Abstract

The Yudnamutana Subgroup is a thick succession of upper Proterozoic (Sturtian) glacigenic rocks preserved in the Yudnamutana trough in the northeastern part of the Adelaide "geosyncline." In ascending order, the subgroup comprises the Fitton, Bolla Bollana, and Lyndhurst Formations. The name "Hamilton Creek Member" is proposed for a locally developed conglomeratic facies at the base of the Fitton Formation.

These rocks are interpreted as the products of two glacial advances. The first is evidenced by the Hamilton Creek Member, which includes sediment gravity flows derived from subaqueous glacial outwash. The Fitton Formation is composed mainly of basinal siltstones and mudstones with associated coarser-grained rocks that are largely sediment gravity flows. Diamictites in the upper part of the formation signify the onset of a second glacial advance. Thick diamictites of the Bolla Bollana Formation are interpreted as rainout deposits from floating ice derived from ice streams debouching from a steep and high hinterland into a fault-controlled basin. The Lyndhurst Formation is mainly fine-grained rocks. Dropstones and sparse diamictites attest to the presence of some floating ice, but some of the diamictites are probably mass-flow deposits. Paleocurrent data from the Yudnamutana Subgroup indicate transport to the west-northwest. The Sturtian glacigenic rocks are overlain by a transgressive shale.

Stratigraphic similarities exist between the Yudnamutana Subgroup and thinner successions to the west. These stratigraphic similarities probably reflect paleoclimatic changes. The twice-repeated sequence of diamictite followed by mudstone is attributed to two glacial advance-retreat cycles. Local variations in thickness and stratigraphic succession are probably related to contemporaneous faulting.

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