Chapter 4: Glaciogenic sediments and structures in the Port Askaig Formation on the Garvellachs and Islay

In this Chapter, we conduct a thorough reappraisal of evidence for glaciation in the Port Askaig Formation (PAF) based on comprehensive documentation of all exposures on the Garvellachs and parts of Islay. Diamictites and associated facies record a range of terrestrial, lacustrine and marine environments. Most massive diamictites on the Garvellachs contain structures indicating shear beneath grounded ice, including ‘stoss-lee’ clasts, strong preferred a-axis orientations, glaciotectonism of subjacent sediments, and streamlined and folded rafts, and are interpreted as subglacial traction tills and glaciotectonites. The Great Breccia on the Garvellachs and the Persabus Complex on Islay consist of folded and thrust megablocks and diamictite, interpreted as the product of proglacial glaciotectonic deformation in which permafrozen foreland materials were excavated and upthrust before being over-ridden. The PAF also contains intervals of laminated mudstones with dropstones, stratified diamictites, and interbeds of sandstone and conglomerate, interpreted as the products of subaqueous deposition in glaciolacustrine and glaciomarine environments. Glaciomarine deposition is most spectacularly recorded in the ‘Disrupted Beds’, including subaqueous fans, submarine landslides, and rain-out of ice-rafted debris. Glaciogenic facies associations exhibit systematic evolution through time, revealing the interplay of climate, glacier advance-retreat cycles and sea-level change over the course of the Sturtian panglaciation.