Acraman impact structure

{{Short description|Impact crater in South Australia}}

{{other uses|Acraman (disambiguation)}}

{{Use Australian English|date=May 2024}}

{{Use dmy dates|date=April 2022}}

{{Infobox terrestrial impact site

| name = Acraman crater

| other_name =

| photo = Acraman crater australia.jpg

| photo_size =

| photo_alt =

| photo_caption = Acraman lake (circle), ringed by Lake Gairdner and others, outlining the Acraman crater depression

| map = South Australia

| map_alt =

| map_caption = Location of the impact structure in South Australia

| map_size =

| location = Gawler Ranges

| label =

| label_position =

| coordinates = {{coord|32|1|S|135|27|E|display=inline,title}}

| coordinates_ref =

| confidence = Confirmed

| diameter = up to {{convert|90|km|mi|abbr=on}}

| depth =

| rise =

| imp_size =

| age = ~590 Ma
Late Ediacaran

| exposed = Yes

| drilled = No

| bolide = Chondrite

| translation =

| language =

| pronunciation =

| topo =

| access =

| country = Australia

| state = South Australia

| province =

| district =

| municipality =

}}

Acraman impact structure is a deeply eroded impact crater in the Gawler Ranges of South Australia.{{Cite Earth Impact DB |name=Acraman |access-date=2017-10-09}} Its location is marked by Lake Acraman, a circular ephemeral playa lake about {{convert|20|km|mi}} in diameter. The discovery of the impact structure and independent discovery of its ejecta were first reported in the journal Science in 1986 by George E. Williams{{cite journal | author = George E. Williams | date = 1986 | title = The Acraman Impact Structure: Source of Ejecta in Late Precambrian Shales, South Australia | journal = Science | volume = 233 | issue = 4760 | pages = 200–3 | doi = 10.1126/science.233.4760.200 | pmid = 17737291 |bibcode = 1986Sci...233..200W | s2cid = 30059305 }} and Victor A. Gostin.{{cite journal | author = Gostin, V.A. |author2= Haines, P.W.|author3= Jenkins, R.J.F.|author4= Compston, W.|author5= Williams, I.S. | date = 1986 | title = Impact Ejecta Horizon Within Late Precambrian Shales, Adelaide Geosyncline, South Australia | journal = Science | volume = 233 | issue = 4760 | pages = 198–200 | doi = 10.1126/science.233.4760.198 | pmid = 17737290 | bibcode=1986Sci...233..198G|s2cid= 11307364|author-link= Victor Gostin}} The evidence for impact includes the presence of shatter cones and shocked quartz in shattered bedrock on islands within Lake Acraman.

The impact structure is deeply eroded, and its original size must be inferred by indirect means. Some authors estimate an original diameter of up to {{convert|85|to|90|km|mi}},{{cite journal | author = Williams, G.E. |author2= Gostin, V.A. | date = 2005 | title = Acraman-Bunyeroo impact event (Ediacaran), South Australia, and environmental consequences: twenty-five years on | journal = Australian Journal of Earth Sciences | volume = 52 | issue = 4–5 | pages = 607–620 | doi = 10.1080/08120090500181036 |bibcode = 2005AuJES..52..607W |s2cid= 128751351 }} while other suggest a smaller size, perhaps only {{convert|35|to|40|km|mi}}, closer to that of the depression in which Lake Acraman is centred.{{cite journal | author = Shoemaker, E.M. |author2= Shoemaker, C.S. | date = 1996 | title = The Proterozoic impact record of Australia | journal = AGSO Journal of Australian Geology and Geophysics | volume = 16 | pages = 379–398 | bibcode = 1990LPICo.746...47S }} The larger size estimate would imply an energy release of 5.2 × 10⁶ megatons of TNT.

The impact event is estimated to have occurred about 590 million years ago during the Ediacaran; this age is not derived from the crater itself but from the position of ejecta within nearby sedimentary basins.

The Lake Acraman Impact Structure is listed on the South Australian Heritage Register.{{cite web|url=http://maps.sa.gov.au/heritagesearch/HeritageItem.aspx?p_heritageno=24 |title=Lake Acraman Impact Structure |publisher=Department of Environment, Water and Natural Resources |work=South Australian Heritage Register |access-date=26 June 2017 }}