WA’s basic topography has been given a good going over for decades by scientists and geologists.
Hence our fair State has long “ceased to be a blank space of delightful mystery,” as Joseph Conrad put it in The Heart of Darkness.
Or has it?
I have a fascination with one small corner of WA that may still hold a secret.
Back in 2008, when I was visiting Denham, Shark Bay, a friend showed me a collection of rocks, shells and seeds gathered from the region.
“Do you think this is a meteorite?” asked the friend, dropping a smooth, brown-black metallic pebble the size of a walnut into my hand.
It was heavy and certainly looked like a weathered iron-nickel meteorite. On the other hand, it may have been a chunk of terrestrial iron ore, so initially I was sceptical.
We discussed it for a while then put it aside and continued looking through the fascinating collection. Shark Bay is a naturalist’s Nirvana.
A year or so later, we caught up again at Margaret River, and the friend pulled out a few more of the curious little boondies.
“Remember these? I found more of them.” And once again we geo sleuths discussed where they came from and what they might be. By now, I was leaning toward them being meteorites – or at least hoping they were.
It then occurred to me that, if there were meteorite fragments scattered about the region, there might just be a crater somewhere out there.
There is, in fact, a very large well-known impact structure in the region, but this was not the object of my interest.
Experts believe that Shark Bay itself, which juts irregularly out into the Indian Ocean in long, radiating promontories from the WA coastline, may have been partially formed by the impact of an asteroid or comet. The structure has become known as the Woodleigh Impact since it is centred at Woodleigh Station east of the bay.
But that event took place approximately 365 million years ago (Late Devonian period), and the telltale evidence is now long buried beneath sediment and hidden below the surface.
It was accidentally discovered during exploration drilling back in the 1970s. Yet its significance as an impact structure was only realised in 1997 during a gravity survey, and it took until 1999 before underlying rock core samples confirmed the impact.
The original discovery team believe the Woodleigh crater may be up to 160 km in diameter. Now it is thought the object that struck the area was 6 to 12km wide. Either way it was a biggie, and created the third largest know astrobleme in Australia, and arguably, the fourth largest on the planet.
However, I thought it unlikely that the ‘meteorites’ I’d seen would be from Woodleigh, considering the age of that impact. Most ejecta would be long buried or corroded away. These ones appeared to be from a more recent event.
If it was younger than Woodleigh, then it still may be visible.
I went to Google Earth, the online satellite-based earth imaging service, to get an overview from orbit and started casually scanning the imagery of Shark Bay and surrounds.
I searched for about an hour without success, and was about to shut down the system for the night, when something curious caught my eye.
I’d been mostly looking around the bay itself (a stunning aquamarine and red earth vista from space), but then decided to peer further inland.
About 40kms due east of Hamelin Pool at the south end of Shark Bay, and about 12kms east of the Overlander Roadhouse on the Northwest Coastal Highway, there is a very interesting circular geological formation.
It jumped out of the screen at me, because it looked awfully like an eroded impact crater and appeared to be about three or four kilometers across. I noted that it was situated at the far eastern side of Hamelin Station.
While there are ancient watercourses and dried up lakebeds in the area, that can look similar to impact features, this formation seemed too perfectly circular to be one of them. It also had an unbroken wall around it, just like a crater.
Meteoroid impacts are usually very symmetrical, due to the speed at which the impactor hits the ground.
Projectiles enter the earth’s atmosphere at very high speeds, ranging from 11 km/sec to 72 km/sec – or between about 40,000 km/h and 260,000 km/h. Therefore the energy released when they strike the earth is like a bomb going off (large bodies produce the explosive power of many atomic bombs), and the familiar circular, rimmed crater formation is what remains.
Our moon is covered in these highly regular depressions. But the lunar craters remain less eroded due to lack of atmosphere, negligible geological activity and low gravity – hence it looks like Swiss cheese.
The explosive force created by an impact can shatter the impactor and, depending on its size, spread debris far and wide. This is known as proximal ejecta for debris landing close to the impact site and distal ejecta for that landing further afield.
