Chapter 2: Geology of the Garvellachs region and a ‘standard’ stratigraphic column for the Port Askaig Formation

The Port Askaig Formation (PAF), the lowest formation in the Argyll Group of the Dalradian Supergroup, crops out at 38 main localities along a 700 km SW-NE trend from western Ireland to NE Scotland. It is best preserved and thickest in NE Islay and the Garvellachs. A composite ‘standard’ stratigraphic column combining those localities measures ∼1100 m, contains five members and 48 diamictite beds and forms a template for information from the other 36 localities. Bare rock outcrops on the raised rock platform around the Garvellachs provide most of the sedimentological information in Chapters 3 to 10 of this Memoir. Multibeam data around the Garvellachs show the bathymetry and together with four seafloor samples allow construction of a preliminary map of the offshore geology. The map shows that the outcrops of the PAF on the islands continue as ridges for 8 km to the southwest, whereas to the northwest the seabed topography suggests that the Dalradian section is ∼1.5 km thick beneath the PAF. Almost 100 sandstone dykes are seen in the Garvellachs; they occur in swarms in three trends and were intruded after compaction and regional tilting but before diagenetic cementation.