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1A Wudgong St, Mosman Bridge Oil Ltd, Sydney 2088, Australia
Upper Carboniferous and Permian coals of eastern Australia (Bowen, Sydney, Cooper, Galilee and satellite basins) and southern Africa (Karoo Basin) resemble one another in their general composition and associated clastic facies. The Australian examples, however, show more petrographic and geometric variability than their southern African Gondwana counterparts.
Critical controls on the distribution and quality of coal in both regions appear to have been (1) a near-polar location with accompanying periglacial conditions, (2) Permian through early Triassic warming independent of any significant change in palaeolatitude, (3) associated floral changes from tundra vegetation to swamps and deciduous Glossopteris forests, (4) variable subsidence and sedimentation rates in foredeep, rift, and epicratonic basin settings, (5) local palaeotopographic effects, (6) eustatically induced changes in base level and marine transgression, and (7) a range of depositional systems from proximal conglomeratic alluvial fans through fluvial, delta-plain, lake-margin, back-barrier, and blanket peat mires in sediment-starved basins.
In most cases, palaeotopography and depositional environment were the most important factors in determining coal seam geometry, and had considerable influence on coal quality, specifically its maceral content. Most of the commercial seams are contained in fluvial and delta-plain sequences, with thick, dip-elongate interchannel seams and really extensive coals typically overlying abandoned delta lobes. However, some important seams are juxtaposed with conglomerates of wet alluvial-fan origin or are associated with quartzose barrier sandstones; others originated in clastic-starved systems bordering shallow lakes or in blanket swamps that received moisture from groundwater discharge or direct rainfall.
Some seams display characteristics that were primarily controlled by other more regional factors such as rapid tectonic subsidence, changes in groundwater regime, or gradual marine transgression. While influencing the thickness and areal extent of seams, these factors were even more important in determining microlithotypes, maceral content, and inorganic composition of the coals.
Not only is coal mining crucial to the economies of both Australia and S Africa, with further expansion likely in the coming decades, but coal and dispersed coal particles have generated billions of barrels of commercial oil reserves along with large volumes of gas. Australian petroleum reserves are predominantly of land-plant origin, and source-reservoir relationships provide important analogues for exploring other non-marine basins.
Key Words: coal • Gondwana • southern hemisphere • Australia • S Africa • basin analysis • petroleum
