Neoichnology of the De Grey Ephemeral River Delta, Northwest Coast, Western Australia: Proxies for Environments and Physicochemical Conditions in Deep Time

We present results to date on the neoichnology of the wave-dominated De Grey delta, being deposited by the dryland, ephemeral De Grey River, which typically flows only after seasonal rainfall events generated by tropical, monsoonal cyclones. These highly variable events are also associated with significant storm-wave reworking by cyclones. The region has a hot arid climate (Köppen BWh), with annual rainfall falling between December and June of 311.5 mm but is subject to some of the largest variations because of erratic cyclones.

The neoichnology of the alluvial plain, delta plain, and proximal delta front settings clearly show diagnostic patterns of plant and animal traces (and diversity), lithofacies, groundwater profiles, and salinities controlled by the climatic setting. Channels in the alluvial plain are mostly devoid of water and water holes have salinities of fresh (<0.5 ppt) to lower oligohaline (<2 ppt); traces of mole crickets are abundant along the margins of these waterbodies. The alluvial plain itself contains termite nests, ant nests, goanna and smaller reptile and mammal burrows. The delta plain contains traces produced by terrestrial and marine fauna; plants and terrestrial fauna dominates overbank deposits of the delta plain, whereas marine invertebrate and vertebrate fauna dominate the distributary channels, which vary in salinity from upper oligohaline (4–5 ppt) to hypersaline (50 ppt). Supratidal areas contain mangroves and are dominated by a variety of crabs, polychaetes, and other vermiform animals, which increase in diversity to the intertidal areas and have the highest salinities.