Neoproterozoic glaciation in Scotland: the Port Askaig Formation

David Webster

The Port Askaig Formation (PAF) in the Garvellach Islands and Islay is some 1100 m thick and includes 48 diamictites. Many were deposited by grounded ice; a few were ice-rafted. The PAF records 76 climatically-related episodes: 28 glacial, 25 periglacial and 23 non-glacial, and also unusual iron-rich and glaciotectonic intervals. Amongst Cryogenian glacial successions, the PAF is exceptional in its combination of formation thickness, the number of climatically-related episodes and the thickness (25km) of its host supergroup.

PAF studies started with MacCulloch (1819); Thomson (1871) proposed a glacial origin; Pitcher and Shackleton (1961) measured the strata in the Garvellachs, leading to Tony Spencer’s 1971 Geological Society of London Memoir (#6). A large multi-disciplinary team is now preparing a new memoir. Its recent work has led to the proposal that the base of the PAF on Garbh Eileach be a candidate GSSP (Golden Spike) for the Cryogenian world-wide. Evidence for and against a “Snowball Earth” during PAF times will also be discussed.

David graduated with a geology degree from Oxford University in 1976 and then worked in the oil industry. He undertook a master’s in environmental management from Strathclyde University and subsequently worked in local government in Scotland. On retiring, he obtained a master’s degree on the geology of Islay from Stockholm University and has continued to research the geology of Islay, Jura and Colonsay. He has written two guidebooks on the geology of these islands.