Early Cambrian acritarch assemblage zones in South Australia and global correlation

Abstract

Lower Cambrian sediments in the Arrowie and Stansbury Basins were deposited on a rifting continental platform and contain abundant acritarchs, trace fossils, small shelly fossils, archaeocyaths, trilobites and brachiopods. Generally, faunas and acritarchs are abundant in transgressive and highstand deposits, but less common in lowstand fluvial deposits, responding both to sedimentary environments and preservational conditions. Seven acritarch Assemblage Zones are recognised in the lowest Cambrian to Toyonian sediments of South Australia. Assemblage Zone 1 embraces the Precambrian-Cambrian boundary, but a dramatic increase of spinose acritarchs (Assemblage Zone 2) is observed around 1 m above the first appearance of the trace fossil Rusophycus. Assemblage Zone 3 occurs in transgressive siltstones during deposition of the uppermost Parachilna Formation and is marked by the first appearance of several taxa, which are widespread and well correlated in South Australia. Assemblage Zones 4, 5 and 6 are dominated by Skiagia; the appearance of S. ornata followed by S. ciliosa in Zones 4 and 5, respectively, indicates correlation with the nominal acritarch zones on the East European Platform and concurrent Schmidtiellus mickwitzi and Holmia kjerulfi trilobite zones, and the Qiongzhusi Formation in southern China. Assemblage Zone 7 encompasses a major acritarch decrease in South Australia during the late early Cambrian regression and 'red-beds' deposition. Continuous occurrences of acritarch genera Skiagia, Ceratophyton and Corollasphaeridium during the early Cambrian provide valuable evidence of evolutionary development, and therefore are reliable for biostratigraphic zonation. The integration of biostratigraphy and unconformity-bounded sequence stratigraphy provides more precise intercontinental correlation.