An integrated analysis of the Callide Basin, east-central Queensland

Abstract

The Callide Basin formed as a half-graben during an episode of crustal extension affecting Eastern Australia in the Late Triassic (Camian-Rhaetian). Continued subsidence of this structure facilitated the accumulation of a thin alluvial sequence of clastic and organic sediments, plus minor primary volcanic debris, in a wholly continental setting. The interbedded conglomerates, sandstones, mudrocks, coals and volcanic rocks of the presentday Callide Coal Measures now represent the eroded remnants of this basin fill sequence. The Callide Coal Measures succession takes the form of a grossly fining upward megasequence, comprising a lower, conglomerate dominated association which contains virtually no coal, and an upper finer-grained association containing all significant coal seams. The contact between the two associations is an abrupt gradation in most areas. Utilising extensive outcrop, mine highwall and exploration borehole data, eleven lithofacies, consisting of three distinct lithofacies associations, have been recognised in the Callide Coal Measures, and interpreted as the record of sediment accumulation in alluvial fan, channel and floodbasin environments, and debris flows. The lower portion of the coal measures was deposited in high-energy, streamdominated alluvial fan environments, and is dominated by conglomerates and associated finergrained deposits typical of such settings. Supply of coarse detritus then rapidly waned, allowing finer-grained fluvial systems to become established and, under appropriate circumstances, peat mires. The deposits of the upper association accumulated in a variety of environments including broad, shallow channels, crevasse splays and splay complexes, levees, shallow floodbasins and peat mires. A series of facies distribution maps for current mining areas, based on successive clastic partings preserved within the Callide Seam Member (the thickest coal-bearing interval in the coal measures), illustrate the facies relationships of its component lithosomes. The alluvial channels that sourced the splays and other clastic sediments within seam partings were of low sinuosity, braided planform, up to 2.8km wide and dominantly loaded rather than incised into underlying peat-rich substrates. The numerous splays in the Callide Seam Member have dominantly elongate plan geometry (up to 4 km long), with lesser irregular and rare lobate shapes. Small, thin splays were formed during single flood events, whereas the more common larger, elongate to irregularly shaped splay complexes were the product of longer-term construction over several flood cycles. The distribution of splay orientations is similar to the palaeocurrent distribution in major alluvial channel deposits as established from cross-bedding structures. Quantitative analysis o f splay dimensions reinforces the wide variation in size, shape and arrangement o f the bodies. Overbank flooding at high water stages led to the development, of elongate levees parallel to the margins of channel belts, and more irregular shaped deposits of mud-grade sediments in areas distal to channel and splay activity. The unusually thick coal deposits within the Callide Seam Member were evidently formed due to preferential accumulation o f peat materials in actively subsiding, syndepositional depressions located on the hanging-wall side of intrabasinal faults. Two types of debris flow deposits, a typical elastic-dominated form and a unique, coal-rich variety, are preserved within the Callide Coal Measures. These units occur throughout the basin fill but are most common within the coal-bearing portion of the sequence. Initiated along localised intrabasinal fault scarps by tectonic “shocking” of watersaturated, semi-consolidated coal measure sediments, these debris flow deposits were preferentially preserved within many of the same fault depressions as the thick peat/coal deposits of the Callide Seam Member. Recognition of clastic debris flow deposits in the subsurface is aided by the distinctive neutron-gamma responses often recorded from the bodies when surveyed in exploration boreholes with wireline geophysical equipment. It is recognised that the Callide Basin was influenced by active tectonism during much o f its depositional history. Many of the controls on the location, physical dimensions and (in part) composition of the sediment packages comprising the Callide Coal Measures, and particularly the coal deposits of the Callide Seam Member, are now interpreted to have been related to tectonic processes occurring within the basin. As a result of this, the most conspicuous sedimentary features of the basin fill (thick coal sequences, debris flow deposits) display close affinities to the basin structure, and these relationships can be utilised to streamline coal exploration within the basin.