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

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.