The timing of Laramide deformation in the northern Rocky Mountains

Along the western edge of the Bighorn Basin in northwestern Wyoming, geologic mapping of Late Cretaceous-Paleogene sediments has revealed a deep paleovalley that formed in the latest Cretaceous. The paleovalley was a principal control for the timing and type of Paleogene sediments deposited in the area. Tilting in the latest Cretaceous led to the erosion of these Cretaceous rocks, and subsequent deformation occurred in the latest Paleocene-early Eocene that steeply tilted them along with basal late Paleocene-early Eocene Willwood rocks to their present-day orientations. The extent of the paleovalley included the Line Creek, Clarks Fork Canyon, and Kimball Bench areas.;There have been varying interpretations regarding the direction(s) of shortening during Laramide deformation of the Rocky Mountains. Our research has revealed a complex multi-stage Laramide deformational history that resulted in the development of variously oriented structures as well as differently aged angular unconformities within Cretaceous-Paleogene strata along the western edge of the Bighorn Basin. In the northern Rocky Mountains, there were three stages of deformation with differing shortening directions. The three stages of deformation occurred as a result of (1) north-northeast-directed shortening in the latest Cretaceous-early Paleocene; (2) east-northeast-directed shortening in the late Paleocene; and (3) east-directed shortening in the early Eocene.;Deformed late Paleocene-early Eocene Willwood sediments are adjacent to a complex system of Laramide structures and in our study area, the absolute ages of these Willwood sediments are not well constrained. We calculated the age of the youngest deformed Willwood sediments based on chronostratigraphy tied to climate cycles. Our analyses also revealed a significant spectral peak corresponding to a cycle thickness of 13.1 meters, which correlates with a 21-ky climatic precession cycle. This indicates that precession-scale climate variations likely affected paleosol development within the Willwood Formation by way of cyclic changes from mostly overbank mudstone deposition to channel-avulsion deposition. Our study has revealed that deformation began in the early Eocene in the Kimball Bench area along the western edge of the Bighorn Basin. It is apparent that during Paleogene deposition, there was a complex deformational history along the western edge of the Bighorn Basin.