The meteorites I’d held suddenly seemed strangely connected to something real and substantial. Were they ejecta from this feature?
What’s more, with closer inspection of the Google Earth image, it looked as if another formation of a similar size, a couple of kilometers south might also be of meteoric origin, although this one was less well defined.
It appeared to have a watercourse running out of it to the west running towards the coast, where the crater rim had been excised on the northwestern section. So, twin craters, if you will. However I was most interested in the intact northern structure (Marked Crater 1).
My first reaction was Eureka! But one needs to be cautious about such things, as proper analyses of surrounding rocks and/or core samples of underlying rocks are required to confirm impact origin.
I thought, surely if this was a major crater(s), then it would be known to science, especially here in super-surveyed WA – but then again, maybe not.
There are only 27 confirmed impact craters in Australia and 190 in the world. Another eight Australian formations remain unconfirmed.
None of these had the coordinates of the Shire of Shark Bay formations that had grabbed my attention. Hence, no one seemed to know about them, or probably thought they were of more commonplace, earthly origin.
In the next few years I would regularly return to Google Earth to peruse the anomalous, curious structures.
I even pictured the impact event itself: in some long-ago epoch, a large meteor hurtling in from space (maybe even two of them, or one splitting in two as it entered the atmosphere) and slamming into the area.
I estimated from the size of the northern ‘crater’ that the meteor would have been between 300 metres and 400 metres wide when it hit the ground.
The rule of thumb with impact sites is that the crater formed is usually about 10 times as large as the object that made it. However this can depend on trajectory, the speed of the object and whether it is of metallic or rocky consistency.
In the case of the Shark Bay features the resultant explosion and shock waves would have been enormous and devastating, killing and laying waste to any life for many kilometres.
In 2010 I was flying back to Perth from Singapore and peered out the window at Shark Bay and there was the formation. It certainly looked like a crater with the less well-defined second feature and its dry waterway to the south of it.
The niggling need to check it all out gnawed at me, and finally last September, I mounted a Starfish field expedition with my son Rhys to visit the site. National Geographic Society, eat your heart out!
Feeling distinctly like Don Quixote and Sancho Panza chasing meteoric windmills, and leaving behind us many a raised eyebrow, let alone open declarations of our evident lunacy, we drove north to the Gascoyne.
Arriving at Hamelin Pool we set up base at the Hamelin Station Stay, now owned and run by Bush Heritage Australia and being rehabilitated to preserve and protect native flora and fauna.
Livestock and feral animals have heavily degraded the station over the decades; but already great strides are being made at bringing it back to its natural state. The station homestead is situated about 40km west of the area we were chasing.
There are several scientists working on the rehabilitation of the 202,000 hectare former pastoral property, as well as ongoing research into the other worldly stromatilites at Hamelin Pool.
We met with Gregg Suosaari, then landscape manager and general manager at Hamelin Station. Gregg’s wife, Dr Erica Suosaari, is a geologist and resident Science Development Fellow at Hamelin Pool, with much of her work devoted to the stromatolites.
When we told them there might be a giant meteorite crater (maybe two side by side) on the property, I’m sure they thought they had a couple of right ones on their hands, but were extremely obliging assisting our expedition.
Gregg and Erica were unsure of the origin of formation, but were aware that the terrain and topography were different and rugged in that part of the property. They wavered toward the area’s appearance being due to ancient watercourses and lakes.
This is understandable, as due to their considerable size, weathering, and being obscured by scrub and bush, the features are difficult to identify from ground level. The only place to really grasp their symmetrical dimensions is from an aerial perspective.
Gregg kindly offered to drive us out to the site, where he would also check for feral animals, remaining livestock and the state of several windmills and bores in the area.
The terrain and tracks being rough, we visited the site in a 4×4, heading off down Shark Bay Road to the NW Coastal Highway and into the wild and desolate eastern sector of the station. We phoned in our intentions to the local authorities before leaving, as the country can be unforgiving. You don’t want to get a puncture out there, even in the cooler months.
