PALEOCLIMATE ANALYSIS OF TWO PETROLOGICALLY DISTINCT SANDSTONE BODIES OF THE TERTIARY MANASTASH FORMATION, CENTRAL WASHINGTON: POSSIBLE CLIMATE CONTROL ON SANDSTONE PETROLOGY

The Tertiary Manastash Formation is composed of multiple exposures of fluvial sandstone in the central Washington Cascades 8 kilometers south of Cle Elum, WA. There are two main exposures of the Manastash Formation, herein referred to as the Manastash main body and the Taneum Ridge body, along with several minor outcrops. Both the Manastash main body and the Taneum Ridge body unconformably overlie the pre-Tertiary Easton Schist and each is unconformably overlain by the Taneum Formation. Previous work in the 1980’s discovered distinct compositional and textural differences between the Manastash main body and the Taneum Ridge body. The Manastash main body has more quartz than the Taneum Ridge body and has grain sizes ranging from medium sandstone to pebble conglomerate with minor mudstones. The Taneum Ridge body is more arkosic with grain sizes ranging from mudstone to medium sandstone. The minor outcrops in the field area are compositionally similar to the Manastash main body. No paleobotanical work has been conducted since the petrologic differences between the two bodies were recognized, and no quantitative paleoclimate studies have ever been done for the Manastash Formation. Paleoclimate information is important because the compositional differences may have been caused by a change in climate during deposition. The purpose of this study is to compare detailed paleobotanical and paleoclimatic data from the Manastash main body and the Taneum Ridge body to determine if climate could have been the main cause for the change in petrology. These data have been collected by morphotyping fossil leaf impressions stored at the Burke Museum of Natural History and Culture and samples collected during the summer of 2011. Morphotyped samples from each body have been used for paleoclimate reconstructions using Leaf Margin Analysis and Climate-Leaf Analysis Multivariate Program. The paleobotanical and paleoclimatic data are compared against the petrologies of the Manastash main body and the Taneum Ridge body to determine if climate change is responsible for the differences in petrology.