Structural evolution of the Oil Well Flats detachment system, Cañon City embayment, Colorado

Abstract

The Cañon City Embayment, located at the southern end of the Denver Basin, Colorado, is an understudied area with a complex structural history affected by major tectonic events such as the Ancestral Rocky Mountain Orogeny, Laramide Orogeny, regional epeirogenic uplift events, and Cenozoic extension. This area is of interest to the petroleum industry in part due to its proximity to Florence-Cañon City Oil Field, which produces from shallow, heavily fractured Pierre Shale and is the oldest continuously working oil field in the United States. Detailed field mapping identified a ~3 km2 detachment fault system in the Oil Well Flats area, located in the northern part of the Cañon City Embayment, which is currently mapped as Quaternary alluvium and landslide deposits. The structure exhibits several topographically high, repeating blocks of Cretaceous Dakota Group sandstones, separated by topographically low valleys that are likely underlain by shale, and a large master fault, identified by a major scarp with a large damage zone and deformation bands, that likely detaches on the shales of the underlying Jurassic Morrison Formation. In addition to the field data collected, construction of cross-sections through the system, examination of HD Google Earth Pro aerial images, and a high resolution 5 meter DEM that reveals the master fault as a strongly linear feature, support the hypothesis that this is a detachment system exhibiting an extensional, normal faulted updip section and a probable compressional toe downdip. The master fault is associated with an intense band of deformation characterized by an extremely dense zone of cataclasis deformation bands, identified via thin section analysis. These bands cut through various lithologies within the Dakota Group, including lower porosity, higher clay content sandstones not typically associated with this type of deformation band. As a result of our study, I interpret that this structure, possibly a unique outcrop example, is not just a modern feature, but rather a longer-lived structure that was reactivated during a series of structural/geomorphic events, potentially including Laramide compression, regional uplift, Rio Grande rifting, and more recent stream downcutting at the toe of the structure.