SEQUENCE STRATIGRAPHY AND BASIN ANALYSIS OF THE PALEOPROTEROZOIC KOMBOLGIE BASIN, NORTHERN TERRITORY, AUSTRALIA

The Paleoproterozoic Kombolgie Basin is located on the Arnhem Land Plateau and is part of the larger McArthur Basin in northern Australia. This intracratonic basin is filled with thick (1-2 km) relatively undeformed, flat-lying, coarse-grained clastic rocks. It is host to world-class unconformity-type uranium deposits that formed in response to tectonically induced fluid events in aquifers that were greatly affected by the internal stratigraphic architecture of the basin. Analysis of outcrop exposures and new exploration drill cores representing up to 1500 m of continuous record show that the Kombolgie Subgroup is composed of three unconformity-bounded stratigraphic sequences. The lower half of the first sequence is composed of coarse-grained sandstone and conglomerate with abundant trough cross-bedding representing deposition in proximal braided rivers that transported sediment away from basin-margins and intra-basin paleohighs. After this initial stage of basin evolution, coarse to medium-grained quartz arenites were deposited in low-energy, distal braided stream environments. Paleo-currents in the upper half of this lower sequence, as well as those of overlying sequences, indicate that the major intra-basin topographic highs no longer existed. The top of the lower sequence is marked by well sorted quartz arenite that contains salt casts, low-angle planar wedge cross bedding, and deflation surfaces representing marginal marine deposition. The middle sequence has a similar pattern of coarse-grained fluvial facies, followed by distal fluvial, and finally interbedded marine and eolian facies. An interval marked by mud-rich, fine-grained sandstones and mud-cracked siltstones with wave ripple marks and flaser bedding representing tidal flat deposition tops this sequence. The uppermost sequence is dominated by distal fluvial and marine facies that contains halite casts, stromatolites, and glauconite suggesting that a marine transgression occurred.

The repeating pattern of sequence development, paleo-current directions, and the presence of basaltic volcanism at the top of each sequence all indicate that the Kombolgie Basin was tectonically active; it was this regional tectonism that led to the development of the basin and ultimately to the economically important mineral deposits it hosts.