GEOLOGY OF THE NORTHERN HALF OF THE MONTEZUMA 7.5 MINUTE QUADRANGLE, CENTRAL COLORADO FRONT RANGE

A 1:24,000 bedrock and surficial geologic map of the northern half of the Montezuma 7.5-minute quadrangle in Clear Creek and Summit Counties of the central Colorado Front Range was produced as part of a USGS EDMAP project. The primary goals were to investigate the Paleo- and Mesoproterozoic structural histories associated with the Yavapai/Mazatzal and Picuris orogenies, respectively, and to better constrain the influence of Proterozoic structures on Cretaceous-Paleogene mineralization. The latter may be associated with the Colorado Mineral Belt (CMB), a northeast-trending ~25-50 km-wide belt of widespread mineralization throughout Colorado. The study area is predominantly composed of Proterozoic biotite gneiss, biotite-sillimanite schist, quartz-biotite gneiss, and hornblende gneiss in a large, heterogeneous sequence of metasedimentary and metavolcanic rocks, and granitic gneiss. Regional correlations suggest that these rocks are probably ~1.8-1.7 Ga. Local migmatite and sillimanite indicate that these rocks reached upper amphibolite facies metamorphism. Within the Paleoproterozoic units, isoclinal folds plunge shallowly east and west. A distinct generation of folds, found mainly in the northeast of the field area, plunge moderately southwest. The southeast section of the area consists predominantly of the ~1.42 Ga Silver Plume granite, which displays flow foliation proximal and parallel to its intrusive contact but is otherwise internally undeformed. The ~39.7 Ma Montezuma Stock in the northwestern part of the area is a monzogranite porphyry that is a part of the CMB. Joint sets are mostly randomly oriented, with one weak subvertical preferred orientation trending ~323º. Two series of glacial till and outwash comprise the most significant surficial deposits. Based on depositional morphology, pedology, and geomorphic position, these deposits are differentiated and correlated with the Bull Lake (170-120 ka) and Pinedale (30-12 ka) glaciations. While older glacial deposits are observed along topographically higher surfaces, they remain uncorrelated with known glacial periods.