Australian asteroid ejecta/fallout units

Abstract

Geochronological U-Pb zircon dates are increasingly indicative of an episodic nature of the evolution of lithosphere and crust, including tectonic and thermal episodes associated with large asteroid impacts. Documented Archaean and early Proterozoic impacts at ~3.47, ~2.63, ~2.57, ~2.56, ~2.48, ~2.023 Ga (Vredefort) and 1.85 Ga (Sudbury) are considered to represent a minimum impact incidence due to gaps in stratigraphic sequences and the difficulty in identifying impact ejecta/fallout units. Evidence for major dynamic and thermal effects of large impact clusters on the early Precambrian crust is provided by ejecta/fallout units associated with: unconformities, tsunami boulder debris, compositional contrasts between supracrustal sequences that underlie and overlie ejecta units; including an onset of iron-rich sedimentation; and near-contemporaneous intrusion of granitoid magmas. A prime example is a ~3.26–3.24 Ga impact cluster whose fallout units, documented in the Barberton greenstone belt, South Africa, are associated with unconformities. The unconformities constitute abrupt breaks between underlying mafic-ultramafic volcanic sequences and overlying continental sediments which include granitoid detritus, representing granite felsic igneous activity. Geocronologically correlated unconformities and olistostrome mega-breccia are observed in the Pilbara Craton, Western Australia. In these terrains a > 300 Ma-long period of greenstone–granite evolution is abruptly terminated by unconformities overlain by impact ejecta, turbidite and banded iron-formation and associated with major faulting, uplift, erosion, and the onset of high-energy sedimentation including detrital components derived from contemporaneous and older granites. Onset of iron-rich sedimentation, including banded iron-formation, in the wake of these impacts is indicative of weathering and soluble transport of ferrous oxide under low-oxidation atmosphere and hydrosphere conditions, likely representing mafic volcanic activity triggered by the impacts. Depending on the site of the ~2.48 Ga impact, extensive injection of mafic dykes during 2.48–2.42 Ga (Matachewan, Scourie, Karelian, Widgiemooltha, Bangalore, Antarctica dykes) may have been related to deep crust/mantle fractures triggered by mega-impacts.