Petrography and sedimentology of the ~2490 Ma DS4 impact spherule layer revisited, Brockman Iron Formation (Hamersley Group, Western Australia)

Scott HASSLER1, Sandra BILLER2, and Bruce M. SIMONSON3
Meteoritics & Planetary Science (in Press) Link to Article []
1The Wilderness Society, One Kaiser Plaza, Oakland, California 94612, USA
2SNAP-Ed Program Manager, University of Wyoming Extension, 1000 E University Ave Dept. 3354, Laramie,Wyoming 82071, USA
3Geology Department, Oberlin College, Oberlin, Ohio 44074, USA
Published by arrangement with John Wiley & Sons

The ~2490 Ma DS4 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 DS4 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 DS4 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 DS4 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 DS4 layer had a complex depositional history that generated multiple debrites.


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