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Shocked quartz grains in the early Cambrian Vakkejokk Breccia, Sweden—Evidence of a marine impact

Alwmark, Carl LU ; Ormö, Jens and Nielsen, Arne T. (2018) In Meteoritics and Planetary Science
Abstract

Here we present a study of the abundance and orientation of planar deformation features (PDFs) in the Vakkejokk Breccia, a proposed lower Cambrian impact ejecta layer in the North-Swedish Caledonides. The presence of PDFs is widely accepted as evidence for shock metamorphism associated with cosmic impact events and their presence confirms that the Vakkejokk Breccia is indeed the result of an impact. The breccia has previously been divided into four lithological subunits (from bottom to top), viz. lower polymict breccia (LPB), graded polymict breccia (GPB), top sandstone (TS), and top conglomerate (TC). Here we show that the LPB contains no shock metamorphic features, indicating that the material derives from just outside of the crater... (More)

Here we present a study of the abundance and orientation of planar deformation features (PDFs) in the Vakkejokk Breccia, a proposed lower Cambrian impact ejecta layer in the North-Swedish Caledonides. The presence of PDFs is widely accepted as evidence for shock metamorphism associated with cosmic impact events and their presence confirms that the Vakkejokk Breccia is indeed the result of an impact. The breccia has previously been divided into four lithological subunits (from bottom to top), viz. lower polymict breccia (LPB), graded polymict breccia (GPB), top sandstone (TS), and top conglomerate (TC). Here we show that the LPB contains no shock metamorphic features, indicating that the material derives from just outside of the crater and represents low-shock semi-autochthonous bombarded strata. In the overlying, more fine-grained GPB and TS, quartz grains with PDFs are relatively abundant (2–5% of the grain population), and with higher shock levels in the upper parts, suggesting that they have formed by reworking of more distal ejecta by resurge of water toward the crater in a marine setting. The absence of shocked quartz grains in the TC indicates that this unit represents later slumps associated with weathering and erosion of the protruding crater rim. Sparse shocked quartz grains (<0.2%) were also found in sandstone beds occurring at the same stratigraphic level as the Vakkejokk Breccia 15–20 km from the inferred crater site. It is currently unresolved whether the sandstone at these distal sites is related to the impact or just contains rare reworked quartz grains with PDFs.

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epub
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Meteoritics and Planetary Science
publisher
Wiley-Blackwell
external identifiers
  • scopus:85058322274
ISSN
1086-9379
DOI
10.1111/maps.13230
language
English
LU publication?
yes
id
9eac3e7d-2aab-4c58-9cab-56ccb685b589
date added to LUP
2019-01-10 08:59:44
date last changed
2019-03-31 05:05:10
@article{9eac3e7d-2aab-4c58-9cab-56ccb685b589,
  abstract     = {<p>Here we present a study of the abundance and orientation of planar deformation features (PDFs) in the Vakkejokk Breccia, a proposed lower Cambrian impact ejecta layer in the North-Swedish Caledonides. The presence of PDFs is widely accepted as evidence for shock metamorphism associated with cosmic impact events and their presence confirms that the Vakkejokk Breccia is indeed the result of an impact. The breccia has previously been divided into four lithological subunits (from bottom to top), viz. lower polymict breccia (LPB), graded polymict breccia (GPB), top sandstone (TS), and top conglomerate (TC). Here we show that the LPB contains no shock metamorphic features, indicating that the material derives from just outside of the crater and represents low-shock semi-autochthonous bombarded strata. In the overlying, more fine-grained GPB and TS, quartz grains with PDFs are relatively abundant (2–5% of the grain population), and with higher shock levels in the upper parts, suggesting that they have formed by reworking of more distal ejecta by resurge of water toward the crater in a marine setting. The absence of shocked quartz grains in the TC indicates that this unit represents later slumps associated with weathering and erosion of the protruding crater rim. Sparse shocked quartz grains (&lt;0.2%) were also found in sandstone beds occurring at the same stratigraphic level as the Vakkejokk Breccia 15–20 km from the inferred crater site. It is currently unresolved whether the sandstone at these distal sites is related to the impact or just contains rare reworked quartz grains with PDFs.</p>},
  author       = {Alwmark, Carl and Ormö, Jens and Nielsen, Arne T.},
  issn         = {1086-9379},
  language     = {eng},
  month        = {12},
  publisher    = {Wiley-Blackwell},
  series       = {Meteoritics and Planetary Science},
  title        = {Shocked quartz grains in the early Cambrian Vakkejokk Breccia, Sweden—Evidence of a marine impact},
  url          = {http://dx.doi.org/10.1111/maps.13230},
  year         = {2018},
}