Development of Laramide Structure in Laramie Basin, Wyoming: ABSTRACT

The triangular Laramie basin lies between the Medicine Bow Mountains and the Laramie Range, and south of the Como Bluff fold. The stratigraphic column ranges in age from Pennsylvanian to Recent and is more than 15,000 feet thick. Hydrocarbons are produced commercially from fourteen fields and from six stratigraphic zones. The basin is asymmetric with a gently dipping east and southeast flank, a steeply folded and faulted west margin, and an indefinite northern boundary. Faulted northwest-trending folds interrupt the southern margin of the basin. Fault blocks in which Precambrian is exposed delimit the southwest margin of the basin south of Centennial. Faults bounding the blocks dip both east and west; individual blocks are elevated as much as 8,000 feet; and are bounded by fault complexes which might be anticipated in drilling. North of Centennial the basin is delimited by the Arlington thrust fault. The Centennial Valley syncline extends north-northwest parallel with the Rock River line of folding. Asymmetry of the anticline changes along strike from southwest at Seven Mile to northeast at Rock River. The Centennial syncline is in part overridden by the west-dipping Arlington thrust fault which dips 15° near Centennial and steepens to more than 45° near Arlington. The hanging wall plate is segmented by northeast-trending tear faults. Cooper Hill appears to be a gravity-emplaced klippe detached from the front of the thrust plate. The Laramie basin is segmented by the basinward extension of the Mullen Creek-Nash Fork shear zone in Precambrian rocks of the Medicine Bow Mountains. North of this zone the dominant feature is the Rock River line of folding and the Cooper Lake depression. Cross sections of the anticlines reveal a systematic pattern of internal deformation of the anticline, involving Triassic Chugwater and Cretaceous Niobrara and Frontier formations. The position of the major anticline was probably controlled by basement fracturing. Interference of three structural trends--Como Bluff (N. 30° E.), Seminoe fault zone (N. 65° W.), and Rock River (N. 20° W.) results in the complex of folds characterized by recurved fold axes near Medicine Bow. The mutual relationships between existent structural trends suggest that the first stage of development was broad crustal up-arching aligned north and south and probably extending to the base of the crust. The second stage developed along northeast-trending basement features and resulted in segmentation of the basement and in limits for the overthrusting. The third stage deformed the west margin of the basin either by faulting and folding (north half) or by development of fault blocks (south half). In this stage east-dipping faults appear to antedate west-dipping faults. The fourth stage involved complex reaction to compression in the Medicine Bow area, resulting in folds with variable orientation. Rocks correlated with the Hanna formation (Earliest Eocene) but not definitely dated rest with marked angular unconformity on the Cretaceous Medicine Bow formation. The Arlington thrust may be in part overlapped by the Hanna. Rocks of definite Wind River age are slightly folded. Rocks of Oligocene age are offset by normal faults. Younger Cenozoic deposits exist but have not been definitely dated. End_of_Article - Last_Page 115------------