The sedimentary record of extraterrestrial impacts in deep-shelf environments: Evidence from the early Precambrian

Impact ejecta layers in four formations in the Hamersley Basin (Western Australia) and in one formation from the Transvaal Supergroup (South Africa) show striking evidence for impact-related reworking. Each layer contains sand-sized spherules of a former silicate melt that resemble those found in well-documented impact layers. Given available isotopic age dates of associated strata and uncertainties in stratigraphic correlation, these layers represent a minimum of three and a maximum of five impacts between ca. 2.49 and 2.63 Ga. All of these layers were deposited below wave base in deep-shelf environments, yet they show a common suite of sedimentary features indicating deposition and reworking under high-energy conditions. These features occur in a consistent order: (1) extensive erosion, including the transport of meter-scale rip-up clasts, (2) reworking by waves, (3) synwave to postwave offshore-directed bottom return flow, and (4) later reworking by sediment gravity flows. We interpret the consistent association of erosion, wave reworking, and bottom return flow as a result of tsunami triggered by the impact. The sediment gravity flows may have been triggered by impact or may occur much later. The wave features in these layers indicate they are the result of oceanic impacts, and their sedimentological similarities suggest a consistent set of depositional processes that can be used to recognize the distal ejecta layers of marine impacts, particularly those deposited in deep-shelf settings. Given the relatively rapid tectonic recycling of oceanic crust, such layers probably constitute our best source of information on the frequency and effects of large impacts in open-ocean basins.