Petrography, chemostratigraphy, and depositional controls on the Paleoproterozoic Weeli Wolli Iron Formation, Hamersley Basin, Western Australia

The ca. 2.45 Ga Weeli Wolli Iron Formation (IF) of Western Australia, situated between the underlying Brockman Iron Formation and overlying Boolgeeda Iron Formation, remains one of the least stratigraphically and chemostratigraphically resolved units within the Hamersley Basin. Here, we present a comprehensive petrographic and geochemical investigation of a 272-meter-deep drill core that preserves well-developed oxide- and silicate- facies IF, as well as some carbonates. Gradational lithofacies transitions indicate that the Weeli Wolli IF documents evolving seawater chemistry and depositional conditions. Petrographic relationships suggest that ferrihydrite (Fe[OH] 3 ), hematite (Fe 2 O 3 ), and magnetite (Fe 3 O 4 ) represent the earliest formed phases, as supported by the preservation of primary sedimentary textures. Chemostratigraphic patterns reveal that silicate-facies deposition corresponded with increased hydrothermal input, intermittently overprinted by pulses of continental runoff delivering detrital material. Relative to the underlying Joffre Member of the Brockman IF and the overlying Boolgeeda IF, Mn concentrations are consistently low, suggesting that the redox-buffering effect of dissolved Fe(II) limited MnO 2 precipitation. Elevated Cr/Ti ratios imply enhanced oxidative weathering and the delivery of solubilized Cr to the oceans, consistent with rising atmospheric O 2 levels. These seemingly contradictory signals, together with transient geochemical excursions, support a model of episodic rather than sustained oxygenation during deposition of the Weeli Wolli IF. Lithofacies distributions and sedimentary structures are consistent with accumulation during a modest marine transgression. Carbonates host various iron-oxide and iron-silicate inclusions, and were developed in relatively shallow settings where higher biomass likely stimulated dissimilatory Fe(III) reduction (DIR), whereas deeper-water environments favored diminished biological activity and the preservation of oxide-facies IF. Collectively, these data indicate that deposition of the Weeli Wolli IF coincided with dynamic redox fluctuations and shifting marine conditions at the onset of the Great Oxidation Event (GOE).