Integrated Sequence and Chemostratigraphy of the Neoproterozoic Krol Platform, Lesser Himalaya, India

本文对印度北部小喜马拉雅地区新元古代（可能还有下寒武统）Krol地台进行了综合层序和化学地层学研究。Krol是一个超过6公里厚的序列的一部分，其中包括底部（Blaini组）可能是Varanger时代的冰川-海洋状态和顶部（Tal组）早寒武纪（Tommotian到Botomian）的陆源岩石。新元古代-早寒武世段在全球范围内以（13C）偏移明显为特征，碳酸盐碳在+12 ~ -5之间。这些变化被用来反映海水同位素组成的长期变化，并且这些变化可以用于全球对比。然而，碳酸盐（13C低至-5）的偏移很难用整个海洋化学来解释，并且可能表明a)表面到海底的同位素梯度大于现代海洋的~2 / mil， b)地层趋势可能部分归因于沉积古环境（水深）的变化。Krol台地可以用来解决对新元古代化学地层学和我们对同位素漂移起源的理解有重要意义的两个问题：1)13C的横向变化作为相（和古水深）的函数是什么？2)克罗尔已公布的13C测量值与西伯利亚、中国南方、纳米比亚和麦肯齐山脉（加拿大）潜在相关地层的数据之间的差异，是由于以前的研究对克罗尔的成岩作用关注不足、不相关，还是因为克罗尔代表了一个异常完整和厚的碳酸盐岩序列（2公里），因此可以获得样本？初步的工作表明，克罗尔可能特别适合层序地层学，因为可用的露头似乎包含了从浅斜坡到深斜坡的过渡，以及碳酸盐岩与近岸砂岩的“交错”。化学地层学与层序地层学的结合将使我们能够建立一个独立的时间地层格架，用它来观察（13C）的“垂直”和“横向”变化，并研究成岩作用在台地不同部位的作用。研究还应确定前寒武纪-寒武纪界线是否位于磷虾的顶部（如已公布的碳同位素数据所表明的）；它可以让我们对（13C）的几个层序边界进行“指纹”，从而将这些表面的时间与其他大致相同年龄的富碳酸盐层序解释的层序边界进行比较。

9614070 Christie-Blick We propose an integrated sequence and chemostratigraphic study of the Neoproterozoic (and possibly lower Cambrian) Krol platform in the Lesser Himalaya of northern India. The Krol is part of a succession more than 6 km thick that includes glacial-marine stata of probable Varanger age at the base (Blaini Formation) and terrigenous rocks of early Cambrain age (Tommotian to Botomian) at the top (Tal Group). The Neoproterozoic and early Cambrian interval is characterized globally by marked excursions in (13C, ranging from +12 to -5 for carbonate carbon. These have been taken to reflect secular variations in the isotopic composition of sea water, and that the excursions can be used for global correlation. However, excursions in carbonate (13C as low as -5 are difficult to explain in terms of whole-ocean chemistry, and may indicate a) a surface to seafloor isotopic gradient that was larger than the ~2 per mil of the modern ocean and b) that stratigraphic trends may be due in part to changes in the paleoenvironment of deposition (water depth). The Krol platform can be used to address two questions of importance to Neoproterozoic chemostratigraphy and our understanding of the origin of the isotopic excursions: 1) What are the lateral variations in (13C as a function of facies (and paleowater depth)? 2) Are differences between published (13C measurements for the Krol and data from potentially correlative strata in Siberia, southern China, Namibia and the Mackenzie Mountains (Canada) due to inadequate attention to diagenesis in the Krol in previous studies, miscorrelation or the fact that the Krol represents an unusually complete and thick succession of carbonate (2km) from which to obtain samples? Preliminary work shows that the Krol may be particularly amenable to sequence stratigraphy because available outcrop appears to encompass the transition from shallow to deep ramp settings, as well as "interfingering" of the carbonate rocks with nearshore sandstones. The co mbination of chemostratigraphy with sequence stratigraphy will allow us to establish an independent time-stratigraphic framework with which to look at both "vertical" and "lateral" variations in (13C, and to investigate the role of diagenesis in different parts of the platform. The study should also resolve whether the Precambrian-Cambrian boundary is located at the top of the Krill (as suggested by published carbon isotopic data); and it may allow us to "fingerprint" several sequence boundaries with respect to (13C, and hence to compare the timing of these surfaces and sequence boundaries interpreted in other carbonate-rich successions of broadly the same age.