Stratigraphy and sedimentology of Lower Cretaceous Sykes Mountain Formation, Bighorn Basin, Wyoming

摘要

During Early Albian time, and after the beveling of the nonmarine Cloverly Formation, continued subsidence of the foreland basin east of the Sevier Fold-Thrust Belt resulted in the advancement of the Early Cretaceous seaway southward. The transitional marine sediments deposited during this time include the Sykes Mountain Formation in the Bighorn Basin, Wyoming and its equivalents throughout the Western Interior, while the marine sediments deposited in this sea include the Thermopolis Shale and its equivalents. The Sykes Mountain Formation in the Bighorn Basin is correlated with the transitional marine sediments of the Fall River Formation (Black Kills), the First Cat Creek Sandstone (central Montana), the Plainview Sandstone Member (Colorado Front Range), the Lower Naturita Formation (Colorado Plateau), and the nonmarine Smoot Formation (western Wyoming). The Sykes Mountain Formation is composed of rust-colored, thinly bedded and laminated sandstones and siltstones, and dark gray mudstones and shales. Sandstones and siltstones are dominant in the lower part of the formation and mudstones and shales increase upward forming an overall fining upward sequence. The sediments of the Sykes Mountain Formation were derived from the east and northeast as well as from the west and northwest. The eastern source was likely the Precambrian and Paleozoic rocks of the Sioux Arch, whereas the western source may have been the older sediments of the Cordilleran region as suggested by the paleocurrent and mineralogical studies. Two sedimentary facies associations are recognized within the Sykes Mountain Formation; the Quartz Arenite Facies Association (QA) and the Heterolithic Facies Association (SM). The two facies associations are arranged in a fining upward sequence, in which the QA Facies Association grades upward into the SM Facies Association. The Sykes Mountain Formation sediments represent intertidal, shallow subtidal, and shallow marine environments.