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

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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.