5:15 PM - 7:15 PM
[BCG06-P15] Upper Cryogenian stratigraphy and sponge-like fossils in South Australia
Keywords:Neoproterozoic, sponge, carbon isotopes
The Proterozoic was a period of environmental changes and biological evolution. The late Neoproterozoic (720–540 million years ago) is the time when animals became multicellular, oxygen concentrations increased, and the whole Earth was frozen. Molecular clock records suggest that the most primitive multicellular animals (sponges) evolved around the late Tonian (ca. 800 Ma), and more complex animal phyla appeared after the Cryogenian. In contrast, the fossil records of multicellular animals are limited and often uncertain. The most reliable evidence for sponges comes from a Cryogenian biomarker, 24-isopropylcholestane (Love et al., 2009), and fossil evidence supporting this has been searched worldwide.
In this study, we aimed to estimate the surface environmental conditions during the late Cryogenian by conducting geological surveys and sample collections from a sedimentary sequence in the Flinders Ranges region of South Australia, including the Trezona Formation. Previous studies of the Trezona Formation have reported fossils that are considered primitive multicellular animals, specifically sponges (Maloof et al., 2010). We analyzed approximately 200 samples collected from a sequence of about 1200 meters thick, including the Trezona Formation and the underlying Eratina Formation. The Eratina Formation primarily consists of laminated mudstones and contains thin calcareous layers. The Trezona Formation, deposited before the Marinoan glaciation, consists primarily of alternating carbonate rocks and black shale. The sponge-like fossils occur in the lower part of the formation, together with stromatolites, indicating a shallow water environment. The Trezona Formation presents a negative anomaly of carbon isotopes which sustains until 10 m below the contact with the Marinoan glacial deposits.
In this study, we aimed to estimate the surface environmental conditions during the late Cryogenian by conducting geological surveys and sample collections from a sedimentary sequence in the Flinders Ranges region of South Australia, including the Trezona Formation. Previous studies of the Trezona Formation have reported fossils that are considered primitive multicellular animals, specifically sponges (Maloof et al., 2010). We analyzed approximately 200 samples collected from a sequence of about 1200 meters thick, including the Trezona Formation and the underlying Eratina Formation. The Eratina Formation primarily consists of laminated mudstones and contains thin calcareous layers. The Trezona Formation, deposited before the Marinoan glaciation, consists primarily of alternating carbonate rocks and black shale. The sponge-like fossils occur in the lower part of the formation, together with stromatolites, indicating a shallow water environment. The Trezona Formation presents a negative anomaly of carbon isotopes which sustains until 10 m below the contact with the Marinoan glacial deposits.