Paper No. 5
Presentation Time: 10:00 AM
DEPOSITION OF IRON FORMATION IN NEARSHORE PALEOPROTEROZOIC SHELF ENVIRONMENTS, FRERE FORMATION, EARAHEEDY BASIN, WESTERN AUSTRALIA
AKIN, Sara1, PUFAHL, Peir K.
1, HIATT, Eric E.
2 and PIRAJNO, Franco
3, (1)Department of Earth and Environmental Science, Acadia University, 12 University Ave, Wolfville, NS B4P 2R6, Canada, (2)Department of Geology, University of Wisconsin-Oshkosh, 800 Algoma Blvd, Oshkosh, WI 54901, (3)Geological Survey of Western Australia, 100 Plain Street, East Perth, 6004, Australia, 104320a@acadiau.ca
The Paleoproterozoic Frere Formation (ca. 1.80 Ga) of Western Australia is a 600 m-thick sedimentary succession of iron formation and siltstone that accumulated in peritidal and shallow water environments on an unrimmed continental margin. Lithofacies suggest that deposition occurred during a marine transgression punctuated by higher-frequency relative sea-level fluctuations that produced parasequences. Parasequences are 35-85 m-thick and defined by a basal flooding surface overlain by interbedded magnetite-bearing siltstone and hummocky cross-stratified sandstone that grade into microbially laminated, hematite-rich mudstone and trough cross-stratified, chert grainstone. Each cycle is interpreted to record progradation of intertidal and tidal channel sediments over middle shelf deposits as accommodation space filled. The presence of eolian dunes, beach facies, mud cracks, absence of coarse clastics, and preponderance of siltstone indicate deposition along an arid coastline marked by input of windblown sediment.
Iron formation in the Earaheedy Basin is restricted to peritidal and nearshore paleoenvironments, contrasting with older Paleoproterozoic iron formations in North America, which accumulated in the full spectrum of shelf environments. These large, economically important iron formations precipitated in an oxygen-stratified ocean where upwelling provided a sustained supply of anoxic bottom water rich in dissolved Fe and Si. Lithofacies associations within the Frere Formation also suggest deposition in seawater with a prominent oxygen chemocline, but with outboard environments dominated by storm-reworked, wind-blown sediment. Dilution by such clastic input is interpreted to have prevented iron formation from accumulating in deeper settings. The absence of pyrite in middle shelf lithofacies further implies upwelled bottom water was not euxinic, supporting recent studies that demonstrate the Paleoproterozoic transition to a sulfidic ocean was not as widespread as previously surmised.
