The Collisional Orogeny in the Scandinavian Caledonides (COSC) project focuses on processes related to the closure of the Iapetus Ocean, causing the Ordovician–Silurian continent–continent collision between Baltica and Laurentia. The rock succession in the second drill core (COSC-2) from the Jämtland County, central Sweden, provides the base for detailed sedimentological, stratigraphic, geophysical, geochemical, geothermal and structural studies. The basement, comprising 1.66–1.65 Ga Transscandinavian Igneous Belt porphyries intruded by 1.47 Ga and 1.27–1.26 Ga mafic dykes and sills, is heavily weathered towards the top. Here it grades into typical saprock and saprolite (including immature soil reflecting the sub-Cambrian peneplain). The overlying sedimentary sequence starts with basal conglomerates and heterogeneous sediments with shell fragments, indicating an early Cambrian rather than a Neoproterozoic age for the marine transgression in the area. The developing early Cambrian basin was rapidly filled, initially by mostly coarse-grained sediment gravity flows. These strata are covered by sandstone turbidites that show an upward transition into the Alum Shale Formation, representing a tectonically quieter period (mid-Cambrian/Maolingian to Early Ordovician/Tremadocian). The upper part of the Alum Shale Formation is overlain by a late Early Ordovician turbidite succession. Local sources of sediments below the Alum Shale Formation and the extended deposition period may indicate continuous sedimentation in a pull-apart basin preserved in a window beneath the Caledonian thrust sheets.
Andersson, A., Claesson, S. and Kooijman, E. 2022. Age and crustal affinity of Precambrian basement nappes and underlying basement in the east central Scandinavian Caledonides. Geologiska Föreningen Special Publication, 1, 242–243.
Bian, L. B., Schovsbo, N. H., Chappaz, A., Zheng, X. W., Nielsen, A. T., Ulrich, T. et al. 2021. Molybdenum-uranium-vanadium geochemistry in the lower Paleozoic Alum Shale of Scandinavia: implications for vanadium exploration. International Journal of Coal Geology, 239, 103730.
https://doi.org/10.1016/j.coal.2021.103730
Corfu, F., Andersen, T. B. and Gasser, D. 2014. The Scandinavian Caledonides: main features, conceptual advances and critical questions. Geological Society, London, Special Publications, 390, 9–43.
https://doi.org/10.1144/SP390.25
Gee, D. G., Fossen, H., Henriksen, N. and Higgins, A. K. 2008. From the early Paleozoic platforms of Baltica and Laurentia to the Caledonide orogen of Scandinavia and Greenland. Episodes, 31(1), 45–51.
https://doi.org/10.18814/epiiugs/2008/v31i1/007
Gee, D. G., Juhlin, C., Pascal, C. and Robinson, P. 2010. Collisional Orogeny in the Scandinavian Caledonides (COSC). GFF, 132(1), 29–44.
https://doi.org/10.1080/11035891003759188
Greiling, R. O., Oszczypko, N. and Garfunkel, Z. 2013. A comparison of two orogenic margins: central Scandinavian Caledonides and western Outer Carpathians.Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, 164(1), 9–32.
http://dx.doi.org/10.1127/1860-1804/2013/0013
Hedin, P., Juhlin, C. and Gee, D. G. 2012. Seismic imaging of the Scandinavian Caledonides to define ICDP drilling sites. Tectonophysics, 554–557, 30–41.
http://dx.doi.org/10.1016/j.tecto.2012.05.026
Heuwinkel, J. and Lindström, M. 2007. Sedimentary and tectonic environment of the Ordovician Föllinge greywacke, Storsjön area, Swedish Caledonides. GFF, 129(1), 31–42.
https://doi.org/10.1080/11035890701291031
Juhlin, C., Hedin, P., Gee, D. G., Lorenz, H., Kalscheuer, T. and Yan, P. 2016. Seismic imaging in the eastern Scandinavian Caledonides: siting the 2.5 km deep COSC-2 borehole, central Sweden. Solid Earth, 7(3), 769–787.
https://doi.org/10.5194/se-7-769-2016
Karis, L. 1998. Jämtlands östliga fjällberggrund. In Beskrivning till berggrundskartan över Jämtlands län. Del 2: Fjälldelen (Karis, L. and Strömberg, A. G. B., eds). Sveriges Geologiska Undersökning, Uppsala, 6–184.
