Revolution in earth history

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, while alternations between microbanded chert mesobands and the adjacent non‐microbanded chert‐matrix are thought to reflect a 25‐year environment cyclicity. There is also a higher‐order cyclicity. The microbands are chemical evaporitic varves. There are many published accounts of modern non‐glacial varves of a similar order of thickness whose identity as varves is established by direct evidence. Many of these are couplets of laminae, one half being largely organogenic, and similarly structured couplets have been widely accepted as varves in descriptions of Phanerozoic rocks. Evaporitic laminae have also been so accepted, although there are no exact modern analogues. Microbands are closely similar to these in geometry, and all varves are characteristically regular. Secular variations in all depositional environments are likely to be related, in the absence of major tectonic changes, to variations in the total annual receipt of solar radiation. The controls of insolation are well known, but their past variation is only usefully calculable back to 1 m.y. Variations of these controls, and especially of rotational obliquity, outside the Pleistocene limits, seems to be the most likely source of an explanation for the various depositional cyclicities of the Dales Gorge Member. The history of Earth's revolution around the Sun (Revolution in Earth History) is a topic concerning which much evidence still remains to be read directly from the stratigraphic record.