Foster Lookout

foster1.JPG	Welcome to Foster Lookout. From here, on a clear day, you can see the South Gippsland hills, Corner Inlet and Wilsons Promontory. Foster Lookout is a great place to stop because you can get an overview of the geology and geomorphology of South Gippsland and Wilsons Promontory. The Warragul 1:250,000 geology sheet (pictured below) is extremely handy at this stop. Foster Lookout sits on the eastern side of the Strzelecki Ranges (in the Gelliondale Block), an uplifted package of Early Cretaceous volcanoclastic sediments of the Strzelecki Group. The Alberton Depression (lowland or graben) in front of us includes Corner Inlet, a large estuary, and the Yanakie Isthmus, a sandy bar connecting South Gippsland to Wilsons Promontory. Foster Lookout is separated from the Alberton Depression by the Gelliondale Monocline and the Waratah Fault. The Depression is filled with sediments from the Cenozoic Era (between 65 and 0 million years old) derived from weathering and erosional stripping of the surrounding South Gippsland rocks, mostly Cretaceous Strzelecki Group and Tertiary rocks including the Latrobe Group. Click here to download the entire Warragul 1:250,000 geological map from Geoscience Australia (6.1 Mb jpeg).
foster2.JPG	Foster is the centre of the universe!
foster3.JPG	In this view we're looking eastwards across South Gippsland and the northern part of Corner Inlet towards Port Franklin, Port Welshpool and Snake Island. You can see the low-lying lands of the Alberton Depression graben, filled with relatively recent sandy sediments.
foster4.JPG	This view looks more or less south towards Wilsons Promontory. Corner Inlet is clearly visible in the mid-ground but Yanakie Isthmus is hidden behind the trees to the right. Hills and mountains visible on Wilsons Promontory include (from left to right) Mt Singapore, Mt Roundback and Mt Vereker.
geology.jpg	This is an extract from the Warragul 1:250,000 geological sheet. The immediate area of Foster Lookout (slightly to the west of "Foster"), the southeastern Strzelecki Ranges, Corner Inlet and northern Wilsons Promontory are depicted. The important things to note on this extract are the prominent fault lines that divide up the landscape, the red-coloured Devonian granites on Wilsons Promontory, the purple Devonian sedimentary rocks, the deep green-coloured Cretaceous rocks, the yellow Tertiary sediments and the light green to yellow Quaternary sediments.
rad.jpg	This is an extract of a gamma-ray spectrometric image from the Warragul Geoscientific Data CD by the Victorian Department of Natural Resources and Environment. It covers roughly the same area as the geological map above. What is this image showing us? A gamma-ray spectrometric image depicts the naturally-occurring radioactivity in the top 30-50 cm of the Earth's surface by mapping gamma radiation from the decay of the radioelements potassium (K), thorium (Th, actually, its daughter product ²⁰⁸thalium) and uranium (U, actually, its daughter product ²¹⁴bismuth) as red, green and blue, respectively. Bright colours indicate a relatively high amount of one or more of the radioelements and dark colours indicate little or none of the radioelements. Combinations of two or more radioelements results in different colours (i.e., yellow = red + green, or relatively large amounts of K and Th in the regolith) Compare this image to the geological map above, what does it tell us? The Strzelecki Ranges (bright pink-red) are relatively K-rich, meaning that they contain a lot of K-feldspar (which we know from looking at chunks of them). The Devonian and Tertiary rocks (green-blue) are relatively Th-rich, with some U. This can be interpreted to mean that they either contain a lot of resistate minerals like monazite (a Th-rich phosphate) and zircon (U-bearing zirconium silicate) or they are quite weathered and Th and U are adsorbed to iron oxyhydroxides like goethite. The granites of southern Wilsons Promontory are bright white to yellow indicating large amounts of all three radioelements (white) or large amounts of K and Th (yellow). This can be interpreted to mean that these rocks are relatively fresh (white colour indicating all three radioelements in abundance) or that they are slightly weathered (yellow colour indicating lots of remnant K and Th, and U has been somewhat leached). The granites of northern Wilsons Promontory are bright to dull pink, indicating variable amounts of K and little Th and U. This can be interpreted to mean that these granites are quite weathered relative to the granites of the southern Prom and that Th and U have been leached away. Alternatively, these could be fresh K-rich and Th- and U-poor granites! You'd need to go and look. The Quaternary sediments in Corner Inlet are dull pink indicating low to moderate amounts of K. These sediments are sourced largely from Mesozoic and Cenozoic deposits in the Strzelecki Ranges and Latrobe Valley, which we know are K-feldspar-rich. It stands to reason that small amounts of K-feldspar and K-bearing clays also occur in the coastal sediments that are derived from these rocks. The Quaternary sand dunes are largely black, indicating low concentrations of all three radioelements. These dunes are composed largely of pure quartz sand, which has none of the three radioelements. Of course, all of these interpretations are based on knowledge from other areas. You really need to go and look for yourself to verify the interpretations.
You can learn more about Foster Lookout and interpreting radiometric images for regolith by reading: W.D. Birch ed. 2003. Geology of Victoria. Geological society of Australia Special publication No. 23. 842 p. J.G. Douglas 1979. Explanatory notes on the Warragul 1: 250,000 geological map. Geological Survey of Victoria, 27 p. B.R.S. Minty 1997. Fundamentals of airborne gamma-ray spectrometry. AGSO Journal of Australian Geology & Geophysics 17(2), 39-50. J.R. Wilford, P.N. Bierwirth & M.A. Craig 1997. Application of airborne gamma-ray spectrometry in soil/regolith mapping and applied geopmorphology. AGSO Journal of Australian Geology & Geophysics 17(2), 201-216. Go back