Labrousse, L., Hetényi, G., Raimbourg, H., Jolivet, L. and Andersen, T. B. 2010. Initiation of crustal-scale thrusts triggered by metamorphic reactions at depth: insights from a comparison between the Himalayas and Scandinavian Caledonides. Tectonics, 29(5), 1–14.
https://doi.org/10.1029/2009TC002602
Lehnert, O., Anderson, M., Cuthbert, S., Almqvist, B. S. G., Andersson, J., Callegari, R. et al. 2023a. COSC-2 and the importance of scientific drilling: discovery of an unexpected Proterozoic igneous and lower Palaeozoic sedimentary succession beneath the Caledonian nappes. EGU General Assembly, EGU23-12123.
https://doi.org/10.5194/egusphere-egu23-13822
Lehnert, O., Calner, M., Klonowska, I., Ziemniak, G., Cuthbert, S., Meinhold, G. et al. 2023b. The unexpected sedimentary contact between Proterozoic igneous rocks and its lower Palaeozoic sediment cover in the COSC-2 core. In Abstracts: IODP/ICDP Colloquium, Hannover, August 29–31, 2023. Hannover, 35–36.
Lescoutre, R., Almqvist, B., Koyi, H., Berthet, T., Hedin, P., Galland, O. et al. 2022a. Large-scale, flat-lying mafic intrusions in the Baltican crust and their influence on basement deformation during the Caledonian orogeny. GSA Bulletin, 134(11–12), 3022–3048.
https://doi.org/10.1130/B36202.1
Lescoutre, R. Söderlund, U., Andersson, J. and Almqvist, B. 2022b. 1.47 Ga and 1.27–1.26 Ga dolerite sheets within the basement underneath the east-central Scandinavian Caledonides. Geologiska Föreningen Special Publication, 1, 433–434.
Lorenz, H., Rosberg, J.-E., Juhlin, C., Klonowska, I., Lescoutre, R., Westmeijer, G. et al. 2022. COSC-2 – drilling the basal décollement and underlying margin of palaeocontinent Baltica in the Paleozoic Caledonide Orogen of Scandinavia. Scientific Drilling, 30, 43–57.
https://doi.org/10.5194/sd-30-43-2022
Nielsen, A. T., Schovsbo, N. H., Klitten, K., Woollhead, D. and Rasmussen, C. M. Ø. 2018. Gamma-ray log correlation and stratigraphic architecture of the Cambro-Ordovician Alum Shale Formation on Bornholm, Denmark: evidence for differential syndepositional isostasy. Bulletin of the Geological Society of Denmark, 66, 237–273.
https://doi.org/10.37570/bgsd-2018-66-15
Schweizer, N., Rast, M., Madonna, C., Almqvist, B. and Wenning, Q. 2023. Linking laboratory seismic velocity measurements with the mineralogical content and (micro)structures of the COSC-2 drill core, central Scandinavian Caledonides. EGU General Assembly, EGU23-11329.
https://doi.org/10.5194/egusphere-egu23-11329
Söderlund, U., Elming, S.-Å., Ernst, R. E. and Schissel D. 2006. The Central Scandinavian Dolerite Group – protracted hotspot activity or back-arc magmatism? Constraints from U–Pb baddeleyite geochronology and Hf isotopic data. Precambrian Research, 150(3–4), 136–152.
https://doi.org/10.1016/j.precamres.2006.07.004
Strömberg, A. G. B. 1998. Tektonik, petrografi och regional geologi i den västliga fjällberggrunden. In Beskrivning till berggrundskartan över Jämtlands län. Del. 2: Fjälldelen (Karis, L. and Strömberg, A. G. B., eds). Sveriges Geologiska Undersökning, Uppsala, 185–363.
Wijk, J. van, Axen, G. and Abera, R. 2017. Initiation, evolution and extinction of pull-apart basins: implications for opening of the Gulf of California. Tectonophysics, 719–720, 37–50.
https://doi.org/10.1016/j.tecto.2017.04.019
Zhao, Z. F., Ahlberg, P., Thibault, N., Dahl, T. W., Schovsbo, N. H. and Nielsen, A. T. 2022. High-resolution carbon isotope chemostratigraphy of the middle Cambrian to lowermost Ordovician in southern Scandinavia: implications for global correlation. Global and Planetary Change, 209, 103751.
https://doi.org/10.1016/j.gloplacha.2022.103751
Ziemniak, G., Klonowska, I., McClelland, W., Lehnert, O., Cuthbert, S., Carter, I. et al. 2023. Detrital zircon geochronology of lower Paleozoic sedimentary rocks from COSC-2 borehole. EGU General Assembly, EGU23-12123.
https://doi.org/10.5194/egusphere-egu23-12123
