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 ch aracterized by the transition from simple to complex multicellular org anisms and by extreme variations in climate, eustasy and possibly tect onics. Understanding Sinian stratigraphy may help interpret the co-evo lution of life and environment during the period. Though Sinian succes sions are poorly dated and difficult to correlate, preservation, facie s and sequence models based on Mesozoic-Recent successions may be used to interpret them. Preservation models (based on basin subsidence) pe rmit estimates of how thick and complete successions can be in differe nt tectonic settings, and how long it took for them to be deposited. F acies models for rift, craton, shelf, slope and basin environments hel p in identification of these different tectonic settings. Sequence mod els 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. G reat shallowing upwards megacycles occur in Sinian successions and eac h ends with regional glaciations. Deep rift basins formed around 850-8 00 Ma preserve the oldest megacycles. But these basins soon were fille d with rapidly deposited glaciogenic sediments. Relatively complete se quences are found only in deep basins subject to repeated rifting. Cra tons and shallow shelves preserve only the youngest megacycles. Older deposits have been removed by younger glacial advances. Deep shelves a nd slopes preserve one to three megacycles depending on the timing of the start of passive margin subsidence. Megasequences can be correlate d based on limited radiometric dating and inferred regional distributi on of unique rock types, but these sequences are not developed to the same extent in all blocks. Problems in correlating the four great mega cycles recognized elsewhere with those of Asia will persist until the requisite facies analysis and U/Pb dating of interbedded volcanics are done.