60-45 Post Flood Sedimentology of the De Grey River, Pilbara Region, Western Australia

Session: 2YC and 4YCU Geoscience Student Research Poster Showcase

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Abstract:

The De Grey River in Western Australias Pilbara region, characterized by its dryland, ephemeral, and tidally influenced nature, is renowned for preserving single flood deposits. Most recently, in February 2025, Cyclone Zelia reconfigured the fluvial environment in proximal, medial, and distal areas. This historic cyclone and the ability of the riverbed to preserve these deposits allow for a detailed aftermath analysis of storms and their mass transport of coarse bed load on dryland rivers. 

We compared pre and post flood satellite imagery to define storm-specific deposits. The satellite images provided a diurect and decisive conclusion of the pre and post flood changes to the channel belt due to imaging from years prior to the most recent, being July 2025. This allowed for multiple target areas on certain cyclone deposits that were post flood. Upon the careful selection of target areas from the satellite footage, 1 m deep trenches were dug to observe and assess the post flood sediment deposition, grain size distributions, sedimentary structures, and textures.

Our results showed that after the flood, large amounts of lower flow regime structures were preserved rather than the anticipated upper flow regime structures tied to large variuable discharge rivers like the De Grey. Predominantly coarse, well-sorted, clean sands and channel lag gravels were the main types of sediment deposited. Upper flow regime unit bars were identified but were atypical. Clay and silt were largely flushed farther into the delta or the distal splays and overbanks rather than being deposited in the main channel belt due to the large volume of water discharged by Cyclone Zelia. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9839

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