THE FACIES ASSOCIATION TILLITE, BOULDER BEDS, BOULDER PAVEMENTS, LIQUEFACTION STRUCTURES AND DEFORMED DRAINAGE CHANNELS IN THE PERMO-CARBONIFEROUS DWYKA GROUP, ELANDSVLEI AREA, SOUTH AFRICA

摘要

The Elandsvlei area is situated in the western part of the glaciomarine Karoo basin. A previous investigation established the stratigraphic succession of six cyclic units. Each cycle consists of a basal tillite capped by a retreat facies association. The basal tillite units are characterized by the recurrence of the facies association tillite, boulder beds, boulder pavements and deformed drainage channels. In some basal tillite units liquefaction structures are present. Understanding this facies association in glaciological context is the purpose of this investigation. Systematic follow-up fieldwork focusing on relevant field relationships, was conducted. The observations were compiled and interpreted utilizing the more recent advances made by the theoretical modeling of subglacial groundwater flow and drainage. The facies association is shown to have common cause with respect to the subglacial deforming-bed process. Evidence is presented showing that boulder pavements form at the ice-bed interface by the abrasion of hard rock on hard rock. A simple shear model is utilized to show that deforming-bed shearing can produce displacement without the development of discrete, visible slip surfaces. The deformation of drainage channel-fills is interpreted as the manifestation of subglacial shearing. The liquefaction structures are explained by a water pressure differential between the ice-bed interface and the boulder beds. Sticky spots that may have been streamlined as drumlins or mega-scale glacial lineations, are related to variable drainage development along one particular stratigraphic horizon. The boulders' composition indicate that the onset area of the Dwyka glaciers, due eastwards, is underlain by a Neoproterozoic igneous province now covered by younger platform sediments.