Sedimentology of the Neoproterozoic Acraman impact-ejecta horizon, South Australia

The Acraman ejecta horizon is a thin (0-40 cm ) sand/breccia unit consisting almost entirely of angular volcanic fragments. The horizon occurs within Neoproterozoic (~590 Ma) deep-water (below storm-wave-base) shales of the Adelaide Geosyncline (Bunyeroo Formation) and Officer Basin (Rodda beds). Much evidence points to the derivation of the horizon from Australias largest meteorite impact feature, the Acraman structure, in the Gawler Ranges. The ejecta horizon contains anomalous quantities of Ir, Au, Pt, Pd, Ru, and Cr, consistent with a meteoritic source for the horizon. Other evidence for an impact-related origin is the presence of shattered mineral grains, multiple sets of planar shock lamellae within quartz phenocrysts, the occurrence of small shatter cones on clasts, and the presence of altered melt particles. Two major sedimentological styles of ejecta occur in the Adelaide Geosyncline: type 1 ejecta sequences are characterised by a distinctive lonestone/breccia-sandy mudstone-graded-sandstone sequence whose clasts are almost perfectly sorted and normally graded; and type 2 sequences are characterised by weak normal grading, poor sorting, and common cross-lamination. Type 1 ejecta sequences are widespread in the central and eastern portions of the Adelaide Geosyncline, while type 2 sequences are more common in the western part of the Geosyncline. In the Officer Basin, the ejecta horizon consists of thin (0-0.7 cm) layers of sand-size angular volcanic clasts. We interpret type 1 sequences as primary fallout deposits whose perfect sorting and normal grading reflects suspension settling; and type 2 sequences as ejecta that have been completely reworked by impact-induced tsunamis. When treated as a single sediment sample, the grain size distribution in the type 1 ejecta sequences displays two quite distinct populations: the basal breccia forms a very poorly sorted population; and the graded sand constitutes a moderately well sorted population. These two grainsize populations might represent different transport modes: the breccia by fireball processes or ejecta flows, and the sand by atmospheric processes such as impact-induced air-blasts.

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1996-01-01T00:00:00
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Geoscience Australia Persistent Identifier/https://pid.geoscience.gov.au/dataset/ga/81457

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Australian Geological Survey Organisation

 Canberra
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Wallace, M.W.

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Gostin, V.A.

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Keays, R.R.

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AGSO Journal of Australian Geology and Geophysics

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16:4:443-451

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2018-11-01T00:00:00
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urn:uuid/fae9173a-718e-71e4-e044-00144fdd4fa6

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2018-04-20T06:07:17
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