Subglacial and proglacial glacitectonic deformation in the Neoproterozoic Port Askaig Formation, Scotland

The recognition and interpretation of glacigenic sediments in the ancient rock record has important implications for understanding processes and patterns of climate change. Diamictite facies from the Neoproterozoic Port Askaig Formation, Scotland, have been variously interpreted as evidence for grounded glacier ice, mass-movements on a glacially influenced submarine slope, and subaqueous mass movements in a non-glacial environment. These contrasting interpretations imply widely differing degrees of ice cover, and have contrasting implications for Neoproterozoic (“Snowball”) glaciations. In this paper, we focus on the Great Breccia and the Disrupted Beds, two controversial units in the Port Askaig Formation. We present evidence that they were formed by the proglacial and subglacial deformation, respectively, of pre-existing shallow marine facies during an advance of grounded ice. Quaternary examples provide remarkably exact analogues for the Great Breccia and Disrupted Beds, illustrating the potential value of applying litho- and tectono-facies models to ancient sedimentary successions.