The palaeozoic Koonenberry Fold and Thrust Belt, Western NSW: a case study in applied gravity and magnetic modelling

Abstract

Geophysical modelling combined with recent geological mapping indicates that the Koonenberry belt of far western NSW is a Mid Palaeozoic fold and thrust system. This new interpretation relies upon the different petrophysical and structural attributes of four distinct tectonostratigraphic packages. Package I comprises mixed, multiply deformed, Late Neoproterozoic-Late Cambrian rift and continental margin sequences. Package II comprises Late Cambrian-Early Ordovician mixed carbonate-siliciclastic facies, and has been subjected to two minor and one major deformation event. Package III contains fluvial-lacustrine red beds and volcanics with one major and one minor set of folds. Package IV is the Late Devonian Mulga Downs Group, a generally flat-lying fluvial cover sequence with restricted folding. Fault kinematics are constrained by analysis and modelling of geophysical data, which indicate across-strike repetitions of various sequences. High spatial frequency magnetic linear features require steep surface dips, but listric character is demanded by lower frequency magnetic anomalies that are best fitted by sub-horizontal bodies at mid-crustal depths. Detailed analysis shows many anomalies are skewed to the east near the positions of suspected faults. However, the major Koonenbeny Fault is a west-dipping back-thrust. These features strongly suggest that the Koonenberry belt is a west-vergent thrust package that detaches in the mid-crust. The principle fold-thrust deformation occurred during the Silurian, prior to deposition of package III , and probably corresponds to the Benambran event in the Lachlan Fold Belt. This deformation overprints a Late Cambrian event that has deformed package I. These conclusions indicate that the Koonenberry belt is a zone of overlap betaleen the Lachlan and Delamerian Orogens.