The late Eocene House Range Flora, Sevier Desert, Utah: Paleoclimate and paleoelevation

Analysis of the late Eocene House Range flora can shed light on the climatic and tectonic history of the Sevier Desert, Utah. It is the first described flora from the middle Tertiary of western Utah, and it grew on the hanging wall of both the Sevier-age Canyon Range thrust and the middle-Tertiary Sevier Desert detachment fault. The flora, which is preserved in a lacustrine limestone on the east flank of the House Range, is dated as Chadronian (late Eocene) on the basis of an associated gastropod. This age determination is consistent with a single-crystal Ar-40/Ar-39 date of 31.4 +/- 0.5 Ma from a nearby outcrop of the Windous Butte tuff which contains clasts of the limestone Physiognomic analysis of the fossil angiosperm leaves implies a mean annual temperature (MAT) of 13.2 +/- 2.9 degrees C and a growing season precipitation of 85 +/- 30 cm. The vegetation is physiognomically most similar to modern warm-temperate floras from Japan. MAT-based and enthalpy-based altimeters derive paleoelevations of 3.6 +/- 0.7 km or 2.9 +/- 1.5 km for the flora, compared to a modern elevation of 1.7 km. This paleoelevation, if taken to be representative of regional paleoelevation, implies that the Sevier orogen was high, but then collapsed to modern elevations during middle and late Cenozoic extension.