PALEOCLIMATE ANALYSIS OF THE CHUCKANUT FORMATION USING FOSSIL PLANTS, WESTERN WASHINGTON, USA

Paleoclimate temperates, which can be derived from physiognomic characteristics of dicotyledon leaf margins using Leaf Margin Analysis (Wilf, 1997, Paleobiology, 23(3), p. 373–390), are given below for the Chuckanut Formation and outline the most detailed assessment of northwestern Washington to date. The Chuckanut Formation members, from oldest to youngest, are the Bellingham Bay, Governor’s Point, Slide, Warnick, Padden, and Maple Falls Members. Combined, they record 8300 meters of paleovegetative history during the late Paleocene to Eocene. Mustoe and Gannaway (1997, Wash. Geology, v. 23, p. 22-27) gave an average mean annual temperature (MAT) for the Bellingham Bay Member at 15°, the Slide Member at 16°, and the Padden Member at 12°C using Climate Leaf Analysis Multivariate Program (CLAMP; Spicer et al, 2005 Palaios, 20: p. 429-438.). Donohoo-Hurley, (2002, GSA Ab. w. Prog., 34 (6), 22-23; 2004 unpub. thesis) used the same methods and provided a more detailed assessment of three localities within the Bellingham Bay Member at 21.5°, 17.5° and 19°C.

In this study, paleo-MAT’s are provided for each member using Leaf Margin Analysis. Fossils collected from conformable localities within the western-most Bellingham Bay Member are, from oldest to youngest: 16°, 18.5°, 17.5°, 19°, 20.5°, and 16.5°C, respectively. Average temperature for the entire section is 18°C. An average of 17°C was derived for the eastern section of the Bellingham Bay Member. MAT values for the Slide Member were 20° and 19°C. Although the dataset was limited for the Warnick Member, the MAT given was 21°C. Lastly, a re-evaluation of the localities that Mustoe and Gannaway used for the Padden Member was 14° and 15.5°C. In contrast to Mustoe and Gannaway’s (1997) and Donohoo-Hurley’s (2004) MAT’s, these results are several degrees higher. Wilf (1997) pointed out that CLAMP has the potential to decrease MAT values by about 2°C in moist warmer settings, which would account for this discrepancy. Nevertheless, these results still place the Bellingham Bay, Slide, and Warnick Members in the paratropical to broadleaved evergreen subtropical forest regime, and the Padden Member in the mixed coniferous to broadleaved evergreen temperate forest regime.