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

The age of the Chuckanut Formation of Northwest Washington lacks marine biostratigraphic constraints. No marine fossils have been identified from the Chuckanut Formation but leaf fossils of varying morphologies indicate a climatic cooling. Placing the climate shift in the appropriate chronostratigraphic context will aid our understanding of the significance of the climate shift with respect to Tertiary history and provide further insight into the nature of Eocene cooling.

The Climate Leaf Assemblage Multivariate Program (CLAMP) uses variations in leaf morphology as a proxy for climatic changes and yields a mean annual temperature (MAT); mean annual range in temperature from winter to summer (MART), and an average precipitation per year. Results from Mustoe and Gannaway (1997) suggest the basal Bellingham Bay Member was deposited in a tropical environment (MAT=15º, MART=10º, and an average precipitation rate of 150-200 cm per year) but the overlying Padden Member was deposited in a more temperate environment (MAT=12º, MART=18º, and an average precipitation rate of 100-200 cm per year). Excellent exposures of approximately 3000m of fluvial sandstones and siltstones of the Bellingham Bay Member provide an opportunity to analyze climate variations in greater detail. Additionally an age for the continuous section will be established using magnetostratigraphy.

Initial CLAMP data from two sites of the basal beds of the Bellingham Bay Member yield varied but tropical climates. Site CD4 yields a MAT=19º, MART=0º, and an average precipitation rate of 250-300 cm per year, and lies approximately 200 m above site CD5 which yields a MAT=21.5º, MART=0º, and an average precipitation rate of 250-300 cm per year. Although site CD4 is 2.5º warmer than site CD5 they are consistent in MART and precipitation rates. However these CLAMP values are 4º to 6.5º warmer than the previously suggested for the whole member, therefore the depositional climate of the younger beds must be significantly cooler then the MAT (15º) for the whole member. Paleomagnetic results throughout the section indicate remanence varies between normal and reverse polarity. Correlation of polarity zones to the global polarity time scale will establish a temporal framework for the basal member of the Chuckanut Formation.