New40Ar/39Ar dates for Cretaceous Chauna Group tephra, north-eastern Russia, and their implications for the geologic history and floral evolution of the North Pacific region
New40Ar/39Ar dates for Cretaceous Chauna Group tephra, north-eastern Russia, and their implications for the geologic history and floral evolution of the North Pacific region
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俄罗斯东北部白垩纪 Chauna Group tephra 的新 40Ar/39Ar 年代及其对北太平洋地区地质历史和花卉演化的影响
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
A. Herman
中科院分区：
文献类型：
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作者：
S. Kelley;R. Spicer;A. Herman
摘要含植物化石的白垩纪陆相火山-陆源沉积岩广泛分布于俄罗斯东北部，植物地层学已成为区域填图的主要工具。然而，超纳群的植物群在成分上是异常的，因此其适当的年龄分配是不确定的。它被认为是阿尔卑斯晚期，与阿拉斯加北部钱德勒河序列的第二花区同时代，尽管存在着相对先进的针叶树和稀有的被子植物，具有晚白垩世的典型特征。关于喷出的火山岩的新的放射性数据严格限制了植物群的年龄，表明它比一些生物地层学家所说的要年轻。超纳群以前的Rb-sr年龄范围为87~100 Ma。而斜长石、黑云母、角闪石的新40Ar/39Ar(激光熔融和激光阶梯加热)年龄均在2σ误差范围内，均接近88 Ma，均属上白垩统的锥花期。即使在年龄较小的情况下，植物区系的组成仍然是独特的，并表明在为地层和古气候目的确定区域植被时，考虑海拔、土壤条件和系列发育的重要性。
Abstract Cretaceous nonmarine volcanogenic-terrigenous sedimentary rocks that contain plant fossils are widespread in north-eastern Russia and phytostratigraphy has been a primary tool for regional mapping. The flora from the Chauna Group is, however, compositionally anomalous, making its proper age assignment uncertain. It has been considered to be late Albian in age, coeval with floral Zone II of the Chandler River sequence in northern Alaska, despite the presence of relatively advanced conifers and rare angiosperms typical of the Late Cretaceous. New radiometric data on extrusive volcanic rocks sharply constrain the age of the flora showing that it is younger than some biostratigraphers had suggested. Previous Rb-Sr dates from the Chauna Group ranged from 87 to 100 Ma. However, new 40 Ar/ 39 Ar (laser fusion and laser stepped heating) dates of plagioclase feldspar, biotite and amphibole all lie within 2σ errors of each other, close to 88 Ma, and all ages fall within the Coniacian stage of the Upper Cretaceous. Even with the younger age assignment the flora remains compositionally distinct and demonstrates the importance of accounting for elevation, edaphic conditions, and seral development when determining regional vegetation for stratigraphic and palaeoclimatic purposes.