STRUCTURAL GEOLOGY OF THE MOUNT DECEPTION QUADRANGLE, TELLER AND EL PASO COUNTIES, COLORADO

The Mount Deception 7.5-minute quadrangle is located in Teller and El Paso Counties, Colorado, in the southern part of the Colorado Front Range. The quadrangle is topographically characterized by a central, north-south trending valley composed of Phanerozoic sedimentary rocks and bordered on the east and west sides by mountainous, granitic terrain of the Mesoproterozoic Pikes Peak batholith. The type of deformation in the quadrangle is dominated by brittle rock behavior in the form of faulting, imbricates, slickensides, fracturing, and emplacement of sandstone dikes. Two regional reverse fault systems transect the length of the quadrangle: (1) the east-directed Ute Pass fault zone which strikes north-south through the west-central part of the quadrangle and (2) the west-directed Mount Deception fault system, which strikes north-south through the east-central part of the quadrangle. Both fault zones have field evidence of low-angle reverse faults with the hanging walls composed of Proterozoic crystalline rocks and the footwalls composed of Paleozoic and Quaternary sedimentary rocks. These two fault systems are responsible for the downthrown, north-south-trending, low-relief area in the center of the quadrangle known as the Woodland Park/Manitou Park graben. The term “graben” generally implies extension accomplished through normal faulting. This central area can still be referred to as a graben since it is downthrown between the two oppositely directed systems. However, this suggests a compressional rather than an extensional stress field. Numerous sandstone dikes are present in the granite of the hanging wall of the Ute Pass fault. The lithology of these sandstone dikes is suggestive of a Cambrian Sawatch Formation source. The dikes proximity to the Ute Pass fault in this quadrangle may also suggest their emplacement during Late Pennsylvanian Ancestral Rockies deformation or an earlier faulting episode that predated lithifaction of the Cambrian Sawatch Formation sands.