INVESTIGATING EVIDENCE OF HIGH FREQUENCY (104-105YR) GLACIAL-EUSTASY IN THE LOWER MISSISSIPPIAN LODGEPOLE FORMATION OF SOUTHWEST MONTANA USING OXYGEN ISOTOPES OF CONODONT APATITE

It has been traditionally accepted that onset of the Late Paleozoic ice age began in the Late Mississippian (Serpukhovian). Recent evidence from the age of South American glacial deposits and oxygen isotope values from conodont apatite suggest that the onset of cooling and glaciation began ~25 My earlier in the Early Mississippian (Tournaisian). This research is testing the Tournaisian early cooling/glaciation hypothesis by analyzing δ¹⁸O values of conodont apatite from high-frequency (10⁴-10⁵ yr) upward-shallowing cycles in the Lower Mississippian Lodgepole Formation of southwest Montana. The cycles (2-8 m thick) are typically composed of fine rhythmically interbedded limestone-marl (sub-storm wave base deposits), overlain by graded skeletal limestone-marl (distal-storm deposits), and capped by ooid, peloidal, or coarse skeletal grainstones (proximal deposits). Abrupt juxtaposition of facies representing such contrasting water depths suggests that the magnitudes of high-frequency sea-level oscillations were on the order of at least a few tens of meters, which implies substantial glacial ice growth and melting. Individual cycles were sampled from late highstand, lowstand, and transgressive systems tracts of two different My-scale depositional sequences to test whether high-frequency sea-level change was driven by glacial-eustasy, and if so, whether the magnitude oscillations changed over the course of My-scale sequence development. Results from oxygen isotope studies will aid in substantiating and quantifying the magnitudes of paleoclimate change during the Tournaisian.