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Carbon isotope chemostratigraphy and sea-level history of the Hirnantian Stage (uppermost Ordovician) in the Oslo–Asker district, Norway
Geological Magazine ( IF 2.3 ) Pub Date : 2021-07-19 , DOI: 10.1017/s0016756821000546
Mikael Calner 1 , Christian M.Ø. Rasmussen 2 , Hanna Calner 3 , Oliver Lehnert 4 , Michael M. Joachimski 4
Affiliation  

We present a δ13Ccarb chemostratigraphy for the Late Ordovician Hirnantian Stage based on 208 whole-rock samples from six outcrops in the Oslo–Asker district, southern Norway. Our data include the Norwegian type section for the Hirnantian Stage and Ordovician–Silurian boundary at Hovedøya Island. The most complete record of the Hirnantian Isotope Carbon Excursion (HICE) is identified in a coastal exposure at Konglungø locality where the preserved part of the anomaly spans a c. 24 m thick, mixed carbonate–siliciclastic succession belonging to the upper Husbergøya, Langåra and Langøyene formations and where δ13Ccarb peak values reach c. +6 ‰. Almost the entire HICE occurs above beds containing the Hirnantia Fauna, suggesting a latest Hirnantian age for the peak of the excursion. The temporal development of the HICE in southern Norway is associated with substantial shallowing of depositional environments. Sedimentary facies and erosional unconformities suggest four inferably fourth-order glacio-eustatically controlled sea-level lowstands with successively increased exposure and erosion to the succession. The youngest erosional unconformity is related to the development of incised valleys and resulted in cut-out of at least the falling limb of the HICE throughout most of the Oslo–Asker district. The fill of the valleys contains the falling limb of the HICE, and the postglacial transgression therefore can be assigned to the latest part of the Hirnantian Age. We address the recent findings of the chitinozoan Belonechitina gamachiana in the study area and its relationship to the first occurrence of Hirnantia Fauna in the studied sections, challenging identification of the base of the Hirnantian Stage.

中文翻译:

挪威奥斯陆-阿斯克地区赫南阶(奥陶系上层)的碳同位素化学地层学和海平面历史

我们提出一个 δ13C碳水化合物基于来自挪威南部奥斯陆-阿斯克地区六个露头的 208 个全岩样本的晚奥陶世赫南阶化学地层学。我们的数据包括赫南阶的挪威型剖面和 Hovedøya 岛的奥陶纪-志留纪边界。最完整的 Hirnantian 同位素碳偏移 (HICE) 记录是在 Konglungø 地区的沿海暴露中确定的,该异常的保存部分跨越C。24 m 厚的混合碳酸盐-硅质碎屑层序,属于上 Husbergøya、Langåra 和 Langøyene 地层,其中 δ13C碳水化合物峰值达到C。+6‰。几乎整个 HICE 都发生在包含赫南蒂亚动物区系,表明远足高峰期的最新赫南特时代。挪威南部 HICE 的时间发展与沉积环境的显着变浅有关。沉积相和侵蚀不整合表明四个可推断的四级冰川-海平面控制的海平面低位,连续增加暴露和侵蚀的演替。最年轻的侵蚀不整合与下切山谷的发展有关,并导致至少在奥斯陆-阿斯克地区大部分地区的 HICE 下落肢被切断。山谷的填充物包含了 HICE 的下降分支,因此冰河后的海侵可以被分配到赫南特时代的最新部分。我们解决了几丁质动物的最新发现白毛蛉在研究区域及其与首次出现的关系赫南蒂亚研究部分的动物群,对 Hirnantian 阶段底部的识别具有挑战性。
更新日期:2021-07-19
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