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Shocked quartz grains in the polymict breccia of the Granby structure, Sweden-Verification of an impact

Author:
  • Carl Alwmark
Publishing year: 2009
Language: English
Pages: 1107-1113
Publication/Series: Meteoritics and Planetary Science
Volume: 44
Issue: 8
Document type: Journal article
Publisher: Wiley-Blackwell

Abstract english

The Middle Ordovician Granby structure in Sweden is generally considered the result of an asteroidal or cometary collision with Earth, although no hard evidence, i.e., shock metamorphic features or traces of the impactor, have been presented to date. In this Study, drill core samples of a sedimentary breccia from the Granby structure have been investigated for microscopic shock metamorphic evidence in an attempt to verify the impact genesis of the Structure. The finding of multiple sets of decorated planar deformation features (PDFs) in quartz grains in these samples provides unambiguous evidence that the structure is impact derived. Furthermore, the orientation of the PDFs, e.g., omega{101 (3) over bar}, pi{101 (2) over bar} and r, z {101 (1) over bar}, is characteristic for impact deformation. The fact that a majority of the PDFs are decorated implies a water-bearing target. The shocked quartz grains can be divided into two groups; rounded grains found in the breccia matrix likely originated from mature sandstone,, and angular grains in fragments from crystalline target rocks. The absence of melt particles provides ail estimated maximum shock pressure for the sedimentary derived quartz of 15-20 GPa and the frequency distribution of PDF orientations in the bedrock quartz implies pressures of the order of 10 GPa.

Keywords

  • Geology

Other

Published
  • ISSN: 1086-9379
Carl Alwmark
E-mail: carl [dot] alwmark [at] geol [dot] lu [dot] se

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Lithosphere and Biosphere Science

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