Upper Proterozoic evaporites in the Amadeus basin, central Australia, and their role in basin tectonics

摘要

The Amadeus basin, an isolated intracratonic basin in the center of the Australian continent, contains an upper Proterozoic to mid-Paleozoic stratigraphic succession of shallow-marine sediments that, in places, exceeds; 14 km in thickness. The Gillen Member of the Bitter Springs Formation, which occur toward the base of the upper Proterozoic succession, includes evaporites which are among the world's earliest (0.8 to 0.7 Ga). Because of their age, the evaporites have been cited in discussions of sea-water chemistry and have been the focus of scrutiny for early life forms. In spite of their importance, the evaporites are poorly known, particularly from the viewpoint of their depositional and tectonic setting. In an attempt to rectify this deficiency, more than 6,000 km of seismic data were analyzed, in conjunction with a field and well-log study of the unit. The Late Proterozoic Amadeus basin appears to have consisted of two major poorly circulated anoxic sub-basins, which perhaps opened to the ocean to the southeast through the Adelaide geosyncline. Data relating to facies are limited but suggest that deposition of the evaporites was cyclic and followed the patterns identified in other major evaporite basins, the carbonates and sulfates being closer to the basin margins and later stage halite and possibly potassium salts being toward the basin center. The evaporites were deposited in a shallow-marine setting at the time of a relative sea-level high stand. The apparent sea-level high, may relate to basin dynamics, whereas the cyclicity of the evaporites may be due to eustatic sea-level controls acting on the barrier to allow intermittent inflow of sea water. Salt tectonism began shortly after the evaporites were deposited and continued throughout basin development. Consequently, most of the major anticlinal structures have salt cores. The geometry of the salt structures suggests that during their growth, the mean strain rate was 10–16 s–1, a rate typical of large salt structures. Growth on these salt structures has played an important role in controlling later sedimentation, particularly during the Cambrian.