Abstract

A petrographic study of acetate peels made from approximately 500 carbonate rock samples from the early Middle Cambrian Wirrealpa and Aroona Creek Limestones led to the identification of nine major rock types. Carbonate grains include well-rounded ooids, oncolites, intraclasts, peloids and fossil fragments. Textures range from grainstones to mudstones. Nodular and semi-nodular layers, regularly interbedded with thin, wavy, calcareous silts, are common throughout. Two megafacies, composed of eight facies, were identified in most of the 22 measured sections. Interpreted depositional environments for these two megafacies arc--(1) an ooid bank area close to the oceanward margin of a shallow intracratonic platform, characterized by mobile ooid shoals intertonguing between areas of lesser turbulence, and (2) a shallow, marine lagoon, behind the ooid shoals characterized mainly by less turbulent depositional conditions, calcareous silts and stromatolites. The detailed analysis of facies in one outcrop area provides a clear record of a marine transgression-regression cycle. Extrapolation to four other outcrop areas enables tentative paleogeographic reconstructions for the whole Arrowie Basin to be hypothesized. These suggest that the transgression came from and probably retreated to the northern areas of the basin.

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