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

“斯堪的纳维亚加里多尼德山脉的碰撞造山作用”(COSC)钻探项目是国际科学钻探计划(ICDP)和瑞典科学钻探计划(SSDP)的组成部分。COSC的重点是早古生代伊阿普图斯海洋的闭合和波罗的海和劳伦蒂亚之间的斯堪的纳大陆-大陆碰撞，导致中晚志留世劳伦蒂亚之下的巴尔托斯珊瑚边缘部分俯冲，形成喜马拉雅型造山带。在奥雷地区COSC-1期间(2014年钻探与俯冲相关的下塞夫推复和下伏的较低等级Särv推复)期间，COSC-1研究了高级变质异地复体的侵位，同时COSC-2将研究实质性的古生代序列(下四肢推覆和自生推覆)、主要的加里东期拆解和位于约尔本东南部利滕湖的芬诺斯坎迪亚地盾的前寒武纪基底。拟议的DFG项目将专注于在外陆架和岩心上部1200米的前陆盆地背景下发育的寒武纪-志留纪沉积序列。伊阿普图斯洋的相继闭合、波罗的海与劳伦西亚的碰撞、大陆边缘的挠曲加载以及挪威-瑞典变形前缘逆冲片的叠加，改变了巴尔托斯珊瑚盆地的形态，并在中奥陶世至志留纪与造山带平行的前陆盆地中创造了不同的深度环境。将对目标区的沉积盖层进行详细的相和海平面变化研究。上奥陶统潜在的地层空白和石灰岩沉积(Llandovery)中可能的岩溶作用可能反映了中奥陶世晚期至上志留世冰库时期的冰川事件和剧烈的气候波动。对深水环境中沉积的上奥陶统沉积物的调查将包括在封闭的伊阿普图斯地区寻找强烈的火山活动(钾膨润岩)的痕迹，以及在加里东前深部寻找靠近陨石撞击的痕迹。在地热学和地球物理学等其他研究专题上与私营部门密切合作是该项目的一个主要重点。详细的岩性研究对于与地热模型相关的采样是重要的。用沉积记录校准钻孔地球物理数据，将允许讨论以前在为确定COSC-2钻探地点而调查的横断面上进行的详细反射地震研究基础上的地下解释。沉积层序是了解COSC目标区地下地震资料的关键。

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.