AN EXCEPTIONALLY PRESERVED PALEOCENE FLORA FROM THE MIDDLE PARK FORMATION, COLORADO: IMPLICATIONS FOR THE LARAMIDE PALEOTOPOGRAPHY OF THE CENTRAL ROCKY MOUNTAIN REGION

The heterogeneity of paleotopography, paleoelevation and paleoclimate in the central Rocky Mountain region during the Paleocene are key variables to deciphering the timing and maximum relief of the Laramide orogeny and its effect on regional climate and evolution of the earliest documented rainforests. The Paleocene Kauffman Creek flora, an exceptionally well-preserved flora from the fluvial Middle Park Formation, occurs in an axial Laramide basin in north central Colorado that presently lies approximately 3 km above sea level. When compared with the nearby (~150 km SW), well-collected, and temporally calibrated lowland floras from the foreland Denver Basin, the Kauffman Creek flora provides an excellent opportunity to test for the presence of a Paleocene east–west climatic and thus topographic gradient at the location of the present-day Front Range. Initial macrofossil collections of approximately 1,000 field and museum specimens from crevasse-splay and stream-side deposits of reworked ash beds and fine-grained sands yielded a diverse assemblage of ferns, horsetails, conifers, and angiosperm leaf species. Angiosperms dominate the flora, which consists of more than forty species, and they comprise more than 90% of the total floral diversity. The dicot leaves show a marked size range: Examples fall in the micro- to macrophyll size categories with the modal leaf size being mesophyll. The large leaf size plus the occurrence of several species with acuminate apices indicates high rainfall. Approximately two-thirds of the dicot species exhibit teeth, indicating a MAT of about 10ºC. The combination of the much cooler temperatures (~6–10ºC) but similar rainfall or leaf size to the synlatitudinal and mountain-proximal Denver Basin floras suggests that the Kauffman Creek flora grew at a significantly higher elevation while also experiencing high, orographically initiated precipitation. Thus, by the Paleocene, the Laramide orogeny had developed a significant topographic gradient (~1–2 km using a terrestrial lapse rate of 5ºC/km) in the present-day location of the Colorado Front Range.