Fergusson, Chris1 & Colquhoun, Gary2
1School of Earth, Atmospheric and Life Sciences, University of Wollongong, Wollongong, Australia, 2Geological Survey of New South Wales, Mining, Exploration & Geoscience, Department of Regional NSW, Maitland, Australia
The timing of the regional deformation of the turbiditic Silurian-Devonian Hill End Trough in the northeastern Lachlan Orogen has been a contentious issue with one view ascribing the regional north-south folds and axial planar foliation to Middle Devonian basin inversion. Alternatively, it has been argued that given the low-angle discordance between Lower and Upper Devonian units on the Capertee and Molong highs, and the development of a dominant episode of folding mapped from the Hill End Trough into the adjoining highs, the major regional deformation is latest Devonian to early Carboniferous and predates intrusion of Bathurst-type granites at 358–314 Ma. We have approached the problem by analysing the gentle, upright, southeast-trending folds and deformation patterns in Devonian units of the northern Capertee High (Cudgegong area), drawing a cross-section across the Hill End Trough and reviewing the structure of two areas in the Molong High (west of Orange, and south of Wellington). Upper Devonian units on the highs are affected by fold patterns that extend into the underlying successions and have been interpreted as continuous into the central Hill End Trough, confirming the extent of the latest Devonian to early Carboniferous regional deformation in the northeastern Lachlan Orogen. Constraints from timing of deposition and radiometric ages are ambiguous and imply deformation was overlapping with sedimentation, as has also been inferred from sedimentary features of the Upper Devonian succession. Steeply dipping faults along the highs predated the Late Devonian and were probably formed in extension during formation of the Hill End Trough and were reactivated during basin inversion and potentially during Upper Devonian sedimentation. Our conclusion is that the simplest hypothesis is that the regional north-south deformation was of latest Devonian to early Carboniferous age, although poorly understood basin inversion occurred in the Middle to Late Devonian.
Chris Fergusson graduated in geology in the 1970s/early 1980s and has been lecturing in geology at the University of Wollongong up until retirement earlier this year. He has maintained long-standing interests in the tectonics of orogenic belts (including the Tasmanides) but also in deep-sea drilling, and Sydney Basin geology.