LARAMIDE THROUGH NEOGENE STRUCTURAL INHERITANCE OF ANCESTRAL ROCKIES FAULT AND FOLD TRENDS NEAR ALMONT, COLORADO

The Almont area contains a record of four principal phases of mountain building in Colorado including Proterozoic intrusion and metamorphism, Pennsylvanian-Permian Ancestral Rockies uplift, early Tertiary Laramide faulting and folding, and late Tertiary intrusion along Laramide faults. Of particular note, the Almont 7.5' quadrangle (Colorado Geological Survey Open File Report 05-5) contains a northeastern boundary fault to the Ancestral Uncompahgre uplift, which has been designated the Roaring Judy fault. The fault was reactivated as a steeply (70º) southwest-dipping Laramide reverse fault, but Ancestral Rockies displacement along the fault is evident from contrasting levels of unconformity beneath Jurassic strata across the fault. The Paleozoic section is absent from the western hanging wall block, but Cambrian through Permian strata are preserved in the footwall. Locally-derived conglomerates of the Maroon and Gothic formations in the footwall contain Precambrian clasts that support a Pennsylvanian-Permian age for uplift of the western block. East of the Roaring Judy fault, palinspastic restoration of the Jurassic angular unconformity delineates an Ancestral Rockies anticline along the trend of the Laramide Cement Creek fault, a steeply (70º) northeast-dipping reverse fault. The west limb of the Ancestral Rockies anticline is recognized within the Almont quadrangle by the 20-35 o westward dip of Paleozoic strata beneath the restored unconformity of the Cement Creek footwall. The crest and east flank of the anticline are restored from the angular unconformity relationships in the Cement Creek hanging wall exposed in the Crested Butte and Cement Mountain 7.5' quadrangles to the north. Both the Roaring Judy and Cement Creek faults are also candidates for Neogene extension. Both fault planes are intruded by 10 my rhyolite plugs that imply dilation across the faults during plug emplacement.