The Neoproterozoic-Cambrian record in Australia: A stable isotope study

Shallow intracratonic basins developed across a large area of the central part of the Australian craton during the Neoproterozoic and continued to accumulate marine sedimentary successions until well into the early Palaeozoic-a half billion year sedimentary record. Carbonate successions ranging in age from Neoproterozoic (Cryogenian) to ostensibly Early Ordovician have been sampled at eleven sites across four of the northernmost of these basins (Amadeus, Georgina, Wiso and Daly Basins). A total of 996 samples were then analyzed for stable carbon (delta C-13(carb)) and oxygen (delta O-18(carb)) isotopes to develop a regional secular stable carbon isotopic curve. The curve complements earlier work and provides an important framework for global correlation at a critical time in Earth history during which the biosphere and atmosphere were evolving rapidly. At one site in the Amadeus Basin, diagenesis proved to have significantly modified the isotopic composition of the carbonates and the data were eliminated. Analysis of the remaining samples suggests that they reflect the global secular stable carbon isotope curve and tie well with established palaeontological timescales. In particular, key isotopic excursions can be identified in the earliest part of the Neoproterozoic succession, notably the negative excursion following the Sturtian glaciation. In the early Palaeozoic record a major excursion during Ordian-early Templetonian (509-506Ma) time can be identified across the region as can the so-called SPICE excursion which has been associated globally with the Steptoean Stage of Laurentia (497.5-495 Ma). A smaller late Templetonian-Floran (506-503 Ma) positive excursion has also been recognized. The stable carbon isotope record provides a strong regional correlation in both fossiliferous and unfossiliferous Neoproterozoic and early Palaeozoic successions and allows a global correlation to be established. (c) 2005 Elsevier B.V. All rights reserved.