Abstract

Six contrasting alluvial sequences are described from the Lower Old Red Sandstone (ORS) of Central Scotland. These are distinguished on the basis of a combination of structural, textural, compositional and paleocurrent criteria. They suggest that the development of ORS basins may have been more complex than has previously been recognized. The presence of a flanking Caledonian mountain chain, implicit in many earlier interpretations, cannot now be reconciled with many features of ORS sedimentation. Initially, a series of small, strike-slip controlled basins were generated along a suture now concealed by the post-ORS Highland Boundary Fault. These early basins received large volumes of coarse recycled detritus from both the north and south, implying that source terrains in these regions were blanketed by pre-existing gravels. Those recycled conglomerates, dispersed from a source to the north, were emplaced by braided rivers of a scale sufficiently large to suggest they represented antecedent rivers captured by the basins. In the east first-cycle detritus was shed from a cryptic source within the Midland Valley. The later evolution of the Strathmore Lower ORS saw the development of a much enlarged basin which appears to drape the earlier basins. This was largely filled by sediment associated with a major axial river system whose scale implies a drainage basin that must have extended far outside Scotland and may have included the equatorial mountain belt produced by Scandian collision. Local first-cycle fans marginal to this axial system were fed by rejuvenation of the flanking metamorphic basement to the northwest. Coeval volcanoes were one of the most important sources of detritus throughout Lower ORS sedimentation. The strike-slip regime which produced early Lower ORS basins in Central Scotland may have been the inboard response to continuing oblique accretion along a destructive continental margin which lay some distance to the south.