Paper No. 25-2
Presentation Time: 8:15 AM
LARAMIDE THRUST FAULT-RELATED FOLDING ACCOMMODATED BY SLIPPED DEFORMATION BANDS IN DAKOTA GROUP SANDSTONES, CAÑON CITY EMBAYMENT, COLORADO
The Cañon City Embayment (CCE) in south-central Colorado contains large-scale folding comprised of Ordovician to Tertiary sedimentary rocks overlying Precambrian basement that is affected by several major thrust faults attributed to the Laramide orogeny. Such thrust faulting may be accompanied by a damage zone that, when affecting highly porous rocks, manifests itself as arrays of deformation bands that increase in density with proximity to the fault. Hence, deformation bands are precursors to faults, with geometric and kinematic properties related to the fault that they are associated with. The Cretaceous Dakota Group consists mainly of sandstones that are pervaded by dense arrays of deformation bands, with the highest density of structures in the southwestern section of the CCE. We conducted structural field work along a 1.75 km-long and ~200 m-wide ridge with spectacular exposures of deformation bands to better characterize their orientations, kinematics, and the general structural setting. The ridge shows an abrupt bend with shallowly dipping beds striking NE-SW in the west and moderately dipping beds striking N-S to the east. Six locations along the ridge were mapped in fine detail, focusing on the kinematic relationships between cross-cutting deformation bands. We discovered several dozen steeply dipping to vertical deformation bands striking NE-SW and NW-SE with well-preserved slip surfaces and offsets of several decimeters to up to 1 m. The slipped bands in the eastern portion show dextral strike-slip fault kinematics, whereas the western portion of the ridge contains sinistral strike-slip kinematics. We interpret the layering of the Dakota Group sandstones, the presence of the deformation bands, and their strike-slip kinematics to have accommodated folding caused by a thrust fault at depth and uplift of Precambrian basement during the Laramide orogeny. The bend in the ridge shows the highest density of deformation bands, which may represent the core of a damage zone within a large-scale anticline overlying a blind thrust fault. The growth of this deformation band damage zone allows us to better analyze the fault kinematics related to the Laramide orogeny and understand the evolution of brittle fault zones in other porous sandstone units of the CCE.