Alexandr Buzilov

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.

The ring structure of Siljan, located in the central part of Sweden, is considered by many researchers to be a meteorite (impact) crater. Impact craters are among the most complex geological objects on the Earth. The origin and formation of these structures still raises many questions. To find answers to these questions we need reliable geological information about the structure of the crater and the composition of the rocks. Information about the thickness and geological structure of the Siljan Ring area sedimentary cover will help to understand the process of the Siljan Ring structure’s formation as well as other similar geological formations on the Earth. Here, we present the results of laboratory studies of sedimentary rock samples taken from four exploration wells drilled in the vicinity of the Siljan Ring crater, which made it possible to compile their detailed lithological description. The laboratory studies included a structural analysis of the samples, and a texture and mineralogical analysis in thin sections. A structure analysis was carried out visually, while structural and mineralogical analyses were carried out on thin sections using a polarizing microscope and a scanning electron microscope. The main components of the rocks (minerals and fragments), along with their ratio and secondary transformations, were determined. The results of the structural analysis of the samples, and the textural and mineralogical analysis of the rocks in thin sections, showed that the sediments’ composition in the sedimentary cover near the Siljan Ring structure changes in different areas in accordance with their facies and stratigraphic characteristics. Furthermore, a change in the thickness of the sections and the succession sequences of rock units was established. A change of this nature is presumably caused by tectonic disturbances of an endogenous or impact source.