PROBLEMS IN APPLYING PRESERVATION, FACIES AND SEQUENCE MODELS TO SINIAN (NEOPROTEROZOIC) GLACIAL SEQUENCES IN AUSTRALIA AND ASIA

The Sinian (800-530 Ma) is an extraordinary period of earth history characterized by the transition from simple to complex multicellular organisms and by extreme variations in climate, eustasy and possibly tectonics. Understanding Sinian stratigraphy may help interpret the co-evolution of life and environment during the period. Though Sinian successions are poorly dated and difficult to correlate, preservation, facies and sequence models based on Mesozoic-Recent successions may be used to interpret them. Preservation models (based on basin subsidence) permit estimates of how thick and complete successions can be in different tectonic settings, and how long it took for them to be deposited. Facies models for rift, craton, shelf, slope and basin environments help in identification of these different tectonic settings. Sequence models facilitate interpretation of the interplay of tectonic subsidence, eustasy, rates of sedimentation and climate. Such models, applied to the Sinian successions of Australia and Asia indicate the following. Great shallowing upwards megacycles occur in Sinian successions and each ends with regional glaciations. Deep rift basins formed around 850-800 Ma preserve the oldest megacycles. But these basins soon were filled with rapidly deposited glaciogenic sediments. Relatively complete sequences are found only in deep basins subject to repeated rifting. Cratons and shallow shelves preserve only the youngest megacycles. Older deposits have been removed by younger glacial advances. Deep shelves and slopes preserve one to three megacycles depending on the timing of the start of passive margin subsidence. Megasequences can be correlated based on limited radiometric dating and inferred regional distribution of unique rock types, but these sequences are not developed to the same extent in all blocks. Problems in correlating the four great megacycles recognized elsewhere with those of Asia will persist until the requisite facies analysis and U/Pb dating of interbedded volcanics are done.