Redefining the Tonto Group of Grand Canyon and recalibrating the Cambrian time scale
Redefining the Tonto Group of Grand Canyon and recalibrating the Cambrian time scale
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DOI：
10.1130/g46755.1
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发表时间：
2020-02
期刊：
影响因子：
5.8
通讯作者：
K. Karlstrom;M. T. Mohr;M. Schmitz;F. Sundberg;S. Rowland;R. Blakey;J. Foster;L. Crossey;C. Dehler;J. Hagadorn
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文献类型：
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作者：
K. Karlstrom;M. T. Mohr;M. Schmitz;F. Sundberg;S. Rowland;R. Blakey;J. Foster;L. Crossey;C. Dehler;J. Hagadorn
我们应用碎屑锆石(DZ)的串联U-Pb测年方法重新确定了美国亚利桑那州大峡谷地区的托托群，并对寒武纪的时间尺度进行了修正。根据Tapeats砂岩最年轻的同位素稀释DZ年龄计算的最大沉积年龄，东部大峡谷为≤508.19±0.39 Ma，内华达州为≤507.68±0.36 Ma，亚利桑那州中部为506.64±0.32 Ma。塔皮茨砂岩下局部整合的Sixtymile组具有类似的丙二醛(≤)(508.6±0.8 Ma)，并被添加到托托群。我们将这些精确的MDA与通托群中三叶虫生物带的生物地层学结合起来。Tapeats砂岩约508-507 Ma；Bright Angel组包括Olenellus、Glossopleura和Ehmaniella生物带，约507-502 Ma；Muav组包括Bolaspidella和Cedaria生物带，约502-499 Ma。法国山白云岩整合于Muav组之上，是同一海侵的一部分；它取代了McKee的未分异白云岩，成为托托群的一部分；它包含Crepicephalus生物带，年龄为498-497 Ma。托托群自东向西增厚，从250m到830m，这是由于次泥盆世不整合之下∼300m的碳酸盐向西增厚和∼300m的向东倾斜所致。三叶虫属Olenellus出现在大峡谷西部而不是东部；它最后一次出现的基准面(LAD)是在∼507.68 Ma地平线以上45m的光明天使组(Bright Angel Form≤)。这次大灭绝事件的时间估计约为506.5 Ma，位于系列2-苗林纪界线以下的两个生物带，我们估计这一界线约为506 Ma。地层背景中碎屑颗粒的连续串联测年，结合三叶虫生物地层学，为重新校准寒武纪地球系统的速度和动力学提供了丰富的潜力。
We applied tandem U-Pb dating of detrital zircon (DZ) to redefine the Tonto Group in the Grand Canyon region (Arizona, USA) and to modify the Cambrian time scale. Maximum depositional ages (MDAs) based upon youngest isotope-dilution DZ ages for the Tapeats Sandstone are ≤508.19 ± 0.39 Ma in eastern Grand Canyon, ≤507.68 ± 0.36 Ma in Nevada, and ≤506.64 ± 0.32 Ma in central Arizona. The Sixtymile Formation, locally conformable below the Tapeats Sandstone, has a similar MDA (≤508.6 ± 0.8 Ma) and is here added to the Tonto Group. We combined these precise MDAs with biostratigraphy of trilobite biozones in the Tonto Group. The Tapeats Sandstone is ca. 508–507 Ma; the Bright Angel Formation contains Olenellus, Glossopleura, and Ehmaniella biozones and is ca. 507–502 Ma; and the Muav Formation contains Bolaspidella and Cedaria biozones and is ca. 502–499 Ma. The Frenchman Mountain Dolostone is conformable above the Muav Formation and part of the same transgression; it replaces McKee’s Undifferentiated Dolomite as part of the Tonto Group; it contains the Crepicephalus Biozone and is 498–497 Ma. The Tonto Group thickens east to west, from 250 m to 830 m, due to ∼300 m of westward thickening of carbonates plus ∼300 m of eastward beveling beneath the sub-Devonian disconformity. The trilobite genus Olenellus occurs in western but not eastern Grand Canyon; it has its last appearance datum (LAD) in the Bright Angel Formation ∼45 m above the ≤507.68 Ma horizon. This extinction event is estimated to be ca. 506.5 Ma and is two biozones below the Series 2–Miaolingian Epoch boundary, which we estimate to be ca. 506 Ma. Continued tandem dating of detrital grains in stratigraphic context, combined with trilobite biostratigraphy, offers rich potential to recalibrate the tempo and dynamics of Cambrian Earth systems.