GSV Report 117 - Creswick 1:100 000 map area geological report

Category: Geological Survey Reports	Product Code: MP-R-29044
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Abstract This report describes the geology, geophysics and mineral potential of the Creswick 1:100 000 mapsheet area (7623) covered by the accompanying 1:50 000 geological and 1:100 000 geophysical maps. It provides a comprehensive account of the geological history, landscape evolution, rock units, geological structure, geophysical character and economic geology of the map area. The report also outlines the regional geological setting and provides a summary of previous geological work. Photographs, diagrams and tables complement the text and include compilation of radiometric ages and a summary of previous mineral exploration. There is a comprehensive reference list and a detailed excursion guide, designed to show the major geological features of the area.

The map area lies in the central part of the western Lachlan Fold Belt, covering parts of the Stawell and Bendigo geological zones which are separated by the Avoca Fault. The oldest rocks are Palaeozoic deep marine turbidites differentiated into the Cambrian St Arnaud Group of the Stawell Zone and the Ordovician Castlemaine Group of the Bendigo Zone. Fault slices of Cambrian Pitfield Volcanics (metavolcanics of tholeiitic and ultramafic affinity known as greenstones) occur along the Avoca Fault between these sequences. The St Arnaud Group consists of the thin-bedded turbidites, mudstone and black shale of the Beaufort Formation and thick-bedded turbidites of the Pyrenees Formation. The Castlemaine Group forms most of the bedrock in the Creswick 1:100 000 mapsheet area and consists largely of thick-bedded turbidites and mudstone that yield sparse graptolite fossils.

The Palaeozoic rocks were strongly deformed during regional deformation in the Late Ordovician - Early Silurian by the Benambran Orogeny. The rocks were folded into upright, tight chevron folds with an associated penetrative cleavage, cut by numerous steeply dipping reverse faults and metamorphosed to greenschist facies grade. The Avoca Fault has an intensely strained hanging wall of multiply deformed schist and represents a major detachment fault that was active during regional deformation.

Post-tectonic I-type granitic plutons of both Early and Late Devonian age have intruded the deformed bedrock, some to shallow levels. Numerous dykes of various compositions and unknown age also intrude the bedrock. Small remnants of Permian glacial tillite lie scattered on the bedrock.

A long period of tectonic quiescence followed in post-Palaeozoic times. Erosion intensified after Cretaceous uplift related to Australia-Antarctica break-up. Several cycles of Cainozoic erosion and voluminous outpourings of basalt strongly influenced the evolution of the present-day landscape.

Significant gold mineralisation was introduced into the bedrock in quartz reefs during regional deformation. Weathering and erosion has remobilised substantial amounts of gold into some of the Cainozoic units to form placer deposits that were extensively mined in the past. The widespread occurrence of basalt has shielded large areas of prospective bedrock from conventional exploration. Significant non-metallic resources of clay, sand, gravel and basalt exist and there are well developed groundwater aquifer systems.

Bibliographic reference Taylor, D.H., Wohlt, K.E., Simons, B.A., Maher, S., Morand, V.J. & Sapurmas, P., 2000. Creswick 1:100 000 map area geological report. Geological Survey of Victoria Report 117.