Smith L H, Kaufman A J, Knoll A H, Link P K
Botanical Museum, Harvard University, Cambridge, MA 02138, USA.
Geol Mag. 1994 May;131(3):301-14. doi: 10.1017/s0016756800011079.
Isotopic chemostratigraphy has proven successful in the correlation of carbonate-rich Neoproterozoic successions. In successions dominated by siliciclastic rocks, chemostratigraphy can be problematic, but if thin carbonates punctuate siliciclastic strata, useful isotopic data may be obtained. The upper Pocatello Formation and lower Brigham Group of southeastern Idaho provide an opportunity to assess the potential and limitations of isotopic chemostratigraphy in overwhelmingly siliciclastic successions. The 5000 m thick succession consists predominantly of siliciclastic lithologies, with only three intervals that contain thin intercalated carbonates. Its depositional age is only broadly constrained by existing biostratigraphic, sequence stratigraphic and geochronometric data. The lowermost carbonates include a cap dolomite atop diamictites and volcanic rocks of the Pocatello Formation. The delta 13C values of these carbonates are distintly negative (-5 to -3), similar to carbonates that overlie Neoproterozoic glaciogenic rocks worldwide. Stratigraphically higher carbonates record a major positive delta 13C excursion to values as high as +8.8 within the carbonate member of the Caddy Canyon Quartzite. The magnitude of this excusion is consistent with post-Sturtian secular variation recorded elsewhere in the North American Cordillera, Australia, Svalbard, Brazil and Nambia, and exceeds the magnitude of any post-Varanger delta 13C excursion documented to date. In most samples, Sr-isotopic abundances have been altered by diagenesis and greenschist facies metamorphism, but a least-altered value of approximately 0.7076 supports a post-Sturtian and pre-Marinoan/Varanger age for upper Pocatello and lower Brigham rocks that lie above the Pocatello diamictite. Thus, even though available chemostratigraphic data are limited, they corroborate correlations of Pocatello Formation diamictites and overlying units with Sturtian glaciogenic rocks and immediately post-Sturtian successions in western North America and elsewhere.
同位素化学地层学已被证明在富含碳酸盐的新元古代地层对比中是成功的。在以硅质碎屑岩为主的地层中，化学地层学可能存在问题，但如果薄碳酸盐层穿插在硅质碎屑岩层中，则可能获得有用的同位素数据。爱达荷州东南部的波卡特洛组上部和布里格姆群下部提供了一个机会，来评估同位素化学地层学在以硅质碎屑岩为主的地层中的潜力和局限性。这个5000米厚的地层主要由硅质碎屑岩岩性组成，只有三个间隔含有薄的夹层碳酸盐。其沉积年龄仅受到现有生物地层学、层序地层学和地质年代学数据的大致限制。最下部的碳酸盐包括波卡特洛组冰碛岩和火山岩之上的盖帽白云岩。这些碳酸盐的δ13C值明显为负（-5至-3），类似于全球新元古代冰川成因岩石之上的碳酸盐。地层较高处的碳酸盐记录了一个主要的正δ13C偏移，在卡迪峡谷石英岩的碳酸盐段内，值高达+8.8。这种偏移的幅度与北美科迪勒拉山脉、澳大利亚、斯瓦尔巴群岛、巴西和纳米比亚其他地方记录的斯图尔特冰期后的长期变化一致，并且超过了迄今为止记录的任何瓦兰吉尔冰期后δ13C偏移的幅度。在大多数样品中，锶同位素丰度已因成岩作用和绿片岩相变质作用而改变，但一个最低改变值约为0.7076支持波卡特洛组上部和布里格姆群下部位于波卡特洛冰碛岩之上的岩石具有斯图尔特冰期后和马里诺冰期/瓦兰吉尔冰期前的年龄。因此，即使可用的化学地层学数据有限，但它们证实了波卡特洛组冰碛岩及其上覆单元与北美西部和其他地方的斯图尔特冰川成因岩石以及斯图尔特冰期后紧接着的地层之间的对比。
