Geoheritage significance of the deltas of the Pilbara Coast, north-western Australia

Semeniuk Trudi A1,4, Semeniuk Vic2,3

1Western Sydney University, The College, Quakers Hill, Australia, 2V & C Semeniuk Research Group, , , 3Notre Dame University, Fremantle, Australia

The Pilbara Coast, one of the most arid coasts in the world, has a consistent ENE orientation. It has a diverse hinterland geology and a sharp rainfall gradient on the landward side, and progressing south to north a distinct progressively microtidal to macrotidal, wave-dominated oceanographic setting. On the seaward front, processes are wave-dominated, and interior to the deltas, processes are tide dominated.  Along this unique coastal stretch, a large number of rivers discharge to the sea. This gives rise to an array of deltas, having variable size, shape, and active processes, and variable occurrence of barriers, lagoons, and landforms on the deltaic plain.  They represent a plethora of delta types that that can be characterised as active deltas, and inactive deltas. The largest drainage systems, such as the Ashburton River and De Grey River, build large classic active deltas, reflecting their large catchments and large supplies of terrigenous sand and mud that interfaces with a wave-dominated environment. Shorter rivers, such as the Robe River, have largely inactive deltas, which are heavily reworked by marine processes. Inactive deltas with shoreline retreat leave a line of shore-parallel ridges in their wake and, with beach-rock cementation, result in a series of cemented ridges that form nearshore shore-parallel rocky ridges.  The Fortescue delta and the Yannarie delta represent a special cases in the region, the former being a reworked edge of an alluvial fan system, and the latter where fluvial discharges interact with a dune field, forming unique deltaic elements. The arid tropical setting of these deltas gives rise to several distinct sediment facies, including low tidal sand flat sheets, dune sand or beach-ridge ribbons, mangrove-structured sand sheets and muddy sand sheets, muddy ribbons and high tidal mud sheets. The range of traditional and non-traditional, active and inactive delta types along this arid coast makes it globally unique ensemble of deltas and a globally significant set of deltas from a geoheritage perspective.


Trudi Semeniuk is a multidisciplinary scientist in the fields of general geology, metamorphic geology, geoheritage, wetland-, mangrove-, foraminiferal-, and tidal-flat sedimentology and ecology.  Her work experience is manifold ranging from fieldwork for VCSRG (a Research & Development Corporation), and a Research Officer for ANU, CSIRO, and Kings Park Botanical Gardens. More recently Trudi has focused on reviewing sites of geoheritage significance listed on, the now archived, Register of the National Estate (NSW) and work as a scientific editor.  Trudi was awarded a PhD from the Institute of Mineralogy and Petrography, ETH Zurich in 2003 in a study of Alpine mylonites, and completed a Postdoctoral Fellowship in 2004-2006 in aerosol chemistry at Arizona State University.  Trudi is active in Geoheritage, and is the co-convenor for the NSW Division for Geoheritage in the Geological Society of Australia.

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