AN EARLY PALEOGENE PALEOBOTANICAL PALEOCLIMATIC ANALYSIS OF THE NORTHERN SIERRA NEVADA

The terrestrial climate record of the early Paleogene is well documented in the continental interior of North America (e.g. Bighorn Basin, Wyoming) via both paleobotanical and oxygen isotope records. We examine the well preserved Susanville and Moonlight floras of the northern Sierra Nevada southwest of Susanville, California in comparison to this mid-continental record. Both floras preserve leaves in mudstone to sandstone lenses within coarser, often cobble-dominated, deposits, here interpreted as reflecting quieter environments within a larger river system. The floras are approximately 10 km apart and were possibly deposited in the same paleochannel, referred to as the Jura River within the gold-bearing Auriferous Gravels. Biostratigraphy constrains the floras to the Paleocene and/or Eocene based on the presence of the genus Macginitea, known only from those epochs. Further field work studying possible volcanic interbeds, together with systematic paleobotany, will be conducted to refine this assignment. Using Leaf Margin Analysis (LMA) as a method for estimating mean annual paleotemperature (MAT), a preliminary MAT of 21°C (+/- 3.5°C) has been estimated for the Moonlight locality, based on 17 morphotypes. Additional specimens were collected in August 2007, more than doubling the size of the existing Moonlight collection, and may reveal new morphotypes to help refine this estimate. Wolfe (1978) has suggested a MAT of 27°C for Susanville, a conclusion which we have yet to re-test. If correct, these MAT estimates are warm, given paleoelevation estimates for the Paleogene Northern Sierra similar to the present day elevation. These paleotemperatures suggest that the Moonlight and Susanville floras fall within the warm interval of the early Eocene or the Paleocene-Eocene Thermal Maximum (PETM), and are similar to estimates of the mid-continent for these warm intervals.