Structure of some Lower Old Red Sandstone conglomerates, Kincardineshire, Scotland: deposition from late-orogenic antecedent streams?

Abstract

A thick succession of early Lower Old Red Sandstone (ORS) conglomerates and subordinate sandstones are preserved in a small fault-bounded basin in the NE Midland Valley of Scotland. The conglomerates associated with the northern margin of this basin are distinguished by a complex provenance which involved both reworking of existing gravels and addition of contemporary volcanic and plutonic detritus. This may explain their characteristic bimodal texture. Deposition was in large, probably permanent, low sinuosity channels with evidence for both periods of uniform high stage flow and repeated flow stage fluctuations. Variable palaeoflows in the basal part of the succession suggest deposition on wet-type alluvial fans supplied by feeder channels with a high sediment and water discharge. The thickness of the succession, the poorly developed cyclothems (despite active syndepositional faulting), evidence for rapid aggradation, and the inferred scale of channel bars and coarsening-upward units produced by bar migration suggest the basin may have been externally drained. Transverse antecedent streams are invoked to account for the structure of alluvium. During the early Devonian, central Scotland lay close to the boundary between regions suffering Scandian orthogonal shortening and Acadian strike-slip shear. The tectonic controls which governed the preservation of early Lower ORS sediments may have been largely uncoupled from those determining the supply and dispersal of the sediment.