Kaufman A J, Knoll A H, Awramik S M
Botanical Museum, Harvard University, Cambridge, Massachusetts 02138.
Geology. 1992 Feb;20:181-5. doi: 10.1130/0091-7613(1992)020<0181:baccon>2.3.co;2.
Recent advances in Proterozoic micropaleontology and sedimentary isotope geochemistry suggest that improved interbasinal correlation of Neoproterozoic (1000-540 Ma) successions is possible. Because widely varying interpretations of its age have been suggested and no reliable radiometric dates or paleomagnetic data are available, the upper Tindir Group of northwestern Canada provides an opportunity to test this hypothesis. The age of these strata is of paleontological importance because silicified carbonates near the top of the group contain disc-shaped-scale microfossils that may provide insights into the early evolution of biomineralization. A reinterpretation of upper Tindir microfossil assemblages suggests a late Riphean age. Although diagenesis and contact metamorphism have altered the isotopic compositions of some carbonates, least altered samples indicate that delta 13C of contemporaneous seawater was at least +4.7%, typical of Neoproterozoic, but not Cambrian, carbonates. Strontium isotopic compositions of the least altered samples yield values of approximately 0.7065, which can be uniquely correlated with late Riphean seawater. Together, micropaleontology and the isotopic tracers of C and Sr constrain the upper Tindir carbonates and their unique fossils to be late Riphean, likely between 620 and 780 Ma.
元古代微古生物学和沉积同位素地球化学的最新进展表明，新元古代（1000 - 540百万年前）层序的盆地间对比有了改进的可能。由于对其年龄的解释差异很大，且没有可靠的放射性测年或古地磁数据，加拿大西北部的廷迪尔群上部提供了检验这一假设的机会。这些地层的年龄具有古生物学意义，因为该群顶部附近的硅化碳酸盐中含有盘状鳞片微化石，可能为生物矿化的早期演化提供见解。对上廷迪尔微化石组合的重新解释表明其为晚里菲纪年龄。尽管成岩作用和接触变质作用改变了一些碳酸盐的同位素组成，但受影响最小的样品表明，同期海水的δ13C至少为+4.7%，这是新元古代碳酸盐的典型特征，而非寒武纪碳酸盐的特征。受影响最小的样品的锶同位素组成产生了约0.7065的值，这可以与晚里菲纪海水唯一对应。微古生物学以及碳和锶的同位素示踪剂共同将上廷迪尔碳酸盐及其独特化石限定为晚里菲纪，可能在620至780百万年前之间。
