Towards a Model of Scrolled Point Bar Sedimentology in Tropical Savannah Channel Belts: A Case Study of the Upper Delta Plain of the Mitchell River, Australia

The Mitchell River catchment is situated in the wet-dry tropical savannah of northern Australia. Flows are strongly seasonal: 80% of flow is received in 4 months, with warm and arid conditions for the remainder of the year. Field investigation of the composition and structure of channel belts on the Mitchell River upper delta has shown that the architecture of these deposits deviates substantially from described scroll sequences in humid regions. Few published studies of scroll-bar sedimentology exist; this research has important implications for interpretation of similar ancient deposits, and corresponding hydrocarbon reservoirs.

Mapping of the upper delta reveals that channel belts are comprised of single-thread channels alternating between confined and wider zones of scrolled floodplain. Most of these palaeochannel belts receive flow during large flood events and demonstrate tidal influence in downstream reaches. Modification of these palaeo-features is ongoing and may be a function of the confinement of most overbank flow to the incised palaeo-valleys.

Sediment logs were recorded across 1km transects spanning 2 representative channel belts. The water table was encountered almost universally 2m below the surface, indicating excellent fluid connectivity across each belt. The surface 0.5 - 1m are comprised of silty material, generally thicker in the swales, underlain by fine to medium sand. The upper surface of sand beds within the scrolls dip away from crests at angles of 10-18°. While the surface topography generally corresponds to the subsurface stratigraphy, surface relief is more subdued than the original scroll-swale forms; subsequent deposition of finer material over these features has partially infilled the swales and thinly capped the scroll bars resulting in surface dip angles of only 1-2°.

In the humid scroll model, more distinct scroll topography persists through time because vegetation growth favours drier scroll crests and encourages sediment deposition during overbank flows on these features. Similarly paced sedimentation occurs in the swales during falling flow stages. Conversely, the seasonally arid conditions in the Mitchell River encourage the densest vegetation to grow in moister swales; maximum rates of sedimentation are focused there. OSL dates for these features will be used to refine cross-cutting relationships and internal architecture for the purposes of improved modelling of reservoir deposits in similar settings.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California