INTERNAL STRATIGRAPHY OF THE PALEOGENE TO EOCENE COLORADO HEADWATERS BASIN, JACKSON AND GRAND COUNTIES, COLORADO

Paleogene sedimentary rocks of the Coalmont Formation (Jackson County) and the Middle Park Formation (Grand County) were deposited in a single, large, Laramide basin now preserved in northern Colorado. The section is thick and contains internal unconformities that document multiple tectonic deformation and subsidence events.

The Middle Park Fm. of Middle Park is at least 3km thick and includes arkosic and volcaniclastic sandstone and conglomerate, recording changes in provenance with time. The Windy Gap Volcanic basal member lies unconformably on the Cretaceous Pierre Shale, and is volcaniclastic, derived dominantly from a volcanic field nearby to the east and south. Beginning about 60Ma, basin fill was rapid as abundant sediment was delivered to a deep lake. Basal sediments are dominantly sublacustrine debris flows with some turbidites. Upward in the section, pro-delta conditions persist through a thick section that includes turbidites and soft-sediment slumps. Younger parts of the section are dominated by arkosic sediment, and are mainly deltaic and fluvial; paleocurrent data from these younger strata indicate sources all around the basin.

The coeval Coalmont Fm. of North Park is up to ~2km thick and contains less and finer volcaniclastic detritus than the Middle Park Fm. but, in places, preserves abundant and coarse crystalline detritus. Middle Paleocene lacustrine sediments are thickest in south North Park, and fluvial and deltaic deposits built east from sediment sources along the west flank of the basin. Paleocene-Eocene fine-grained lacustrine strata dominated the central North Park basin, but eventually fluvial-deltaic sediments filled the basin mainly from the north and west, making up the youngest part of the preserved section.

Considered as one, large sedimentary system, the stratigraphy records a complex subsidence history and uplift of multiple, disparate sediment sources. Most of these sources can be identified today around the area of the modern North Park and Middle Park geomorphic basins. This provides a challenging opportunity to determine synorogenic Laramide uplift areas and correlate stratigraphy across the Colorado Headwaters Basin.