COLLABORATIVE RESEARCH: Integrated Biostratigraphy and K-Bentonite Chronostratigraphy in the Ordovician of the Northern Appalachian Basin: Geochemical and Geochrono......

阿巴拉契亚前陆盆地北部特伦顿群中、晚、早奥陶世晚期碳酸盐岩和硅质碎屑岩包裹着大量的蚀变火山灰层(K-膨润岩)，这些蚀变火山灰层是在近海火山弧上由普林尼期喷发沉积的。这些地层还构成了北美奥陶纪时间尺度的标准参考剖面的一部分，以及该间隔中通常用于时间对比的牙形刺和笔石生物分带的参考标准。这项研究的目的是利用K膨润土中未改变的斑晶的地球化学指纹，以及对笔石和牙形刺物种范围的新的和详细的研究，将这些独立的地层整合到一个统一的年代地层网络中。我们建议这样做：1)通过详细收集大量测量剖面的范围数据，建立笔石带与北大西洋和中大陆牙形刺带之间的精确联系，每个测量剖面跨越从特伦顿群碳酸盐岩到尤蒂卡页岩的相变；2)在横向相当的硅质碎屑相和碳酸盐相之间建立精确联系。钾基膨润土地球化学指纹的同步时间平面钾-膨润土对比所采用的技术包括：1)石英斑晶中熔体包裹体的主量元素和微量元素组成的分析。此外，我们还建议使用高精度的U-Pb锆石测年来校准我们的年代地层学。由此产生的年代地层学将提供一个以年为单位校准的时间框架，将直接测试现有笔石和牙形刺生物地层学的准确性，以及对这些岩石的层序地层学和核心对比研究。这一年代地层学将构成必要的时间框架，用于研究研究区段中笔石和其他生物之间的演化速度。同时，钾膨润土的地球化学和年代学数据将提供有关这些蚀变火山灰岩层的构造来源以及塔康造山事件的性质和持续时间的独特见解。

The late Middle and early Upper Ordovician carbonate and siliciclastic rocks of the Trenton Group in the northern Appalachian foreland basin enclose a large number of altered volcanic ash beds (K-bentonites) deposited from Plinian eruptions on an off-shore volcanic arc. These strata also form part of the standard reference section of the North American Ordovician time scale as well as the reference standard for the conodont and graptolite biozonations commonly used for temporal correlation in this interval. The goal of the research is to employ geochemical fingerprinting of unaltered phenocrysts within the K-bentonites together with new and detailed studies of the ranges of graptolite and conodont species to integrate these separate stratigraphies into a unified chronostratigraphic network. We propose to accomplish this by 1) establishing precise ties between the graptolite zones and the North Atlantic and midcontinent conodont zones based on detailed collecting of range data from numerous measured sections that each span the facies transition from Trenton Group carbonates into the Utica Shale and 2) establishing precise ties between the laterally equivalent siliciclastic and carbonate facies based on the identification of synchronous time planes through geochemical fingerprinting of the K-bentonites. The techniques to be employed for K-bentonite correlation include analysis of 1) major and trace element composition of melt inclusions within quartz phenocrysts. Additionally, we propose to use high precision U-Pb zircon geochronometry to calibrate our chronostratigraphy. The resulting chronostratigraphy will provide a temporal framework, calibrated in years, that will provide a direct test of the accuracy of the existing graptolite and conodont biostratigraphies as well as sequence stratigraphic and coenocorrelation studies of these rocks. This chronostratigraphy will constitute the temporal framework necessary for studies of the rate of evolution among graptolites and other organisms that occur in the studied sections. Simultaneously, the K-bentonite geochemical and geochronological data will provide unique insights about the tectonic provenance of these altered volcanic ash beds and the nature and duration of Taconic orogenic events.