VIMP Report 82 - St Arnaud 1:250 000 map area: A geological interpretation of the geophysical data
 |
| |||||||||||||||
Product description:AbstractThis report uses the geophysical and geological signatures of the rock units to describe them and their relationships with each other. It is part of a 1:250 000 series that is intended to cover the areas of concealed Palaeozoic geology in Victoria. The ST ARNAUD region lies on the southern edge of the Cainozoic sediments of the Murray Basin and the northern edge of the Western Victorian Uplands, of Palaeozoic sedimentary rocks and granites. The magnetic and gravity responses of the Palaeozoic units in the south allow them to be traced north under the Murray Basin cover and to establish their structural and stratigraphic relationships. The oldest rocks are the basalt slivers present in the Moornambool Metamorphic Complex and the hanging wall of the Avoca Fault. These are ocean floor tholeiitic basalts formed during the Neoproterozoic to Early Cambrian Rodinian break-up. Although only a few allochthonous occurrences are known, the entire area is believed to be floored by these basalts. West of the Moyston Fault (in the southwest of the area) there is little outcrop, but the rocks below the Murray Basin cover have been correlated with the Glenthompson Sandstone and the Mount Dryden volcanic belt, part of the Mount Stavely Volcanic Complex. These are island arc volcanic rocks with associated volcaniclastic sandstones, deformed late in the Delamerian Orogeny. Using the magnetic character of the rocks east of the Moyston Fault, the Cambro-Ordovician turbidite pile has been subdivided into 7 units. The most prominent magnetic features are a) a continuous magnetic layer with a response of up to 30 nT that can be traced as a continuous feature in the hanging wall of the Percydale Fault for about 70 km. Because it is so persistent along strike, there is some confidence that correlations across strike may be valid, particularly as the enveloping surface of the whole package has previously been seen to be flat. If the unit is coherent, it may mark a time of less sedimentary input into the region, either because of a major change in the discharge patterns of sediments further west or a generally quiescent period. b) the transition from magnetic to nonmagnetic rocks. The easternmost part of the area has a major transition from magnetic to nonmagnetic metaturbidites. This transition correlates well with the first appearance of graptolites in the rocks, a critical characteristic of the Castlemaine Group. It may also mark a significant internal boundary in the turbidite pile since it seems to have truncated some of the magnetic stratigraphy. The other units have been separated using these two markers. One implication of this is that some rocks up to 50 km east of the Avoca Fault east may be better classified as part of the St Arnaud Group, since they are magnetic and are not known to contain graptolites. Another implication is that the boundary between the magnetic and nonmagnetic metaturbidites may be a widespread stratigraphic boundary, and so can be used to define structural domes beneath the Murray Basin. This is important for gold exploration, since gold has long been known to concentrate in anticlinoria. The gold distribution in the present study is consistent with this proposition. The regional deformation of the area took place in the Benambran Orogeny, from 460 to 420 Ma. The trends of deformation are different either side of the Avoca Fault, but this may be related to the shapes of the cratonic basement either side of the deforming sedimentary pile as much as to different ages of deformation resulting in different stress fields. Deformation was followed by gold mineralisation and then by granite intrusion at about 400 Ma. Later, deformation during the Tabberabberan Orogeny caused significant brittle faulting and fault reactivation. Over 200 volcanic plugs have been identified below the Murray Basin. These may be the source of the volcanic detritus in the Eumeralla Formation of the Otway Basin. Since the volcanic detritus in the Eumeralla formation is essentially coeval with the deposition of the unit, it would suggest an age of volcanism of about 110 Ma. It would also suggest that the extension in the Otway Basin came as far north as ST ARNAUD, allowing streams carrying the detritus to flow south. At about 50 Ma, when the Otway Basin rifting broke through to the mantle and that part of Australia finally split off from Antarctica, the West Victorian Uplands started to form. This reversed the southward flowing streams and the Murray Basin started to collect sediment, the Olney Formation and its upstream equivalents the White Hills Gravel and the Calivil Formation. The Olney Formation is only known in the subsurface and is nonmagnetic, but three upstream units can be recognised geophysically. The oldest, the White Hills Gravel, is strongly dissected and has low radiometric responses. The other two sets of channels are both magnetic; the younger channels follow the present stream patterns whilst the older does not. The magnetic responses are considered to have formed when non-magnetic iron-bearing minerals were replaced by a magnetic mineral in laterite-forming events at about 10 Ma and 3 Ma, the Mologa and Karoonda surfaces. The older magnetic channels are correlated with the Calivil Formation and the younger with the Shepparton Formation. The lower part of the Shepparton Formation is the nonmarine equivalent of the marginal marine Parilla Sand. Since the last regression, at about 5 Ma, the Parilla Sand in the south of ST ARNAUD has been raised about 60 m when compared to the northern part. Quaternary sediment has partly covered the Parilla Sand, and some formerly west-flowing streams, like the Richardson and Avon rivers, may have changed course in response to the tilting. Interpreting the high-resolution airborne magnetic data has led to the discovery of the mineral sands. It has also raised interest in the Cambrian magnetic basalts, which have become targets for gold exploration. Bibliographic reference Moore, D.H., 2004. St Arnaud 1:250 000 map area: a geological interpretation of the geophysical data. Victorian Initiative for Minerals and Petroleum Report 82. Department of Primary Industries, Victoria. Download The downloadable version of this report is supplied in PDF format: 34462 VIMP82 Report (PDF 23.5MB) Please be patient when downloading large files as this might take some time depending on your internet connection. Related products:
| ||||||||||||||||
