David I Gravestock and John H Shergold

Australian Early and Middle Cambrian Sequence Biostratigraphy with Implications for Species Diversity and Correlation

This description of Lower and Middle Cambrian strata from the Stansbury, Arrowie, Amadeus, and Georgina basins combines elements of biostratigraphy and sequence stratigraphy. The record of some South Australian Lower Cambrian sequences is missing, or has not been recognized, in central Australia. Deposition in the Middle Cambrian of the central Australian basins and the Stansbury Basin reflects subsidence-induced transgression, but these sequences cannot be differentiated in the almost unfossiliferous clastic deposits of the Arrowie Basin. Trace fossil assemblages in basal siliciclastic rocks are most diverse in lowstand half-cycles of relative sea level. Archaeocyath species diversity is highest in transgressive tracts, whereas lowstands are accompanied by extinction on shallow to emergent carbonate shelves. Trilobite species diversity is likewise highest in transgressive tracts but is seemingly unaffected by lowstand conditions. Duration of the Early and Middle Cambrian is 25—35 m.y. and 10-15 m.y., respectively, indicating very high rates of trilobite speciation in successive transgressive systems tracts.

AUSTRALIAN LOWER AND Middle Cambrian sedimentary rocks contain rich assemblages of fossil marine invertebrates, calcified and organic-walled microbial fossils, and traces of organic activity. Knowledge of the taxonomy and affinities of Australian Cambrian invertebrate fossils has increased significantly in the past decade, but at present only the archaeocyaths and trilobites have been studied in detailed strati-graphic successions. Progress is being made in the further study of mollusks and other small skeletal fossils, superbly described by Bengtson et al. (1990).

In this chapter we document the species distribution of archaeocyaths in the Lower Cambrian and trilobites in the Middle Cambrian of the Stansbury and Arrowie basins in South Australia and the Amadeus and Georgina basins in the Northern Territory and western Queensland (figure 6.1). Upper Cambrian trilobite faunas are well preserved

Figure 6.1 Cambrian and undifferentiated Cambrian-Ordovician sedimentary basins of central and eastern Australia. Source: Modified after Cook 1988.

in the Georgina and Warburton basins, but are beyond the scope of this study because correlative strata in the Stansbury, Arrowie, and Amadeus basins have yielded few fossils.

Trace fossils occur in basal Cambrian siliciclastic rocks beneath archaeocyath-bearing carbonates in all of these basins (Daily 1972). For completeness the occur rences of trace fossils are investigated, together with archaeocyaths and trilobites, in a sequence stratigraphic context (sensu Vail et al. 1977; van Wagoner et al. 1988). On the basis of our analysis, we discuss three key attributes of the Cambrian radiation in Australia: species diversity and relative sea level change; correlation of sequences between basins; and rates of speciation, assisted by the increasing number and accuracy of radiometric ages of Cambrian successions.

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