Sedimentological and micromorphological examination of a Late Devensian multiple diamicton sequence near Moneydie, Perthshire, east-central Scotland

Abstract

Synopsis Re-investigation of a site at Shochie Burn near Moneydie in east-central Scotland suggests that earlier interpretations need to be modified. At this site, in the past, two tills were recognized as indicative of two separate ice advances. Previously, this site was known as a stratotype for the Perth Readvance in eastern Scotland. The evidence for a regional readvance is no longer accepted but the two-till problem remains unexplained. Present investigations suggest that the two tills are part of a large package of sediments deposited as stacked layers of a subglacial deforming bed that have been tectonized by a very short and local ice readvance perhaps of the type that occurs as a winter readvance. Sedimentological and micromorphological evidence points to the tills as not being separate but of the same depositional phase undergoing subglacial deformation and/or local marginal tectonization following subglacial transport in a deforming bed environment resulting in the incorporation of proximal outwash sediments. The till at Shochie Burn exhibits evidence, both at the macro- and microscale, of intense deformation under high porewater pressures that are attributable to the strong plasmic microstructures, fold structures, shear zones, Riedel shears, and of possible submarginal freezing conditions resulting in localized brecciation and foliation structure development. This is the first site in this area to indicate the presence of subglacial deforming bed conditions followed by marginal glaciotectonism, therefore continued research is necessary to establish this modified view of subglacial conditions in this part of Scotland.