A robust age model for the Cryogenian Pocatello Formation of southeastern Idaho (northwestern USA) from tandem in situ and isotope dilution U-Pb dating of volcanic tuffs and epiclastic detrital zircons
A robust age model for the Cryogenian Pocatello Formation of southeastern Idaho (northwestern USA) from tandem in situ and isotope dilution U-Pb dating of volcanic tuffs and epiclastic detrital zircons
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DOI：
10.1130/ges02437.1
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发表时间：
2022-02
期刊：
影响因子：
2.5
通讯作者：
V. Isakson;M. Schmitz;C. Dehler;F. Macdonald;W. Yonkee
中科院分区：
文献类型：
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作者：
V. Isakson;M. Schmitz;C. Dehler;F. Macdonald;W. Yonkee
美国西北爱达荷州Pocatello组火山碎屑火山岩和冰川及非冰川沉积地层中锆石的原位和同位素稀释U-Pb串联分析为北美科迪勒拉地区冰川期冰川作用提供了新的时代约束。在Scout山段下辉长岩段的冰成地层中采集的2个英质凝灰岩样品获得了高精度的化学磨损同位素稀释U-Pb锆石喷发和沉积年龄分别为696.43±0.21和695.17±0.20 Ma。通过对下辉长岩不整合上覆的滨面和近海砂岩的高精度碎屑锆石最大沉积年龄≤670 Ma的补充，这些数据与下辉长岩与早低温世（约717-660 Ma）斯图特冰川作用的对比一致。这些670-675 Ma的锆石持续存在于上部二晶岩和帽状白云岩单元之上的地层中，直至并包括所谓的“重新加工的沉降凝灰岩”，我们得出的结论是，从火山碎屑碎屑中得出的最大沉积年龄≤673 Ma。658 Ma的稀有碎屑锆石为上部辉长岩和上覆盖白云岩单元提供了最大沉积年龄。这一新的地质年代学框架支持了Pocatello组Scout山段下部和上部双晶岩段与Sturtian （716-660 Ma）和Marinoan（≤650-635 Ma）低纬冰川作用的岩性和化学地层对比。因此，波卡特洛组包含了比先前假设的更完整的冰川期记录。
Tandem in situ and isotope dilution U-Pb analysis of zircons from pyroclastic volcanic rocks and both glacial and non-glacial sedimentary strata of the Pocatello Formation (Idaho, northwestern USA) provides new age constraints on Cryogenian glaciation in the North American Cordillera. Two dacitic tuffs sampled within glacigenic strata of the lower diamictite interval of the Scout Mountain Member yield high-precision chemical abrasion isotope dilution U-Pb zircon eruption and depositional ages of 696.43 ± 0.21 and 695.17 ± 0.20 Ma. When supplemented by a new high-precision detrital zircon maximum depositional age of ≤670 Ma for shoreface and offshore sandstones unconformably overlying the lower diamictite, these data are consistent with correlation of the lower diamictite to the early Cryogenian (ca. 717–660 Ma) Sturtian glaciation. These 670–675 Ma zircons persist in beds above the upper diamictite and cap dolostone units, up to and including a purported “reworked fallout tuff,” which we instead conclude provides only a maximum depositional age of ≤673 Ma from epiclastic volcanic detritus. Rare detrital zircons as young as 658 Ma provide a maximum depositional age for the upper diamictite and overlying cap dolostone units. This new geochronological framework supports litho- and chemostratigraphic correlations of the lower and upper diamictite intervals of the Scout Mountain Member of the Pocatello Formation with the Sturtian (716–660 Ma) and Marinoan (≤650–635 Ma) low-latitude glaciations, respectively. The Pocatello Formation thus contains a more complete record of Cryogenian glaciations than previously postulated.