GEOLOGY OF THE SOUTHERN HALF OF THE MOUNT EVANS 7.5 MINUTE QUADRANGLE, CENTRAL COLORADO FRONT RANGE

The purpose of this study was to produce a 1:24,000 bedrock geologic map of the southern half of the Mount Evans 7.5-minute quadrangle, Clear Creek County, Colorado, which is mostly composed of Paleoproterozoic and Mesoproterozoic rocks. This area was selected for investigation of Paleoporoterozoic structure away from Proterozoic shear zones such as the Idaho Springs – Ralston shear zone, and from Cretaceous and younger overprinting structures that are strongly developed near the Colorado Mineral Belt and Rio Grande Rift to the west. The oldest rocks include a heterogeneous unit of mafic and intermediate gneiss, a granitic gneiss, and an amphibolite and metasedimentary sequence. The latter includes quartzite, calc-silicate gneiss, amphibolite and biotite-sillimanite schist. Based on regional correlations these units are likely to be Paleoproteozoic, ~1.8-1.7 Ga. Isoclinal folds are overprinted by open to isoclinal folds that plunge ~40° towards ~350°. Poles to axial planes plot along a great circle suggesting a third generation of folds plunging moderately to the northeast. Sillimanite and the presence of local partial melt indicate metamorphism under upper amphibolite facies conditions. The youngest rocks are mainly granodiorite and fine- to coarse-grained granite. Mesoproterozoic plutonic bodies are known to exist immediately north (~1.4 Ga Mount Evans batholith) and southeast (~1.1 Ga Pike’s Peak batholith) of the map area. The granodiorite and granite, which are largely undeformed but locally contain flow foliation, are therefore likely to be Mesoproterozoic in age. Orientations of various joint sets can be divided in structural domains of unknown origin. The most pervasive joint set trends 355°-020° and is subvertical. It is best developed in the western to southwestern part of the map area, and may be related to the Rio Grande Rift. Joint orientations are generally consistent with topographical lineaments in the same structural domains. The most significant surficial deposits are two series of glacial till and outwash. They correlate with the Bull Lake (170-120 ka) and Pinedale (30-12 ka) glacial periods based on deposit characteristics, pedology, associated landform morphology, and geomorphic position. Possible older glacial deposits have been observed along topographically higher surfaces.