Abstract
The results from the geological and geophysical investigations of the Siljan Ring impact structure (central Sweden) have shown that the Paleozoic sedimentary succession and the Precambrian basement were strongly affected by complex deformational processes. Studies of a new drill core from the C-C-1 well provide valuable additional information necessary for the reconstruction of the geological setting in the southwestern part of the Siljan Ring. It was found that the contact between the basement and the sedimentary cover is tectonic, not normal sedimentary, in origin. The basement interval comprises Precambrian metavolcanic and metasedimentary rocks with a single mafic intrusion (gabbro-dolerite) in the upper part. The rocks have only been partially metamorphosed. The intercalation of calcareous mudstones, skeletal wackstones, and black shales in the sedimentary cover interval is not consistent with the regional lithostratigraphy scheme. Thus, more likely that the sedimentary sequence is not complete as a result of tectonic displacements, and a significant part of the Lower and Middle Ordovician succession is missing. The Post-Proterozoic tectonic reactivation and impact event also caused the formation of four types of fracture. The third type of fracture is accompanied by cataclastic zones and probably have an impact-related nature. In the highly fractured basement rocks, a dissolution along the second type of fracture has resulted in the development of open vugs. Open vugs and microporosity in cataclastic zones have been considered to be an effective storage space for hydrocarbons.
Highlights
The Siljan Ring is considered to be one of the largest impact structures in the world and has a long history of investigations, for both scientific and applied purposes
This study summarizes the results of detailed petrologic investigations of the drill core from a new C-C-1 well drilled in the southwestern part of the Siljan Ring
The results of the C-C-1 drill core investigation allow the following conclusions to be drawn: 1. The structure of the basement interval reveals some features of the geological evolution of the Proterozoic basin in the Siljan Ring area
Summary
The Siljan Ring is considered to be one of the largest impact structures in the world and has a long history of investigations, for both scientific and applied purposes. The results of geophysical studies, a more detailed recent analysis [2,3,4,5,6], provide valuable information about the geological complexity of the area, which includes structural and stratigraphic disturbance. This complexity is a result of the high paleo-tectonic activity in the area and superimposed deformation due to Caledonian orogenic events. The published results of two core section investigations (Mora 1 and Solberga 1) [14] provided important sedimentological and stratigraphic information about the sedimentary cover structure in the western and eastern parts of the Siljan Ring. Comprehensive studies of the Stumsnäs 1 core [15,16] highlighted some questions about fracture distribution and mineralization in the basement rocks
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