Revolution in earth history

Abstract

Abstract The 2,400‐metre thick Hamersley Group is characterised by its 900 m of banded iron formation, and is the middle of three constituent groups of the Mt Bruce Supergroup, which forms the contents of the 2,300–1,800 m.y. old Hamersley Basin. The Hamersley Basin initially covered about 150,000 km2 of northwestern Australia, and in its present widespread outcrop, the Mt Bruce Supergroup is mostly little disturbed or metamorphosed. Within one iron formation unit 142 m thick, the Dales Gorge Member of the Brockman Iron Formation, within the Hamersley Group, there are three scales of stratification, termed macrobanding, mesobanding (the normal ‘banding’ of banded iron formation) and microbanding. Microbands are thin (0.2–2.0 mm) regular laminae, alternately rich and poor in iron, within chert mesobands. Microbands, mesobands and macrobands may all be correlated over the whole of the present outcrop. Microbands are believed to result from annual seasonal control of the primary precipitation in the basin, w...