Two-stage development of the Wind River Basin, Wyoming: Laramide shortening followed by post-Laramide regional extension, localized backsliding, and arch collapse

Abstract

This study addressed mechanisms and timing of fracture development in basement-involved foreland basins. Fracture analyses were used to test hypotheses for the structural development of the Wind River Basin in central Wyoming. Current debates in the Rocky Mountains of North America include the mechanisms and timing of fracture development and how the fractures relate to the Laramide Orogeny, which was responsible for the bulk of Phanerozoic deformation in the region. These fracture systems have important implications, especially for hydrocarbon recovery. This study tested hypotheses for fracture mechanisms and timing, including fracturing due to: 1) pre-Laramide distal compression or forebulge migration during the Sevier Orogeny; 2) syn-Laramide east-northeast directed horizontal compression; and 3) multiple post-Laramide hypotheses for fracture development including: i) near-surface topographic collapse and exhumation; ii) elastic strain release following Laramide shortening; iii) left-slip faulting resulting in the basin's trapezoidal shape; iv) broad regional extension due to extensional plate interactions or epierogenic uplift; and v) localized extension coupled to backsliding on listric thrust faults and collapse of basin-bounding arches. Multiple datasets of fractures were analyzed and eigenvectors were used to calculate the inferred axes of the stresses responsible for the deformation in the Wind River Basin. These datasets included 1447 fault strike segments digitized from geologic maps, 9107 subsurface fractures previously interpreted by log analysts from micro-resistivity image logs from 23 wells, and 1833 joints and minor faults measured at 42 stations in outcrops of Cambrian to Eocene units from throughout the basin. Timing relationships and ideal σ1 and inferred σ3 results indicate two distinct fracture sets. The first set consists of 135 strike-slip faults and 330 thrust faults and was found in Cambrian through lower Eocene units. Using ideal σ1 analysis for this set, a mean N66°E-trending, bedding-parallel maximum compressive stress was calculated, consistent with shortening due to the Laramide Orogeny. The second set consists of 546 normal faults, one recent earthquake's previously calculated fault plane solution, 617 systematic joints, and the 9107 subsurface fractures. Timing relationships observed in the field indicate that these fractures are younger than those in set one. They were found in Cambrian through upper Eocene units and include modern fractures. Two distinct fracture strikes were observed in both the natural and modern fractures in this set, northwest-southeast and east-west. Eigenvector analyses indicate a N44°E- trending minimum compressive stress from fractures with northwest-southeast strikes and a N7°E-trending minimum compressive stress from fractures with east-west strikes. Previous workers have documented northwest-southeast-striking joints across Wyoming and Colorado, supporting the hypothesis of regional post-Laramide extension for these fractures. East-west-striking fractures, however, are parallel to, and localized in proximity to the northern and southern basin-bounding faults. The correlation between set two fracture strikes and the strike of the closest basin-bounding fault suggests a coupling of these features. In conclusion, the thrust and strike-slip faults in set one are consistent with ENE-WSW Laramide horizontal shortening, and the normal faults and systematic joints in set two are consistent with post-Laramide extension. The two calculated stress axes from the post-Laramide fractures supports a combination of regional northeast-southwest extension, which may have unloaded Laramide arches bounding the basin and localized north-south extension coupled to backsliding on listric thrust faults and collapse of basin-bounding arches.