Geological evidence for a strewn field of impact spherules in the early Precambrian Hamersley Basin of Western Australia


Predominantly sand-sized spherules of K-feldspar with spherulitic, vesicular, and other crystalline textures help to define a unique marker bed in the 2.6 Ga Wittenoom Formation which persists for more than 300 km across the Hamersley Basin of Western Australia. These spherules usually occur as a minor constituent of a single turbidite, but locally they are restricted to centimeter-thick lenses of pure spherules along a single horizon in the argillite close beneath it. In the shallower northeastern part of the Basin, similar spherules (as well as larger, more heterogeneous particles) likewise occur at only one horizon, but here they are a minor constituent of a thick dolomitic debris-flow deposit. As the Wittenoom Formation and the Carawine Dolomite are thought to be stratigraphically equivalent, this debris-flow deposit is believed to be a proximal equivalent of the spherule-bearing turbidite. On the basis of their similarity to microtektites and microkrystites in shape, size, and mode of occurrence, the spherules are interpreted as droplets of silicate melt that were generated and dispersed across the Hamersley Basin by a major bolide impact. Moreover, the mass of the spherules preserved in the Hamersley Basin is of the same order of magnitude as the estimated masses of microtektite glass in major Cenozoic strewn fields, despite the fact that the spherules cover an area which is 2 to 3 orders of magnitude smaller. The layers which host the spherules are interpreted as the deposits of a major sediment gravity flow which exhumed and redeposited most of the spherules after shallow burial. The flow, however, is not believed to have been a direct result of the proposed impact. If the spherule-bearing layers constitute a chronostratigraphic marker, as suggested here, there are important implications for the deposition of carbonates and banded iron formations (BIF) within the Hamersley Basin. Similar spherules also occur at a single horizon ~250 m higher stratigraphically in the Brockman Iron Formation, suggesting that two impacts occurred near the Hamersley Basin in relatively rapid succession. Correlative strata may be present in the Transvaal Basin of South Africa.


Geological Society of America

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Geological Society of America Bulletin



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