SNAPSHOTS IN TIME: FLORAL CHANGE IN THE DENVER BASIN FROM 68-64 MY

The Denver Basin, an asymmetric foreland basin covering roughly 7,000 mi2, preserves pre- and synorogenic sediments spanning Late Cretaceous through early Paleocene time. Although outcrops are rare, the rocks are often fossiliferous. To date, >15,000 megafloral specimens have been collected from >290 localities found in strata that are between 69-64 Ma. Data from the individual quarries has been compiled into a single Denver Basin flora comprised of over 270 unique dicotyledonous leaf types whose occurrences document patterns of floral change both spatially and temporally.

By mapping this floral data onto the Denver Basin time map, landscapes can be compared through time to document environmental responses to discrete, basin-wide geologic events. During deposition of the Laramie Formation (69-68 Ma), lowland areas of the basin were dominated by swamps. The floral diversity was not high and composition was similar across the basin. As the Front Range developed, floral composition proximal to the Eastern flank of the mountains began to diverge from the central basin. These proximal floras exhibit both higher diversity and unique taxa when compared to coeval floras from the central basin. The end-Cretaceous extinction left the basin with higher floral diversity proximal to the mountain front and much lower diversity in the central basin. Although this low diversity, recovery flora stretched from the Denver Basin to Saskatchewan during the earliest Paleocene, it shares few taxa with forests that grew proximal to the mountain front.

By ~64 mya, rainforests thrived along the eastern flank of the mountains, which by this time had risen to height of ~2 km above the basin floor. Although plant diversity was now high proximal to the mountains, the forest was just beginning to recover in the central basin. A possible explanation for the disparity in diversity seen along the basinal transect is that the localities proximal to the mountain front uniquely preserved an ecotone, offering a rare glimpse of upland forests that are not typically preserved in Paleocene lowland assemblages. Using the data management tools that we have developed to describe and calibrate the Denver Basin flora, we are now beginning to incorporate other Western Interior Basins to look at patterns of plant migration and effects of topography on landscapes.