The development of dark shales from the middle and late Cambrian to early Ordovician on the East European Platform – with focus on Gotland

ABSTRACT

By compiling data from literature and unpublished reports a more detailed description is presented on the geographical and stratigraphic distribution of the Alum Shale Formation (ASF) and correlateable units on the East European Platform. In the northern part of Gotland, downfaulted patchy beds of the ASF indicate a former wider extension of the formation. Lower Ordovician (Tremadocian) examples of downfaulted patchy beds are the contemporaneous Sepopol and Nivenskaya formations in northeastern Poland and the Kaliningrad area, as well as the Salantai Formation in the east Baltic area. It is indicated that Furongian-Tremadocian beds, contemporaneous with the Kallavere, Türisalu, Tosna and Koporye formations, in the area of northern Estonia and the northwestern part of the Moscow Basin, extended to the Gotland and the South Bothnian Basin areas. South of Gotland, in the Swedish sector of the Baltic Basin, drill cores show evidence of tectonic movements through the presence of erosional surfaces indicating occasional subaerial exposure. In this region, variations in the areal extent and thickness of the ASF and coeval formations are suggested to be the result of epeirogenic and tectonic block movements. Tremadocian ASF is also indicated to be present south of Gotland. On Gotland, at least 5 m of the ASF is presumed to have been eroded. The Moscow Basin contains 19 m of dark shales (Koporye Formation) which is significantly thicker than in surrounding areas.

KEYWORDS:

• Cambrian
• Miaolingian
• Furongian
• Ordovician
• Tremadocian
• ASF
• Gotland
• Baltic basin
• Åland basin
• South bothnian basin
• Moscow basin
• East European platform

Acknowledgements

Svenska Petroleum Exploration AB is thanked for giving access to unpublished reports. The Geological Survey of Sweden (SGU) is also very much thanked for letting us have access to the Grötlingbo-1 and B-7 cores. Determination of the material and the completion of the paper was undertaken by SH. All the authors contributed to editing and refinement of the paper. We thank two anonymous reviewers for their work in providing constructive critism and suggestions to improve the article. We also thank Marina Amantova for the help of correcting the figures. Finally, we thank Chris Mays for linguistic corrections of the manuscript. The project was supported by Svenska Petroleum Exploration AB, Sweden.

Disclosure statement

No potential conflict of interest was reported by the authors.