Planet killer: world’s largest asteroid impact crater discovered in Deniliquin, Australia

Jan 20, 2024
Asteroid approaching planet Earth, meteorite in orbit before impact Image:iStock

Evidence for a 520 km diameter impact crater, 3 times larger than the Chicxulub crater left from the asteroid that killed the dinosaurs, has been discovered in Deniliquin, Australia.

Close analogies exist between the atmospheric effects of large asteroid impacts and those of greenhouse-induced climate change in terms of initial clouding by aerosols, consequent cooling, release of carbon gases and warming, and extensive fires (Glikson and Groves, 2016).

Present studies indicate very large asteroids have impacted Earth through much of its history. About 20 years ago Tony Yeates, a highly experienced geologist with the Australian Geological Survey, suggested circular gravity and magnetic anomalies beneath the Murray Basin, New South Wales, could represent a very large asteroid impact structure (Yeates et al., 2000).

More recently geophysical evidence, including total magnetic intensity, magneto-telluric records (Kirkby et al., 2020) and deep seismic reflection data (Kennett et al., 2015) indicate a mantle dome beneath the centre of the Deniliquin structure, outlining a 520-km diameter multi-ring structure (Figure 1) (Glikson and Yeates, 2022).

Figure 1. A. Location of the Deniliquin structure in southwestern New South Wales; B. Total Magnetic Intensity image (TMI) of the Deniliquin multi-ring structure, southern New South Wales (Glikson and Yeates, 2022);

When an asteroid impact produced a crater larger than about 4 km in diameter the underlying crust responds through an elastic rebound, producing a central dome which subsequently disintegrates or is subjected to erosion. With large craters the eroded dome may be most or all that is preserved from the original structure. Such scars represent the deep-seated root zone of large impacts, such as the 170 km-diameter-large Chicxulub impact structure in the Yucatan peninsula of Mexico, famed for extinction of the dinosaurs.

The 520 km-large diameter of the Deniliquin structure exceeds the 298 km diameter of the Vredefort impact structure in South Africa, long considered the largest impact structure identified on Earth. The impact is likely to have occurred in the Late Ordovician (Hirnantian) (445.2 to 443.8 Million years ago) and is likely responsible for triggering the large-scale glaciation and mass extinction during this period, an event which eliminated about 85% of species (Glikson, 2023). The Hirnantian, second in scale only to the Permian-Triassic boundary extinction at ~251 Ma, and more than double the scale of the K–T impact and extinction event (66 Ma ago), thus likely represents the effects of the Deniliquin impact event (Glikson, 2023). A search for impact-deformation effects in proximal and distal Ordovician strata around the DMS is required.

The Australian continent and its Gondwana precursor have been the target of numerous asteroid impacts, including at least 38 confirmed and 43 potential impact structures (Figure 2), including partly to completely buried root zones of large impact structures, cf. Woodleigh (diameter ~120 km), Gnargoo (d~75 km), Mount Ashmore (d>50 km), Talundilli (d~84 km) Tookoonooka (d~55 km), Warburton East (d~200 km). Warburton West (d~200 km) and Acraman (d<90 km) (Glikson and Pirajno, 2018)).

The history of the asteroid bombardment of Earth is partly concealed due to subduction, deep erosion of craters, impact central uplifts, and ejecta units (Glikson and Pirajno, 2018; Byerly et al., 1996). New geological and geophysical discoveries are unearthing signatures of impacts structures and impact ejecta of asteroids tens of kilometre-large, heralding a paradigm shift in the understanding of key stages of evolution of the Earth. This includes the discoveries of ~3.2 Ga-old impact ejecta in South Africa and Western Australia, the oldest ones possibly signifying the tail-end of the Late Heavy Bombardment of the Earth (Lowe and Byerly, 2018). Furthermore, large impacts are correlated with the mass extinction of species, as does the Cretaceous-Tertiary (KT) impact (Alvarez et al. 1980) and likely end-Devonian impact clusters with mass extinction of species (Figure 3).

Figure 2. Impact structures and ring structures of possible to probable impact origin and offshore; Large circles – confirmed impact structures; green circles – impact craters; yellow circles – possible to probable ring structures; red circled – impact structures >~100 km in diameter; white circled red circles – impact structures of >50 km diameter.

Figure 3. Mass extinctions through the Phanerozoic history of Earth. The extinction event abruptly affected all major taxonomic groups and caused the disappearance of one third of all brachiopod and bryozoan families, as well as numerous groups of conodonts, trilobites, echinoderms, corals, bivalves, and graptolites.

The present study indicates the root cause of the extinction stems from an impact by a the largest asteroid impact known to date, likely in the late Ordovician (Hirnantian, 445.2 to 443.8 Million years ago), with consequent abrupt climate crisis, glaciation and mass extinction of numerous species.

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