Restricted shallow-water sedimentation of Early Archean stromatolitic and evaporitic strata of the Strelley Pool Chert, Pilbara Block, Western Australia

Abstract

Abstract The 3.4 Ga-old Strelley Pool Chert is a 25-m thick sedimentary unit near the top of the predominantly volcanic Warrawoona Group in greenstone belts of the eastern Pilbara Block, Western Australia. It is here subdivided into 5 members containing 13 lithofacies. The basal Member, I, is composed of quartzose sandstone deposited in a high-energy wave- or tide-dominated shallow-water system. Overlying this are Members II and III, which make up the bulk of the formation and were deposited in a low-energy, partially restricted hypersaline basin. They record a predominantly regressive succesion of deposits including subaqueous laminite, stromatolite and evaporite; stromatolite, carbonaceous laminite, black-and-white banded chert, evaporite and intraformational detrital units deposited under intermittently to predominantly exposed conditions; and subaerially deposited windblown sand, evaporite and evaporite-solution layers. Members IV and V record the progradation of a volcaniclastic alluvial fringe. The Strelley Pool Chert represents an association of sedimentary environments directly comparable to that observed in modern, low-energy, shallow-marine carbonate-evaporite systems, such as along the Trucial Coast of the Persian Gulf, and abundantly developed in Phanerozoic carbonate platform deposits. There is no evidence, however, that uniquely identifies the environment as having been marine. Deposition may have taken place in either a large hypersaline lake or a restricted marine basin. Evidence of predominantly low energy depositional conditions and a paucity of terrigenous detritus indicate that sedimentation was dominated by orthochemical and biological processes. Silicified evaporites, including coarsely crystalline layers resembling Messinian selenite, are widespread and similar to younger evaporite deposits. They clearly indicate that evaporites were common within shallow-water Archean sequences. The presence of an assemblage of biogenic deposits, including organic laminite, stromatolites, encrusting carbonaceous mats, carbonaceous granules and oncolites, deposited under conditions ranging from fully subaqueous to nearly subaerial and locally evaporitic, points to the existence of an ecologically and probably biologically diverse microbial community 3.4 Ga ago.