The origin of the iron-formation-rich Hamersley Group of Western Australia — deposition on a platform

Abstract

With only minor exceptions, the 1.5 km thick sediments of the 2.5 Ga Hamersley Group are either chemical/biological (iron-formation, chert and carbonates) or pyroclastic/chemical (“shales”). Terrigenous clastics are sparse or absent. Palinspastic reconstructions indicate that the sediments were deposited on a submarine, essentially volcanogenic platform or bank (the Fortescue Group) built on an older Archaean, sialic, northwest-trending shelf protruding into, or marginal to, an ocean. A deep ocean basin is precluded by the geologic setting. Deposition in a barred basin is considered unlikely in the combined absence of terrigenous clastics, a defined shoreline or lateral facies changes. Upwelling of marine bottom currents resulted in precipitation of iron, silica and other components derived under anoxic conditions, largely from the pulsed output of a large oceanic rift or hot spot, possibly supplemented by normal continental drainage. The currents generally persisted during sedimentation of the Hamersley Group, temporarily interrupted or perhaps diverted by eustatic changes, growth of barrier reefs or the oscillating emergence and submergence of intervening volcanic chains. Ash emissions from the latter, combined with chemical precipitates, were largely responsible for the “shales” in the succession.