Quantitative, multi-scale characterization of modern depositional analogues of dryland deltas in response to increasing tidal range (Gascoyne, Ashburton & De Grey River deltas, Western Australia).

Subsurface modelling of dryland fluvial-deltaic systems requires that a range of uncertainty is considered when determining the 2D and 3D geometry and spatial juxtaposition of facies. In dryland systems, the rivers typically flow only following seasonal rainfall events (a few weeks of the year either following cyclones or winter storms). This study investigates the impact of increasing tidal range and storm- generated flooding associated with seasonal river flashy flows and wave erosion events on the facies distribution of three deltas along the remote arid to semi-arid coast of Western Australia (Gascoyne, Ashburton, and De Grey River deltas). Facies distribution and quantitative geometric measures from element to complex scale are presented for the three systems, outlining the potential application as useful analogues for subsurface modelling of comparable ancient systems as the tidal influence increases. Quantitative data to characterize the deposits of these depositional systems are presented, including results from satellite image analysis, airborne LiDAR, drone based hi-resolution photogrammetry, automated satellite-derived bathymetry, shallow auger holes, river sediment sampling, grain-size analysis. As the deltas grow, distributary channels undergo avulsion to build new sub-areal splays and flood deposits, interacting with tidal flats on the lower delta plain, and growing mouth-bars at the delta front that become extensively reworked by tides and minor waves to build asymmetric deltas, increasing sand connectivity. Tidal reworked fine-medium sands are pumped up the distributary channels, especially in the dry seasons. Laterally, storm-wave generated beach ridges accrete up-drift from the mouth-bars and become moulded into aeolian dunes along their crests. High evaporation rates lead to high salinity in the lower delta plain lagoons and distributary channels, and at the De Grey, calcareous, brown ooids form within a vast delta-front tidal shoal attached to the mouth-bar complex, reworked into distributary channels and beach ridges.