Acraman
Key facts
- Country
- Australia
- Diameter
- 90 km
- Age
- about 580 million years
- Location
- 32.02° S, 135.45° E
Lake Acraman, a salt lake about 20 km across in the Gawler Ranges, marks the eroded centre. The crater's outer form is hard to see on the ground.
Acraman, in the Gawler Ranges of South Australia, is a crater whose story was pieced together from two places about 300 km apart. It is one of the best examples of how geologists can match a crater with the debris it threw out.
The crater. Around 580 million years ago, during the Ediacaran period, an asteroid struck volcanic rocks in what is now South Australia. The resulting crater is thought to have been very large, perhaps around 90 km wide, though its exact size is still debated because the area has been heavily eroded. Today its centre is marked by Lake Acraman, a salt lake about 20 km across. Rocks around the lake show shatter cones and shocked quartz, the tell-tale signs of an impact shock wave.
The ejecta. In the early 1980s the geologist Victor Gostin and colleagues were studying layers of ancient sedimentary rock in the Flinders Ranges, about 300 km to the east. Within a formation of fine-grained shale called the Bunyeroo Formation they found a thin layer of broken volcanic fragments, out of place among the surrounding mud. The fragments contained shocked quartz. At about the same time, George Williams identified the Acraman structure. The volcanic fragments matched the rocks of the Gawler Ranges, and the two finds were published together in 1986. The layer in the Flinders Ranges was material blasted out of Acraman and thrown hundreds of kilometres before settling on the sea floor.
Since then, traces of the same ejecta layer have been found across a wide area of southern Australia, including in rocks now lying hundreds of kilometres from the crater. This makes Acraman one of the few ancient impacts where crater and ejecta can be linked so clearly.
A link to early life? The Ediacaran period saw some of the first large, complex living things, soft-bodied organisms found as fossils in the Flinders Ranges and elsewhere. Some researchers have suggested that the Acraman impact affected the evolution of early life, pointing to changes in fossil microorganisms in rocks above the ejecta layer. This idea is interesting but unproven, and it remains a subject of research rather than settled fact.
Visiting. The Gawler Ranges are a remote region of ancient volcanic hills, much of it within a national park and pastoral land. Lake Acraman lies on pastoral lease land, so permission is needed to visit. The ejecta layer in the Flinders Ranges can be seen in some road cuttings and creek beds, but collecting in national parks is not allowed.
Did you know?
Debris from Acraman was found 300 km away, in the Flinders Ranges, and matched to the crater.
The impact happened when some of Earth's earliest large, complex organisms were living in the sea.
The rocks struck by the impactor were themselves ancient volcanic rocks more than a billion years old.