A Comparison of Clast Fabric and Shape in Late Precambrian and Modern Glacigenic Sediments

Abstract

ABSTRACT Northeastern Svalbard contains a well-preserved glacigenic sequence of Vendian (Late Precambrian) age in which terrestrial, lacustrine, intertidal, and marine glacial facies are represented. Diamictite facies are of two main types, massive and weakly to strongly bedded. Diamictite field appearance and relationships with other facies, however, provide insufficient information to allow a clear interpretation of the precise mode of glacial deposition. By combining this information with data concerning the fabric of clasts in modern glacigenic sediments (where the mode of glacial deposition is better known), a wider understanding of the processes of glacial deposition in Vendian time is obtained. Several diamictite samples were interpreted to be of lodgement tillite facies, and material o water-laid and glacigenic sediment flow origin was also recorded. Clast shape analyses indicate that most glacial debris was transported, at least for a time, at the glacier bed. Little angular supraglacial debris was present, implying that the ice mass was of ice-cap or ice-sheet dimensions, with few subaerial rock exposures. Clast shape in the Late Precambrian deposits was not useful in distinguishing between the variety of lithofacies derived from basally transported glacial material. Too few systematic studies of clast striation in modern glacigenic sediments are yet available for this criterion to be helpful in distinguishing between tillite facies. Lithofacies analysis, together with clast fabric and shape studies, provide useful and complementary lines of evidence concerning the ode of deposition of ancient diamictite sequences. An understanding of the depositional processes and patterns of modern glacial sedimentation provides a fundamental framework for such studies.