Results of Petrographic Investigations of Core Samples from the Basement of the Siljan Ring Structure

Among the large ring structures considered meteorite (impact) craters, the Siljan structure is one of the best-known in central Sweden. Samples of basement rocks were collected and studied from four sections penetrated by exploration boreholes along the periphery of the Siljan crater. Based on the results of laboratory studies, detailed petrographic characteristics of the rocks were obtained, which included a description of their structure and texture, mineral composition, and secondary transformations. Significant differences in the composition of the rocks in the sections located in the eastern and western parts of the structure were revealed. In the western part, boreholes exposed volcanic and metamorphosed volcanic-sedimentary rocks, while the section in the eastern part of the structure is composed of igneous rocks of predominantly acidic composition. This is probably due to structural shifts as a result of the influence of intense tectonic processes.