REGIONAL PALEOVEGETATION RECORDS OF THE EOCENE-OLIGOCENE TRANSITION OF MONTANA

The Eocene-Oligocene transition (EOT; 34.2–33.5 Mya) was a dramatic turning point in the climatic history of the Earth, representing a shift from a Mesozoic-early Cenozoic “hothouse” to the “icehouse” of the modern era. While the broad consensus is that global climate became cooler and drier during the EOT, intermontane continental basins in Montana were less impacted by global EOT climate fluctuations than lower paleoelevation or near-coastal sites. Paleobotanical evidence suggests that this area was forested across the EOT, while paleosols and trace fossils indicate open grass-dominated habitats instead. This project investigates the response of vegetation in southwestern Montana to the EOT and specifically aims to constrain the timing of the transition from forest to more open grassland habitat. Evidence from phytoliths (plant opal) and paleosols are used to create a broad regional record of local vegetation and climate from four biostratigraphically dated sites in southwestern Montana across the EOT: Little Spring Gulch (Chadronian), North Hough Draw #1 (middle/late Chadronian), Little Pipestone (latest Chadronian), and Matador Ranch (Orellan). Morphotypes were combined into plant functional groups (e.g., forest indicators, grasses) and ecologically significant morphotypes (e.g., palms) were noted for vegetation reconstruction. Preliminary phytolith results indicate that each of the sites are dominated by forest indicators with only a minor/moderate component of grass morphotypes. Paleosols at these sites were recognized based on a combination of root traces, burrows and/or mottled color and all of the paleosols are classified as either Entisols or Inceptisols. While the paleosols are insufficiently developed to confirm the phytolith results, they may be used to provide local paleoclimatic context. Based on this preliminary evidence—and constrained by the time interval studied—the transition from forested to open-grassland ecosystems in southwestern Montana was after the early Oligocene. This is consistent with previous phytolith studies from late Eocene and Early Oligocene sites in Nebraska, Wyoming, Montana, Idaho and Colorado.