Abstract

The Sand Wash Fault Zone (SWFZ) in the southern Uinta Basin is a significant extensional fault zone exhibiting complex deformation patterns. Our study integrates detailed structural analyses with a broader tectonic model to elucidate the tectonic evolution of the region. The SWFZ, extending over 34 km with a NW-SE strike, is underlain by an extensional decollement. Substantial deformation is observed in the upper layers, notably within the S2 sandstone of the Eocene Green River Formation. The deformation diminishes with depth, terminating at the oil-rich Mahogany oil shale zone, below which minimal deformation is observed. Our tectonic model, developed primarily through outcrop analysis utilizing Eocene-age outcrops, borehole breakouts, and dike and joint systems, highlights three key stages in the development of extensional fault zones related to the Uncompahgre Uplift within the Uinta Basin. Initially, eastward-directed contraction beginning during the late Cretaceous reactivates Precambrian oblique strike-slip faults, coinciding with the latest Uncompahgre Uplift event. Regional Laramide stress develops NW-SE joint patterns throughout the Uinta basin. Subsequently, reactivated Precambrian faulting leads to the formation of tight drape folds, with extensional stresses enhancing the regional Laramide stress regime deformation patterns along the axial plane of the drape folds. Finally, late Eocene orthogonal flexure results in extensional faulting along Precambrian faults. Maximum stress (σ1) of the SWFZ during its development was oriented vertically, while minimum stress (σ3) was perpendicular to the fault zone at approximately NE-SW, matching the trend of the Laramide stress regime. In the Devils River Uplift region, the Laramide stress regime resulted in the formation of 0.5m scale drape folds and anticlines, whose axes ultimately experienced orthogonal flexure, thereby resulting in the development of neutral-surface extensional features (Ferrell et al., 2022). Results from the regional tectonic model provide additional insights into stress orientations and timing, elucidating the role of reactivated Precambrian faults and orthogonal flexure in shaping the structural evolution of the southern Uinta Basin. Integration of these findings increases our understanding of fault zone development and serves as an analog for similar systems in the western USA.

Degree

College and Department

Geological Sciences; Computational, Mathematical, and Physical Sciences

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