LARAMIDE UPLIFT, OROGRAPHIC PRECIPITATION, AND BASIN-MARGIN RAINFORESTS: AN EARLY PALEOCENE TEST CASE FROM THE COLORADO FRONT RANGE

Diverse angiosperm-dominated fossil floras allow for the estimation of ancient mean annual temperature and mean annual precipitation by analysis of leaf margin (LMA) and size (LSA) of species or morphotypes of woody dicotyledons. The Denver Basin is a Laramide synorogenic structure that contains two unconformity bounded packages of fossiliferous strata. The lower package, the D1 sequence, is composed of rocks previously mapped as the Arapahoe Conglomerate, the Denver Formation, and the Dawson Arkose (part). Clast composition from the base of the D1 sequence documents the unroofing of the Laramide Front Range and magnetostratigraphy and palynostratigraphy of two cored wells constrain its age to ~68 Ma. The upper portion of the D1 sequence is dated by these methods and by a number of sanidine-bearing tuffs at ~64 Ma. In this context, the surprising occurrence of an early Paleocene (~63.8 Ma) tropical rainforest site at Castle Rock, Colorado initiated the hypothesis that Laramide uplift was responsible for increased orographic precipitation along its eastern flank. Construction activities have presented opportunities to excavate numerous fossil leaf sites along the Front Range urban corridor and three major excavations have been supported by the Colorado Department of Transportation, allowing us to test this hypothesis. Each of these sites is placed in spatial-temporal context using the geochronological basin framework and the distance east of the present mountain front. The analyzed floras include Scotty’s Palm, Chopping at the Gap, Sick of Sycamores, Castle Rock, and Baptist Road. These floras show a MAT range of 17.5-24.6° C and a MAP range of 157-256 cm/year. Collectively, they support the concept of a band of rainforest vegetation along the eastern margin of the front from shortly after the K-T boundary at 65.51 Ma to 63.8 Ma when the record ceased. Ongoing work in the central and eastern basin is underway to determine the eastern margin of this vegetation.