Abstract
The Neoproterozoic Port Askaig Formation is a thick succession of interbedded diamictite, conglomerate, sandstone and mudstone that records glacially influenced marine sedimentation in the extensional Dalradian basin. Giant cross-beds (individual units up to 11 m thick) are observed in sandstone at the base of Member III of the Port Askaig Formation. The cross-bedded sandstones are closely associated with other sandstone, diamictite and conglomerate facies and are characterized by both planar and trough-shaped foreset laminae with an average maximum dip angle of 14°. The cross-bedding has compound internal structure and indicates a predominant paleocurrent direction towards the south. The sandstone is medium-grained, well sorted and quartzitic. The giant cross-beds are interpreted to have formed as a result of the migration of large subaqueous simple and compound dunes in a marine setting. Based on sedimentological analysis of the sandstone and associated facies as well as the tectonic setting of the Port Askaig Formation, the giant dunes are thought to have formed under the influence of tidal currents and indicate open marine, ice-free depositional conditions. The sedimentology of the cross-bedded sandstone and its close association with diamictite and conglomerate indicate repeated ice margin advance and withdrawal, with periods of both ice-free and ice-influenced conditions. The environmental conditions recorded in these Port Askaig deposits appear inconsistent with ‘deep freeze’ or rapid meltback associated with a snowball Earth.
Published Version
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