COLLABORATIVE RESEARCH: Integrated Biostratigraphy and K-bentonite Chronostratigraphy in the Middle Ordovician of the Northern Appalachian Basin

阿巴拉契亚北部前陆盆地中奥陶统晚期和上奥陶统早期特伦顿群的碳酸盐岩和硅质碎屑岩包裹着大量由滨海火山弧喷发形成的蚀变火山灰层（k -膨长岩）。这些地层也构成了北美奥陶纪时间尺度的标准参考剖面的一部分，也是该段牙形石和笔石生物分带的参考标准，通常用于时间对比。该研究的目标是利用k -膨润土中未改变的斑晶的地球化学指纹图谱，以及对笔石和牙形石物种范围的新的详细研究，将这些单独的地层整合到一个统一的年代地层网络中。为了实现这一目标，我们建议：1)在笔石带与北大西洋和大陆中部牙形带之间建立精确的联系，这是基于从特伦顿群碳酸盐岩到尤蒂卡页岩的许多测量剖面中详细收集的范围数据；2)在横向等效的硅屑和碎屑岩之间建立精确的联系通过钾基膨润土的地球化学指纹图谱识别同步时间平面，确定了碳酸盐岩相。k -膨润土对比的技术包括：1)石英斑晶熔体包裹体的主微量元素组成分析。此外，我们建议使用高精度的U-Pb锆石年代学来校准我们的年代地层。由此产生的年代地层学将提供一个时间框架，以年为单位进行校准，这将为现有笔石和牙形石生物地层学的准确性以及这些岩石的层序地层学和共生研究提供直接测试。这种年代地层学将构成研究笔石和在研究剖面中出现的其他生物的进化速率所必需的时间框架。同时，k -膨润土的地球化学和年代学数据将为这些蚀变火山灰床的构造物源以及Taconic造山事件的性质和持续时间提供独特的见解。

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.