It was a fascinating mission.
The features are situated in scrub and low tree terrain in a remote corner of the property. Coordinates: North formation: 26°25’48.59”S 114°35’37.31”E South formation: 26°28’21.69”S 114°34’33.56”E
On the way in we crossed a few clay large pans, which were still a little soft following the good winter rains, and were strewn with millions of small dark rocks. One of these had even been converted into an airfield by the previous station owner, who had shaped the runway by clearing and lining up the scattered stones.
Approaching the formations, they appear as low hills covered in scrub and trees, albeit it scattered with rubble and breccia.
Both have heavily eroded walls with a width ranging from 500m to 600m, and form bowl-like structures about 3.5 kms across.
One must drive up and over the ‘rims’ and into the central zones. Pastoralists have established windmill bores and troughs for livestock at the centre of the depressions. These central areas have filled with sand and sediment.
The bedrock in the region is primarily cretaceous calcilulite and this is very much in evidence in the surface rubble, and seems to have been extruded only in the area of the features. Harder rocks are also strangely embedded in softer rock in many places, forming odd-looking breccia.
The presence of these calcareous rocks in the features suggests they were formed after the bedrock was laid down, so the features would appear to be younger than this bedrock sediment.
The elevation varies between about 65m above sea level at the centre of the depressions to an average of about 75 metres above sea level around the top of the circular rims. Hence the top of the ‘crater walls’ are about 10 metres higher than the lowest points.
The terrain in the ‘walls’ is very rough and damaged, consisting of smashed and broken limestone, silica and sandstone rocks. This is quite different from the surrounding sand and red clay flats, suggesting it may have been forcefully raised and ejected from lower and older strata.
The sense is that these weathered rims were a lot higher in the past – indeed, more crater-like – and are situated on a gentle incline that rises approximately 20m over a 5km line west to east.
Posing a hypothetical, it is conceivable both craters may have filled with water in a wetter epoch to become lakes after they were formed. Eventually the west wall of the southern crater was breeched by the water and created a periodic creek that in wet periods still runs all the way to Hamelin Pool, 40kms to the west.
We wanted to drive south over the rim of the northern feature and into the second feature, as the two more or less back up against one another. However, even though there is a very crude track passing from rim to rim, it was too rough for the vehicle.
So after visiting the centre of the northern formation we had to drive out again, then back in up and through the dry water course the emanates from the northwest side of the southern formation. Once more we had the sense we were standing in a giant shallow bowl.
Unfortunately we only had a couple of hours to move between the two depressions so didn’t have a chance to collect rock samples on our visit – but what we had seen was fascinating.
Gregg was happy with the way the station rehabilitation was going (we saw a few feral goats, kangaroos and emus, but no remaining livestock). We crater hunters were delighted with what we had seen and managed to get some good photographs of the terrain.
Soon after getting back to Perth in early October I contacted some of the state’s foremost experts in meteoritics and earth sciences to run our story by them. I also sent some of the pictures featured here off to them.
The response has been encouraging. Comments from the scientists range from “This sounds interesting” to “It does look pretty intriguing!” Each requested further photographs and was keen to know if we had collected any rock samples.
The samples would be tested for shock deformation features in their structure, indicting the extremely high pressures exerted by an impact. Such features are often found in rocks at impact sites, as well as where nuclear explosions have been conducted.
Other experts thought the depressions might be part of an ancient system of lakes or watercourses, rather than of impact origin.
The jury is still out.
Is this strange set of features protruding from the red earth of the Gascoyne of extraterrestrial origin?
To me, the circular symmetry of the depressions, uplifting of substrata, crater-like features, iron-nickel fragments found in the region, and an irregular watercourse running out of one of the features suggests they could easily be eroded impacts.
When the weather cools in 2017, Don Quixote and Sancho Panza might just have to pop back up to the site for more ratting around and come back with some rock samples for the boffins to zap.
After all, there really are windmills at the centre of these ‘craters’.
Starfish Photographs: Rhys Rigby & Peter Rigby