FACIES ARCHITECTURE OF ALLUVIAL FLOODBASIN DEPOSITS - 3-DIMENSIONAL DATA FROM THE UPPER TRIASSIC CALLIDE COAL MEASURES OF EAST-CENTRAL QUEENSLAND, AUSTRALIA

A detailed investigation of floodbasin facies architecture was underta ken in the Upper Triassic (Carnian-Rhaetian) Callide Coal Measures in east-central Queensland, Australia, using extensive highwall and explo ration borehole data from ongoing mining activities. The composite Cal lide Seam Member varies up to 23 m in thickness and is locally split b y a number of elastic partings up to several metres thick, ranging fro m claystone to coarse sandstone. A subset of the nine lithofacies reco gnized in surface exposures was identified fi om geologists' logs of c ored and uncored drillholes through the Callide Seam Member. Facies ma pping of each elastic parting (split) was then undertaken using all av ailable highwall and drilling data. Sequential maps of facies and inte rval thickness for each coal body and elastic parting over the mine ar ea (6000 x 2500 m) record sediment accumulation in alluvial channel an d floodbasin environments (including levees, splays and splay complexe s, and mires). The maps indicate that the numerous splays have dominan tly elongate plan geometry (up to 4 km long), with lesser irregular an d rare lobate shapes. Small, elongate splays were evidently formed dur ing single flood events, whereas larger, elongate bodies and more irre gularly shaped complexes were the product of longer-term splay constru ction over several flood cycles. Quantitative summaries of splay dimen sions indicate a wide variety of shape and size. The distribution of s play orientations is similar to the palaeocurrent distribution in majo r alluvial channel deposits as established from cross-bedding. Alluvia l channels that sourced the splays and other elastic sediments within seam splits were of low sinuosity, braided planform, constructed sedim ent bodies up to 2800 m wide and were dominantly loaded rather than in cised into underlying peat-rich substrates.