Stratigraphic age of the Ordovician sedimentary succession in Lumparn Bay, Åland Islands, Finland

The depression of the ancient Lumparn meteorite impact structure is partly infilled by Cambrian and Ordovician sediments, lying nowadays below the seawater. The Ordovician carbonate succession at this site, recognised through erratic boulders and by the drilling project in the late 1950s, is of particular interest due to its isolated and distant location from other areas with Ordovician sediments in the Baltoscandian Palaeobasin. In this study, we present new data on ostracod biostratigraphy and stable carbon isotope chemostratigraphy obtained from three old drillcores. Comparison of ostracod distribution with sections in Estonia generally supports the previous interpretations of a Darriwilian and Sandbian age for the lowermost Tranvik Limestone (which we propose renaming the “orthoceratite limestone” in the Åland Islands area). The uppermost Östersjö Limestone spans almost the entire Katian age. We also observe the global MDICE and GICE carbon isotopic excursions, as well as Katian Rakvere and Saunja excursions, within the Lumparn succession. The Ordovician succession in Lumparn Bay is facially resembling the Estonian Shelf facies of the Baltoscandian Palaeobasin.

古代兰帕恩（Lumparn）陨石撞击构造的凹陷现今被海水覆盖，其内部部分充填有寒武纪与奥陶纪沉积物。该位点的奥陶纪碳酸盐岩序列可通过漂砾与1950年代末的钻井项目识别，由于其在波罗的斯堪的纳维亚古盆地（Baltoscandian Palaeobasin）中与其他奥陶纪沉积物分布区域隔绝且位置偏远，因而受到特别关注。本研究基于三份老旧钻井岩芯，报道了介形虫（ostracod）生物地层学与稳定碳同位素化学地层学（stable carbon isotope chemostratigraphy）的全新数据。将介形虫分布与爱沙尼亚的相关剖面进行对比后，研究结果总体支持此前关于最底部特兰维克灰岩（Tranvik Limestone）的形成时代为达瑞威尔阶与桑比阶的推断——我们提议将奥兰群岛（Åland Islands）区域的该套地层重新命名为「直角石灰岩」。最顶部的厄斯特舍灰岩（Östersjö Limestone）几乎覆盖了整个凯迪阶（Katian）地层序列。此外，我们在兰帕恩的地层序列中还观测到全球尺度的MDICE与GICE碳同位素偏移，以及凯迪阶的拉克韦雷（Rakvere）和桑亚（Saunja）同位素偏移。兰帕恩湾的奥陶纪序列在岩相特征上与波罗的斯堪的纳维亚古盆地内的爱沙尼亚陆架相具有相似性。