Dryland Coastal Deltas of Western Australia—Reservoir Analogues for Mixed-Influenced Fluvial-Deltaic Depositional Systems

The impact of increasing tidal range and storm-generated flooding on deltas is investigated using three dryland deltas along the arid to semi-arid coast of Western Australia (Gascoyne, Ashburton, and De Grey river deltas). New element complex scale maps are presented based on satellite data, LiDAR, drones, augers, coring, laser particle size analysis and carbon-dating. The element complex scale maps reflect reservoir scale geobodies produced by F-, W-, & T-dominated processes.

Upstream avulsion during major flood events produce fluvial-dominated distributor channels in the upper delta plain, feeding tide-influenced to tide-dominated distributaries on the lower delta plain, and asymmetric wave- to tide-dominated mouth bars on the mixed-influenced delta front. As the tidal range increases, the individual mouth-bar elements become increasingly amalgamated into a very broad sand-prone delta front, increasing lateral sand connectivity at the element complex scale. Tidal reworked sands are pumped up the distributary channels, especially in the dry seasons, where they are highly bioturbated, and homogenized. Laterally, waves generate highly elongate beach-ridges accreting up-drift from the mouth-bars and becoming moulded by aeolian processes. Down-drift, tidal flats pre-dominate, stabilised by mangroves that also line the mud-prone tidal creeks.

High evaporation rates lead to high salinity in the lower delta/coastal plain. On the De Grey delta, brown ooids (aragonite-coated terrigenous fine sand) develop on the tide-dominated mouth bars and shoreface, then washed into beach ridges and tide-dominated channels. 2D and 3D geometry and spatial juxtaposition of facies has implications for the range of uncertainty in subsurface reservoir modelling of dryland fluvial-deltaic reservoirs.