Acritarchs in polydeformed and highly altered Cambrian rocks in western Victoria

Abstract

The paucity of microfossils in the Cambrian volcanic and sedimentary succession west of the Avoca Fault in western Victoria is generally attributed to their low preservation-potential in the polydeformed and highly altered early Palaeozoic host rocks. The resulting lack of biostratigraphic age control has made temporal correlations between Cambrian successions difficult, leading to a relatively poor understanding of the Cambrian palaeogeographic and palaeotectonic evolution of the region. Here, we present evidence of a diverse range of acritarchs and a rare possible conodont from siliceous mudstone (chert) of the middle Botoman (ca 515 Ma) Albion Formation at Stawell and the middle Middle Cambrian (ca 505 Ma) Glenronald Shale Member near Mt Stavely. Acritarchs are polyphyletic organisms that include the vegetative and resting cysts of unicellular protists, mainly phytoplankton, which underwent rapid diversification during the Early Cambrian. The morphologically heterogeneous populations of Early Cambrian acritarchs enable them to be used as biostratigraphic indicators. Therefore, the discovery of several populations of acritarchs in successions that have relatively good age control represents an important advance, because these fossils may be used to assist with temporal correlations of successions in southeastern Australia. Moreover, the well-preserved nature of these microfossils means that a variety of biota may be preserved in chert in highly deformed and altered Cambrian successions elsewhere in East Gondwana that currently have poorly constrained ages.