Structure of the Silurian Quidong Basin: New observations on a microcosm of Eastern Lachlan Orogen tectonic and metallogenic problematica

Hood, David I. A.1, Durney,Dr David W.2, Parkes, Dr Ross A.2

1R&D Dept., Ardex Australia, Seven Hills, Australia, 2Earth & Environmental Sciences, Macquarie University, , Australia

The Quidong Basin is a small (~20 km2) structural basin made up of sediments of Wenlock to Ludlow age within the in-faulted Ordovician–Silurian Tombong Block in the Delegate area of far south-eastern New South Wales. It is a true microcosm of several contested tectonic and metallogenic problems that are common to other parts of the Lachlan Orogen of New South Wales. (1) It displays an angular unconformity with older Silurian sediments attributed to a localised convergent orogeny: the “Quidongan Orogeny”.  (2) There have been competing syngenetic and thrust-related models for the origin of stratbound and faultbound massive sulphide mineralisation in the Basin. (3) It is affected by complex superposed folding and cleavage development whose sequence has not been resolved in the published literature.  (4) Interpreted time relations between folding and prominent faulting in the area have been ambivalent.  (5) The area lacks a direct time constraint on the age of the convergent fold deformation.

A better understanding of the deformational structures and how the unconformity may have formed is also important for palaeogeogrphical reconstruction and stratigraphical correlation of fossil horizons in the faulted and folded sediments. It was for this purpose that DH carried out field observations and analysis of structures at selected sites in the area in support of palaeontological and sedimentalogical work by Ross Parkes for his PhD study at Macquarie University in the early 2000s.

We have since reviewed the structural data of DH to examine their implications for the broader questions listed above. These data have the benefit of being observational and detailed.  The methods of analysis  are well known but not often used in rocks of similar age elsewhere in the Orogen: domain analysis of fold directions and whether a time sequence can be determined in individual superposed folds, and incremental strain-axis analysis of kinematically significant minor fault, vein and stylolite associations.  We also describe a new type of structure—incoherent fault-related folds (DD)—which provides unambiguous criteria to determine the time relation between sub-parallel faults and minor folds.

From these observations and analyses we report (a) the time relation between at least two of the three known fold systems in the area, (b) the same three fold systems above and below the unconformity, which argues against a convergent deformation origin of the unconformity, (c) minor thrust and wrench kinematic data consistent with sinistral wrench reactivation of prominent NNW faults, (d) pre-fold normal movement on the prominent NNW faults, (e) a pre-fold or syngenetic origin of the massive-sulphide mineralisation, (f) no detected map-scale thrusts or inversions and (g) regional correlations which suggest a post-Late Devonian or Kanimblan age of the multiple folding.


David Hood obtained Hons in Earth Sciences at Macquarie University (1996), specialising in multiple deformations.  Employment includes James Hardie and ARDEX Australia as an Industrial Chemist.

David Durney taught structural geology and geomechanics at Macquarie University to 2006.

Ross Parkes obtained Hons then PhD (2005) in Palaeontology at Macquarie University

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