GEOLOGIC MAP OF THE HOMESTAKE RESERVOIR, COLORADO 7.5-MINUTE QUADRANGLE

The Homestake Reservoir 7.5’ quadrangle lies west of Leadville, Colorado in the northern Sawatch Range. This geologic map updates previous work by Tweto (1974) to address structural relations of 1.7 Ga metamorphic fabrics, 1.4 Ga intracontinental deformation, and Quaternary glacial landforms. Structural analysis of 1.7 Ga metamorphic fabrics specifies distinct gneissic foliation domains. Geologic field mapping identifies structural relations between 1.4 Ga Metalamprophyre dike intrusion; St. Kevin batholith emplacement; and mylonite, ultramylonite, and pseudotachylite shear zone deformation. Field relations show that Metalamprophyres are truncated by and rafted within the St. Kevin batholith. Mylonite, ultramylonite, and pseudotachylite mutually cross-cut each other suggesting deformation at the mid-crustal brittle/ductile transition. Pseudotachylite occurs within the 1.7 Ga gneiss as well as the 1.4 Ga St. Kevin batholith. Timing and kinematics of 1.4 Ga deformation in the Homestake Reservoir quadrangle correlates to intracontinental deformation of the newly identified 1.4 Ga Picuris Orogeny in northern New Mexico. Newly mapped Last Glacial Maximum (LGM) ~20-22 ka extent and cosmogenic sampling suite attempt to quantify glacial incision rates and timing of deglaciation ~13-14 ka for the northern Sawatch Range.