Petrography and sedimentology of the ~2490 Ma DS 4 impact spherule layer revisited, Brockman Iron Formation (Hamersley Group, Western Australia)

The ~2490 Ma DS 4 impact layer in the Dales Gorge Member is the only bed in the Brockman Iron Formation (Hamersley Group, Western Australia) known to contain “splash form” impact spherules. At a newly discovered site in Munjina Gorge ( MG ), the internal stratigraphy of the DS 4 impact layer differs from previously known occurrences; it ranges from 36 to 57 cm in total thickness and consists of two distinct subunits. The lower subunit contains abundant cobble‐ to boulder‐scale intraclasts and spherules supported by a finer matrix. We interpret this subunit as the product of poorly cohesive debris flows. The upper subunit is 11–15 cm of low‐density turbidites. The DS 4 layer also consists of two newly recognized subunits at Yampire Gorge ( YG ). The lower subunit is rich in well‐sorted spherules, 0–22 cm thick, and comprises an unstratified bedform with an irregular or swaley upper surface. This is overlain by 2 dm‐scale, fine‐grained, irregularly laminated beds that we interpret as low density turbidites laterally equivalent to the upper subunit at MG . The bedform at YG could be the lateral equivalent of the debrite at MG , genetically related to the overlying turbidites, or a product of impact tsunami‐induced bottom return flow. Other DS 4 layer sites that have debrites similar to the one at MG are geographically separated from one another by sites that both lack debrite facies and feature well‐sorted spherules like YG . These characteristics suggest the DS 4 layer had a complex depositional history that generated multiple debrites.