Debris-flow deposits in an alluvial-plain succession: The Upper Triassic Callide Coal Measures of Queensland, Australia

The Carnian-Rhaetian Callide Coal Measures are preserved in a small (22.5 k m by 8 km), partially fault-bounded basin remnant in east-central Queenslan d, Australia. The < 150 m thick coal-measure succession is interpreted to h ave accumulated during a phase of mild crustal extension that formed a seri es of discrete, intermontane basins in eastern Australia. The succession fi nes upward from a conglomerate-rich lower part into a finer-grained and coa l-bearing upper section (including coal seams < 34 m thick), and is interpr eted as the deposits of an alluvial-plain environment. Anomalous, matrix-ri ch diamictites, breccias, and conglomerates have been recognized within the succession at several localities, in many cases interbedded with coals. Th ese are interpreted as the product of debris flows. Two debris-flow lithofacies are recognized: (1) mixtures of fine carbonaceo us material, clay, silt, sand, gravel, and volcaniclastic debris, and (2) b reccias consisting principally of coal clasts in a coaly matrix with minor elastic and volcaniclastic debris. Facies 1 is found in sharp-based, simple or composite bodies (< 15 m thick) that are elongate to lobate in plan (wi th some multi-lobate), and as much as at least 1500 m long and 600 m wide. Facies 2 is less common and occurs in sharp-based units as much as 1.5 m th ick, at least 200 m in length, and 60 m in width. Both facies are character ized by abundant fine matrix and a very poorly sorted clast population, and show little internal organization or sedimentary structure other than vari ably developed imbrication of coarse clasts. Parts of some bodies show an e rosional base (with up to several meters of coal removed), although a flat, apparently non-erosional contact is more common. The distribution of debris flows in the Callide Coal Measures shows a coinc idence with mapped faults and interpreted structural lineaments, The debris flows may have been triggered by fault movements, which formed rupture top ography on the flat alluvial plain, and caused destabilization of water-sat urated elastic and organic sediments. Some debris-flow bodies may have been mounded, such that subsequent peat formation was restricted until those bo dies were buried. The preservation of debris-how units at different stratig raphic levels along mapped structures suggests multiple paleoseismic events or multiple debris-how events along those structures, The mixing of volcan iclastic debris into debris-flow facies suggests that seismic events were c oincident with (or perhaps caused by) nearby, explosive volcanic activity. The close relationship between debris-flow deposits and thick coal bodies o n the inferred downthrown sides of faults at Callide further suggests that periodic, tectonic subsidence may have facilitated thick coal accumulation.