Integrated Biostratigraphy and K-Bentonite Chronostratigraphy in the Middle Ordovician of Northeastern Appalachian Basin: Geochemical and Geochronologic Constraints

Bergstrom层序地层相发育解释已得到广泛应用。然而，这个模型很少用一组独立的高精度年代地层对比来检验。提出的研究旨在直接检验层序地层学的两个中心假设：不整合界的上/下旋回在大范围内是同步的，旋回内的最大淹没面可以作为区域等时线。我们建议通过比较北美东部两个区域广泛的样带中奥陶世晚期沉积序列的时间来验证这些预测（图1）。第一个将延伸到塔科尼克前陆盆地的西南部，从纽约海岬穿过宾夕法尼亚再进入到弗吉尼亚海岬。沿着这条横断面，我们将检查宾夕法尼亚州中东部、弗吉尼亚州西部、田纳西州东部和阿拉巴马州北部的部分。第二个样带将从塔科尼克前陆盆地开始，穿过肯塔基州中北部的Jessamine Dome，进入克拉通内陆，在那里我们将研究爱荷华州、伊利诺伊州和密苏里州的部分。这些地区在地理上相距很远，在这段时间内经历了不同的构造历史。沿着这两个样带，我们将相对于独立构建的高精度年代地层学来研究中奥陶统上上部三级层序的时空分布。我们将特别关注1)下特伦顿组-尤蒂卡页岩海侵和高地体系域，这是在美国圆锥虫纪期间在纽约塔尼克盆地发育的；2)随后的低地扇（Dolgeville组），这是在ruedemanni纪期间发育的。该循环的预测序列等效将取自已发表的文献。我们之前对纽约州中部中奥陶统特伦顿群和尤蒂卡页岩的研究，将k -膨润土的地球化学指纹数据与笔石和牙形石的生物地层学数据相结合，得出了一个高分辨率的年代地层学。我们建议通过1)笔石和牙形石生物地层研究，以及2)对研究地点k -膨润土石英斑晶中磷灰石斑晶和玻璃包裹体的高精度电子探针分析来扩展这一年代地层框架。基于该体系内图形对比的年代地层学分辨率应该很高（远远超过单独的生物地层学或放射性测年），并足以对快速重叠的海侵体系轨迹、覆盖的最大洪水面/海相裂谷和上层序边界/低洼扇的同步性提供明确的测试。通过将这组时间线延伸到相对稳定的克拉通内部，并向南延伸到在Taconic造山运动的早期Blountain阶段经历过碰撞负荷的地区，将有可能1)获得在北美东部这段时间内记录的海侵-海退旋回之间的精确和独立的同步测量，因此，2)利用这一关联体系考察了海平面上升变化与区域构造事件对中奥陶世晚期地层层序形成的相对贡献。这项工作将为克拉通和主要造山带上的地质事件之间的物理和时间联系提供重要的、否则无法获得的信息。

9616347 Bergstrom Sequence stratigraphic interpretations of facies development have come into wide application. Yet this model has seldom been tested against an independent set of high precision chronostratigraphic correlations. The proposed research seeks to directly examine two of the central assumptions of sequence stratigraphy: that unconformity bounded onlap/offlap cycles are synchronous across broad regions, and that maximum flooding surfaces within cycles can serve as regional isochrons. We propose to test these predictions by comparing the timing of late Middle Ordovician depositional sequence along two regionally extensive transects across eastern North America (Fig 1). The first will extend to the southwest within the Taconic foreland basin, from the New York Promontory through the Pennsylvania Reentrant to the Virginia Promontory. Along this transect, we will examine sections in east central Pennsylvania, western Virginia, eastern Tennessee, and northern Alabama. The second transect will extend from the Taconic foreland basin through the Jessamine Dome, in North central Kentucky, and into the cratonic interior, where we will examine sections in Iowa, Illinois, and Missouri. These areas are widely separated geographically and are known to have experienced different tectonic histories during this interval. Along these two transects, we will examine the time-space distribution of the upper-most Middle Ordovician third order stratigraphic sequence relative to an independently constructed high-precision chronostratigraphy. We will focus in particular on the equivalents of 1) the lower Trenton Group-Utica Shale transgressive and highstand systems tract that developed in the Taconic basin of New York during the Corynoides americanus Chron and 2) the succeeding lowstand fan (Dolgeville Formation), which developed during the Orthographus ruedemanni Chron. Predicted sequence equivalents of this cycle will be taken from published literature. Our previous research on the Middle Ordovic ian Trenton Group and Utica Shale in central New York State has produced a high resolution chronostratigraphy that integrates data from geochemical fingerprinting of K-bentonites with data from graptolite and conodont biostratigraphy. we propose to extend this chronostratigraphic framework by a combination of 1) graptolite and conodont biostratigraphic studies, and 2) high precision electron microprobe analysis of apatite phenocrysts and glass inclusions within quartz phenocrysts obtained from K-bentonites at the study sites. Chronostratigraphic resolution based on graphic correlation within this system should be high (greatly exceeding that of biostratigraphy alone or of radiometric dating) and sufficient to provide a definitive test of the synchroneity of the rapidly onlapping transgressive system track, the overlying maximum flooding surface/marine hiatus, and the upper sequence boundary/lowstand fan. By extending this set of time lines into the relatively stable cratonic interior and south into regions that experienced collisional loading during the earlier, Blountain Phase, of the Taconic Orogeny, it will be possible 1) to gain a precise and independent measure of synchroneity among the transgressive-regressive cycles documented in this interval in eastern North America and, therefore, 2) to use this system of correlations to examine the relative contributions of changing eustatic sea-level and regional tectonic events to the formations of late Middle Ordovician stratigraphic sequences. The work will provide important and otherwise inaccessible information about he physical and temporal connections between geological events on the craton and those along a major orogenic belt.