Captains Flat Explanatory Notes

Abstract

The Captains Flat Special 1:50 000 map sheet area covers a segment of the eastern Lachlan Orogen in the Southern Tablelands of New South Wales. The map area hosts extensive exposures of the lower (middle to Late Silurian) parts of rift basins (Goulburn Basin and Hill End Trough) which contain carbonate sequences and deep-water felsic to bimodal volcanic centres which are known to host VAMS deposits (e.g. Woodlawn Volcanics and Currawang Basalt of the Mount Fairy Group, Kohinoor Volcanics of the Hoskinstown Group). Extensive Late Silurian bimodal plutonic rocks (Thurralilly Suite) intrude basement beneath the basin sequences. The Silurian basin sequences developed in a back-arc setting on a substrate of Ordovician siliciclastic turbidite and black shale (Adaminably, Bendoc and Margules groups) that was deformed and thickened during the preceding Benambran Orogeny. The development of this back-arc basin resulted in the creation of ~2 km of depositional space that was rapidly filled by thick, post-rift turbidite and mass-flow sequences in the latest Silurian (Sinclair Conglomerate Member and Carwoola Formation of the Hoskinstown Group, Covan Creek Formation of the Mount Fairy Group). The termination of rifting and filling of the basin was possibly the result of localised uplift during the latest Silurian Bowning event. A transition to subaerial conditions and renewed magmatism in the Early Devonian is preserved to the south and east of the CaptainsFlat50, which saw plutons of the Glenbog* and Candelo* suites intrude close to the calcareous base of the Silurian successions along the eastern margin of the basin, along with deposition of extensive ignimbrite deposits. The entire area underwent compressional deformation during the Middle Devonian (Tabberabberan Orogen) and early Carboniferous (Kanimblan Orogen), producing regional-scale folding and complex fault systems. The Captains Flat Special 1:50 000 mapping follows the completed second editions of the Braidwood 1:100 000 and Goulburn 1:250 000 geological maps and the acquisition of detailed airborne geophysics over the southeastern part of NSW. The project focused solely on, and completes the recent mapping of the mineralised middle Silurian to Early Devonian Goulburn Basin of the southeastern Lachlan Orogen.

Reference

Fitzherbert J.A., Deyssing L., Sherwin L., Bruce M.C. & Carlton A.A. 2017. Captains Flat Special 1:50 000 geological map. Explanatory Notes. Geological Survey of New South Wales, Maitland, NSW.