Indicators of relative completeness of the glacial record of the Port Askaig Formation, Garvellach Islands, Scotland

The Port Askaig Formation (PAF) is a diamictite-bearing succession in the Dalradian Supergroup of Scotland that provides an excellent archive of a Cryogenian glaciation in the Garvellach Islands and Islay, Argyll. The formation is ∼1100 m thick, comprises 5 members and includes 47 diamictite beds, interbedded with siltstones, dolostones and sandstones. Here we document seven features of the PAF that indicate its relative stratigraphic completeness. There are gradual, progressive changes up-section in the lithologies of the diamictites, their interbeds, and clast lithologies. The sharp basal surfaces of the diamictites each show the same, repeated pattern of environmental change, from non-glacial to glacial. Many of the top surfaces of the diamictites show evidence of periglacial conditions. The succession in the PAF records a total of 76 climatically-related stratigraphic episodes: 28 glacial episodes, 25 periglacial episodes and 23 non-glacial episodes. Parts of Member 1 (Diamictites 1–12 and Diamictites 16–18) and Member 2 (Diamictite 31 to the base of Member 3) are most compete on the east coast of Garbh Eileach. The PAF in the Garvellach Islands occurs within a succession that is several kilometres thick, as newly revealed by sea-floor mapping. Compared with other Cryogenian and Phanerozoic glacial successions, the PAF is exceptional in its combination of formation thickness, the number of climatically-related stratigraphic episodes, and the considerable thickness of its host supergroup. Furthermore, these indicators of relative stratigraphic completeness provide evidence that the base of the PAF on the east coast of Garbh Eileach is a succession without a major break in deposition, supporting the account of the strata at and below the base of the PAF in the companion article by Fairchild et al. (2018)