COSC-2 and the importance of scientific drilling: discovery of an unexpected Proterozoic igneous and Lower Palaeozoic sedimentary succession beneath the Caledonian nappes

The COSC (Collisional Orogeny in the Scandinavian Caledonides) project is an integral of the International Continental Scientific Drilling Program (ICDP), performed by a multidisciplinary and international team of geoscientists. It focuses on processes related to the Early Palaeozoic continent-continent collision between Baltica and Laurentia. The collision resulted in the final closure of the Iapetus Ocean in the Middle-Late Silurian when the Baltoscandian margin was partially subducted beneath Laurentia, forming a Himalayan-type orogen. In west-central Sweden this collisional mountain belt is deeply eroded and COSC-2 successfully recovered a continuously cored succession to a depth of 2276 m..

Based on seismic profiling, geophysical models and the resulting interpretations, COSC-2 predicted a continuous Lower Palaeozoic allochthonous sedimentary succession, the main Caledonian décollement in the Cambrian Alum Shale Formation, and a Fennoscandian basement. The unexpected core record therefore perfectly underlines the importance of deep continental drilling. Logging and early studies show that the succession intruded by dolerite dykes involves a thick porphyry sequence instead of Paleoproterozoic granitic basement. Drilling shows that an imbricate zone with Proterozoic and Cambrian sandstones, formed in different settings, covers the basement. The basal sandstones are overlain by deformed Alum Shale comprising the main décollement and by Lower Palaeozoic siliciclastics formed in more outboard and deeper environments. This differs significantly from interpretations based on the preliminary site investigations, which also suggested a main detachment hosted in Alum Shale, but close to the top of the basement, overlain by a zone of imbricates.

New detailed core descriptions show that there is a continuous sedimentary succession on top of a weathered basement (saprock and saprolith) covered by regolith (level of the Sub-Cambrian Peneplain?) which is overlain by basal conglomerates and a few meters of heterogeneous sediments (Lower Cambrian?), displaying the unusual development of a basin filled initially by mostly coarse-grained sediment gravity flows grading into finer-grained turbidites. This sedimentation was interrupted by a longer period of Alum Shale deposition (Middle Cambrian through Tremadocian), which transitioned into turbidite sedimentation again. This higher turbidite sequence (Tremadocian and younger) shows fining upward indicating a general deepening and was previously regarded as a much younger foreland basin fill (Föllinge greywackes). However, local sources of the turbiditic sediments below the Alum Shale and the extended time of deposition may rather point to a continuous sedimentation in a long-lived pull-apart basin preserved in a window beneath the Caledonian thrust sheet.

After many delays caused by Covid pandemic restrictions, the core was logged in fall 2021 and afterwards by the sampling party at the BGR Core Repository in Berlin/Spandau (summer 2022). Dating of the sedimentary units is the base of a stratigraphic framework for further correlations of geotectonic events, sea-level fluctuations, evolutionary pulses, climate changes, and the re-interpretation of seismic models. The continuous COSC-2 sequence provides various possibilities for interdisciplinary collaborations and studies performed by the COSC science team. The first scientific results are presented in session TS6.4 "The Caledonian Orogen of the North Atlantic region: insights from geological and geophysical studies".