A DIVERSE MEGAFLORA FROM EARLY PALEOCENE SYNOROGENIC STRATA IN THE DENVER BASIN, COLORADO SPRINGS, COLORADO

The Denver Basin was filled during the Late Cretaceous and Paleogene as Laramide uplift created structural relief along a series of faults that denote the present eastern margin of the Colorado Front Range. Fossil plants preserved in these synorogenic strata provide an opportunity to assess gradients of elevation and local climatic conditions from the distal to proximal portions of the Denver Basin. Scotty’s Palm is a fossil flora site located in proximal synorogenic strata on the north side of the Air Force Academy in Colorado Springs. The site occurs in strata mapped as Dawson Arkose. Recent work suggests that these rocks represent an unconformity-bounded unit of latest Cretaceous to early Paleocene age known as D1 synorogenic strata. Palynological sampling at Scotty’s Palm indicates an early Paleocene age. The site was extensively quarried in 1992-3. Fossils are preserved in claystone layers that are interbedded with arkosic sandstone layers. Depositional setting appears to have been the marginal portion of a large fluvial channel. Whole fossil leaves densely cover bedding surfaces, indicating that the site of deposition was not far from the source forest. Large Sabalites palm fronds and a few species of ferns are common but the flora is taxonomically and numerically dominated by dicotyledonous angiosperms representing at least 40 morphotypes. Pieces of charcoal document the presence of conifers not seen in the compression flora. The angiosperms are characterized by large leaf size and acuminate apicies (driptips) that suggest high rainfall. This assemblage is physiognomically similar to the 64.1 Ma Castle Rock Rainforest site which is located 40 km to the north and slightly higher in the D1 synorogenic strata. Together, these localities begin to document the existence of a high-diversity rainforest vegetation along the western margin of the Denver Basin about 1.5 Ma after the K/T boundary. The low-diversity, small-leafed Baptist Road flora occurs 3 km north and stratigraphically above the Scotty’s Palm flora but below the top of the D1 strata, suggesting that the rainforest vegetation may have been short-lived or patchy. Collectively, these sites are beginning to provide a better understanding of post K/T boundary biotic recovery, paleoclimate, and paleogeomorphology of the Denver Basin during the early Paleocene.