Lessons from a case study of a geophysical interpretation in Western Victoria

Abstract

This paper compares the predictions and results of the Victorian Initiative for Minerals and Petroleum drilling programme, mostly over the Horsham 1:250 000 map sheet area of western Victoria. Sixteen holes were drilled beneath thin cover of the Cainozoic Murray Basin at targets identified in Moore's (1996) geological interpretation of the airborne magnetic and ground based gravity data. This integrated interpretation divided the region into the Stawell and Glenelg Zones. The Stawell Zone metaturbidites and metabasalts form the eastern edge of the Lachlan Fold Belt. The Glenelg Zone included weakly metamorphosed andesites of the Dimboola Subzone, greenschist to amphibolite facies metaturbidites and metavolcanics of the Miga Subzone and amphibolite grade rocks of the Ozenkadnook Subzone. All were deformed in the 500 Ma Delamerian Orogeny. Drilling depths were estimated both from magnetic modelling and previous drilling in the region. Drilling targeted basement depths between 30 m and 250 m. Where magnetic modelling was the main depth control, basement depths were within 20% of predicted depths, slightly more reliable than those estimated from nearby drilling. Of the sixteen holes, nine intersected the predicted lithologies and another three intersected lithologies that substantially agreed with the predictions. While four drill holes intersected quite different lithologies to those predicted, no major geological reinterpretation was needed. The drilling intersected both Cambrian continental island arc volcanic rocks and Upper Proterozoic metasedimentary basement, strengthening the temporal and tectonic correlations between western Victoria and western Tasmania. The study showed that a careful interpretation can give a reliable indication of the basement, even in areas of total cover. Integrating geological and geophysical data and using standard geological logic and techniques, like drawing sections to test plan interpretations, are powerful aids. Drilling complemented the interpretation by providing samples that could be used to test the interpretive hypothesis.