Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v13b2123q&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #V13B-2123
Physics
3625 Petrography, Microstructures, And Textures, 5225 Early Environment Of Earth, 5420 Impact Phenomena, Cratering (6022, 8136), 5455 Origin And Evolution, 9619 Precambrian
Scientific paper
Understanding the intense meteorite bombardment hypothesized to have dominated the early evolution of the Earth is a fundamental goal of planetary geology. While the surface of the Moon preserves a record of early impacts, terrestrial evidence remains elusive; no Hadean impact structures have been identified on Earth. The vast majority of terrestrial impact structures are removed by erosion shortly after crater formation, however corresponding sedimentary records have not been identified. In this study we report the occurrence of detrital grains of shocked metamorphosed quartz and zircon in siliciclastic sediments from the channel of the Vaal River, a large meandering river actively eroding the 2023 Ma Vredefort Dome. The sediment samples analyzed all contain numerous detrital grains of shocked quartz which preserve a single orientation of decorated planar deformation features (PDFs), and also detrital shocked zircon grains that contain one to three sets of parallel planar fractures (PF). The recognition that impact evidence in the form of shocked minerals can persist in siliciclastic sediments up to 2 billion years after an impact provides a new investigative tool for identifying ancient impact-related detritus from structures that are long since eroded. The Hadean rock record is fragmentary, however the existence of Hadean detritus has been demonstrated by the documentation of >4000 Ma zircons in Australia, China, USA, and Canada. The geochemistry of the Hadean zircons strongly suggests they originated in quartz-saturated granitoids that likely contained abundant quartz. We hypothesize that large volumes of shocked quartz and zircon were produced during Hadean impact events. While the Hadean impact structures have long since eroded, shocked detritus may be preserved in Archean sediments. The results from the Vredefort Dome demonstrate the viability of this potential record, and in particular offers promise that traces of the lost record of impacts on the Early Earth may be preserved in siliciclastic rocks.
Cavosie Aaron J.
Moser Danielle E.
Quintero R. R.
Radovan H. A.
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