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

最近的地层学和同位素研究揭示了石炭纪冰川作用比先前认为的更复杂的性质，并突出了进一步研究贡德瓦南冰川作用开始的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的必要性。此外，一些同位素替代数据表明，中石炭世(切斯特里期)甚至可能是冰川幕间的全球变暖和冰川消退时期。我们假设，伴随着冰盖动力学的重大变化而发生的海平面变化、大陆风化模式以及海洋和古气候条件的变化，无论是与密西西比河晚期主期冰川的开始或潜在的间歇性冰川消融相关，都应该记录在切斯特尔地层的地层学和同位素组成中。我们进一步假设，上述两种情景中的每一种都将赋予切斯特河沉积一套独特的地层和地球化学特征，可以用来对假设的贡德瓦南冰川演化路径施加定量限制。为了验证这些假设，我们提议对爱达荷州中东部和蒙大拿州西南部的切斯特河碳酸盐和硅质岩进行综合的、高分辨率的旋回和层序地层学和同位素研究，重点是建立切斯特河世在潘塔拉萨这一部分的环境变化记录。将从研究区的近端和远端环境对切斯特纪大化石和微化石的生物地层学进行详细的区域重新评估，以便在拟议的层序地层框架内最大限度地提高年代地层对比的分辨率。我们将对切斯特岩进行层序地层学分析，以破译其沉积历史，并在局部和区域尺度上确定潮周和潮下准层序、主要层序边界不整合和准层序界面的性质和分布。这将允许评估准层序和表面如何与微妙的构造运动或记录的海平面波动有关。古生物分析将对每个准层序或层序中记录的相对海平面上升和/或下降的数量提供重要的定性和定量约束。我们建议在最近得到的石炭系稳定和放射性同位素曲线的基础上，开发一套切斯特河系的综合稳定(d13C，d18O和d34S)和放射性(87Sr/86Sr)同位素记录，这将通过以下方式做出独特贡献：(1)提炼现有同位素曲线的一部分，(2)定义d13Ccarb和d13Corg耦合记录，以及(2)测试重建鹿角前陆盆地O和C同位素垂直梯度的潜力。通过从同一组年代地层良好约束的地层开发一系列高分辨率替代，沉积我们将能够评估地层和同位素替代记录的相对速率和阶段(即超前-滞后)关系，这一方面在以前的盆地内研究中已受到不同程度的影响。这种方法应该能够区分该盆地中长期的构造事件和频率较高的海平面事件，从而对构造强迫对切斯特河地层的地层和同位素特征的作用提供相当大的洞察力。反过来，这将有助于洞察构造强迫在控制盆地容纳能力以及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在潘塔拉萨这一部分发生的环境变化的光谱，包括全球古海洋和古气候扰动。最后，这项研究的结果应有助于深入了解洋流、物理和化学海洋条件与古气候之间的任何机械联系的性质，从而最终有助于限制冈德瓦南冰川形成的时间和模式。

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.