Lang, Prof. Simon1, Paumard, Victorien1, O’Leary, Mick1, Goodwin, Ian1, Cousins, Victoria1, Lebrec, Ulysse1, Jian, Andy1, Holbrook, John2, Smith III, Pomeroy2, Hasiotis, Stephen3, Vakarelov, Boyan4 & Krapf, Carmen5
1University Of Western Australia, , Australia 2Texas Christian University, Dallas, Texas, USA 3Kansas University, Lawrence, Kansas 4Sedbase, OOD, Sophia, Bulgaria 5Geological Survey of South Australia, Department of Energy and Mining, Adelaide
Marine deltas are controlled by the dominance of fluvial outflow (F) relative to the influence of waves (W) and tides (T) that control facies distribution. However, in dryland deltaic systems, the rivers typically flow only following ephemeral or seasonal flooding events (i.e., a few weeks of the year either following cyclones or winter storms).
This study focusses on the influence of increasing tidal range and wave power on the coastal geomorphology of three coastal deltas along the arid to semi-arid coast of Western Australia (Gascoyne, Ashburton, and de Grey river deltas), and the role of distributary channel avulsion that build large distributary fluvial systems. Satellite image-derived bathymetry, dGPS transects, digital elevation models, ground penetrating radar, auger holes and outcrops are used for facies mapping of the surficial stratigraphy. Initial results show internal geometries and facies distribution of channel bar forms (dominated by coarse- and medium grained downstream and lateral accretion macroforms), alluvial and delta plain silt- and mud-prone oxidised overbank facies, sandy and silty coastal plain tidal-flats, tidal channels, well-sorted fine-grained sandy strandplain beach ridges, and fine-medium grained distributary mouth bars.
Fluvial distributary channels undergo upstream avulsion and new fluvial mouth-bars grow, and alluvial flood deposits accrete or are eroded on the upper and lower delta plain. Most of the year, the mouth-bars are reworked by waves and tides to build asymmetric, mixed-influenced deltas, and aeolian processes rework the delta/coastal plain. As the tidal range increases from micro-tidal to meso- and macro-tidal, the individual mouth-bar elements become amalgamated into a very broad sand-prone delta front, increasing sand connectivity. Tidal reworked sands are pumped up the distributary channels in the lower delta plain, 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 delta plain distributary channels, coastal lagoons and salinas. Calcareous ooids in foreshore deposits are preserved in some of the beach-ridges.
The 2D and 3D geometry and spatial juxtaposition of facies has implications for the range of uncertainty in subsurface reservoir/aquifer modelling of dryland fluvial-deltaic reservoirs.
Professor Simon Lang is a sedimentologist and stratigrapher with global experience including regional geological mapping, sedimentology/stratigraphy research, and petroleum exploration & development. He is Director of the Centre for Energy Geoscience, University of Western Australia, leading industry-funded research on quantitative seismic stratigraphy and reservoir analogues.