CONTROLS ON OPENING-MODE FRACTURE FORMATION ON A LARAMIDE-STYLE FOLD AND IMPLICATIONS FOR SUBSURFACE FRACTURE PREDICTION

Empirical studies of natural fractures often rely on geometric relationships, such as cross-cutting relationships, orientation, and host age, to infer relative timing and states of stress. Genetic links between fracture populations and potentially causative geologic events such as diagenesis, pore pressure evolution, tectonics, and/or burial history are more difficult to establish. To better constrain the relationship between causative events and fracture formation, we combine traditional geometric fracture analyses with geothermometry and textural analyses of crystalline fracture fills hosted by the Woodside anticline in eastern Utah. We consider these fracture analyses in the context of a detailed P-T-t history of this Laramide-style fold that is locally calibrated via outcrop apatite fission track and borehole cutting vitrinite reflectance analyses. Orientation, abutting relationships, and kinematic indicators of fractures observed in sandstones of the Jurassic Morrison Formation identify two distinct opening mode fracture populations. One fracture set pre-dates folding while the second set appears to be influenced by fold growth and is interpreted have formed early in fold development. The abundances of both fracture populations are primarily controlled by the mechanical thickness of host sandstone beds, although the syn-folding set demonstrates enhanced fracture development related to structural position. In the context of our locally described time-temperature history, optical fluid inclusion homogenization temperatures of ~80-100 C from calcite fracture fills are consistent with the formation of both fracture sets at depths equal to or greater than ~70% of maximum burial. We interpret the pre-folding set to have formed in the late Cretaceous, during a period of rapid loading due to Sevier foreland sedimentation. The syn-folding set could have formed either relatively soon after the first fracture set, or during rapid exhumation as late as the Oligocene. Importantly, these results suggest that even in a folded setting, vertical stress changes brought on by burial and exhumation must be considered along with tectonic loading in order to understand the dominant controls on fracture development.