TECTONIC AND CLIMATIC SIGNIFICANCE OF A LATE EOCENE LOW-RELIEF, HIGH-LEVEL GEOMORPHIC SURFACE, COLORADO

New paleobotanical data suggest that in the late Eocene the erosion su rface which capped the Front Range, Colorado was 2.2-3.3 km in elevati on, which is similar to the 2.5-km present elevation of surface remnan ts. This estimated elevation casts doubt on the conventional belief th at the low-relief geomorphic surface was formed by lateral planation o f streams to a base level not much higher than sea level and that the present deeply incised canyons must represent Neogene uplift of Colora do. Description of the surface, calculations of sediment volume, and i sostatic balance and fluvial landsculpting models demonstrate that whi le the high elevation of the erosion surface was due to tectonic force s, its smoothness was mostly a result of climatic factors. A sediment balance calculated for the Front Range suggests that from 2 to 4 km of material were eroded by the late Eocene, consistent with fission trac k ages. This amount of erosion would remove a significant portion of t he 7 km of Laramide upper crustal thickening. Isostatic modeling impli es that the 2.2-3.3 km elevation was most likely created by lower crus tal thickening during the Laramide. A numerical model of fluvial erosi on and deposition suggests a way that a late Eocene surface could have formed at this high elevation without incision. A humid climate with a preponderance of small storm events will diffusively smooth topograp hy and is a possible mechanism for formation of low-relief, high-level surfaces. Paleoclimate models suggest a lack of large storm events in the late Eocene because of cool sea surface temperatures in the equat orial region. Return to a drier but stormier climate post-Eocene could have caused the incision of the surface by young canyons. By this int erpretation, regional erosion surfaces may represent regional climatic rather than tectonic conditions.