FLUVIAL IRON FORMATION: A NEW PRECAMBRIAN BIOGEOCHEMICAL SEDIMENTARY SYSTEM, EARAHEEDY BASIN, WESTERN AUSTRALIA

Precambrian iron formation provides a record of ocean biochemistry and circulation on the early Earth. Large, economically important continental margin iron formation reflects creation of extensive continental shelves and oxygenation of the ocean-atmosphere system. Exhalative iron formation contains a record of hydrothermal vent chemistry through time. We present a new type, fluvial iron formation, preserved in the Paleoproterozoic Chiall Formation (ca. 1.8 Ga), Earaheedy Basin, Western Australia. This Fe-rich, biochemical sediment is interpreted to have precipitated by mixing Fe derived from continental weathering in river discharge and oxygenated seawater in coastal paleoenvironments.

Hematitic iron formation occurs only where braided rivers emptied into the Earaheedy Basin. Laminated and granular iron formation is interbedded in delta channel deposits and laterally adjacent peritidal facies containing abundant hematitic oncoids. Oncoids are formed of microbially precipitated hematite that bound quartz grains during growth. As in modern cyanobacterial oncoids, cortical layers are discontinuous, suggesting the microbes included photosynthetic forms, and grew on the top, sunlit surfaces, until overturned by currents, to form oncolitic hematite rudstones. Interidal deposits are marked by flaser-bedded hematitic sandstone. Supratidal facies contain microbially-laminated sandstone and ripped up microbial mats transported from subtidal environments. Iron formation is absent in all lithofacies that accumulated away from paleoriver mouths; these are composed entirely of sandstones and microbialites. Iron formation is also lacking in deep-water sediments in the basin center. Here, turbidites are interbedded with hemipelagic mudstones. The conspicuous absence of pyrite further suggests input of terrestrially derived Fe into non-ferrugenous basin waters.

Fluvial iron formation expands the locus of known abiotic and microbial Fe precipitation processes to estuarine settings, emphasizing the importance of interpreting iron formation in proper depositional context. Because it bridges the terrestrial-marine interface it shifts the iron formation realm landward and provides a new window into ocean-atmosphere evolution.