The present geological architecture of the Gloucester Basin suggests that it has experienced a complex structural history. Early normal and syn-depositional faults are widespread and extensive, and in many cases have been reactivated by the later Hunter-Bowen Orogeny (AGL, 2010). The main tectonic episodes proposed for the Permian development of the basin are summarised below (Grieves and Saunders, 2003; SRK Consulting, 2005, 2010):
- Early to Late Permian extension resulted in normal and strike-slip reactivation of older pre-Permian faults, particularly around the margins of the basin and the development of syn-depositional faults (north and east). This coincides with the phase of extension and sedimentation defined by Cawood et al. (2011) in the Sydney-Gunnedah-Bowen Basin and coeval with roll back of the Pacific Plate. Fault activity has been interpreted during the deposition of the Early Permian Alum Mountain Volcanics (Roberts et al., 1991, p. 167). Faulting also controlled (to varying degrees) the deposition of most Permian coal-bearing strata in the basin (Roberts et al., 1991, p. 284; Harrington et al., 1989, p. 64).
- Late Permian compression (north-east shortening) and tilting of the basin during the early stages of the Hunter-Bowen Orogeny, resulted in reactivation and inversion of many faults, and new thrust faulting and erosion.
Harrington et al. (1989, p. 64) described the paleogeography of the Gloucester Basin. Deposition in the basin was initiated and controlled by northerly faults active during sedimentary infilling. This began in the Early Permian period, when fluvial gravel and sand deposits formed at the base of the unit and were interbedded with the Alum Mountain Volcanics.
By the late Early Permian period, sedimentation occurred in a shallow near-shore marine environment with a barrier and freshwater lagoon complex and wave-dominated fan delta system (Dewrang Group). Relative uplift of the surrounding highlands led to a change in sedimentary dynamics at the start of deposition of the Gloucester Coal Measures, marked by fluvial valley infill deposits (Avon Subgroup; Hancock, 1974). A marine transgression followed with the deposition of beach and marine sands (Speldon Formation). The remainder of the Gloucester Coal Measures has significantly less marine influence, and includes evidence for a depositional hiatus (Wenham Formation) overlain by advancing alluvial fan deposits of the Wards River Conglomerate, after the reactivation of marginal faults (Harrington et al., 1989, p. 64). The Jilleon, Leloma and Crowthers Road formations were deposited in terrestrial meandering, alluvial plain and braided outwash environments (Harrington et al., 1989, p. 64; Roberts et al., 1991, p. 184).
Figure 24 Permian lithostratigraphy in the Gloucester Basin
1The duplicate use of Avon is not formally recognised in the Australian Stratigraphic Units Database
Source data: (i) Brown et al. (1996), (ii) AECOM (2009), (iii) SRK Consulting (2010), (iv) Pells Consulting (2012) and (v) the Australian Stratigraphic Names Database.
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