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

COSC 2是“斯堪的纳维亚喀里多尼德碰撞造山”(COSC)钻井项目的一部分，是瑞典科学钻探计划(SSDP)在国际科学钻探计划(ICDP)框架内的组成部分。由于早古生代伊阿普图斯洋的闭合和波罗的海与劳伦蒂亚之间相应的陆-陆碰撞，巴尔托斯珊瑚边缘在志留纪中晚期部分俯冲到劳伦蒂亚之下，形成喜马拉雅型造山带。COSC项目研究这些过程，COSC 1号调查了奥雷地区高级变质异地龙的侵位，COSC 2号旨在基于地震解释和地球物理模型研究连续的早古生代沉积序列、寒武纪明铝页岩序列中的主要加里东消融作用和芬诺斯坎迪亚花岗岩基底。然而，科学钻探记录到了一个意想不到但非常有趣的岩心序列，其中包括一个厚厚的火山序列，它是由榴辉岩岩墙侵入的，而不是古元古代花岗岩基底。在钻探过程中，认为该基底覆盖着一条具有不同背景的元古界和寒武纪砂岩的叠瓦带，其上覆盖着变形的明铝页岩，包括主要的滑脱，随后是早古生代硅质碎屑岩序列，形成的环境比以前假设的更外面和更深。然而，详细的测井结果表明，在被原岩覆盖的风化基底顶部存在连续的沉积序列。几米长的非均质沉积，包括一些带有三叶虫的泥灰岩，表明是下寒武统而不是新元古代，显示出“寒武纪盆地的诞生”，该盆地开始快速充填，主要是粗粒沉积物重力流，然后在明铝页岩沉积的大部分时间内，被较长的静止期中断，随后是一个浊积岩序列，表现为细微向上和总体加深。该序列以前被认为是前陆盆地的充填，但当地的物源和漫长的沉积时间涉及到在加里东推覆之下保存完好的拉分盆地中的沉积。这一现存的序列为详细的沉积学、地层学和地球化学研究提供了可能性。沉积单元的测年对于地层格架、与大地构造事件、海平面波动、演化脉动、气候变化以及对地震模型的重新解释是最重要的。该项目的重点为COSC科学小组不同工作组进行的跨学科调查提供了基础，如地质年代学、地热学、地质学和地球物理学。

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.