Stratigraphy and rock type

The Gloucester Basin includes the known extent of the Permian Coal Measures and Alum Mountain Volcanics (Figure 24). The Permian sequences lie unconformably on low permeability Carboniferous strata and no flow boundaries are assumed along the edges of the Permian basin (Merrick and Alkhatib, 2012, p. 37).

The complex interplay of tectonic extensional faulting and high rates of sediment supply produced significant lateral stratigraphic variability in the Permian sequences of the Gloucester Basin (AGL, 2010). Alum Mountain Volcanics

The basal stratigraphic unit of the Gloucester Basin is the Alum Mountain Volcanics, an approximately 200 m thick Early Permian sequence of bimodal volcanic and interbedded sedimentary rocks. Some volcanic flows are separated by thin layers of siltstone, conglomerate and coal (AECOM, 2009, p. 1723). This volcanic unit lies unconformably on Late Carboniferous conglomeratic rocks, such as the Muirs Creek Conglomerate. Dewrang Group

The early Late Permian Dewrang Group, which formed in shallow to marginal marine environments, disconformably overlies the Alum Mountain Volcanics. It consists of coarse- and medium-grained sandstone interbedded with lesser siltstone, shale, conglomerate and coal seams. There are three stratigraphic formations in the Dewrang Group which, from oldest to youngest, are the Durallie Road Formation, Weismantels Formation, and Mammy Johnsons Formation. Coal seams occur in the two uppermost formations. Gloucester Coal Measures

Avon Subgroup

The Late Permian Avon Subgroup is the stratigraphic base of the Gloucester Coal Measures. It is divided into two formations (Waukivory Creek and Dogtrap Creek formations) that between them contain seven coal seams.

  • Waukivory Creek Formation

In contrast to the older Dewrang Group, the deposition of the Waukivory Creek Formation occurred in a terrestrial coastal plain to upper delta plain environment (AECOM, 2009, p. 17–4). This formation contains five discrete coal members which, from oldest to youngest, are the Parkers Road, Valley View, Glen Road, Rombo, and Triple coal members. These coal seams are best developed in the eastern basin (AGL, 2010). The Parkers Road Coal Member is widespread in the basin with thick seams of approximately 5 m (AECOM, 2009, p. 17–4).

  • Dog Trap Creek Formation

The Dog Trap Creek Formation consists of three coal members: the Bucketts Way, Glenview and Marker Two coal members. The formation is characterised by coarsening upward sedimentary sequences, bioturbated mudstone layers and crevasse splay structures. It was deposited in a lower delta plain environment which formed during marine transgression. Minor fluvial-derived deposits are interpreted as evidence for restricted uplift or tectonic activity, with a number of growth faults reported (Roberts et al., 1991, p. 179; AGL, 2010).

Speldon Formation

The deposition of the Speldon Formation is interpreted as the culmination of the marine transgression which formed the earlier Dogtrap Creek Formation. It contains well bedded medium- to fine-grained sandstone with minor siltstone layers. The depositional environment precluded development of significant coal. Growth faults are reported in this unit (AECOM, 2009, p. 17–4).

Craven Subgroup

The Late Permian Craven Subgroup forms the upper part of the Gloucester Coal Measures and is divided into five formations which include nine coal seams. The Craven Subgroup has significantly less marine-derived sediment than the Avon Subgroup or the Speldon Formation.

  • Wenham Formation

The Wenham Formation contains about 25 m of fine-grained sandstone and coal (Bowens Road Coal Member).

  • Wards River Conglomerate

The Wards River Conglomerate is widespread in the Gloucester Basin and contains conglomerate with minor sandstone, shale and rare carbonaceous shale which was deposited in an alluvial fan environment. Although it forms the entire Gloucester Coal Measures in the western part of the basin, it is very much reduced in thickness along the eastern margin where it occurs stratigraphically above the Wenham Formation (AECOM, 2009, p. 17–5).

  • Jilleon Formation

The Jilleon Formation consists of a fine-grained sandstone, shale and mudstone sequence with four coal members. It was deposited in an alluvial plain environment (AGL, 2010). The Jilleon Formation contains the Roseville and the less consistent Tereel (or Fairbairns Lane) coal members in the basal sequence and the Cloverdale Coal Member in the upper sequence. The Cloverdale Coal Member contains a distinct tuff band used for correlation between wells. The formation onlaps and is eventually replaced by the Wards River Conglomerate in the west of the basin (AGL, 2010).

  • Leloma Formation

The Leloma Formation (or Woods Road Formation) contains siltstone, sandstone and numerous thin coal seams, such as the Deards, Bindaboo, Jo Doth and Linden coal members as well as several thin unnamed coals (AGL, 2010). The sediments which formed these rocks were deposited in an upper alluvial plain environment. Correlation of coal seams is particularly difficult in this formation as they vary significantly in thickness and commonly split over relatively short distances (AGL, 2010).

  • Crowthers Road Conglomerate

The Crowthers Road Conglomerate marks the stratigraphic top of the Permian sequence in the Gloucester Basin. It consists of conglomerate and medium- to coarse-grained sandstone deposited in a series of alluvial fans which were sourced from the older Carboniferous rock formations to the west and north of the basin. It is generally confined to the western and northern basin (AGL, 2010). Quaternary sediments

The surficial sediments in the Gloucester Basin consist of unconsolidated alluvial and swamp sediments (sand, gravel, silt and clay) which variably infill the valley floor of the main rivers and creeks. The alluvial sediments do not have a consistent thickness and generally conform to the basin paleovalleys (SRK Consulting, 2010). The sediments are geologically young (Quaternary) and are not formally named.

Last updated:
5 January 2018