COLLABORATIVE RESEARCH: Distinguishing Regional Tectonic, Global Eustatic, and Isotopic Events in Chesterian Rocks of East-Central Idaho and Southwestern Montana

摘要：最近的地层学和同位素研究揭示了石炭纪冰川作用比以前认为的更为复杂的性质，并强调需要进一步研究冈瓦纳冰川作用的起始时间和模式，及其与海平面波动、大陆风化速率变化以及古海洋和气候条件的机制联系。此外，一些同位素代理数据表明石炭纪中期（切斯特纪）甚至可能是一个时期全球变暖和冰川消退之间的现象。我们假设，海平面、大陆风化模式以及海洋和古气候条件的变化，伴随着与密西西比纪晚期主期冰川作用或潜在间歇性冰消作用相关的冰盖动力学的显着变化，应该记录在切斯特地层的地层学和同位素组成中。我们进一步假设，上述两种情况都会在切斯特沉积物上赋予一组独特的地层和地球化学特征，这些特征可用于对冈瓦纳冰川的假设演化路径进行定量限制。为了检验这些假设，我们建议对爱达荷州中东部的切斯特碳酸盐和硅质碎屑进行综合、高分辨率旋回和层序地层和同位素研究。蒙大拿州西南部的重点是开发切斯特纪时期潘萨拉萨地区环境变化的记录。将根据研究区域的近端和远端设置对切斯特大型化石和微化石生物地层学进行详细的区域重新评估，以便最大限度地提高拟议层序地层框架中年代地层相关性的分辨率。我们将对切斯特阶岩石进行分层层序地层分析，以破译其沉积历史，并确定局部和区域尺度的潮间和潮下准层序、主要层序边界不整合面和准层序边界的性质和分布。这将有助于评估准层序和表面如何与微妙的构造运动相关或记录海平面波动。古生物学分析将为每个准层序或层序中记录的相对海平面上升和/或下降的量提供重要的定性和定量约束。我们建议在最近导出的石炭系稳定和放射性同位素曲线集的基础上，开发一套完整的切斯特系稳定（d13C、d18O和d34S）和放射性（87Sr/86Sr）同位素记录，该记录应通过以下方式做出独特贡献：（1）完善现有同位素曲线的一部分，（2）定义耦合的d13Ccarb和d13Corg记录，以及(2) 测试重建 Antler 前陆盆地 O 和 C 同位素垂直梯度的潜力。通过从同一组年代地层约束良好的沉积物中开发一系列高分辨率代理，我们将能够评估地层和同位素代理记录的相对速率和相位（即超前-滞后）关系，这一方面在以前的盆地内研究中一直受到不同程度的影响。这种方法应该能够区分该盆地的长期构造事件和高频海平面上升事件，从而深入了解构造强迫对切斯特地层的地层和同位素特征的作用。反过来，这应该有助于深入了解构造强迫在控制盆地适应性、区域海洋条件和环流方面的作用。此外，我们预计这项研究应该定量地限制切斯特纪期间潘塔拉萨这部分地区发生的环境变化范围，包括全球古海洋和古气候扰动。最后，这项研究的结果应该有助于深入了解海平面、物理和化学海洋条件与古气候之间任何机制联系的性质，从而最终有助于限制冈瓦纳冰川作用的启动时间和模式。

ABSTRACTRecent stratigraphic and isotopic studies reveal a more complex nature to Carboniferousglaciation than previously considered, and highlight a need for further study of the timing and mode of initiation of Gondwanan glaciation, and its mechanistic link to eustatic fluctuations, changes in continental weathering rates, and paleoceanographic and climatic conditions.Furthermore, some isotopic proxy data suggest that the mid-Carboniferous (Chesterian) may even have been a time of global warming and deglaciation between glacial episodes. We hypothesize that the changes in sea level, continental weathering patterns, and oceanographic and paleoclimatic conditions that would have accompanied the significant change in ice-sheet dynamics associated with either onset of main-stage glaciation or potentially intermittentdeglaciation during the Late Mississippian should be recorded in the stratigraphy and isotopic compositions of Chesterian strata. We further hypothesize that each of the two aforementioned scenarios would have imparted, on Chesterian deposits, a unique set of stratigraphic andgeochemical signatures that can be used to place quantitative constraints on the hypothesized evolutionary paths of Gondwanan glaciation.In order to test these hypotheses, we propose to carry out an integrated, high-resolution cycle- and sequence-stratigraphic, and isotopic study of Chesterian carbonates and siliciclastics from east-central Idaho and southwestern Montana focused on developing a record of environmental change during the Chesterian in this portion of Panthalassa. A detailed regional reassessment of the Chesterian macrofossil and microfossil biostratigraphy will be made from proximal and distal settings of the study area in order to maximize the resolution of chronostratigraphic correlation in the proposed sequence stratigraphic framework. We will carry out a hierarchical sequence stratigraphic analysis of the Chesterian rocks in order to decipher their depositional history, and to determine the nature and distribution of peritidal and subtidal parasequences, major sequence-bounding unconformities and parasequence boundaries at a local- and regional-scale. This will allow for evaluation of how the parasequences and surfaces relate to subtle tectonic movements or record eustatic fluctuations. Paleontologic analysis will provide important qualitative and quantitative constraints on the amount of relative sea level rise and/or fall recorded in each parasequence or sequence. We propose to build on the recently derived set of stable and radiogenic isotope curves for the Carboniferous by developing a set of integrated stable (d13C, d18O, and d34S) and radiogenic (87Sr/86Sr) isotope records for the Chesterian that should contribute uniquely by: (1) refining a portion of the existing isotope curves, (2) defining a coupled d13Ccarb and d13Corg record, and (2) testing the potential for reconstructing O and C isotope vertical gradients for the Antler foreland basin.By developing a series of high-resolution proxies from the same set of chronostratigraphically well-constrained deposits, we will be able to evaluate the relative rates and phase (i.e., lead-lag) relationships of the stratigraphic and isotopic proxy records, an aspect that has been variably compromised in previous intrabasinal studies. This approach should allow for differentiation of long-term tectonic events from higher frequency eustatic events in this basin, and thus provide considerable insight into the role of tectonic forcing on the stratigraphic and isotopic character of the Chesterian strata. In turn this should provide insight into the role of tectonic forcing in controlling basin accommodation, and regional oceanographic conditions and circulation. Moreover, we anticipate that this study should quantitatively constrain the spectrum of environmental changes that occurred during the Chesterian in this portion of Panthalassa, including global paleoceanographic and paleoclimatic perturbations. Lastly, the results of this study should provide insight into the nature of any mechanistic links between eustasy, physical and chemical oceanographic conditions and paleoclimate, thus ultimately contributing towards constraining the timing and mode of initiation of Gondwanan glaciation.