Geology and Geomorphology of the Hamilton Research Institute

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The geology of the area has been mapped by the Geological Survey of Victoria at a scale of 1:250,000. The area is almost completely covered by basalt and is referred to as the 'Newer Volcanics' province. These are late Tertiary and Quaternary basalts and they have been extensively weathered. In the Hamilton region, two distinctive basalt types are present: lava plains and plateaus (mapped as Qvn); and lava domes, stony rises and valley flows (mapped as Qvh) from sources such as Mt. Napier. These deposits are near the western limit of Victoria's western volcanic plains.

Tholeiitic, alkaline and sub-alkaline mafic basalts (with agglomerate, scoria and tuff) outcrop in the Hamilton-Portland area, forming the western end of Victoria's basaltic plains landscape. These range in age from early Pliocene (in the order of 4 million years ago) to Holocene, where the Pliocene basalts are the most extensive. This basalt sheet can be more than 60 metres thick near eruption centres and in valley flows. Undissected areas can have kaolinitic weathering profiles more than 10 metres thick, developed on the basalt. These Pliocene basalts overlie Miocene marine limestone and marl of the Heytesbury Group. Only a few of the volcanoes which gave rise to the Pliocene basalts have been identified (Douglas and Ferguson, 1988). Newell (1962) noted that a Mines Department bore near the front gate of the Research Station recorded some 7 metres of clay and weathered basalt rock, overlying some 100 metres of basalt which in turn overlies fossiliferous marine sands and marls.

Buckley's Swamp was formed after drainage was blocked by a recent lava flow from Mt. Napier. Gill and Elmore (1973) reported a radio-carbon date of 7,240 (+/- 140) years for a sample from the base of a peat bed deposited at a depth of 1.4 metres in Buckley's Swamp. This indicates that Mount Napier erupted some 8,000 years ago. The scoria cones and shallow craters of Mount Napier lie on a broad lava shield with a stony rise surface, lava cones and lava channels (Douglas and Ferguson, 1988).

The radiometric (gamma-ray spectrometry) image is derived from regional airborne magnetic and radiometric surveys flown for the Bureau of Mineral Resources (now AGSO) and the Geological Survey of Victoria between 1974 and 1999. The image produced here is a colour (RGB) composite image that has been gridded to a 50 m cell size. In the RGB image the three main natural gamma ray emitters Potassium (K), Thorium (Th) and Uranium (U), have been assigned the colours red, green and blue respectively. Radiometric data are particularly useful for distinguishing soils and near-surface rock types (upper 30-45 cm of the profile) based on their geochemical composition. The RGB image provides a good overview of the radiometric data within the study area. Near pure red colours indicate high potassium rich basalts i.e. Qvh basalts. The bluer/greener colours indicate lower potassium basalts i.e. Qvn.

References

Douglas, J.G and Ferguson, J.A (1998). Geology of Victoria. Geological Society of Victoria.

Geological Survey of Victoria. (1994). Hamilton 1:250,000 SJ 54-7 Geological Map. Mapping by Douglas, J.D. and Spencer-Jones, D. 1970, with recent additions by A.H.M. Vandenberg.

Gibbons, F.R and Downes, R.G (1964). A Study of the Land in South-Western Victoria. Soil Conservation Authority.

Gill, E.D and Elmore, L.K.M (1973). Radiocarbon dating of Mount Napier eruption. Vict. Nat. 90; pp.304-306.

McDonald, P.A (1991). A Preliminary Classification of Basalt Soils using Radiometrics, Hamilton Area, Western Victoria. Unpublished Report. Geological Survey of Victoria.

Newell, J. W. (1962). The Soils of the Hamilton Pastoral Research Station. Technical Bulletin 15. Department of Agriculture.

Slater, K.R (1995). An appraisal of new airborne geophysical data over the Glenelg region, North West VIMP area, Victoria. Victorian Initiative for Minerals and Petroleum (VIMP) Report 14, Geological Survey of Victoria.