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Linked Evolution of the Holocene Mitchell River Megafan and Delta, Gulf of Carpentaria, Australia

Lane, Tessa I.; Nanson, Rachel A.; Ainsworth, Bruce; Vakarelov, Boyan; Amos, Kathryn

The Mitchell River megafan extends east to west across Cape York, Queensland, Australia. Its complex network of modern and palaeo-distributary channel belts drain westward into the Gulf of Carpentaria. Previous workers have demonstrated that while the megafan was initiated in the Pliocene, the modern Mitchell River delta commenced progradation in association with much younger fan units in the last 6 ka BP (Holocene). The timing of the Holocene channel belt avulsions, their sedimentology and linkage to delta progradation are the focus of this research.

Detailed mapping of the cross-cutting relationships of palaeochannel belts has revealed that the evolution of the delta is intimately linked to avulsions on the megafan surface; a series of major avulsions have shifted sediment deposition from the south towards the northern limb of the fan to feed the Mitchell River delta. Abandoned palaeochannels on the southern limb of the megafan have become tidally modified at their seaward extents. Accurate topographic surveys revealed that the channel belts display a distinct ridge and swale topography and are elevated up to two meters above the surrounding floodbasin. Trench and auger data have also demonstrated that these channel belts display a general fining upwards succession of sand capped with mud. Sonic coring of the system is currently underway and detailed facies descriptions of each major depositional element from the delta and lower fan will be presented. Dating evidence (OSL, TL and C14) constrains the relative and absolute timing of channel belt switching and delta progradation.

The Gulf of Carpentaria offers a unique opportunity to study a fluvial megafan supplying a mixed-influence (tide-dominated, fluvial-influenced, wave-affected; Tfw) delta in a low gradient, low accommodation setting. Furthermore, the Mitchell River megafan provides information about the interaction between tidal, wave and fluvial processes in a region with limited anthropogenic influence. The study of the Mitchell River megafan and delta can be used as a modern analogue for similar palaeo-environments to improve subsurface reservoir models. Learnings regarding the linkages between megafan evolution and delta development and their interplay with forcing factors such as climate, sediment supply and accommodation development during the Holocene may also be used to improve predictions in ancient settings.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013