Facies and palaeogeography of Late Precambrian Sturtian glacial sediments, Copley area, northern Flinders Ranges and in the Sturt Gorge near Adelaide, South Australia

Abstract

West of Copley and Leigh Creek, northern Flinders Ranges, South Australia, late Precambrian diamictite of the Merinjina Formation unconformably overlies dolomite and fine sandstone of an unnamed member of the Burra Group. The Merinjina Formation, 130-310 m thick, consists of diamictite (boulder to pebble bearing sandy mudstone), bedded fine sandstone, laminated siltstone, and conglomerate. Maximum clast length is 1.5 m. Clast types include 95% quartzites, siltstones, and dolomites probably derived from the underlying Burra Group, with a minor fraction of volcanic, plutonic and metamorphic rocks. Dropstones, till pellets and till clasts occur and are considered diagnostic criteria for a glacigene origin. No glaciated floor was recognized. Paleocurrents and paleogeographic reconstructions indicate that sediment probably came from a northeasterly source. In the Sturt Gorge near Adelaide, the Sturt Formation, about 220 m thick, is lithologically similar and probably stratigraphically equivalent to the Merinjina Formation. It overlies thin-bedded fine sandstone and siltstone of the Belair Subgroup disconformably. Clasts are up to 1.5 m in length and consist of 75% granite and gneiss and 25% sedimentary rocks, mostly carbonates. Basement clast types were probably derived from a westerly source. In each area the glacigene formations are conformably overlain by thin dolomites and laminated siltstones of the Tapley Mill Formation. Massive clast-poor diamictites are interpreted as waterlaid tillites deposited from a grounded or floating ice shelf. Bedded siltstones, sandstones, and clast-rich diamictites are interpreted to represent reworked till deposited in a glaciomarine environment with weak bottom currents and periodic incursions of floating ice. Cross bedded granule conglomerates represent deposits from high energy traction currents in glacial meltwater. Local subaqueous mass movement deposits occur.