Australia: The Land Where Time Began

A biography of the Australian continent 

The Sydney Basin is part of a major foreland basin, the Sydney-Gunnedah-Bowen Basin, extending from the southern coast of New South Wales to central Queensland. The Sydney Basin is about 350 km long and the width averages about 100 km. The onshore area of the basin is about 44,000 km², a further 5,000 km² extending offshore to the continental shelf.

The basement is comprised of strata of the Lachlan Fold Belt in the southwestern part of the Sydney Basin. The basement is unknown in the central and eastern portions of the basin, though they are believed to consist of strata of the Lachlan Fold belt. To the north the basement was believed to be an extension of the New England Fold Belt strata, located across the Hunter Thrust, further to the north. There is some evidence that most of the basement in the northern part of the Sydney Basin may be strata of the Lachlan Fold Belt. Another change in ideas concerning the Sydney Basin is that the basin was formerly believed to have resulted from rifting, but later interpretations of the structural history of the basin suggest compression dominated in its formation, during foreland loading many cycles of deposition many have been initiated.

Volcanic sand and silts deposited in a marine shelfal environment, basaltic lavas erupting from island volcanoes in the region of the lower Hunter Valley, are the earliest dispositions in the basin. The Dalwood/lower Shoalhaven Group were formed by this marine deposition during the Early Permian. Compression subsequently led to folding of the New England Fold Belt to the north and the development into a foreland basin in the northern Sydney Basin. In the north of the basin the alluvial fan deposits of the Greta Coal Measures received sediments sourced from the New England Fold Belt. At this time the sediments did not reach as far as the southern part of the Sydney Basin.

Towards the top of the Greta Coal Measures increasingly marine conditions resulted from basement sag, and eventually to a major transgression that covered basement as well as sediments of Permian age. The Maitland Group/upper Shoalhaven Group were formed by this transgression, that included a minor regressive phase, the unit being comprised of mixed volcanogenic sands derived from the New England Fold belt and quartz rich sands and silts derived from the Lachlan Fold Belt. In the middle of the group the Muree/Nowra Sandstone was formed from regressive New England derived sands.

The New England Fold Belt and northern Sydney basin was folded and faulted by the  following Hunter-Bowen Orogeny. major long-term regression resulted from the shedding of large amounts of sediment from the uplifted rocks of the New England Fold Belt. The Tomago/Whittingham Coal Measures of the Late Permian were formed in the north under deltaic plain to fluvial conditions. The deposition was interrupted by the transgressive Kulnura Marine Tongue/Bulga Formation. The lower Illawarra Coal Measures in the south, deposited under lower delta plain conditions, were interrupted by the Erin's Vale Formation. A marine incursion that formed the Dempsey Formation/Denman Formation/ Bargo Claystone/Ball Bone Formation terminated the formation of coal measures. The Waratah/Watts Sandstone formed in the north in a beach environment, prograding south to form the Darkes Forest/Angus Place Sandstone. The Newcastle/Wollombi Coal Measures continued to be deposited in a meandering steam that was prograding, dominated by an alluvial plain environment as regression continued. In the south more distal environments formed the upper Illawarra Coal Measures.

The Sydney Basin was effectively filled by the deposition of coal measures. A lacuna followed during which subsidence at the end of the Late Permian, that was more pronounced in the north, continued into the Early Triassic. The deposition of the Narrabeen Group, onlapping the coal measures, that was deposited with material from renewed uplift of the New England Fold Belt, in an environment of alluvial flood plains, grading into estuarine and fluvial flood plain environments. Throughout this period sedimentation varied, beginning with lithic material derived from New England, then mixed load, and eventually with quartz-rich Lachlan-derived sediment. The redbeds of the Bald Hills Claystone, in the south, formed as a result of laterite weathering Gerringong Volcanics.

The northern part of the Sydney basin is believed to have become the site of a frontal fold and thrust belt, compressive tectonism being thought to have continued into the Triassic. Tilting of the basin to the north and deposition of the Hawkesbury Sandstone, that was quartz-rich, in a fluvial environment, was the result of either Lachlan Fold Belt uplift of subsidence of the New England Fold Belt.

Jurassic to Cretaceous sedimentation, at least 1-2 km, and possibly up to 4 km thick, is believed to have occurred before it was shed by erosion as a result of the tectonism associated with the rifting of the Tasman Sea, and/or underplating of the eastern continental margin, beginning in the Aptian.

Source Rocks and Maturity

The dispersed organic matter (DOM) content of the marine and terrestrial sediments of the Sydney Basin have been found to be high, based on reflected light organic petrology and a limited amount of geochemical analysis. The DOM content ranges from 2-26 % in the Denman Formation, averaging 8 %. In the Bulga Formation the average is 11 % and 4 % in the Mulbring Siltstone. In many of these units the total organic carbon content is above 2 %, making them excellent source rocks. The TOC content of the Snapper Point Formation, Rutherford Formation and the Pebbly Beach Formation is above 1 %, so they could all be classed as good source rocks.

It has been found by kerogen typing that humic kerogen dominates in all these source rocks, the DOM being intertinite-rich, with lesser amounts of vitrinite and exinite, that is valuable.

Sources & Further reading

1. Geology of the Murray-Darling Basin - Simplified Lithostratigraphic Groupings
2. Sydney Basin - Geological Overview