Lower-middle Cambrian microbialites from South Australia: construction, biofacies and biogeochemistry

Microbialites are organo-sedimentary deposits, sometimes composed of accumulations of cyanobacteria and other bacteria, which form at the sediment-water interface. Microbialites represent one of the earliest records of life on Earth, with stromatolites - laminated accretionary microbialites - dating back at least 3.7 Ga. Stromatolites became extremely diverse and abundant throughout the Archean era (4-2.5 Ga), causing increased atmospheric oxygen levels on Earth as part of the Great Oxidation Event. The early Cambrian bilaterian radiation coincided with a sharp decline in stromatolite abundance. In this study, microbialites were sampled from lower and middle Cambrian carbonate facies in the Arrowie Basin, South Australia. The appearance, construction, and biogeochemistry of stromatolites from different depositional environments, is described to investigate morphological variation and ecological associations, which may have been altered after the evolution of epifaunal grazing bilaterians. Few research projects have investigated stromatolites in the Arrowie Basin. This project investigates morphological variations in microbialites through petrographic observations, attempting to resolve previously unanswered biogeochemical questions using multi-disciplinary techniques: field relationships, hand and thin section observations, gas chromatography-mass spectrometry, Raman spectroscopy, fourier transform infrared spectroscopy, and X-ray fluorescence. The samples are highly thermally mature. Microbialites are not found in association with any fossils preserved in life position. Depositional environments ranging from shallow supratidal to low-energy subtidal are consistent with previous regional interpretations.