Facies Characterization of The De Grey River’s Upper Delta Plain

Large dryland ephemeral deltas are important reservoirs for oil, gas, and CO2. The De Grey Delta is impacted by a macrotidal range, cyclonic storms, and a high wave base during the storms. This study defines facies characterization and spatial recognition of elements through landforms within the fluvial dominated upper delta plain. The study uses lidar scanning of the delta plain for element-scale mapping, followed by field investigation of outcrops, trenches, and shallow auger holes. The ephemeral, single thread, wide and shallow, lower-flow-regime De Grey channel, comprises medium-to-coarse grained quartz sands in cross bedded bed sets making up larger unit bars and ultimately side-attached compound bars. The trunk channel ultimately narrows after its avulsion node from 1000 to 300 m on average. Thalwegs host cobble-pebble grained planar cross sets. Floodplain elements comprise amalgamated channel splays with a fining upward lithology of pebble to fine sand to silty texture, the coarser grained, basal beds are interpreted as synchronous with the avulsion of the active channel since, the older abandoned channel is back filled and blocks flows, enhancing size of overbank sediments. Exceptionally large levees are upstream adjacent of the avulsion node, and comprise massive-bioturbated sands, silts and clays, and are highly vegetated. Splays and over bank deposits border and combined, extend up to 4 times the width with similar lengths of the channels. As barriers to prevailing seasonal wind directions aeolian dune facies add height to these levees. The floodplain is also characterized by dunes 2–3 meters high with alternating cross bed sets pointing in opposing directions due to the switching patterns of winter/summer seasons. The ephemeral processes affecting the De Grey have evolved depositional elements in critical need of analogue modeling for paleo-systems.