PALEOCENE – EOCENE STRATA REVEAL COMPLEX BASIN EVOLUTION OF THE NORTH PARK-MIDDLE PARK BASIN, COLORADO

The Laramide sedimentary fill of the North Park-Middle Park Basin (also called the Colorado Headwaters Basin, or CHB) offers unique insight into the complex uplift history of the Front Range and the regional paleogeography of this area. Strata preserved in the CHB partly record events overlapping an ~8-Ma hiatus in the Laramide section of the Denver Basin and therefore provide a more complete picture of sedimentation and uplift adjacent to the Front Range.

Field observations are combined with palynology, paleobotany, radiometric dating, well-log analysis and seismic data to develop a stratigraphic framework that reveals basin evolution.

Paleocene Middle Park Formation and Paleocene – Eocene Coalmont Formation make up the over 7000-ft (>~ 2000 m)-thick Laramide basin fill of the CHB. Generally these strata unconformably overlie the Hygiene Sandstone member of the Pierre Shale, suggesting considerable erosion (3000-4000 ft or >~1000 m) before sediment started to accumulate. However, at distinct places within the basin both the Coalmont and Middle Park formations overlie either Niobrara or Dakota outcrops, marking locations where significantly more early uplift and folding occurred. The preserved Laramide section starts with several local basalt flows (63-64 Ma) in the southeast part of the basin, which are overlain by a ~60.5-Ma package of matrix-supported debris flows with predominantly volcanic clasts (Windy Gap Volcanic Member). After this, the basin filled with alluvial (locally), braided and meandering fluvial, paludal and lacustrine deposits of both volcanic and granitic origin. Sediment accumulation rates indicate a rapidly subsiding basin.

The low-energy deposits (shales-coals) preserved at the base of the sequence along the current margins of North Park, now flanked by steep mountains, suggest an originally wider and broader basin. After deposition of the Coalmont Formation, Laramide contraction continued as the basin narrowed significantly.

Neogene and possibly even younger faulting overprint the older deformation, adding further complexity to the geologic history of this basin.