EARLY PALEOCENE LANDSCAPES OF THE ROCKY MOUNTAIN REGION

The K-T boundary and the upper and lower contacts of geomagnetic polarity subchron C29r form a set of three timelines that span about 600,000 years and can be traced throughout the Rocky Mountain region. This time-bounded package of sedimentary rocks ranges from 10 to nearly 1000 meters in thickness depending on local rates of basin subsidence. Sedimentology, stratigraphy and paleontology of the subchron C29r rock sequence provides an opportunity to recreate specific landscapes over a large region. These rocks are synorogenic in the Raton, Denver, and Hanna Basins to the south but represent much more distal deposition in the Powder River, Williston, and Bighorn Basins to the north. In the Williston Basin, marine deposition occurred in the latest Cretaceous and the early Paleocene. Significant regional variation occurs in megaflora, palynoflora, and vertebrate fauna and this variation can be attributed to latitude and position relative to the newly forming Laramide ranges and the retreating Western Interior Seaway. Paleoclimate information from leaf physiognomy and the presence and thickness of coal seams are both suggestive of orographically controlled precipitation with high rainfall of the eastern windward margin of the uplifted ranges. This trend in pronounced in the Denver Basin. From the montane gravelly streams of the western margin of the Denver Basin to the low-lying coastal swamps of the Williston Basin, there is now enough landscape resolution to begin to understand the geography of the biotic response to the K-T boundary extinction event. K-T boundary extinction levels are lower and recovery rates higher in the wet mountainous areas than they are in the coastal lowlands. Low diversity forests characterize all the lowland landscapes while more diverse wet forests occur along the mountain fronts.