In recent research published by myself and my colleague Tony Yeates within the journal Tectonophysics, we investigate what we consider – based on a few years of experience in asteroid impact research – is the world’s largest known impact structure, buried deep within the earth in southern Recent South Wales.
The Deniliquin structure, yet to be further tested by drilling, spans as much as 520 kilometers in diameter. This exceeds the scale of the near-300-km-wide Vredefort impact structure in South Africa, which thus far has been considered the world’s largest.
Related: 10 Earth impact craters you need to see
Hidden traces of Earth’s early history
The history of Earth’s bombardment by asteroids is basically concealed. There are a number of reasons for this. The primary is erosion: the method by which gravity, wind and water slowly wear away land materials through time.
When an asteroid strikes, it creates a crater with an uplifted core. This is comparable to how a drop of water splashes upward from a transient crater while you drop a pebble in a pool.
This central uplifted dome is a key characteristic of enormous impact structures. Nevertheless, it may possibly erode over hundreds to hundreds of thousands of years, making the structure difficult to discover.
Structures will also be buried by sediment through time. Or they may disappear in consequence of subduction, wherein tectonic plates can collide and slide below each other into Earth’s mantle layer.
Nonetheless, latest geophysical discoveries are unearthing signatures of impact structures formed by asteroids that will have reached tens of kilometers across – heralding a paradigm shift in our understanding of how Earth evolved over eons. These include pioneering discoveries of impact “ejecta,” that are the materials thrown out of a crater during an impact.
Researchers think the oldest layers of those ejecta, present in sediments in early terrains all over the world, might signify the tail end of the Late Heavy Bombardment of Earth. The latest evidence suggests Earth and the opposite planets within the Solar System were subject to intense asteroid bombardments until about 3.2 billion years ago, and sporadically since.
Some large impacts are correlated with mass extinction events. For instance, the Alvarez hypothesis, named after father and son scientists Luis and Walter Alvarez, explains how non-avian dinosaurs were worn out in consequence of a giant asteroid strike some 66 million years ago.
Uncovering the Deniliquin structure
The Australian continent and its predecessor continent, Gondwana, have been the goal of various asteroid impacts. These have resulted in at the very least 38 confirmed and 43 potential impact structures, starting from relatively small craters to large and completely buried structures.
As you will recall with the pool and pebble analogy, when a big asteroid hits Earth, the underlying crust responds with a transient elastic rebound that produces a central dome.
Such domes, which may slowly erode and/or turn into buried through time, could also be all that’s preserved from the unique impact structure. They represent the deep-seated “root zone” of an impact. Famous examples are present in the Vredefort impact structure and the 170-km-wide Chicxulub crater in Mexico. The latter represents the impact that caused the extinction of the dinosaurs.
Between 1995 and 2000, Tony Yeates suggested magnetic patterns beneath the Murray Basin in Recent South Wales likely represented a large, buried impact structure. An evaluation of the region’s updated geophysical data between 2015 and 2020 confirmed the existence of a 520 km diameter structure with a seismically defined dome at its centre.
The Deniliquin structure has all of the features that might be expected from a large-scale impact structure. For example, magnetic readings of the realm reveal a symmetrical rippling pattern within the crust across the structure’s core. This was likely produced through the impact as extremely high temperatures created intense magnetic forces.
A central low magnetic zone corresponds to 30-km-deep deformation above a seismically defined mantle dome. The highest of this dome is about 10km shallower than the highest of the regional mantle.
Magnetic measurements also show evidence of “radial faults”: fractures that radiate from the middle of a giant impact structure. That is further accompanied by small magnetic anomalies which can represent igneous “dikes,” that are sheets of magma injected into fractures in a pre-existing body of rock.
Radial faults, and igneous sheets of rocks that form inside them, are typical of enormous impact structures and will be present in the Vredefort structure and the Sudbury impact structure in Canada.
Currently, the majority of the evidence for the Deniliquin impact is predicated on geophysical data obtained from the surface. For proof of impact, we’ll need to gather physical evidence of shock, which may only come from drilling deep into the structure.
When did the Deniliquin impact occur?
The Deniliquin structure was likely positioned on the eastern a part of the Gondwana continent, prior to it splitting off into several continents (including the Australian continent) much later.
The impact that caused it could have occurred during what’s generally known as the Late Ordovician mass extinction event. Specifically, I feel it could have triggered what’s called the Hirnantian glaciation stage, which lasted between 445.2 and 443.8 million years ago, and can be defined because the Ordovician-Silurian extinction event.
This huge glaciation and mass extinction event eliminated about 85% of the planet’s species. It was greater than double the size of the Chicxulub impact that killed off the dinosaurs.
It’s also possible the Deniliquin structure is older than the Hirnantian event, and will be of an early Cambrian origin (about 514 million years ago). The following step will likely be to collect samples to find out the structure’s exact age. This can require drilling a deep hole into its magnetic centre and dating the extracted material.
It’s hoped further studies of the Deniliquin impact structure will shed latest light on the character of early Paleozoic Earth.