Australia: The Land Where Time Began

A biography of the Australian continent 

Gosse's Bluff Crater      Tnorala Conservation Park

Tnorala is the Aboriginal name for a wooden dish belonging to a star ancestor the came down from the sky during the dreamtime. The Aboriginal People regard the site as a sacred site. A permit is required to visit the site.

Gosse's Bluff is the remnant of an impact crater, dated to 130 Ma, situated on the Missionary Plain south of the MacDonnell Ranges (Source 1).

The crater is 6 km across, but at the time of impact it would have been 20-25 km in diameter. It is possibly a unique landform. No other formation of this kind is known anywhere on Earth. With most other impact craters the whole formation is visible, but at Gosse's Bluff the outer wall has been removed by erosion leaving only the former central area that forms a pound surrounded by a circular mountain range. The origin of the formation was uncertain for some time after its discovery, but when the 'shatter cone effect' was found in the rocks comprising the pound it proved that the structure resulted from an explosion. This is the name given to the condition when many of the rocks are deeply fractured with fine conically-radiating fissures, the apexes of which all point to the same central point. After various surveys found a large area of disturbed rock surrounding the formation it was generally accepted that it resulted from the impact of an extraterrestrial body, a meteor, comet, etc. Because no remains of the bolide (impacting object) have been found, many think it was probably a comet that vaporised completely on impact. 

Others claim that if it was a meteorite composed of nickel-iron the heat generated by such a massive impact would have been sufficient to vaporise even the metallic structure and any remaining fragments would be expected to have been completely weathered in the millions of years since the impact.

Studies of the crater have enabled scientists to work out a probably history of formation of the present structure, whatever the bolide was. The bolide struck a flat land surface that was higher than the present surface. The explosion on impact could only be described as cataclysmic, completely vaporising the bolide, sending up a mushroom cloud, similar to that produced by the detonation of a nuclear bomb, thousands of metres into the atmosphere. It has been estimated that the energy of the impact would have been at least 200,000 times that of the Hiroshima atomic bomb. As well as devastating thousands of square kilometres around the impact it is believed it would have been felt globally.

The size of the impacting bolide is unknown but it is thought that it must have been travelling at about 70 km/second. It is thought the bolide penetrated less than 600 m below the surface before vaporising but the impact was so great that breakage of rock strata deeper than 4 km occurred. The impact caused very severe compression followed by rebound which produced a very large crater (20-25 km across) and brought the central core rocks to the surface from thousands of metres below. The rocks of the central section came from 3000+ m below the surface and those of the outer section from about 200 m (Source 1). It has been estimated that the present surface is about 2 km lower than the original impact surface (Source 2).

The crater rim was eroded away and the crater filled with sediments that eventually reached the level of the surrounding plain. At this time the only indication of a crater would have been isolated low hills. Because a stream ran through the crater, the sediments were eventually eroded down to the very resistant rock that resulted from the impact that are present now, appearing to be raised because the level of the surrounding plain has been lowered by erosion.

The rim, 180 m above the surrounding plain, is bounded by steep cliffs. The depression in the centre of the structure is only slightly above the level of the surrounding plain, on the northern edge of the Amadeus Basin, a syncline with a flat floor comprised of strata dated to the Proterozoic and Palaeozoic, about 8 km thick. On the northeast side of the crater the rim has been breached by stream erosion. On the crest of the rim, about 900 m above sea level, there is an ancient erosion surface that is believed to be of Late Cretaceous or Tertiary age (Source 2).

According to Twidale & Campbell, there is evidence implicating a meteorite impact, that includes sediments that are steeply dipping and locally overturned, many shatter cones, deformation lamellae, as well as planar features in quartz, strongly brecciated sediments, that include fragments that up to 10s of metres in diameter, and devitrified glass. There are also gravity and magnetic anomalies over Gosses Bluff similar to those from other structures that are presumed to be impact structures. The most recent rocks deformed by the impact have been dated to the Late Devonian, absolute age determinations giving an age of about 144 Ma, in the Late Jurassic (Source 2).

A return journey from the western ranges could be via Tyler Pass and the western end of Larapinta Drive. In the south, a conservation reserve protects Gosse's Bluff. Although the access track is only short, it can be impassable to conventional vehicles.

1. West MacDonnell National Park
2. Australian National Parks
3. Australian National Parks - Northern Territory

1. Helen Grasswill & Reg Morrison, Australia, a Timeless Grandeur, Lansdowne, 1981
2. Twidale, C.R. & Campbell, E.M., 2005, Australian Landforms: Understanding a Low, Flat, Arid, and Old Landscape, Rosenberg Publishing Pty Ltd. 

Links

1. Tnorala (Gosses Bluff)
2. Gosses Bluff images
3. Disaster Down-Under
4. Gosses Bluff Crater