VIMP Report 39 A geological interpretation of the geophysical data for the Ouyen 1:250 000 map sheet area D.H. Moore July 1997 Bibliographic reference: MOORE, D.H., 1997. A geological interpretation of the geophysical data for the Ouyen 1:250 000 map sheet area. Victorian Initiative for Minerals and Petroleum Report 39. Department of Natural Resources and Environment. © Crown (State of Victoria) Copyright 1997 Geological Survey of Victoria ISSN 1323 4536 ISBN 0 7306 9417 8 Keywords Ouyen 1:250 000, aerial magnetic surveys, magnetic interpretation, geophysical interpretation, Palaeozoic, Cainozoic, granite, greenschist facies, amphibolite facies, basalts, turbidites, Lachlan Fold Belt, Glenelg Complex, Netherby Trough, Murray Basin, gold, copper. This report may be purchased from: Business Centre, Department of Natural Resources and Environment, Ground Floor, 115 Victoria Parade, Fitzroy, Victoria 3065 For further technical information contact: Manager, Geological Survey of Victoria, Department of Natural Resources and Environment, P O Box 2145, MDC Fitzroy, Victoria 3065 GEOLOGICAL INTERPRETATION - OUYEN 1 Contents Abstract 2 1 Introduction 3 2 Access and physiography 5 3 Previous work 6 3.1 Regional studies 6 Geology 6 Geophysics 7 3.2 Mineral exploration 8 3.3 Petroleum exploration 10 4 Regional interpretation 12 4.1 Pre-Delamerian and Delamerian 12 General 12 Glenelg Zone 12 Delamerian intrusions 16 Stawell Zone 17 Devonian granites 19 Carboniferous-Permian 19 Mesozoic and Cainozoic 20 5 Conclusions 23 6 References 25 Appendix 1 29 Specifications of the Horsham airborne survey 29 Appendix 2 30 Geophysical domains - Ouyen 1:250 000 map sheet area 30 Victorian Initiative for Minerals and Petroleum (VIMP) report series 41 List of figures 1 Location map Ä OUYEN 4 2 OUYEN magnetic image 8 3 OUYEN gravity image 9 4 Thermal history of the Murray Basin 10 5 Geophysical domains Ä OUYEN 13 6 OUYEN digital elevation image 21 List of tables 1 Aeromagnetic surveys over OUYEN 7 2 Summary geological history for OUYEN 14 List of maps 1 Magnetic interpretation of basement geology (scale 1:250 000) 2 Magnetic interpretation of post-cratonic features (scale 1:250 000) 3 Calculated depths to magnetic basement (scale 1:250 000) 2 GEOLOGICAL INTERPRETATION - OUYEN Abstract The Grampians Group was deposited mostly on the Dimboola Subzone basement, in the In 1994, as part of the Victorian Initiative for Netherby Trough, some time between the end Minerals and Petroleum, the Ouyen 1:250 000 of the Delamerian Orogeny and the intrusion of map sheet area (OUYEN) was flown with a ?400 Ma granites. These Early Devonian high quality aeromagnetic and radiometric granites intruded both the Grampians Group survey. This report presents a geological and the Stawell Zone. The Netherby Trough interpretation which combines the new also contains significant amounts of the information with older gravity data and the Carboniferous to Permian Urana Formation. geology from drill holes, and is consistent with the regional geology. It contains the first Many of the point magnetic sources have been comprehensive maps of the Palaeozoic attributed to Upper Cretaceous to Eocene basement geology of the area, the interpreted basaltic plugs. post-tectonic features, and depths to basement computed from the magnetic data. The area is generally covered by from 200 m to 500 m of Murray Basin sediments. The The magnetic basement of OUYEN has been shallowest parts are in southwestern and divided into the eastern Stawell Zone and the southeastern OUYEN, and the deepest parts western Glenelg Zone. The geological and cover the Netherby Trough. The present tectonic framework established for the Glenelg interpretation of the radiometric, magnetic and Zone on HORSHAM has been continued digital elevation data in the eastern part of the northwards. The boundary between the area shows a significantly larger area of Stawell and Glenelg Zones is the northern Loxton-Parilla Sands than on previous maps. extension of the Moyston Fault. Both the Loxton-Parilla Sands and a younger palaeolake system show indirect evidence of On OUYEN, the Glenelg Zone has been late Cainozoic faulting. subdivided into the Dimboola, Upson and Ozenkadnook Subzones based on the Recently RGC have announced the discovery of interpreted rock units, metamorphic grades and two potentially economic heavy mineral sand deformation histories. The Dimboola Subzone deposits east of the town of Ouyen. The area is mostly oceanic island arc and ocean floor contains several small occurrences hosted by rocks that have been weakly metamorphosed the Parilla Sand. No presently economic and simply deformed. There is little metalliferous mineral deposits are known, but information on the Upson Subzone, but it there has been no exploration, probably because includes low grade metasediments and mafic of the Murray Basin cover and a lack of volcanic rocks that have previously been understanding of the basement geology. Gold correlated