BEDROCK AND SURFICIAL GEOLOGY OF THE SOUTHERN HALF OF THE MONTEZUMA 7.5 MINUTE QUADRANGLE, CENTRAL COLORADO FRONT RANGE

A geologic mapping project was conducted as part of a USGS EDMAP project to analyze the bedrock and surficial geology of the Montezuma 7.5’ quadrangle in the central Colorado Front Range. The quadrangle is located within Summit, Clear Creek, and Park counties. High-resolution topographic data, Lidar, and satellite imagery was used to delineate various geologic features prior to mapping. The main lithologies in the study area are Proterozoic biotite gneiss, biotite-quartz gneiss, biotite-sillimanite gneiss, and hornblende gneiss. Several Proterozoic granitoid and pegmatite units are also present, as well as small granite intrusions related to the ~39.7 Ma Montezuma stock in the northwestern part of the Montezuma quadrangle. At least two structural domains can be distinguished in the southern half of the quadrangle. Moderately NW- to NNW-plunging lineations in the Snake River Cirque and Handcart Gulch of the western and southwestern parts of the map area comprise one structural domain. The other domain is in the Geneva Creek region of the northeastern part of the quadrangle, where lineations and folds plunge WNW. Joint sets in the southern half of the Montezuma quadrangle show mostly random orientations, with one subvertical joint set dipping steeply towards ~330º and another weakly defined joint set dipping moderately towards ~250º. Glacial till and alluvium sediments comprise the most significant surficial deposits. Based on depositional morphology, pedology, and geomorphic position, the deposits can be differentiated and correlated with the 170-120 ka Bull Lake and 30-12 ka Pinedale glaciation events.