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The Evolution of the Mitchell River Megafan and its Control on the Distribution of Delta Facies

Abstract

The Mitchell River megafan extends east-west across the Cape York Peninsula (Australia) and its network of modern and palaeo-distributary channel belts drain westward into the low gradient Gulf of Carpentaria and have formed the Mitchell River delta. The key foci of this research are to determine: (1) the timing of channel belt avulsions, (2) the control that these avulsions exert on depositional loci on the coastal plain, and (3) the internal sedimentology of their associated facies. Detailed mapping of the cross-cutting relationships of palaeochannel belts has determined the sequence of channel belt activity. A series of major avulsions have shifted sediment deposition progressively northwards during the Holocene. New Optically Stimulated Luminescence (OSL) dating has refined these episodes of channel belt avulsion to discrete periods within the Holocene. These avulsions have been caused by combinations of tides, indurated sediment horizons, the backwater effect and large seasonal variations in discharge resulting from the monsoonal climate setting. Calculations of backwater length demonstrate that avulsion nodes are concentrated at the upstream portion of the estimated backwater zone; the backwater length is at least in part controlling nature of avulsion and the distribution of channel fill facies. The architecture of the delta is intimately linked to avulsions on the megafan surface; as fluvial channels are progressively abandoned, channels below the backwater limit become increasing affected by tidal processes. The result is the infilling of progressively more tidally influenced channels by increasingly heterolithic material, such that fluvial (F) channels through to tidal channels (T) demonstrate decreasing proportions of sandy fill. Furthermore, delta deposits adjacent to active channels (delta lobes) differ significantly from those adjacent to post depositionally incised channels. A new classification system has been used to map these facies associations, herein termed Element Complexes. New core, OSL and ground penetrating radar (GPR) survey data are presented to illustrate the architecture of these Element Complexes and to demonstrate lateral associations between these delta units.