SWAUK IT OUT: CARBON ELEMENTAL AND ISOTOPIC ANALYSES OF SEDIMENT BULK ORGANICS IN THE PALEOGENE SWAUK FORMATION, CENTRAL CASCADES, WASHINGTON

The Swauk Formation is an early Eocene (~52 Ma) fluvial sedimentary sequence located in central Washington east of the Cascade Range crest, near the city of Cle Elum. The Swauk is composed of sandstone, claystone, and siltstone strata, with shale and coal interbeds, and minor conglomerates. We used carbon elemental and isotopic analyses of the bulk organics to investigate the geochemical relationship between the Swauk and Chuckanut Formations. The Chuckanut is a similar deposit located approximately 200 kilometers northwest of the Swauk near the city of Bellingham, WA. Previous research has suggested that these two formations were deposited in the same oblique-slip basin as a result of either incipient extensional tectonics in the region, or oblique subduction and breakup of the Kula oceanic plate. The formations were subsequently separated by the Straight Creek Fault, a large right-lateral strike slip fault. We collected samples of the main Swauk sedimentary facies (sandstone, siltstone, claystone) over approximately six meters of section. To prepare samples for analysis of bulk organic material we removed inorganic carbonates using an acid pretreatment. We first investigated the lithologic and stratigraphic trends in %C (by weight) and δ¹³C values within the Swauk, and then directly compared these datasets to published records from the Chuckanut. The Swauk formation δ¹³C values range from -26.7 to -23.5 permil and the %C values range from 0.05 to 0.55 percent. There is no positive correlation between %C and δ¹³C values of Swauk that might indicate inorganic carbonate contamination of the organic material. Within the Swauk there are no consistent patterns in %C or δ¹³C values among lithologies or across stratigraphy, but detailed analyses were constrained by a small sample size. For all three lithologies the Swauk %C and δ¹³C values fall within the ranges reported for the Chuckanut. Our organic geochemical data suggest a potential common origin for the Swauk and Chuckanut formations. However, a larger dataset and detailed structural context for the Swauk are needed to establish a direct relationship between the two formations. Determining a more clear relationship between these two formations will provide a greater understanding of the regional geologic setting of the Pacific Northwest.