COSC 2 – Constraints on age and setting of Proterozoic igneous and early Palaeozoic sedimentary rocks below and within the Caledonian nappe pile

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COSC 2 as part of the ‘Collisional Orogeny in the Scandinavian Caledonides’ (COSC) drilling project, represents an integral of the Swedish Scientific Drilling Program (SSDP) In the frame of the International Scientific Drilling Program (ICDP). As a consequence of the closure of the Iapetus Ocean and the corresponding continent-continent collision between Baltica and Laurentia during Early Palaeozoic times, the Baltoscandian margin was partially subducted beneath Laurentia in the mid-late Silurian forming a Himalayan-type orogen. The COSC project studies these processes, COSC 1 investigated the emplacement of high-grade metamorphic allochthons in the Åre area and COSC-2 aimed to study, based on seismic interpretations and geophysical models, a continuous Early Palaeozoic sedimentary succession, the main Caledonian décollement in the Cambrian Alum Shale succession and a Fennoscandian granitic basement. However, scientific drilling recorded an unexpected but very interesting core succession involving a thick volcanic sequence intruded by dolerite dykes instead of Palaeoproterozoic granitic basement. During drilling it was suggested that an imbricate zone with Proterozoic and Cambrian sandstones originating from different settings covers this basement, overlain by an deformed Alum shale comprising the main decollement followed by Early Palaeozoic siliciclastics succession formed in more outboard and deeper environments than assumed before. The detailed logging, however, showed that there is a continuous sedimentary succession on top of the weathered basement covered by a Protolith. A few meters of heterogenous sediments including some marls with trilobites, indicating a Lower Cambrian instead of Neoproterozoic age, display the “birth of a Cambrian basin” which starts to fill rapidly by mostly coarse grained sediment gravity flows before an interruption by a longer quiet period during most of the time of Alum Shale deposition followed by a turbitite sequence showing fining upward and general deepening. The succession was previously regarded as a foreland basin fill, but the local sources and the long time of sedimentation involved points to deposition in a pull apart basin which is well preserved beneath the Caledonian nappes. This exiting succession provides the possibility of detailed sedimentological, stratigraphic and geochemical studies. Dating of sedimentary units is most important for a stratigraphic framework and further correlations with geotectonic events, sea-level fluctuations, evolutionary pulses, climate change as well as for the re-interpretation of seismic models. The focus of this project provides the base for interdisciplinary investigations performed by different working groups of the COSC science team such as geochronology, geothermics, geology, and geophysics.