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

摘要近年来的地层学和同位素研究表明，石炭纪冰川的性质比以往认为的更为复杂，并强调需要进一步研究冈瓦纳冰川的开始时间和模式，以及其与海平面波动、大陆风化速率变化、古海洋和气候条件的机制联系。此外，一些同位素代用资料表明，中石炭世（切斯特纪）甚至可能是冰期之间的全球变暖和消冰时期。我们假设，海平面、大陆风化模式、海洋学和古气候条件的变化可能伴随着与密西西比晚期主要冰期开始或潜在间歇冰期相关的冰盖动力学的重大变化，这些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，上述两种情况中的每一种都将赋予切斯特沉积物一套独特的地层和地球化学特征，这些特征可用于对假设的冈瓦纳冰川演化路径进行定量限制。为了验证这些假设，我们建议对爱达荷州中东部和蒙大拿州西南部的切斯特纪碳酸盐和硅酸盐进行综合、高分辨率的旋回、层序地层和同位素研究，重点是在Panthalassa的这一部分建立切斯特纪的环境变化记录。为了最大限度地解决所提出的层序地层格架中的年代地层对比问题，将从研究区近端和远端环境对切斯特系大化石和微化石生物地层学进行详细的区域重新评估。我们将对切斯特岩进行分层层序地层学分析，以解读其沉积历史，并在局部和区域尺度上确定潮旁和潮下副层序、主要层序边界不整合和副层序边界的性质和分布。这将允许评估副层序和表面如何与微妙的构造运动或记录的起伏有关。古生物学分析将对记录在每个准层序或层序中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。我们建议在石炭系稳定同位素曲线和放射性成因同位素曲线的基础上，建立一套完整的石炭系稳定同位素（d13C、d18O和d34S）和放射性成因同位素（87Sr/86Sr）记录，该同位素记录的独特贡献在于：(1)完善部分现有同位素曲线，(2)定义d13Ccarb和d13Corg的耦合记录，(2)测试重建鹿角前陆盆地O和C同位素垂向梯度的潜力。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用物，我们将能够评估地层和同位素代用记录的相对速率和相（即超前-滞后）关系，这方面在以前的基底内研究中一直受到不同程度的损害。这种方法应该能够区分该盆地的长期构造事件和频率较高的起伏事件，从而对构造强迫对切斯特地层的地层和同位素特征的作用提供相当深入的了解。反过来，这将有助于深入了解构造强迫在控制盆地调节、区域海洋条件和环流中的作用。此外，我们期望这项研究能够定量地约束Panthalassa这一部分在Chesterian期间发生的环境变化谱，包括全球古海洋和古气候扰动。最后，本研究的结果将有助于深入了解游动、海洋物理和化学条件与古气候之间的任何机制联系的性质，从而最终有助于限制冈瓦纳冰川开始的时间和模式。

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.