LARAMIDE STRUCTURES AT SHEEP MOUNTAIN AND DELANEY BUTTE, JACKSON COUNTY, CO

Published geologic maps of the Laramide structures in northwestern North Park, Colorado show faults that both cut the Paleocene-Eocene Coalmont Formation and are depositionally overlain by it; this paradoxical relationship was the motivation for new mapping in the area. The North Park Basin is an axial Laramide basin located west of the Front Range and bounded by the Continental Divide to the south and west. The western part of the basin hosts three N trending fault-bounded ridges. The two southernmost ridges, Sheep Mountain and Delaney Butte, are the focus of my study.

Sheep Mountain is a west-verging, basement-cored anticline that folds the Mesozoic section. In the northern portion of the ridge, strata of the gently dipping eastern limb have typical thicknesses and constant dips. The western limb is cut by a steep west-directed reverse fault and is locally overturned. The anticline plunges to the south. Mesozoic units are tectonically thinned where they fold around the southernmost basement exposures.

Delaney Butte is also a west-verging, basement-cored anticline in the hanging wall of a reverse fault but displays more complex geometries and can be broken into two structurally distinct pieces. The northern portion is a south-plunging anticline in the Mesozoic section but no basement is exposed. The southern portion is another south-plunging anticline. Mesozoic strata overlying crystalline basement on the eastern limb are greatly attenuated with no evidence of Triassic units. Coalmont Formation rocks are locally folded into a footwall syncline west of the reverse fault. West-vergent folds clearly formed after deposition of the Coalmont Formation.

West of Delaney Butte and Sheep Mountain, folds in the Cretaceous Pierre Shale are trimmed and unconformably overlain by the Coalmont Formation. These folds are east-vergent and stand in stark contrast to the steep west-vergent anticlines that trend NNW. These relationships document two separate deformation events, each with a distinctive structural style.