View source for 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<br />[[Ediacaran|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]].<ref name=EIDBAcraman>{{Cite Earth Impact DB |name=Acraman |access-date=2017-10-09}}</ref> 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 blanket|ejecta]] were first reported in the journal ''[[Science (journal)|Science]]'' in 1986 by [[George E. Williams (geologist)|George E. Williams]]<ref name=Williams1986>{{cite journal | author = [[George E. Williams (geologist)|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 }}</ref> and [[Victor Gostin|Victor A. Gostin]].<ref name=Gostin1986>{{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}}</ref> The evidence for impact includes the presence of [[shatter cone]]s 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}},<ref name=Williams2005>{{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 }}</ref> 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.<ref name=Shoemaker1996>{{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 }}</ref> The larger size estimate would imply an energy release of 5.2 × 10<sup>6</sup> [[TNT equivalent|megatons of TNT]].<ref name="Williams2005" />

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 basin]]s.<ref name="Williams2005" />

The Lake Acraman Impact Structure is listed on the [[South Australian Heritage Register]].<ref>{{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 }}</ref>

== Ejecta layer ==
A widespread layer of ejecta, believed to be from the Acraman impact structure, is found within Ediacaran rocks of the [[Flinders Ranges]] at least {{convert|300|km|mi}} east of the crater,<ref name="Gostin1986" /> and in [[diamond drilling|drill holes]] from the [[Officer Basin]] to the north.<ref name=Wallace1989>{{cite journal | author = Wallace, M.W. |author2= Gostin, V.A.|author3= Keays, R.R. | date = 1989 | title = Geological Note: Discovery of the acraman impact ejecta blanket in the officer basin and its stratigraphic significance | journal = Australian Journal of Earth Sciences | volume = 36 | issue = 4 | pages = 585–587 | url = http://www.informaworld.com/index/781041341.pdf  | doi = 10.1080/08120098908729511|bibcode = 1989AuJES..36..585W }}</ref> At the time these areas were shallow sea, and the ejecta settled into mud on the sea floor. The ejecta, containing shocked minerals and small shatter cones, is composed of rock similar in age and composition to that at the crater,<ref name="Gostin1986" /> and is associated with an [[iridium anomaly]] suggesting contamination with extraterrestrial material.<ref name=Gostin1989>{{cite journal | author = Gostin, V.A. |author2= Keays, R.R.|author3= Wallace, M.W. | date = 1989 | title = Iridium anomaly from the Acraman impact ejecta horizon: impacts can produce sedimentary iridium peaks | journal = Nature | volume = 340 | issue = 6234 | pages = 542–544 | doi = 10.1038/340542a0 |bibcode = 1989Natur.340..542G |s2cid= 4310570}}</ref> An [[Adaptive radiation|evolutionary radiation]] within marine microorganisms ([[acritarch]]s) occurs just above the level as the ejecta layer, and some authors believe there may be a connection.<ref name=Grey2003>{{cite journal | author = Grey, K. |author2= Walter, M.R.|author3= Calver, C.R. | date = 2003 | title = Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact? | journal = Geology | volume = 31 | issue = 5 | pages = 459–462 | doi = 10.1130/0091-7613(2003)031<0459:NBDSEO>2.0.CO;2|bibcode = 2003Geo....31..459G }}</ref><ref name=Williams2003>{{cite journal | last1 = Williams | first1 = George E. | name-list-style = amp | date = 2003 | title = The Acraman asteroid impact, South Australia: magnitude and implications for the late Vendian environment | journal = Journal of the Geological Society of London | volume = 160 | issue = 4 | pages = 545–554 | doi =  10.1144/0016-764902-142 | last2 = Wallace | first2 = Malcolm W.| bibcode = 2003JGSoc.160..545W | s2cid = 129728586 }}</ref> The proximity of the crater to the type area for the [[Ediacara biota|Ediacara Biota]] is noted, though probably not significant given the likely global consequences of the impact.

==John Acraman==
The Acraman impact structure, Lake Acraman and the nearby [[Acraman Creek]] are named after South Australian colonial businessman [[John Acraman]].<ref> Manning, Geoffrey H. (2012). [https://published.collections.slsa.sa.gov.au/placenamesofsouthaustralia/A.pdf A Glossary of South Australian Place Names - From Aaron Creek to Zion Hill]. Gould Genealogy & History, 2010. Via State Library of South Australia. Retrieved 7 March 2025.</ref>

== Gallery ==
<gallery class="center" mode="packed" heights="200px">
File:Acraman.jpg|Landsat image of Lake Acraman; screen capture from [[NASA World Wind]]
File:Acraman x10.jpg|Oblique Landsat image of Lake Acraman draped over digital elevation model (10x vertical exaggeration), looking east towards the Flinders Ranges where ejecta has been found; screen capture from NASA World Wind.
File:Acraman Impact Structure, South Australia.jpg|Orange and brown soils mix with off-white saltpans, including Lake Gairdner and Lake Everard in this true-colour image.
</gallery>

== References ==
{{Reflist|30em}}

== External links ==
{{portal|Geology|South Australia}}
* [https://web.archive.org/web/20101116124902/http://www.passc.net/EarthImpactDatabase/acraman.html Acraman at Earth Impact Database]
* [https://maps.google.com/maps?ll=-32.015533,135.441513&spn=1.146767,1.714279&t=k&hl=en Satellite image of the region] (from Google Maps)

{{Impact cratering on Earth}}

[[Category:Impact craters of South Australia]]
[[Category:Proterozoic impact craters]]
[[Category:Precambrian Australia]]
[[Category:Far North (South Australia)]]
[[Category:South Australian Heritage Register]]
[[Category:Gawler bioregion]]