IS THE MID-EOCENE CLIMATIC OPTIMUM REPRESENTED IN WESTERN WASHINGTON? A CASE FOR THE MAPLE FALLS MEMBER OF THE CHUCKANUT FORMATION, USA

The late Paleocene to middle-late Eocene Chuckanut Formation consists of five members. From oldest to youngest, the first four are the Bellingham Bay, Governor’s Point, Slide, and Padden members. The stratigraphic positioning of the fifth member, the Maple Falls Member, is uncertain and its placement has been debated in previous literature. Two possibilities exist for its placement in the Chuckanut Formation: 1) between the Slide and Padden members (late early Eocene) or 2) during the last deposition in the Chuckanut Basin (mid-late Eocene).

The second scenario is preferred here for the following reasons: 1) Map patterns suggest that the Maple Falls Member was deposited after the Padden Member. 2) The QFL ratios show a progressive decrease in the proportion of feldspar in the Chuckanut Formation from oldest to youngest deposition. The Maple Falls Member has the least amount of feldspar compared to the other members; such a decrease may represent a maturing of or changing provenance in the basin through time. 3) The Higgins Mountain and Sperry Peak units of the southwestern outcrop belt of the Chuckanut Formation, which are petrographically and depositionally equivalent to the Maple Falls Member in the northwestern outcrop belt, have been dated at 44 Ma. This is the youngest date obtained for the Chuckanut Formation. 4) Previous works show that paleoclimate fluctuated through the Paleocene-Eocene. Although the placement of the Maple Falls above the Padden Member would suggest a complex pattern of warm climate (Bellingham Bay, Governor’s Point, and Slide Members), cooling (Padden Member), and then warming again (Maple Falls Member), such a fluctuation is consistent with what is known about paleoclimate change at the time. The dicotyledonous angiosperm morphotype changes in the Chuckanut Formation reflect these paleoclimate fluctuations.

The above evidence supports that the Maple Falls Member was deposited at approximately 40-44 Ma, close to or during the mid-Eocene Climatic Optimum. This placement suggests that the Maple Falls Member represents one of the very few units in the western US that record this very important paleoclimatic time in Earth history. It records deposition during the last major warming period within an overall greenhouse to icehouse transition.