CLAMP ANALYSIS AND MAGNETOSTRATIGRAPHY OF THE EARLY TERTIARY CHUCKANUT FORMATION OF NORTHWEST WASHINGTON

Precise age dates for the Chuckanut Formation of Northwest Washington have not been established largely due to lack of marine biostratigraphic age constraints. While marine fossils have not been identified from the Chuckanut Formation, its abundant plant fossils do suggest a climatic cooling. The placement of this cooling within the proper chronostratigraphic context will facilitate our understanding of the significance of the climate shift with respect to Paleogene history and provide further insight into the nature of Eocene cooling. Paleoflora of the Chuckanut Formation consist of terrestrial leaves of various morphologies that were analyzed by Climate Leaf Assemblage Multivariate Program. CLAMP analysis, by considering variations in leaf morphology as a proxy for climate change, will provide data on mean annual temperature (MAT), mean annual range in temperature (MART), and an average amount of rainfall per year. Previous work by Mustoe and Gannaway (1997) indicate that deposition of the basal Bellingham Bay Member occurred in a tropical environment (MAT=15°C, MART=10°C, and average rainfall of 150-200 cm per year), where the overlying Padden Member was deposited in a more temperate climate (MAT=12°C, MART=18°C, and average rainfall of 100-200 cm per year). The Bellingham Bay Member is well exposed, with a 3,000m thick continuous section of fluvial sandstone and siltstone. The excellent exposure of this member provides an opportunity to examine variations of climate in greater detail. Additionally, the continuous section should allow for the age of the Bellingham Bay Member to be constrained using magnetostratigraphy. CLAMP data from site CD-5 (approx. 0m) and site CD-4 (approx. 750m) respectively yield a MAT=19°C,21.5°C; MART=0°C,0°C; and average rainfall for both of 200-350 cm per year. Since the MAT of the oldest beds in the member are significantly warmer (19°C -21°C) than the mean paleotemperature for the whole member (15°C), the younger beds of the Bellingham Bay Member must be significantly cooler. The paleomagnetic study of the Bellingham Bay Member indicates zones of both normal and reverse polarity remanence. These polarity zones will be correlated to the global polarity time scale and establish a temporal framework for the basal member of the Chuckanut Formation.