LOOKING FOR EVIDENCE OF A NEOPROTEROZOIC ‘SNOWBALL' IN THE PORT ASKAIG FORMATION, SCOTLAND

The Neoproterozoic Port Askaig Formation consists of a thick succession (over 700 m) of diamictites, conglomerates, sandstones and mudstones. These deposits record marine and glacially-influenced marine sedimentation in an extensional basin on the margins of the proto-Iapetus ocean and are broadly correlative with other successions thought to have formed under ‘Snowball Earth' conditions. Detailed sedimentological and allostratigraphic analysis of the Port Askaig Formation exposed on the Garvellach Islands allows determination of the extent of glacial influence on sedimentation and the importance of climatic and tectonic controls on succession development.

The Port Askaig Formation was deposited in a variety of marine environments ranging from upper slope settings dominated by sediment gravity flow processes to shallow tidally-influenced settings in which cross-bedded sands accumulated. Diamictite facies formed primarily as subaqueous debris flows, making assessment of the extent of glacial influence on their formation difficult. A significant tectonic subsidence event is recorded by the Great Breccia, a diamictite/megabreccia formed by catastrophic mass failure of a faulted carbonate margin. Other units in the Port Askaig Formation show pervasive soft sediment deformation (e.g. the Disrupted Beds) and may also record seismic activity in the basin.

The vertical succession of facies types in the Port Askaig Formation can be subdivided into distinct allostratigraphic packages, each separated by bounding discontinuities. Allostratigraphic analysis indicates an early episode of abrupt basin deepening, followed by progressive basin infilling and a transition to tidally-influenced marine conditions. Relatively small scale changes in relative sea level, recorded by flooding surfaces and facies changes, are superimposed on this shallowing-upward trend. The Port Askaig Formation allostratigraphy records the interplay between changes in accommodation space, eustatic sea level and sediment supply. Given the limited evidence for glacially influenced sedimentation within the Port Askaig Formation, it is difficult to verify the existence of ‘Snowball Earth' conditions at this time.