PALEOFLORA AND PALEOCLIMATE OF THE EARLY PALEOGENE MANASTASH FORMATION, CENTRAL WASHINGTON STATE, USA

This study presents the first detailed assessment of the paleoflora and paleoclimate from the late Paleocene – early Eocene Manastash Formation in central Washington State. Previous studies recognized two distinct sedimentary “bodies” in the Manastash Formation: the older, arkosic Taneum Ridge Body (TRB) and the younger, quartzose Manastash Main Body (MMB). In this study, fossil dicotyledonous angiosperm leaves were examined from both bodies and were assessed using Leaf Area Analysis (LAA), Leaf Margin Analysis (LMA), and Climate Leaf Analysis Multivariate Program (CLAMP). The paleoclimate analyses used leaf morphotypes from three individual localities in the TRB, from the entire TRB, and from the entire MMB. Comparison of the two bodies demonstrate that both the TRB (CLAMP = 17 ˚C; LMA = 17 ˚C) and the MMB (CLAMP = 17.6 ˚C; LMA = 23.4 ˚C) were deposited under similar paleoclimatic conditions. The three localities within the TRB, however, show a possible cooling trend from the oldest locality (CLAMP = 19 ˚C; LMA = 24 ˚C) to the youngest locality (CLAMP = 17 ˚C; LMA = 19 ˚C). Mean annual precipitation results from LAA (TRB=149 cm; MMB=121 cm) and growing season precipitation results from CLAMP (TRB = 201 cm; MMB = 188 cm) indicate that these paleoflora received abundant precipitation. This analysis shows that the Manastash Formation was deposited in a mesic, mesothermal to megathermal setting. These paleoclimate results are compared to paleoclimate results from the Chuckanut Formation in western Washington because the lower Chuckanut Formation is coeval to the Manastash Formation, it records similar depositional environments, and was possibly deposited in the same basin. The Manastash Formation and the lower Chuckanut Formation were likely deposited during several paleoclimatic optima such as the Paleocene-Eocene Thermal Maximum and the Early Eocene Climatic Optimum based on their ages and paleoclimatic signatures derived from the analysis of the leaf morphotypes. Comparing the paleoclimate results from the Manastash Formation and Chuckanut Formation will help to understand paleoclimatic conditions in the Pacific Northwest during these important intervals.