GEOLOGY OF THE STAR BASIN AREA AND UPPERMOST SPRING CREEK BASIN, GUNNISON COUNTY, COLORADO

The area is divided into blocks that show different geologic features. All blocks have at least one fault boundary; smaller blocks are entirely fault-bounded. In Block 6, Paleozoic beds on American Flag Mountain (AFM) dip moderately west; dips steepen to the south where the basement contact is near-vertical. The northern boundary is an E-W vertical fault that cuts basement rocks north of AFM and stops at the eastern edge of the central Italian Mountain pluton. Displacement is probably south side down. Overturned, east-dipping Paleozoic and crystalline rocks form Block 1, the E-W ridge north of the Block 6 boundary fault. These rocks have been rotated about 135º. Block 2, the N-S Hill 12,721 Ridge north of Block 6, consists of right-side-up, tremolitic Leadville marbles and Devonian sandy dolomites close to the northern granite. Block 5 includes vertical Sawatch through Belden strata and basement rocks. The Paleozoic beds are in correct stratigraphic order and tops point away from the basement contact. These rocks extend eastward from upper Cement Creek basin and end against a N-S fault in Star Basin. The Sawatch beds show vertical extension, but folds in Belden shales suggest compression in strata farther away from the basement rocks. A km or so to the north, subhorizontal Paleozoic beds rest atop basement rocks.

The Devonian through Belden strata in Star Basin (Blocks 3a and 3b) are upside down, based on field relations and 2 vertical shafts in Block 3a that show Leadville limestone above black shales. Star Mine Ridge, Block 4, consists of gently dipping Leadville Fm dolostones and limestones that are also probably upside down. Sawatch-basement rock contacts in Blocks 5 and 6 are interpreted to be drape-fold faults rooted at depth in east-dipping reverse faults. Most basement rocks in the area show very low fracture densities and little evidence of plastic deformation. Sharp-folding of the crystalline rocks seems unlikely, given sub-greenschist facies temperatures and the problem of how crystalline rock components originally in the hinge area are redistributed.