EARLY UPLIFT PRIOR TO SUBSIDENCE OF THE NORTH PARK-MIDDLE PARK BASIN, NORTHERN COLORADO, SHOWS COMPLEX LARAMIDE HISTORY

A broad arch measuring > 150 km north-south by > 50 km east-west and 1.5-2.5 km high formed in Late Cretaceous-early Paleocene time, roughly spanning from Fraser, Colorado to Centennial, Wyoming. Surface mapping and subsurface drillhole data show that this contiguous block rose nearly vertically (i.e., little tilting) so that the upper part of the Upper Cretaceous marine Pierre Shale and overlying formations (Fox Hills Sandstone, Laramie Formation) were eroded over this vast area, exposing identifiable sandstone members of the middle sandy part of the Pierre. Total uplift was at least 1.5 km, based on regional stratigraphic thickness of the eroded strata. An additional increment of regional uplift is suggested by paleobotanical data that indicate the Paleocene sediments were deposited about a kilometer above sea level. This regional uplift took place at roughly the same time as initial subsidence of the Denver-Cheyenne Basin 80-130 km to the east and at roughly the time of initial subsidence in the Laramie and Hanna Basins 100-140 km north (starting about 67-66 Ma). The broad extent and vertical nature of the regional uplift that is coeval with subsidence in adjacent basins suggest lithospheric buckling was dominant (c.f. Tikoff and Maxson, 2001) over lithosphere-penetrating low-angle thrusts. Arches and swales of similar age, trends, and dimensions are documented in the subsurface beneath eastern Colorado and Nebraska and Kansas.

This uplift area began to subside after 61 Ma (North Park-Middle Park basin) and filled with >2.5 km of clastic sediment during middle Paleocene through early Eocene time. This basin post-dates the Williams Range thrust, which is overlain by undisturbed Paleocene strata near Kremmling, Colorado. However, the faulted, basement-cored mountain ranges that flank the basin today are mostly younger and do not appear to have contributed significant sediment during Paleogene basin filling.