Degree Name

Doctor of Philosophy


Department of Geology


This thesis reports on a study of the stratigraphy, depositional settings and petrography of the Late Permian Illawarra Coal Measures in the southern Sydney Basin of New South Wales, eastern Australia. The depositional settings of the Sydney Subgroup, the upper of the two subgroups in the coal measures, are detailed as it contains the economic coal seams of the region.

Thirteen modifications to the currently-accepted stratigraphic nomenclature are recommended in this study. These include changes to the definitions or status of some previously-defined units and the naming of several new units.

Six depositional systems have been recognised using an hierarchical approach to facies analysis. The lowermost depositional unit, Deltaic System A, comprises the Pheasants Nest Formation and records a continuation of a regressive trend in the upper part of the marine Shoalhaven Group. This system is represented by widespread deltaic facies which grade up-dip into alluvial fan and braidplain facies, flanking the "Gerringong Volcanics".

The upper part of Deltaic System A was reworked during a transgression which resulted in the deposition of extensive, sandy, subtidal to middle shelf facies in the Shallow Marine System (Erins Vale Formation and Kulnura Marine Tongue).

A regressive phase of sedimentation is recorded in the overlying Fluvial/Shore-zone System in the east and the Braided Fluvial System in the west. Upper shoreface to beach deposits of the Fluvial/Shore-zone System were deposited along a storm-dominated, southwest-northeast trending shore-line. This was periodically charged with sediment from meandering rivers (lower Wilton Formation) and was located in the eastern part of the basin. At a similar time, alluvial fan, braidplain and fan-delta facies of the craton-derived Marrangaroo Conglomerate (Braided Fluvial System) were deposited along the western margin of the basin. The distribution of the overlying Woonona seam above a wide range of depositional units suggests the synchronous development of the Marrangaroo Conglomerate and lower Wilton Formation, and indicates a period of tectonic stability.

Several delta systems are identified in Deltaic System B. They record deposition of widespread interdeltaic facies constructed around a thin framework of distributary channels. The delta systems in the west passed up-dip into an extensive braidplain represented by the Blackmans Flat Conglomerate (Braided Fluvial System). The abandonment of these delta systems is reflected in the blanket geometry of the overlying Tongarra seam.

Progradation of a complex delta-system, Deltaic System C, resulted from expansion of the New England Orogen. The lower half of this system records deposition on a lower deltaic plain. This includes the prodelta, delta front and interdeltaic sequences of the Bargo Claystone, Darkes Forest Sandstone and Allans Creek Formation respectively. Upper deltaic plain sedimentation is recorded in the very lithic Kembla Sandstone which was deposited by a complex network of distributaries that flowed directly across the basin. The thick Wongawilli Coal formed on abandonment of this fluvial system and passes up-sequence into interdistributary lake-fill deposits in the lower Eckersley Formation. Sandstone is the dominant component of the upper Eckersley Formation and records deposition by meandering rivers in extensive fluvial tracts. The coal seams associated with these fluvial sequences (Balgownie and Bulli seams) are laterally extensive and exhibit little evidence for contemporaneous fluvial sedimentation.

Sandstone in the Sydney Subgroup can be categorised into two petrographic suites. The Quartz Suite rocks are characterised by quartz to rock fragment ratios greater than 0.5 whereas the same ratio is less than 0.5 for Lithic Suite rocks. The Quartz and Lithic Suites were derived from the Lachlan Fold Belt, and from the New England Orogen and "Gerringong Volcanics", respectively.

Tuffaceous units in the Illawarra Coal Measures of the southern Sydney Basin have been recognised on the basis of petrographic criteria.