TECTONIC AND CLIMATIC SIGNALS? AN INTEGRATED STRATIGRAPHIC FRAMEWORK FOR THE PALEOCENE TO EOCENE SYNOROGENIC STRATA OF THE COALMONT AND MIDDLE PARK FORMATIONS, NORTH PARK – MIDDLE PARK, COLORADO

The Paleocene – Eocene strata of North park and Middle Park comprise the basin fill of the North Park – Middle Park (or Colorado Headwaters; CHB) Basin during the Laramide orogeny and reveal insights in basin formation and regional paleogeography. The contemporaneous Middle Park and Coalmont Formations are traditionally broken out by general lithostratigraphic and spatial differences: the Middle Park Formation occurs in Middle Park and is volcaniclastic to mixed volcaniclastic – granitic in composition, the Coalmont Formation is mostly granitic and occurs north of the present Continental Divide.

A series of sections was measured and integrated with subsurface data, surface mapping efforts, palynology and radiometric dating to create a 3D picture of timing of basin infill. Sediments range from volcanic debris flows to alluvial, fluvial, paludal, and lacustrine. Sediment accumulation started at about 61Ma, after the basin originated in the latest Cretaceous to early Paleocene. At the onset of deposition, a large volcanic complex was present in the south of the study area and sediments document a gradual unroofing from volcaniclastic provenance to basement of the uplifting Front and Park Ranges that enclosed the basin. Active deformation can be documented, before, during and after sedimentation and influences basin geometry. An apparent cyclicity is present in the alluvial sediment fill, however by lack of time control, an origin is yet unidentified. Generally, clast size decreases upward as the basin becomes more ponded. A short pulse of fluvial and deltaic sedimentation overprints this overall drowning pattern and greatly reduces the lake extent for a brief interval. This thin progradation falls between Paleocene and Eocene pollen sites and appears to fill the basin from several directions. Others have also documented increases in sheet-sand sedimentation in the relatively nearby Bighorn, Piceance and Green River basins at the Paleocene – Eocene thermal Maximum (PETM) disrupting background sedimentation.

Integrating the stratigraphy of both Middle Park and Coalmont Formations and comparing sedimentation records of the CHB, Hanna, Denver, and South Park basins in a larger study allows reconstructing climatic effects and paleogeography for the evolving Front Range during the Laramide orogeny.