Paleoproterozoic regolith beneath wave-cut erosion surfaces preserved on the northern margin of the Yilgarn Craton, Western Australia

Abstract

The Archean rocks in the Joyner's Find Greenstone Belt (JFGB), on the northern margin of the Yilgarn Craton in Western Australia, were protected and undisturbed until the mid-Cenozoic to Recent erosion of a thick cover of Paleoproterozoic sedimentary rocks. The erosion has exhumed the Paleoproterozoic erosion surfaces, and thick underlying saprolite developed on Archean rocks of the JFGB and surrounding granitic rocks of the Yilgarn Craton in Western Australia, providing a clear picture of the state of the Yilgarn Craton's northern margin at 2 Ga BP. During the late Archean to mid-Paleoproterozoic eras, what is now the northeastern margin of the Yilgarn Craton was reduced to a low-relief deeply weathered landscape with large areas of granitic rocks and minor greenstone belts similar to that existing at the present. An extremely arid oxidising climate caused lowering of the water-table to >200 m below the surface, dehydrating goethite in the saprolite to hematite. Minor subsidence and/or a rise in sea-level was followed by a Paleoproterozoic marine transgression at ca 2.2 Ga that removed only the upper part of the weathered profile and eroded wave-cut surfaces on >200 m of saprolite that was inundated by sea water. The Paleoproterozoic Yerrida Basin shelf sediments were only slightly deformed and unmetamorphosed, as the craton margin was tilted at 12° to the northwest during the Glenburgh Orogeny. The area was then uplifted, and owing to the tilting of the craton margin, renewed erosion uncovered the southern part of the wave-cut erosion surface and the previously weathered saprolite, as well as unweathered Archean rocks to the south, again reducing the area to a landscape of low-relief. New oxidised weathering profiles were then overprinted on the previous saprolite. A second transgression at ca 1.8 Ga eroded a new wave-cut surface across the tilted Yerrida Basin and the uplifted Archean rocks to the south. Epeirogenic sediments of the Earaheedy Group were deposited covering the Archean rocks for nearly 2 Ga until a third cycle of erosion completely removed the Earaheedy Group and more of the Yerrida Basin from the JFGB, exposing the Proterozoic saprolite and wave-cut surfaces on deeply weathered silicified jaspilitic banded iron formation to renewed oxidative weathering.