LARAMIDE-AGE STRUCTURE OF LITTLE PARK ROAD MONOCLINE, NORTHERN UNCOMPAHGRE PLATEAU, WESTERN COLORADO

Structural features of the Colorado National Monument (CNM) include Laramide-age, NW-SE striking monoclines cored by NE-vergent, basement reverse faults. The Little Park Road Monocline (new) is a formerly unmapped region extending southeastward from the CNM. The Little Park Road Monocline (LPRM) differs from monoclines found in the CNM. In the CNM, there are multiple, sub-parallel, narrow monoclines. The horizontal distance between the upper to lower fold hinges of these monoclines is less than one kilometer (<500m locally). Also, reverse faults in basement rocks are exposed and strata are locally overturned. In contrast, the LPRM has a much broader distribution of strain, with low-angle dips of monoclinal strata (<20 deg NE) and a horizontal distance between the upper and lower hinges of 4 to 5 km. One hypothesis for the broad width of the LPRM monocline is the presence of multiple, subsurface, small-displacement, blind, basement reverse faults beneath this monocline. Anomalous structural features are also found along the LPRM. These include several areas of oppositely dipping or ‘back-dipping' beds within the Jurassic-age, Tidwell Member (shales, carbonates, and sandstones) of the Morrison Formation. Beneath these back-dipping beds, Tidwell shales display localized deformation such as chaotic slickenlines and small-scale S and Z folds that gently plunge to the NE. These deformed zones are interpreted to represent décollement-style thrust faults, in weak shale layers. The thrust faults ramp up across more competent sandstone beds along well-exposed 30 deg dipping thrust faults. The S and Z folds serve as kinematic indicators and suggest down-monocline gravity sliding orthogonal to the fold hinges. The back-dipping zones are interpreted to represent superficial gravity slides that may have formed during original, Laramide-age monocline development. Seismic shaking and reduction of normal stress in the fluidized weak shale layers may have allowed the beds to slide at a low dip angles (<20 deg) during the Laramide Orogeny.