GIANT CROSS-BEDDED SANDSTONES IN THE NEOPROTEROZOIC PORT ASKAIG FORMATION, SCOTLAND: PALEOGEOGRAPHIC IMPLICATIONS

The Neoproterozoic Port Askaig Formation consists of a thick succession of interbedded diamictites, sandstones, conglomerates and mudstones, which record marine and glacially-influenced marine sedimentation in the extensional Dalradian basin. This glacigenic succession is subdivided into five members defined by changes in lithological characteristics. Member III is characterized by thick units of sandstone (up to 80 m in thickness) interbedded with diamictite.

Giant cross-bedded sandstones occur within the lower portion of Member III. Cross-bed sets are up to 11 m in thickness and contain foresets with both planar and trough-shaped lower bounding surfaces. Internally, they consist of simple and simple-compound second order sets, with foresets bounding concordant and discordant internal bedding. Foresets have a dip angle of approximately 20 degrees with a predominant paleocurrent direction towards the south. Lithologically, the sandstones are medium-grained, well sorted and quartzitic. Giant cross-bedded sandstones are associated with horizontally laminated sandstones, massive sandstones, trough cross-bedded sandstones, diamictites and conglomerates.

These giant cross-bedded sandstones are interpreted to have accumulated as a result of the migration of large subaqueous simple and compound dunes on a marine shelf. Such large dunes can be generated by either tidal or geostrophic currents and indicate open marine, ice-free depositional conditions. The close association of giant cross-bedded sandstones with diamictites suggests that conditions were repeatedly ice-free during accumulation of the Port Askaig Formation and not completely ice-covered as proposed by the Snowball Earth hypothesis. The presence of large subaqueous dune systems in the early Dalradian basin also suggests that the basin was quite extensive and was periodically supplied by large amounts of sand, possibly from fluvial/deltaic sources.