Biostratigraphy and correlation of the Cambrian Series 2 (Stage 4) to Miaolingian (Wuliuan) Thorntonia Limestone, south-eastern Georgina Basin (Queensland, Australia)

The Australian Cambrian stratigraphic record is one of the most complete for any continent. However, there is a general division, with older Cambrian rocks in South Australia (predominantly Terreneuvian, Stage 2 to Miaolingian, Wuliuan) and younger rocks in northern Australia (mostly latest Series 2, Stage 4 and younger), with minimal stratigraphic overlap. The recently ratified boundary between Cambrian Series 2 and Miaolingian occurs in the interval of overlap between these successions, hence investigation of these levels is a priority. The successions in the southern and eastern parts of the Georgina Basin (Queensland and Northern Territory) underpin the Cambrian biostratigraphy of northern Australia. Therefore, targeted, multi-proxy work on these successions is important for developing a fully resolved Australian Cambrian chronostratigraphy. Here we present new insights into this critical interval via biostratigraphic analysis of the Thorntonia Limestone (shown to span the Cambrian Series 2 – Miaolingian boundary) in three recently drilled core holes near Boulia, Queensland, in the south-eastern Georgina Basin. A diverse fossil fauna (>50 species) is described from these cores, including biostratigraphically important trilobites, agnostids, brachiopods, bradoriids, echinoderms, hyoliths, molluscs, sponges and other small shelly fossils (SSFs). These show affinities with Australian Ordian (late Stage 4) to Templetonian (Wuliuan) faunas, with a potential Floran (early Drumian) interval in the uppermost Thorntonia Limestone. Many SSF taxa recovered from the Thorntonia Limestone show close affinities with other East Gondwanan assemblages from North China and Antarctica in particular, as well as with assemblages from Laurentia, Siberia, South China, Kazakhstan, the Himalaya and West Gondwana.