Abstract Detailed mapping of 150 km2 area of pediment south of Glenormiston Station Homestead, western Queensland has shown that the lower Upper Cambrian Georgina Limestone comprises over 400 m of interbedded limestone and clastic lithologies. Limestone accounts for some 30% of the formation and constitutes two distinct facies: abundantly fossiliferous, pellet‐rich sandy limestone, thought to have formed under shallow subtidal conditions, and sparsely fossiliferous silty limestone considered to be of supratidal and intertidal algal‐mat origin. The Sun Hill Arkose previously thought to underlie the Georgina Limestone unconformably is uncon‐formable upon it and likely to be of Mesozoic age or younger. Biostratigraphic analysis of the upper Georgina Limestone has revealed a complete faunal sequence for the lower Upper Cambrian Idamean Stage. Five Idamean trilobite assemblage zones are recognized, of which the zones of Proceratopyge cryptica and Stigmatoa diloma are new. A composite sequence, termed the Brown...