CENOZOIC ALTITUDES AND PALEOBOTANY OF WESTERN-INTERIOR UNITED STATES

A century of geologic research constrains Cenozoic evolution of regional topography. The high Late Cretaceous-early Paleogene continental divide passed near tracts now in central Nevada (Basin-Range province has since doubled in width, and lowered) and lithostatic head drove thin-skinned Sevier foreland thrusting. The east foot of the thrust belt was near sea level almost to the end of Cretaceous time and likely was little above it when thrusting ceased. The divide lay south of since-raised Colorado Plateau in Arizona, which lacked a thrustable stratal wedge. Structural relief of the central and southern Rocky Mountains mostly dates from latest Cretaceous and early Paleogene Laramide crustal shortening, but basins remained low, accumulating sediment, until late Tertiary regional uplift resulted in sluicing of basins and incision of Plateau, mountains, and Great Plains. Floristic determinations of paleoclimates (Axelrod, MacGinnitie, Leopold) accord with this scenario. Voluminous evidence for this history is dismissed by theorists (e.g., Chase, Molnar, England) who assume that present high western-interior regional altitudes could only result from Laramide deformation and who seek support in analysis of fossil-leaf physiognomy by Wolfe and Gregory. Wolfe rejected floristics and assumed that floras have diagnostic mean annual temperatures (MAT), independent of annual distributions of temperature and precipitation, definable by multivariate analysis of rank and polynomial (nonlinear and non-normal) variables of size and shape of woody-dicot leaves in small samples. Fit of this flawed model to modern floras is weak, and many assemblages wholly misfit, but Wolfe applied it to paleofloras. The Paleogene MATs thus deduced for western-interior US frequently are too cold (thus increasing deduced altitudes) for the palms, crocodilians, and large tortoises present but ignored. Global variation of temperature with continental surface altitude is typically 5-6 degrees C per km, and this was used by Axelrod for conversion to paleoaltitude of early Tertiary equable-climate floras; but present variation of MAT for extremely continental (and extended) western US is about 3 degrees per km, which Wolfe used inappropriately for Paleogene floras, thus doubling paleoaltitude conversions.