THE PALEOCENE ENIGMA: HIGH DIVERSITY FLORAS ALONG THE ROCKY MOUNTAIN FRONT RANGE

Traditionally, the Paleocene is described as having a “boring flora” with very low diversity. This view was formed by extensive excavation of Paleocene sites in many Laramide foreland basins. Evidence from the Denver Basin began challenging this concept of a “boring Paleocene” with the discovery of the Castle Rock rainforest, a very diverse fossil assemblage situated <15 km from the rising Rocky Mountains. Subsequent work in the Denver Basin detailed a marked increase (nearly an order of magnitude) in floral diversity proximal to the Front Range when compared with floras that are situated >50 km eastward from the mountain front. In contrast, pre-orogenic Late Cretaceous localities collected in regions proximal and distal to the mountains not only contain similar (low) diversity floras but share many taxa.

The Denver Basin, an asymmetric foreland basin covering roughly 7,000 mi², preserves pre- and synorogenic sediments ranging from Late Cretaceous to early Paleocene. Although outcrops are rare, the rocks are abundantly fossiliferous. To date, >200 fossil plant localities have been collected, producing >15,000 specimens and >250 (dicot) holomorphotypes, spanning 68-64 Ma. Initially, these sites were analyzed on an individual quarry basis and identified to the level of morphotype. To enable cross-basin comparison and reconstruction of regional rather than local vegetation, all of the holomorphotypes have now been combined into a single Denver Basin Flora, which is currently being photographed, described and loaded into an on-line digital prodromous, to be accessible via www.dmns.org.

To enable spatial and temporal analysis of the Denver Basin Flora, a generalized geologic map of the basin was created and linked to a database of fossil localities, pollen samples, dated ashes, well log picks and surface geologic observations. The K-T boundary was modeled on this map by intersecting a digital elevation model with data from 2 cored wells, 4 surface K-T Boundary locations and >700 subsurface water, oil and gas well electric logs. Since the taxa at each locality is linked by the map, this interwoven data matrix will allow us to resolve the proximally high and distally low diversity enigma by locating the Denver Basin taxa in time and space on an evolving synorogenic landscape.