Title

Discovery of a layer of probable impact melt spherules in the Late Archaean Jeerinah Formation, Fortescue Group, Western Australia

Abstract

Individual layers rich in sand-sized spherules interpreted as distal ejecta from Late Archaean to Early Palaeoproterozoic impacts have already been reported from three stratigraphic units in the Hamersley Group of Western Australia: the Wittenoom Formation, the Carawine Dolomite, and the Dales Gorge Member of the Brockman Iron Formation. Here we report the occurrence of a similar layer near the top of the Jeerinah Formation, the uppermost unit in the underlying Fortescue Group. This layer, which we informally name the Jeerinah Impact Layer, was deposited ca 2.63 Ga below wave-base in a marine deep shelf to upper slope environment. As in all other spherule layers, the spherules in the Jeerinah Impact Layer are sand-sized (up to 0.83 mm across), dominantly spheroidal, and show a mix of crystallisation and devitrification textures internally, indicating they are droplets of former low-silica silicate melt. The morphologies of some of the crystallites indicate they were originally plagioclase, yet they now consist of K-feldspar; this, too, is common in all of the other spherule layers. Despite the fact that it is ≤6 mm thick, the Jeerinah Impact Layer pinches and swells laterally and consists of several different subunits internally. This indicates that more than the passive settling of particles in still water was involved in the formation of the Jeerinah Impact Layer, which probably involved traction of coarse sand-sized grains. Despite its thinness, the Jeerinah Impact Layer still represents a thicker accumulation of spherules than all but the most proximal parts of spherule layers formed by some of the largest Phanerozoic impacts, suggesting the Jeerinah Impact Layer represents a major impact. If the Jeerinah Impact Layer and the other spherule layers in the Hamersley Basin did form by impacts, this also means that ejecta from a minimum of three distinct impacts are preserved with age dates suggesting a recurrence interval similar to the inferred rate of impacts in the Phanerozoic. The discovery of the Jeerinah Impact Layer also reconfirms that basinal successions such as black shale are among the most favourable environments for the preservation of distal layers of impact ejecta. It also raises intriguing questions about stratigraphic correlations in the Hamersley Basin of Western Australia and to another spherule layer discovered recently in the Transvaal Supergroup of South Africa.

Publisher

Taylor & Francis

Publication Date

4-5-2000

Publication Title

Australian Journal of Earth Sciences

Department

Geology

Document Type

Article

DOI

10.1046/j.1440-0952.2000.00784.x

Keywords

Correlation, Fortescue Group, Hamersley Basin, Impact ejecta, Jeerinah Formation

Language

English

Format

text

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