Early Phanerozoic outer shelf succession of Baltica beneath the Caledonian nappes: a first evaluation of the sedimentary record from the COSC 2 core hole, Sweden

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The ‘Collisional Orogeny in the Scandinavian Caledonides’ (COSC) drilling project represents an integral part of the International Scientific Drilling Program (ICDP) and the Swedish Scientific Drilling Program (SSDP). COSC is focusing on the Early Palaeozoic closure of the Iapetus Ocean and the Scandian continent-continent collision between Baltica and Laurentia resulting in partial subduction of the Baltoscandian margin beneath Laurentia in the mid-late Silurian and the formation of a Himalayan-type orogen. While the emplacement of high-grade metamorphic allochthons was studied during COSC-1 in the Åre area (drilling of the subduction-related Lower Seve Nappe and underlying lower-grade Särv Nappe in 2014), COSC-2 will investigate the substantial Palaeozoic succession (Lower Allochthon and Autochthon), the main Caledonian décollement and the Precambrian basement of the Fennoscandian Shield at Liten Lake southeast of Järpen. The proposed DFG project will focus on the Cambro-Silurian sedimentary succession developed in outer shelf and foreland basin settings in the upper >1200 m of the core. The successive closure of the Iapetus Ocean, the collision between Baltica and Laurentia, flexural loading on the continental margin and stacking of thrust sheets along the Norwegian-Swedish deformation front changed the configuration of the Baltoscandian Basin and created different depth-related environments in a foreland basin parallel to the orogen during Middle Ordovician through Silurian times. The sedimentary cover in the target area will be studied in detail for facies and sea-level changes. Potential stratigraphic gaps in the Upper Ordovician and possible karstification in the limestone deposits (Llandovery) likely reflect glacial events and drastic climate fluctuations during the late Middle Ordovician through Upper Silurian Icehouse Period. The investigation of the Upper Ordovician sediments deposited in deeper water environments will include the search for traces of the intense volcanic activity (K-bentonites) in the closing Iapetus region and of near-by meteorite impacts in the Caledonian fore-deep. Close cooperation with the PIs on other research topics such as geothermics and geophysics are a main focus of the project. A detailed lithological study is important for the sampling related to a geothermal model. Calibration of the borehole geophysical data with the sedimentary record will allow discussing former subsurface interpretations based on detailed reflection seismic studies in the transect investigated for deciding on the COSC-2 drill site. The sediment succession is the key for an understanding of the seismic data in the subsurface across the COSC target area.