HR: 11:35h
AN: U22A-06    [Abstracts]
TI: Geophysical detection of eroded and buried impact structures in layered sedimentary and sedimentary-igneous terrains: Foelsche impact structure and relatives, Australia
AU: * Haines, P W
EM: peter.haines@doir.wa.gov.au
AF: University of Tasmania, Sandy Bay Campus, Hobart, TAS 7001, Australia
AB: Weakly deformed Proterozoic and early Paleozoic basins of Australia contain perhaps the worlds best record of old impact structures, many being of Proterozoic age. Early discoveries resulted from identification of anomalous circular features on geological maps and aerial photographs. Recently, high quality aeromagnetic surveys are revealing circular and concentric magnetic anomalies in these basins. In most cases such anomalies occur in areas where sedimentary successions contains interbedded magnetic volcanic units or sills. Subsequent investigation of several sites has revealed evidence for impact. The best example, Foelsche impact structure, is used here as a model for impact-induced circular aeromagnetic anomalies in flat-lying, layered, sedimentary- igneous terrains. Foelsche lies within flat-lying Paleoproterozoic to Mesoproterozoic rocks of the McArthur Basin, Northern Territory (16 40 S, 136 47 E). It was discovered after an aeromagnetic survey revealed a prominent concentric magnetic anomaly. An outer ring 5.5 km in diameter surrounds a 4 km in diameter annular magnetic trough. An inner somewhat discontinuous annular magnetic ridge 2.5 km in diameter surrounds a central magnetic low. Most of the structure is buried beneath a sub-circular hill of post-impact Neoproterozoic sandstone, with relics of deformed Mesoproterozoic sandstone around the edge, interpreted as remnants of the collapse rim. The structure is interpreted as a partly buried complex impact crater with an original rim diameter about 6 km. The lowest exposed levels of infilling sediments comprise conglomeratic sandstone bearing common shocked clasts (planar fractures and planar deformation features - PDFs - in quartz), and clasts of possible melt rock. This material may be redeposited from fall-back breccia, or eroded from the central uplift. On stratigraphic grounds, Foelsche is inferred to be Neoproterozoic, was probably buried soon after impact, and only recently partly exhumed. The aeromagnetic anomaly, which was instrumental in discovery of the site, is mostly due to the disruption, displacement and/or local removal of thin magnetic dolerite sills in the target stratigraphy, but inner parts of the anomaly may include a contribution from buried melt rocks.
DE: 5420 Impact phenomena, cratering (6022, 8136)
DE: 6022 Impact phenomena (5420, 8136)
DE: 8136 Impact phenomena (5420, 6022)
SC: Union [U]
MN: 2007 Fall Meeting