PALEOCLIMATE AND PALEOECOLOGICAL FEATURES OF THE EARLY TO MIDDLE MIOCENE MOHAWK VALLEY FLORA, NORTHERN SIERRA NEVADA

         Early Neogene floras are rare in the higher elevations of the Northern Sierra Nevada. Those that are known are poorly studied, depauperate, dominated by leaves, or a combination of these features. The Mohawk Valley Flora occurs in fluvial and lacustrine sediments within the dominantly volcaniclastic Bonta Formation [18.1 (+2 - 3) Ma to 22.8 (+1.6) Ma]. The flora contains well-preserved leaves, fruits, seeds and wood; preliminary study suggests the presence of nine identified angiosperm species and 10-25 further morphologically distinct forms, including the first reports of Staphylea and Zanthoxylum in the Miocene of the Sierra Nevada and Great Valley. The fossils suggest affinities with present-day floras of the southeastern United States and eastern Asia, indicating a warmer, wetter environment than currently occurs in this region. The Mohawk Valley Flora may represent an upland tongue of vegetation associated with the coeval coniferous-hardwood to mixed mesophytic floras to the north while floras east and west suggest warmer, drier habitats.

 

         Future research will focus on refining identifications, detailing local stratigraphy, and providing an interpretation of paleoclimate based on the Nearest Living Method and the Climate-leaf Analysis Multivariate Program (CLAMP). While both methods have limitations, each provides a check for the other allowing interpretation of overall climatic conditions at the time of deposition. Full interpretation of the climatic setting may permit the interpretation of paleoelevation using leaf physiognomy and stomatal densities.