TWO-STAGE CRETACEOUS-EOCENE DEFORMATION HISTORY IN NORTH PARK, JACKSON COUNTY, COLORADO

Map relationships and outcrop-scale structures suggest overprinted folds rather than a single "Laramide" deformation event in rocks of Cretaceous - Eocene age in North Park (Jackson County), Colorado. North Park is a topographic basin west of the Medicine Bow Mountains of the Colorado Front Ranges and east of the Continental Divide. The Paleocene-Eocene sedimentary section is part of the Colorado Headwaters depositional basin, and in North Park is known as the Coalmont Formation. This study incorporated mapping, structural analysis and subsurface data to distinguish two folds sets with distinctive geometries, kinematics, geographic distributions and age constraints.

The older fold set consists of east-vergent fold trains that occur in rocks as young as the Cretaceous Pierre Shale. Fold axes are sub-horizontal and trend north. The best-exposed example, at Pitchpine Mountain on the west edge of North Park, includes four asymmetric folds with wavelengths of 1-2 km and amplitudes of several hundred m. These folds are erosionally trimmed and overlain by the Coalmont Formation. Folds in the Mesozoic section on the east side of North Park in the McCallum oil field are also erosionally trimmed and overlain by the Coalmont Formation.

The younger fold set is characterized by large asymmetric anticlines associated with steep reverse faults. These fault-bounded folds deform both the Mesozoic section and the overlying Coalmont Formation. Fold axes trend north-northwest, parallel to the strike of the faults. Both folds and faults are typically limited in lateral extent, and often occur in en echelon sets. Stratigraphic separation can be on the order of 2 km. Near the northwest edge of North Park, asymmetric anticlines of this fold set are basement-cored and west-verging; they form the hanging walls of en echelon, right-stepping reverse faults. Another west-directed thrust fault at Buffalo Ridge, along strike to the southeast, is farther from the basin margins and, in spite of significant structural relief, does not expose basement rocks at the ground surface. Near the northeast edge of North Park, asymmetric folds of this set are east-verging. These do not bring basement rocks to the ground surface.