PRELIMINARY OBSERVATIONS ON THE FLORAL DIVERSITY IN THE EARLY TERTIARY CHUCKANUT FORMATION, NORTHWEST WASHINGTON, USA

The Chuckanut Formation is one of the thickest non-marine sedimentary sequences in North America. Deposition began before the uplift of the Cascade Mountain magmatic arc when the Farallon/Kula/Pacific triple junction began subducting beneath the North American Plate. The subducting plates were aligned obliquely to the coast, resulting in the break-up of the Kula Plate into blocks separated by large dextral strike-slip faults (Dickinson, 1979, Cenoz. Paleogeog, W US, Soc. of Econ. Paleontol. and Mineral., p. 1-13). The Chuckanut Formation was being dismembered during deposition by these faults and subsequently folded by the Cowichan Fold and Thrust Belt post-lithification, resulting in a complex, discontinuous stratigraphic arrangement. This stratigraphy is in the process of being revised and the definitions of the members contained within the northwestern outcrop belt are being revisited by the author. The Chuckanut, Swauk, and Manastash Formations may have been deposited within the same transtensional fluvial basin but were disconnected syndepositionally and scattered throughout western Washington (Frizzell, 1979, PhD thesis, Stanford Univ.; Mustoe et al, 2007, GSA Field Guide 9, p. 123-124).

Lowland fluvial environments are rarely preserved along the western margin of North America during the early Tertiary. The Chuckanut Formation is a lowland fluvial environment in which significant paleoclimatic changes occurred during the Eocene (Mustoe and Gannaway, 1997, Wash. Geol., v. 23, p. 22-27). Abundant paleontological evidence collected for the current study gives new insight into the changing ecosystem within the structurally complex basin. More than 200 leaf morphotypes that occur in time-transgressive sequences document angiosperm diversity within the Chuckanut Basin. This is part of a larger project that includes leaf assemblage correlation within the Swauk, Manastash, and Chuckanut Formations.