Bedforms produced by impact-generated tsunami, ∼2.6 Ga Hamersley basin, Western Australia

Late Archean strata in the Hamersley basin of Western Australia contain a layer of anorbital wave ripples composed largely of impact-generated melt spherules. Sedimentologic evidence indicates that these ripples formed in a deep marine shelf setting well below the effects of meteorological processes. Combined with the impact-related origin of the spherules, this suggests that the ripples were formed by an impact-generated tsunami. To test if this geologic interpretation is reasonable, we have modeled anorbital ripple formation by impact-generated tsunami waves in the Hamersley basin. We assume a deep ocean target area, and that tsunami waves traveled from the target area unhindered to the Hamersley basin. Using melt spherule radius as an indicator, we infer a bolide diameter of 4.9 km. Assuming a bolide density of 2.6 and a velocity of 20 km/s, this impact would produce a crater 60.4 km in diameter. We modeled tsunami waves generated by the collapse of this crater, and propagated them into the Hamersley basin using shallow water wave theory. The wave models indicate tsunami waves created at impacts between 2000 and 8000 km from the Hamersley basin could have generated the observed anorbital ripples. (C) 2000 Published by Elsevier Science B.V.