Environmental analysis of the Late Precambrian Appila Tillite equivalent at Depot Flat, southern Flinders Ranges, South Australia

Abstract

The Appila Tillite equivalent at Depot Flat near Quorn in the southern Flinders Ranges belongs to the younger phase of Sturtian glaciation recorded in the sediments of the Adelaide Geosyncline. This glacial sequence rests unconformably on a regolith formed on Burra Group sediments and consists of 112 to 146 metres of bedded and massive diamictites interbedded with sandstones and lesser amounts of conglomerates, breccias and laminated shales containing dropstones. Lenticular units of pebbly and non-pebbly ankeritic dolomites are concentrated near the top of the sequence and probably represent deposition in small saline lakes formed during deglaciation. A prominent varve unit containing till pellets and dropstones can be traced laterally across the map area for approximately one kilometre. It occurs near the base of the glacial sequence and records a period of glaciolacustrine sedimentation probably resulting from temporary damming of the area by glacial ice. The marked dissimilarity between closely spaced stratigraphic sections testifies to rapid lateral and vertical facies changes. Evidence from these rapid transitions and the association of diamictites with dropstone laminates, varves, ankerites and coarse clastics suggests that the environment of deposition was proglacial to ice-contact superimposed on shallow marginal marine conditions. The tectonic setting was probably that of a labile shelf thus representing a continuation of this type of setting from preglacial times. Approximately 75% of the clasts were derived from the sediment of the underlying Burra Group and extrabasinal lithologies such as granites, metamorphics and volcanics were probably derived from the basement rocks of the Gawler Craton to the west. The presence of a disconformity separating the glacial sequence from the post-glacial Tapley Hill Formation is suggested by the removal of some of the upper beds in the glacial sequence. The newly discovered volcanic tuff unit which occurs very near to or at the top of the glacial sequence is a useful marker horizon, however, it's presence may be of far greater importance. The similarities in stratigraphic position and lithology with the Beda Volcanics, dated at 1076± 34Ma, suggest a possible correlation. If this correlation proves correct, the age of the Sturtian Tillites would be set back approximately 350 million years.