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

9616347 Bergstrom 层序对相发育的地层解释已得到广泛应用。然而，该模型很少针对一组独立的高精度年代地层相关性进行测试。拟议的研究旨在直接检验层序地层学的两个中心假设：不整合面限制的上超覆/超脱旋回在广泛区域内是同步的，并且旋回内的最大洪水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横断面中奥陶世晚期沉积序列的时间来检验这些预测（图1）。第一条将延伸到塔科尼克前陆盆地内的西南部，从纽约海角穿过宾夕法尼亚折返角到弗吉尼亚海角。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。第二条横断面将从塔科尼克前陆盆地延伸，穿过肯塔基州中北部的杰萨明圆顶，进入克拉通内部，我们将在其中检查爱荷华州、伊利诺伊州和密苏里州的部分。这些地区在地理上相距甚远，并且在此期间经历了不同的构造历史。 沿着这两条样线，我们将研究中奥陶世最上部三级地层序列相对于独立构建的高精度年代地层的时空分布。我们将特别关注 1) 下特伦顿群-尤蒂卡页岩海侵和高位体系域，其在 Corynoides americanus Chron 期间在纽约塔科尼克盆地发育，2) 随后的低位扇（Dolgeville 组），在 Orthographus ruedemanni Chron 期间发育。该循环的预测序列当量将取自已发表的文献。我们之前对纽约州中部中奥陶世特伦顿群和尤蒂卡页岩的研究已经产生了高分辨率年代地层学，它将钾膨润土地球化学指纹数据与笔石和牙形刺生物地层学数据相结合。我们建议通过结合 1) 笔石和牙形刺生物地层学研究，以及 2) 对从研究地点的钾膨润土中获得的石英斑晶中的磷灰石斑晶和玻璃包裹体进行高精度电子探针分析来扩展这一年代地层框架。基于该系统内图形相关性的年代地层分辨率应该很高（大大超过单独的生物地层学或放射性测年的分辨率）并且足以提供对快速重叠的海侵系统轨迹、上覆最大淹没表面/海洋裂隙以及上部层序边界/低位扇的同步性的明确测试。 通过将这组时间线延伸到相对稳定的克拉通内部和南部，延伸到在塔科造山运动的早期布隆坦阶段经历过碰撞载荷的区域，将有可能 1) 获得北美东部这一区间记录的海侵-海退循环之间同步性的精确和独立测量，因此，2) 使用这个相关系统来检查海平面变化和区域构造事件对中奥陶世晚期地层序列的形成。这项工作将提供有关克拉通地质事件与主要造山带沿线地质事件之间的物理和时间联系的重要且难以获得的信息。

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.