LARAMIDE DEFORMATION IN THE STAR MINE AREA, GUNNISON COUNTY, COLORADO; A ROTATED BASEMENT NONCONFORMITY, UPSIDE-DOWN STRATA, AND THE ROLLER/BALL BEARING THEORY

Star Mine Basin, a 7 km2 area in the headwaters region of Italian Creek, lies astride the western margin of the Laramide Sawatch Uplift. Rocks include Sawatch SS (Cambrian) through Belden Fm (Pennsylvanian) and a remarkably unfractured, undeformed Proterozoic granite. Many of the Paleozoic strata are overturned or upside down. The Sawatch-Belden section on an E-W ridge at the S margin of the basin dips ~ 45° E and is overturned. At the basement nonconformity, Sawatch quartz pebble conglomerate is seen to lie below undeformed Proterozoic granite. These rocks, including an unknown thickness of granite, have undergone a 135° rotation that left no discernible deformation within the block. Proterozoic granite-Paleozoic contacts elsewhere in the area are steeply-dipping faults.

Along the NW perimeter of the basin, Belden strata are overlain by limestone and dolostone, the expected order for an upside-down section of Devonian, Leadville, and Belden beds. In the North Star Mine area and along the N perimeter, Belden strata are overlain by limestone, suggesting an upside-down Leadville-Belden section. In the area between N Italian Creek and its divide with S Italian Creek, outcrops and old shaft dumps indicate that the Paleozoic section (Devonian dolostones, Leadville, and Belden) is upside down, and south of the Star Mine Buildings, dolostone resting on Leadville Lm is consistent with upside down beds. The postulated upside down strata require rotations of 180° or more.

These rotated blocks support the idea that small, fault-bounded blocks formed in the Laramide deformational zone could have acted like bearings to facilitate translational movements of larger adjoining slabs or sheets. A deep keel of strong Proterozoic granite would help the blocks remain intact and protect attached Paleozoic strata. The original bottom fracture of the granite keel for the block containing the rotated nonconformity might be found in the Proterozoic rocks farther east.