Catastrophic mass failure of a Neoproterozoic glacially influenced continental margin, the Great Breccia, Port Askaig Formation, Scotland

The Great Breccia is a thick (over 50 m) diamictite that occurs close to the base of the Neoproterozoic Port Askaig Formation in Scotland, and contains matrix-supported clasts up to 100 m across in a poorly sorted dolomitic matrix. Detailed logging and mapping of cliff and shoreline exposures through the Great Breccia on the Garvellach Islands show that it is not a simple ‘megabreccia’ but consists of three sedimentary units including a megabreccia (Unit 1), diamictites interbedded with dolomitic conglomerate, sandstone and mudstone (Unit 2) and a diamictite with metre-sized dolomite clasts (Unit 3). The sedimentological characteristics of the Great Breccia indicate that it formed by a variety of subaqueous sediment gravity flow processes, including matrix-rich debris flows and turbidites. Clasts up to 100 m across contained within the megabreccia unit record catastrophic collapse and failure of local calcareous siliciclastic deposits; other diamictite units were deposited by debris flows. Conglomerates were deposited from high-concentration turbulent flows, and sandstones and fine-grained facies also exhibit characteristics typical of turbidites. Continued downslope remobilization of sediment in the basin following deposition of the Great Breccia is recorded by overlying deformed diamictites, detrital dolomites and siltstones of the Disrupted Beds. The Great Breccia and associated facies within the lowermost part of the Port Askaig Formation are similar to those found in carbonate megabreccia successions that record catastrophic mass failure of carbonate platforms. Failure events in the Port Askaig Formation most likely relate to localized faulting and subsidence of the Dalradian Basin. Although the Port Askaig Formation is considered a Neoproterozoic ‘glacial’ succession, there is little direct evidence for glacial influence on sedimentation in the Great Breccia, making it difficult to fully assess paleoclimatic conditions.