PALEOGEOGRAPHY AND BASIN EVOLUTION OF THE COLORADO HEADWATERS BASIN (NORTH PARK-MIDDLE PARK) BETWEEN 70 AND 55 MA

The Colorado Headwaters Basin (CHB), underlying North Park and Middle Park in north-central Colorado, is a folded remnant of a Laramide depositional basin that subsided between two basement uplifts. The stratigraphic record of this basin in comparison to the nearby strata of the Denver and Cheyenne Basins gives insights into the Laramide evolution of the Front Range. This study is based on outcrop information, subsurface geophysical well logs, palynology, paleobotany and radiometric dating.

Correlations in the sandy part of the Cretaceous Pierre Shale are used to determine depth of erosion in this area, before the CHB was formed and started to rapidly receive sediment at about 60 Ma. These correlations reveal subtle folding, tilting and extensive erosion prior to deposition of the Paleocene-Eocene Coalmont and Middle Park Formations (shortened to Coalmont Fm). In general, 3000-4000 ft of Pierre Shale and latest Cretaceous sediment were eroded over most of the CHB between about 68 and 60 Ma, prior to the onset of Coalmont deposition. Several thousand additional feet of Pierre Shale and older strata were also eroded near internal basement faults and the basin margins.

Fossil leaf margin analysis shows that at 58 Ma this area was elevated significantly higher than the nearby Denver Basin. Within the CHB synorogenic strata angular unconformities are present, and combined with the other observations, indicate multiple phases of deformation and fault activity before, during and after deposition of the Coalmont Formation. The sedimentary record suggests that in parts of the basin the current basin margins do not necessarily coincide with the Laramide basin geometry.