Due to a new scientific discovery, the largest impact crater on the planet may have just
been found within mainland Australia.
But, if the reported feature is truly an impact crater, its existence raises many more questions.
This is the story of Australia's Deniliquin Structure.
For several years, people who looked at the available aeromagnetic maps of Australia which
display the variable abundance of magnetic minerals in the Earth's crust might have noted
a large and circular region which contained less magnetic minerals than the surrounding
rock.
And, at the edge of this feature there appeared to be a structure, likely 200 kilometers in
diameter which had a higher concentration of magnetic minerals than adjacent rock.
Due to how much of a perfect circle this region represented, some scientists suggested that
it might represent a buried impact crater.
After all, such magnetic surveys have been used for several decades to discover buried
impact structures which might otherwise go unnoticed.
For example, the exposed Lonar impact crater in India displays the hallmarks of such a
structure.
In its center is a decreased amount of magnetic minerals while its rim contained a temporary
increase.
Due to the resemblance, a new higher quality magnetic survey was performed within New South
Wales and Victoria.
This study, which was published in 2022 not only confirmed the existence of this ring
structure in higher detail, but also found another even larger ring measuring 520 kilometers
or 323 miles across!
This structure is known as the Deniliquin Structure, which is named after its approximate
center near the town of Deniliquin in New South Wales.
It encompasses a vast area which stretches all the way down to Melbourne in the south,
Robinvale in the west, Hillston in the north, and Albury in the east.
This suspected impact feature is not only the largest impact crater in Australia, beating
out Bedout Crater, but also the largest impact crater in the world, comprising a little more
than 2.5% of Australia's landmass, with a little more than 14% of the country's population
living within its rim.
So, what evidence other than high resolution magnetic surveys do we have?
For one, the 200 kilometer wide ring which initially rose suspicions was not the outline
of the entire impact structure, but rather represented only the diameter of its innermost
ring, within which is a central peak.
Why, is this evidence important?
Simply put, at every single impact crater larger than 4 kilometers in diameter on Earth,
material during the impact will be pushed towards the center and then pushed upwards,
being a hallmark of what is known as a complex crater.
Also, there appears to be a notable ring surrounding the Deniliquin Structure's outer ring, which
is most notable via the location of a series of lakes north of Mossgiel.
Since this suspected structure is buried by more than 10,000 feet thick of rock, it should
come as no surprise that it is suspected to be more than 400 million years old.
In fact the Deniliquin Structure is dated to somewhere between 417 and 525 million years
in age.
If this is indeed an impact crater, it goes without saying that much like the 180 kilometer
wide Chicxulub Crater that wiped out the non avian dinosaurs 66.04 million years ago, the
vastness of the Deniliquin Structure certainly implies that whenever it formed also created
a deadly mass extinction event.
During the current estimate we have for its age range, 6 minor mass extinctions and 1
major mass extinction occurred.
The major mass extinction is known as the Late Ordovician mass extinction, and is considered
to be the second deadliest mass extinction in Earth's history, where 443 million years
ago 85% of all of the planet's species went extinct in a brief period.
This was associated with both falling sea levels and an ocean which suddenly became
largely devoid of oxygen.
As a reminder, the element iridium is more common in asteroids because these largely
represent poorly differentiated bodies than in the Earth's crust, as on Earth the vast
majority of iridium long ago sunk into the Earth's core.
Thus, if an impact event was to have occurred, a distinctive layer or iridium enriched material
would have settled around the planet via what is known as an iridium spike.
Yet, thorough searches have been performed on the planet's rock units, and no iridium
spikes were found at the layers representing the time of the Late Ordovician mass extinction
event.
So, if there is no iridium spike, is it possible for this structure to be truly an impact crater?
While the evidence we currently have does support the assertion that the Deniliquin
Structure is an impact crater, it is far from conclusive.
Conclusive evidence would include finding shatter cones or what are known as shocked
quartz.
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