Lithological correlations of Adelaidean glaciogenic rocks in parts of the Amadeus, Ngalia, and Georgina Basins

Adelaidean glaciogenic sediments, representing two major glaciations, are widespread in three central Australian basins. Recognition of distinguishable marker dolomites, identical to those found above lower and upper tillites in the Adelaide Geosyncline, South Australia, has led to a revision of the Adelaidean stratigraphy of the Amadeus Basin. There the earlier glaciation is represented by the Areyonga Formation. The term Areyonga Formation is here restricted to the previously unnamed lower member (diamictite, conglomerate, sandstone, silt-stone, dolomite) as originally defined in the central-western sections, where it is discontinuous; and corresponds to the whole of what has been mapped as Areyonga Formation in the extreme east. In each area it is capped by a sequence of thinly laminated, dark grey dolomite and silt-stone. The former upper sandstone member of the Areyonga Formation lies with inferred disconformity on various older units. It is probably a non-glacial lateral equivalent of the glaciogenic Olympic Formation (formerly Member) in the east, which records the younger of the two glaciations. The Olympic Formation is overlain by a typical upper marker dolomite, which is pink to buff-coloured, laminated and flaggy. A thin lower tillite, with typical lower marker dolomite, was recognised for the first time in the Ngalia Basin. It lies below a shale sequence beneath the upper tillite (Mount Doreen Formation), which has a typical upper marker dolomite. In the Georgina Basin, the diamictites and varved siltstones of the Field River Beds, recording the earlier glaciation, are capped by an identical lower marker dolomite. Glacials at Central Mount Stuart and at Nancy Hill are of uncertain stratigraphic position, but lithological associations suggest a comparison with the earlier of two Sturtian (lower) tillites in South Australia. These correlations indicate remarkable persistance of certain marker lithologies, and the recurrence of extremely similar lithologic successions, across much of the late Precambrian platform. Presently available isotopic and biostratigraphic data are imprecise and subject to many doubts and qualifications, but are consistent with the view that each glaciation was approximately synchronous in the basins considered. New nomenclature for some upper Adelaidean units in the Amadeus and Ngalia Basins is formalised in the Appendix.