sack‐like bulges, terraces and small flat‐bottomed channels. Large channels are also known. Scoured surfaces occur also in the Lower Old Red Sandstone amongst sandstones, within which they may delineate major groups of sedimentation units as well as individual units in trough cross‐stratified compilations.The basal features of the cyclothems (scoured surfaces, intraformational conglomerates) record periods when sediments of the cuvette were reworked. This probably took place as the rivers instrumental in deposition of the Lower Old Red Sandstone underwent major changes in their courses, perhaps as the result of crevassing directly onto their floodplains. By reference to modern river sands, the trough cross‐stratified sandstones appear to have accumulated on sandbanks in the stream channels of the cuvette through the action of megaripples.