Fracture analysis of Circum-Bighorn Basin anticlines, Wyoming-Montana

Abstract

The Bighorn Basin (BHB), Wyoming-Montana, is an oval Laramide intermontane basin (80-45 Ma) in the Rocky Mountain foreland province. Regional thrust and backthrust faults related to the Laramide orogeny uplifted basement rocks around the basin forming the present Beartooth, Bighorn and Owl Creek Mountains. Within the basin, weak Paleozoic stratigraphy succumbed to the same regional shortening, transforming the once sub-horizontal strata into a series of thrust fault related anticlines around the basin margin. Due to the oblique orientation to Laramide shortening direction of anticlines at the north and south side of the BHB, this study examines local stress regimes through Google Earth Pro lineament analysis and ground trothing these measurements with outcrop fracture analysis. Both lineament and fracture orientations were collected at nine anticlines around the basin shoulder and classified based on their relationship to the hinge line of the anticline: dip (AC), strike (BC) and oblique fractures and lineaments. Data collected was analyzed through a combination of rose diagrams and stereonets to classify the orientations of lineaments and fractures in each anticline. Results indicate that the orientation of the three lineament and fracture sets are strongly influenced by the orientation of the fold axis. Based on the results, the BHB can be divided into two tectonic domains. The first tectonic domain, anticlines adjacent to the east and west side of the basin, displays an average fold axis trend of 154°, aligning well with the suggested Laramide shortening direction of 060° N. Tectonic domain two is situated at the southern extent of the basin and defined by Thermopolis anticline. This domain shows strong influence of the Owl Creek Mountains sinistral accommodation zone to the south by the counterclockwise rotation of the anticline hinge line to 114°. The new anticline orientation is clearly oblique to the Laramide shortening direction and determined by a more local stress regime. Fracture orientations within this anticline also display a counterclockwise rotation indicating the strong relationship to the hinge line trend mentioned above. Overall, the orientations of Laramide structures identified in this study align well with previous research across the Rocky Mountain foreland.