Glaciomarine model for upper Precambrian diamictites of the Port Askaig Formation, Scotland

Descriptive techniques (lithofacies codes) and vertical profile analysis emphasizing lithofacies contacts and geometry allow objective characterization and comparison of diamict(ite) sequences irrespective of geologic age. Upper Precambrian diamictites (?650−600 m.y.) of the Middle Dalradian Port Askaig Formation in Scotland are traditionally interpreted as products of repeated ice-sheet advances over an emergent shallow marine shelf and deposition by in situ basal melt-out. The Dalradian sequences are, however, fundamentally different from modern and Pleistocene diamicts deposited by either lodgment or basal melt-out and fail to show evidence of tractional processes associated with basal ice movement. Scottish diamictite sequences are similar to assemblages of massive and stratified diamict lithofacies, of late Pleistocene age, deposited subaqueously below floating ice and recently described from the Lake Ontario Basin of North America. A glaciomarine depositional model may have implications for the origin and paleogeographical setting of other upper Precambrian diamictites of similar age and stratigraphic context in East Greenland, Spitsbergen, and Scandinavia.