Palaeoecological interpretations of South Australian Precambrian stromatolites

Abstract

Abstract Despite the palaeoecological emphasis of recent stromatolite research outside the USSR, controversy still exists regarding the environmental restriction of these organosedimentary structures and some analogies with modern occurrences have been maintained too rigidly. Palaeo‐environments should be determined where possible by a combination of all biological and sedimentological evidence. In South Australia, Precambrian carbonate rocks contain widespread stromatolites. The absence of independent biological environmental indicators necessitates a reliance on general stratigraphic and sedimentological data for the interpretation of regional palaeogeography and local environmental conditions. The Skillogalee Dolomite contains one predominant form of stromatolite almost throughout its extent, and it chiefly grew as biostromes in littoral and lagoonal settings on an extensive, level, carbonate‐depositing platform, probably of restricted access to the open sea. The interspace and associated sediments show evidence of varying energy conditions, degree of oxidation and possibly salinity, but the same stromatolite persists, although there is some modification of morphology in the higher energy environments. The Umbera‐tana Group has basinal shale and siltstone in its lower part, but during a subsequent widespread regression, the marginal parts of the basin were shallowed sufficiently to allow the growth of stromatolites, probably in water depths ranging from shallow subtidal to low supratidal. In addition, in the northern central part of the basin, offshore banks of shallow water limestone may have been related to areas of diapiric activity. The stromatolites of the Umberatana Group most often occur as bioherms of varying sizes intimately associated with ooid and intraclast grainstones, cross‐bedded sandy limestone, and, in marginal areas, penecontemporaneous dolomite. Different forms may occur in sequences of seemingly indistinguishable litho‐facies. These observations strengthen the view that stromatolite morphology is at least partly biologically controlled, the defined taxa being largely independent of the local environment.