Geology & geophysics of the Koonenberry Belt, far western New South Wales, and eastern Australian correlates : timing, development and prospectivity of a sector of the Late Proterozoic-Palaeozoic Gondwana margin

Consideration of the latest geophysical datasets and new mapping and stratigraphic data shows the Koonenberry Belt of far western NSW, to be an Early to Mid Palaeozoic foldand-thrust belt. This study presents the results of geologically-controlled qualitative interpretation and twodimensional modelling of gravity and magnetic data which permit a new tectonic interpretation, relying upon the different petrophysical, structural and metamorphic attributes of three distinct tectonostratigraphic packages. The oldest known sequence in the belt comprises shales, sands, dolomites and strongly magnetic transitional alkaline basalts, which are believed to exist in large volumes at depth, producing a major magnetic anomaly. These rocks are disconformably overlain by a regionally extensive ?Lower to ?Middle Cambrian turbidite sequence. The two packages represent Late Neoproterozoic continental rifting, dated at 587 Ma, and Early Cambrian volcanic passive margin formation. Dense, rifted blocks of Neoproterozoic sedimentary sequences are believed to underlie this remnant margin at depth, on the basis of high gravity signatures. A second widespread rifting or crustal attenuation event is represented by marine shelf sequences interbedded with minor volumes of calc-alkaline volcanic rocks. This event has been dated around 525 Ma. Comparison of turbidite sequences in the belt and South Australia indicates fewer similarities than previously thought; equivalent passive margin sequences to those in western NSW may lie in westernmost Victoria. Equivalents to the Cambrian shelf sequences exist within the Warburton Basin of northern South Australia, and indicate that the second episode of rifting dated at 525 Ma was widespread. The late Middle Cambrian-Late Cambrian thrusting event and subsequent post-collisional volcanism in the belt is also believed to be represented in western Victoria and western Tasmania. In these locations, it is represented by allochthonous slices of intra-oceanic arc mafic-ultramafic complexes and overlying 'in situ' post-collisional volcanics. The structural and metamorphic character of these sequences, and their common timing, contrast strongly with the exposed sequences of the Adelaide Fold Belt and other portions of the Ross-Delamerian Orogen in Antarctica, and strongly suggest that the orogen is divided into internal and external zones. Analysis of differences in timing and character of deformation between the two zones suggests that the Delamerian Orogeny was a long lived, mantle-driven process with overprinting effects from short-lived plate-geometries. This model contrasts with earlier suggestions of a rigid arc-continent style collision. Subsequent development of the Koonenberry and related fold-belts in an in-board position is related to long-lived accretion at the free plate margin to the east. Effects include major fold and thrust deformation in the Ordovician-Silurian, and widespread high crustal-level strike-slip deformation in the Devonian and Carboniferous. A second major mantle-driven thermal event in the Devonian found elsewhere in the Lachlan Orogen is absent within the Belt. The tectonostratigraphic sequences and deformation style of the belt suggest significant prospectivity for a wide variety of base and precious metals, and diamonds. Correlations with equivalent sequences elsewhere support this conjecture. Petroleum prospectivity is also high, with known mature sources and reservoir facies; however, the structural complexity of the belt suggest all plays will have a high element of risk.