Abstract

The shaly upper part of the Wittenoom Dolomite, which belongs to the 2.5 Ga Hamersley Group of Western Australia, locally contains thin layers to lenses of ferruginous chert arenite that are very similar to cherty arenites found in granular Precambrian iron formations. Where they are best preserved, the clasts in the Wittenoom arenites consist of chert with finely disseminated inclusions of hematite or iron silicate. In some layers, the clasts are elongated and petrographically very similar to thinly laminated cherts in the surrounding layers, suggesting that they are rip-up clasts redeposited locally. In other arenites, however, the chert clasts are peloidal to oolitic, suggesting a shallower source and a longer distance of transportation. Both types were probably transported during short-lived, high-energy events such as turbidity currents and/or bottom return flow induced by storms. Restriction of the arenites to the northeastern part of the Hamersley Basin and other sedimentological data suggest that these currents were moving toward the south and west. These occurrences of ferruginous chert arenite provide new evidence that the textural differences between large banded iron formations (such as those of the Hamersley Group) and large granular iron formations (such as those of the Lake Superior region and Nabberu Basin) primarily reflect differences in hydrodynamic conditions.

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