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Microscopic shock-metamorphic features in crystalline bedrock : a comparison between shocked and unshocked granite from the Siljan impact structure

Mroczek, Robert LU (2014) In Dissertations in Geology at Lund University GEOL01 20142
Department of Geology
Abstract
During the Devonian, 377 Ma ago, the Earth was impacted by a large celestial body, striking ground in the Siljan area in Dalarna, Sweden. The impact created what is today the largest confirmed impact structure in western Europe. Since its formation, the structure has however been heavily eroded, leaving no trace of the original crater. Today, it is instead recognizable due to the central plateau and a partly lake-filled annular depression. The central plateau presents a chance to see what has happened to the bedrock which, at the time of impact, was situated hundreds to thousands of meters beneath the crater floor.
As the majority of shock-features created when an impact event occurs are on the microscopic scale, using a microscope is a... (More)
During the Devonian, 377 Ma ago, the Earth was impacted by a large celestial body, striking ground in the Siljan area in Dalarna, Sweden. The impact created what is today the largest confirmed impact structure in western Europe. Since its formation, the structure has however been heavily eroded, leaving no trace of the original crater. Today, it is instead recognizable due to the central plateau and a partly lake-filled annular depression. The central plateau presents a chance to see what has happened to the bedrock which, at the time of impact, was situated hundreds to thousands of meters beneath the crater floor.
As the majority of shock-features created when an impact event occurs are on the microscopic scale, using a microscope is a necessity when studying them.
This study has focused on 14 thin sections, taken from granites in the Siljan area. They have been analyzed using a polarization microscope. Site 11 showed by far the most signs of being shocked, with PDFs in the quartz showing in almost all (95-100%) of the grains, while site 67 showed very little (5-10%) signs despite being located at roughly the same distance from the center of the structure. PDFs were also observed in the K-feldspars of site 11. The low percentage of quartz-grains in site 67 which displayed PDFs can however be explained by large grain size and the fact that a U-stage was not used.
PDFs are the only type of shock-metamorphic effect which was definitively identified, however other effects which are not directly accredited to the shock-waves, but rather to long-term secondary effects (due to the cracking of the bedrock) were also found. The bedrock in the area has been heavily chloritized, with about 90% of the biotite in the shocked samples having been converted to chlorite, while the estimate for the non-shocked samples is set to about 60% conversion. This is probably an effect from the impact (although not a direct one), which would have given rise to cracks, potentially speeding up the process of chlorititization considerably, and increasing the amounts of recrystallized clay minerals, which seem to be slightly more abundant in the shocked samples. (Less)
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author
Mroczek, Robert LU
supervisor
organization
course
GEOL01 20142
year
type
M2 - Bachelor Degree
subject
keywords
impact structure, Siljan, planar deformation features
publication/series
Dissertations in Geology at Lund University
report number
418
language
English
id
4732729
date added to LUP
2014-11-05 11:22:30
date last changed
2014-11-05 11:22:30
@misc{4732729,
  abstract     = {During the Devonian, 377 Ma ago, the Earth was impacted by a large celestial body, striking ground in the Siljan area in Dalarna, Sweden. The impact created what is today the largest confirmed impact structure in western Europe. Since its formation, the structure has however been heavily eroded, leaving no trace of the original crater. Today, it is instead recognizable due to the central plateau and a partly lake-filled annular depression. The central plateau presents a chance to see what has happened to the bedrock which, at the time of impact, was situated hundreds to thousands of meters beneath the crater floor.
As the majority of shock-features created when an impact event occurs are on the microscopic scale, using a microscope is a necessity when studying them.
This study has focused on 14 thin sections, taken from granites in the Siljan area. They have been analyzed using a polarization microscope. Site 11 showed by far the most signs of being shocked, with PDFs in the quartz showing in almost all (95-100%) of the grains, while site 67 showed very little (5-10%) signs despite being located at roughly the same distance from the center of the structure. PDFs were also observed in the K-feldspars of site 11. The low percentage of quartz-grains in site 67 which displayed PDFs can however be explained by large grain size and the fact that a U-stage was not used.
PDFs are the only type of shock-metamorphic effect which was definitively identified, however other effects which are not directly accredited to the shock-waves, but rather to long-term secondary effects (due to the cracking of the bedrock) were also found. The bedrock in the area has been heavily chloritized, with about 90% of the biotite in the shocked samples having been converted to chlorite, while the estimate for the non-shocked samples is set to about 60% conversion. This is probably an effect from the impact (although not a direct one), which would have given rise to cracks, potentially speeding up the process of chlorititization considerably, and increasing the amounts of recrystallized clay minerals, which seem to be slightly more abundant in the shocked samples.},
  author       = {Mroczek, Robert},
  keyword      = {impact structure,Siljan,planar deformation features},
  language     = {eng},
  note         = {Student Paper},
  series       = {Dissertations in Geology at Lund University},
  title        = {Microscopic shock-metamorphic features in crystalline bedrock : a comparison between shocked and unshocked granite from the Siljan impact structure},
  year         = {2014},
}