ABSTRACT

The Amadeus basin is an isolated intracratonic basin at the center of the Australian continent which, because of its location and geometry, provides an ideal opportunity to investigate depositional controls. To this end, more than 6,000 km of seismic data, in conjunction with a field and well-log program, have been used in a study of the late Proterozoic-Early Cambrian Arumbera Sandstone.

The Arumbera Sandstone was deposited as two major depositional sequences in a shallow marine and deltaic or coastal-plain setting, similar to that of the Devonian Catskill delta of North America. Sediment was supplied from the southwest by heavily laden, braided streams. During the deposition of the first sequence, when sea level was at its maximum, sedimentation appears to have been largely aggradational as sediment supply balanced or exceeded basin subsidence. The sediments were probably deposited in a coastal or delta plain setting, where small-scale deltas prograded across the carbonate platform sediments of the Julie Formation. During the accumulation of the second sequence, sea level fell rapidly, exposing large areas of sediment deposited during the first cycle and restricting sedimentation to two steep-sided, rapidly subsiding subbasins. Deposition was largely progradational, as major deltaic complexes developed on the southern and southwestern margins of the subbasins. Between the major delta complexes, a depositional environment similar to that of the first sequence predominated. In the deep subbasins, the Arumbera Sandstone forms a continuous depositional unit across the Proterozoic-Cambrian boundary. The unconformable nature of that boundary over much of the Earth possibly could be the result of a major eustatic sea level drop.

The final geometry of the Arumbera Sandstone and the nature of its sediments were determined largely by local subsidence rates. Subsidence briefly outstripped sediment supply during a stage of crustal extension as the basin evolved during deposition of sequence 1. Depositional space and facies distribution were, however, fine tuned by salt tectonics and major sea level cycles. As at passive margins, sequence boundaries appear to be controlled by sea level cycles. Other variables, such as climate, the lack of vascular plants, and the uplift of the source area were also factors in controlling deposition.

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