Laramide Basin Subsidence and Basement Uplift in Rocky Mountain Foreland of Wyoming: ABSTRACT

Abstract

The basement-cored ranges of the Rocky Mountain foreland have attracted geologists' attention since the time of early exploration of the western United States and have been the subject of numerous structural studies since early in this century. Until the advent of hydrocarbon exploration, however, the basins got little attention. Even today, our knowledge of the uplifts far exceeds that concerning basin genesis. The most recent work shows that uplift of the ranges and subsidence of the basins are intimately related and suggests that viewing the subsidence-uplift couple as the unit of deformation in the foreland is most likely to lead to a better understanding of the timing and kinematics of the Laramide orogeny. The Wind River Range in western Wyoming is an excellent natural laboratory for studying a Laramide uplift. A COCORP seismic profile provides geometric control, and tectogenic sediments record the history of uplift and erosion. The stratigraphy and provenance of these sediments indicate a complex Laramide and later tectonic history for the range and identify the timing and position of individual faulting events. These events are (1) main uplift of the range by motion on the Wind River fault and the formation of an erosion surface of low relief (Late Cretaceous through early Eocene), (2) elevation of this erosion surface as much as 3,000 ft (914 m) by motion on imbricates and associated tear faults in the hanging wall of the Wind River fault (end of early Eocene), (3) collapse of the ti of the Wind River fault into sedimentary fill of the Green River basin (between middle Eocene and late Oligocene), (4) uplift of the crest of the range by nearly 3,000 ft (914 m) forming the highest peaks in the Wyoming foreland (late Oligocene), and (5) collapse of the southern part of the range along normal faults (Neogene). Basin modeling in two distinctly different structural settings points to several driving mechanisms for subsidence in Laramide basins. Subsidence of the northern Green River basin was a flexural response to sediment loading and the intracrustal and topographic loads imposed by uplift of the adjacent Wind River Range. In contrast, the Hanna basin subsided when a rigid crustal block rotated downward as the Rawlins uplift was raised on the other end. Both flexure and rigid block rotation likely are operative to varying degrees in most Laramide basins. A schematic cross section through central Wyoming suggests that deep basins, where both rotation and tectonic loading are important, support structurally low ranges. Where rotation is not an important component of subsidence, basins support structurally high ranges. Furthermore, because basin subsidence and basement uplift are genetically linked, both indicate the timing of Laramide deformation. End_of_Article - Last_Page 1523------------