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.

澳大利亚西部Yilgarn克拉通北缘Joyner’s Find 绿岩带（JFGB）的太古代岩石一直因覆盖而未遭搅扰，直至新生代中期到最近古元古代沉积岩的厚覆盖层发生了剥蚀，暴露出古元古代 剥蚀面，以及发育在西澳大利亚Yilgarn克拉通JFGB太古代岩石和周围的花岗质岩石之上的厚层下伏腐泥土，提供了Yilgarn克拉通的北部边缘在2Ga时的状态的清晰画面。在太古代晚期到古元古代中期，Yilgarn克拉通现在的东北缘被缩减到起伏度低的深度风化的地形，大面积花岗岩和少量绿岩带与现存的相似。一个极端干旱氧化型气候造成地下水位降低到地面之下> 200微米，​​将腐泥土中的针铁矿脱水成赤铁矿。海平面轻微下降和/或上升之后是在约2.2Ga的古元古代海侵，仅移走风化剖面的上半部，剥蚀曾由海水淹没的大于200米的腐泥土的海蚀面。古元古代Yerrida盆地大陆架沉积物只是轻微变形、未经变质，因为在Glenburgh造山运动过程中，克拉通缘向西北向倾斜12度。该地区随后抬升，并且由于克拉通缘的倾斜，再次侵蚀暴露了的海蚀面的南部和此前风化的腐泥土，以及南部的未风化太古代岩石，同样将该地区降低成起伏度低的地形。新的氧化风化剖面，叠加到以前的腐泥土之上。在约1.8Ga时的第二次海侵将倾斜的Yerrida盆地和南部的太古代岩石隆起的新海蚀面剥蚀掉。Earaheedy群的造陆沉积物

沉积并覆盖太古代岩石近2Ga年，直到第三个剥蚀周期彻底去除Earaheedy群和大部分的Yerrida盆地，暴露出深度风化硅化碧绿铁质岩的条带状含铁地层上的元古宙腐泥土和海蚀面，使之遭受又一次氧化型风化作用。

KEYWORDS:

• Paleoproterozoic wave-cut surfaces
• Paleoproterozoic regolith
• marine transgressions
• weathering cycles
• Yilgarn Craton
• Joyner's Find greenstone belt
• Yerrida Basin

ACKNOWLEDGEMENTS

The paper is based on fieldwork during employment by Golden West Resources Ltd who generously gave access to data and permission to publish findings. Laboratory studies were conducted at the University of Western Australia and funded by Golden West Resources Ltd. The author is solely responsible for the views expressed in this paper but wishes to thank colleagues and reviewers who contributed helpful criticism and discussions.