SEQUENCE STRATIGRAPHY AND THE INTERPRETATION OF NEOPROTEROZOIC EARTH HISTORY

The application of sequence stratigraphy to Neoproterozoic successions is important for improving the resolution of time-correlation within individual sedimentary basins and potentially at a global scale. The m ethodology is illustrated in this paper by reference to two contrastin g examples from the Flinders Ranges (Adelaide geosyncline) of South Au stralia, where the younger part of the Neoproterozoic to earliest Camb rian succession (similar to 770 Ma to similar to 540 Ma) has been divi ded into thirteen unconformity-bounded depositional sequences. One of the most prominent sequence boundaries, at or near the base of the Won oka Formation, is characterized by a series of buried canyons as much as 1 km deep. High-resolution sequence-stratigraphic studies at Umbera tana syncline continue to support the view that the canyons were cut s ubaerially and filled by fluvial and shallow-water sediments. In contr ast to the Wonoka canyons, sequence boundaries interpreted at the base of the Nuccaleena Formation/Seacliff Sandstone and near the top of th e ABC Range Quartzite are relatively subtle, with only limited evidenc e for erosion and valley incision. Four sequence boundaries, at the le vel of the Sturtian and Marinoan (Varanger?) glacial deposits and in t he vicinity of the Precambrian-Cambrian boundary, are thought to corre late with surfaces in the Amadeus basin of central Australia. Other pr ominent sequence boundaries, including the Wonoka canyons and surfaces within the upper part of the Wonoka Formation and at the base of the Ediacara Member of the Rawnsley Quartzite, correspond with a relativel y condensed section in the Amadeus basin, and their lateral persistenc e beyond the Adelaide geosyncline is therefore difficult to evaluate. Given the lack of precision in biostratigraphy and isotope geochemistr y in Neoproterozoic rocks, and in a marked departure from Phanerozoic practice, we recommend placement of a terminal Proterozoic GSSP at a s equence boundary. A prime candidate in Australia is the sequence bound ary at the base of the Nuccaleena Formation/Seacliff Sandstone, immedi ately above the Marinoan glacial rocks in the Adelaide geosyncline, an d its likely correlative at or near the base of the Gaylad Sandstone i n the Amadeus basin.