with the Kanmantoo Group. The could possibly be targeted in the Stawell Zone. Ozenkadnook Zone includes rocks that have Copper-gold deposits may be associated with been metamorphosed to amphibolite facies and the intrusions in the southwest corner of the show complex deformation. Other lower area. metamorphic grade rocks have been fault emplaced over the amphibolite grade rocks. The Glenelg Zone was deformed and intruded during the Late Cambrian to Early Ordovician Delamerian Orogeny. The Ozenkadnook Subzone may also have been deformed by an earlier orogenic event. The turbidites and oceanic basalts of the Stawell Zone are typical of much of the Lachlan Fold Belt in western Victoria, although more metamorphosed. They are weakly magnetic, with most trends striking north to northwest. The Stawell Zone was undeformed before about 440 Ma. GEOLOGICAL INTERPRETATION - OUYEN 3 1 Introduction This report is the second of a series that gives new interpretations of the basement geology of northwestern Victoria and which are part of the Victorian Initiative for Minerals and Petroleum (VIMP). Large areas have now been flown with high quality magnetic and radiometric surveys. Major gravity surveys are also being carried out on a 1.5 x 1.5 km grid, with several traverses at 250 m station spacing. The new interpretations combine the available geological and geophysical data to give a coherent interpretation of the "basement" beneath the Murray Basin. While other publications have generally described the surface geology of the region, this report is the first to detail the OUYEN1 basement and to provide a comprehensive basement map. Section 4 is a synthesis of the geology as it is now understood, and includes discussion on both the basement and "cover". Appendix 2 gives formal descriptions of domains not previously described in the earlier report on HORSHAM. Three 1:250 000 scale maps are included with the report. One outlines the domains, and gives an interpretation of the basement geology. A second map outlines the post-cratonic rocks and includes a reinterpretation of the surface geology based on the radiometric and digital terrain data collected. Twyford (1997) has outlined the depth of the Murray Basin cover as calculated on the magnetic data using the Naudy method. A 1:250 000 enlargement of the Ouyen part of her image is also included. The report also includes a brief discussion of the previous work carried out in the region. The technical details of the flying undertaken over OUYEN are included as Appendix 1. The images and digital data from the surveys used to make the interpretation can be purchased from Minerals and Petroleum Victoria. 1 All 1:250 000 map sheet areas in this report are referred to by capitalising their names. Thus the Ouyen 1:250 000 map sheet area appears as OUYEN. 4 GEOLOGICAL INTERPRETATION - OUYEN Figure 1 Location map Ä OUYEN. GEOLOGICAL INTERPRETATION - OUYEN 5 2 Access and physiography OUYEN lies about 400 km northwest of Melbourne. Access is via the Western and Henty Highways (Fig. 1). Within the sheet area, access is variable, ranging from very good in the southeastern parts to highly restricted within the Big Desert Wilderness Park. Extensive flooding from winter rains also causes problems away from formed roads. The area is a plain which slopes gently from about 140 m ASL in the southwest to about 50 m ASL in the northeast. On this plain are parabolic dunes with up to 20 m relief and east- west dune chains with up to 10 m local relief (Lawrence, 1975). The larger parabolic dune fields form the Big Desert and Little Desert. Salinas and ephemeral swamps are present in low interdune areas. Lake Hindmarsh and Lake Albacutya are more permanent, filling successively from the Wimmera River. Lunettes are common on the eastern sides of the larger lakes. Prior to settlement, most of the area was covered with mallee gum. More luxuriant tree growth was present along Outlet Creek, an ephemeral extension of the Wimmera River. Much of the area outside the parabolic dune fields has now been cleared. Annual rainfall is generally less than 350 mm. 6 GEOLOGICAL INTERPRETATION - OUYEN 3 Previous work Other significant hydrological studies include those by Thorne et al. (1990) and Jayatilaka et al. (1993). 3.1 Regional studies Although the regional Palaeozoic basement Geology does not crop out, evidence of it can be extrapolated from outcrops to the south and east on BALLARAT, HAMILTON and All of OUYEN is covered by sediments of the ST ARNAUD, and from the recent geophysical Cainozoic Murray Basin. The basement to the interpretation of HORSHAM. Several authors, Murray Basin does not crop out; it is known including Gibson and Nihill (1992) and Gray only from 7 drill holes. Five are recorded as et al. (1988), have subdivided the basement having penetrated Cambrian St Arnaud Group further south into two zones, the Glenelg Zone rocks or their equivalents. The other two to the west and the Stawell Zone to the east.