TECTONIC AND PALEOCLIMATIC SIGNIFICANCE OF EARLY PLIOCENE PALYNOFLORAS FROM THE SOUTHEASTERN ALASKA RANGE

The 650 km long Alaska Range is a significant orographic barrier, but the effects of uplift on the spatial and temporal distribution of Neogene vegetation in central and southern Alaska are poorly understood. This is due to the lack of constraints on the uplift history of the Alaska Range and the scarcity of paleobotanical data from Late Miocene and Early Pliocene strata adjacent to the Range. The Nenana Gravel, which lies ~20 m uncomformably above the 6.7 Ma Grubstake Tephra, provides evidence of uplift and unroofing north of the Alaska Range (1). Fission track dating corroborates a Late Miocene initiation of surface uplift, providing a date of 6 Ma for the onset of rapid exhumation on Denali (2). On the south side of the Alaska Range, a 30 m section of siltstone, sandstone, and lignite is exposed along Hoo Doo Creek and Dan Creek (63° 13.0 N; 145° 26.06 W). A basal tephra from this site was previously assigned a 5.26 Ma K/Ar date (3). 40Ar/39Ar dating of a second white tephra (Hoo Doo Tephra), exposed approximately 6 m above the 5.26 Ma tephra, is being conducted in an attempt to confirm the earliest Pliocene age of the section. Geochemical analyses will permit comparison with the Grubstake Tephra and, possibly, identification of an eruptive source. The Hoo Doo Creek section is overlain by ~1000 meters of conglomerate, petrologic evidence for the onset of rapid uplift south of the Alaska Range. Palynological analysis of underlying siltstone and coal horizons will thus provide a record of vegetation cover and prevailing climate prior to, and potentially during, initial surface uplift in the Hoo Doo Creek region. Ultimately, our results will allow us to assess the impact of elevation change on Early Pliocene floras from the southern foothills and address the possibility of time-transgressive uplift across the Alaska Range.

1. Fitzgerald, P.G., Stump, E., and Redfield, T.F., 1993, Late Cenozoic uplift of Denali (Mt McKinley) and its relation to relative plate motion and fault morphology. Science, v. 259, p. 497-499. 2. Thoms, E., 2000, Late Cenozoic Unroofing Sequence and Foreland Basin Development of the Central Alaska Range: Implications from the Nenana Gravel: M.S. Thesis, University of Alaska, Fairbanks. 3. Weber, F.R. and Turner, D.L. A late Tertiary thrust fault in the central Alaska Range; U.S.G.S. Survey in Alaska: Accomplishments 1976, Geological Circular 751-B.