Wolfe, J.A., Gregory-Wodzicki, K.M., Molnar, P., and Mustoe, G., 2000, Paleobotanical evidence for the development of high altitudes during the early Eocene in northwestern North America [abs.]: Goteborg, Sweden.

Abstract.--Analyses of fossil leaf assemblages of late Paleocene through middle Eocene age is underway to estimate paleoaltitudes to constrain geophysical theory relating to the tectonic development of interior southern British Columbia and adjacent Washington (the Okanagan). According to recent theory, during this age interval pressure created by the westward movement of North America against oceanic plates was relieved by thrust faulting and consequent crustal thickening in the interior. This thickening should lead to some increase in altitude and, as the lower part of the thickened crust was heated and melted, some intrusives should appear. Increasing thrusting, thickening, and melting (with extensive volcanism) should ultimately result in the crustal root dropping off; with little anchoring crust remaining, the upper part of the crust should then be released to move substantially upward to form a high plateau. This high-altitude plateau, however, would have little physical support and be inherently unstable; consequently, the upper crust should extend, essentially flowing away from the topographic high. Normal faulting associated with the extension should create numerous grabens, which should be altitudinally lower than the preceding high plateau.

The paleobotanical analyses are carried out with the CLAMP methodology, which is based on ~170 modern samples of leaves. Collection of these samples proximal to meteorological stations provides calibration to environmental parameters such as mean annual temperature (MAT) and mean growing season precipitation. Of importance in the present context is the parameter of moist enthalpy, a concept that involves temperature and specific humidity and is based on conservation of energy. Differences in moist enthalpy values can be translated into differences in altitudes. The modern CLAMP samples yield estimates of altitudes that have a standard error of + 890 m.

The stratigraphic coverage of the paleobotanical record in the Okanagan during the critical time frame is good, and the record can be compared to lowland sequences of floras east (Huntingdon Fm.) and southeast (Chuckanut Fm.) of Vancouver and south of Seattle (Puget Group). In the Okanagan, the oldest studied flora is near Similkameen Dam west of Oroville, Washington, dated by intrusives as >54 Ma; all the radiometric ages cited are based on old K/Ar analyses, but numerous samples are now being dated using 40Ar/39Ar analyses. Comparison of the Similkameen Dam assemblage to late Paleocene leaf assemblages in the Chuckanut suggest a paleoaltitude of ~1.7-2.4 km. Extensive volcanism, including lavas as much as 2 km thick, occurred ~53 Ma. Between 52 and 53 Ma, fossiliferous sediments of the White Lake Formation were deposited on top of the volcanics. At White Lake southwest of Penticton, the lower part of the formation has produced a flora that, when compared to the early Eocene flora of the lowland Huntingdon, indicates an altitude of ~4 km, which represents a marked increase in altitude in a geologically short period of time. By 48-50 Ma, extensive grabens had developed in and around Republic, Washington, and in British Columbia, notably near Princeton (Allenby Fm.), Kamloops (lower Tranquille beds), and along the North Thompson River (Chu Chua beds). Prior analyses of these floras suggest paleoaltitudes of 2.0-2.4 km; paleoaltitudes had considerably lessened but were still substantially higher than today. In general, the paleobotanical analyses are supportive of the newer geophysical model of tectonism.

The microthermal (MAT <13°C) flora of the Okanagan late Paleocene-Eocene uplands changed markedly. At the beginning of this time period