Tectonic and sedimentary controls, age and correlation of the Upper Cretaceous Wahweap Formation, southern Utah, U.S.A.
Jinnah, Zubair Ali
The Wahweap Formation is an ~400 m thick clastic sedimentary succession of fluvial and estuarine channel sandstones and floodbasin mudrocks that was deposited in western North America during the Late Cretaceous. It preserves important mammal, dinosaur, crocodile, turtle and invertebrate fossils that have been the subject of recent palaeontological investigations. The Wahweap Formation can be divided into lower, middle, upper, and capping sandstone members based on sand:mud ratios and degree of sandstone amalgamation. Facies analysis reveals the presence of ten facies associations grouped into channel and floodbasin deposits. Facies associations (FAs) from channels include: (1) single-story and (2) multistory lenticular sandstone bodies, (3) major tabular sandstone bodies, (4) gravel bedforms, (5) low-angle heterolithic cross-strata, and (10) lenticular mudrock, whereas floodbasin facies associations include: (6) minor tabular sandstone bodies, (7) lenticular interlaminated sandstone and mudrock, (8) inclined interbedded sandstone and mudrock, and (9) laterally extensive mudrock. The lower and middle members are dominated by floodbasin facies associations. The lower member consists dominantly of FA 8, interpreted as proximal floodbasin deposits including levees and pond margins, and is capped by a persistent horizon of FA 3, interpreted as amalgamated channel deposits. FAs 4 and 6 are also present in the lower member. The middle member consists dominantly of FA 9, interpreted as distal floodbasin deposits including swamp, oxbow-lake and waterlogged-soil horizons. FAs 1, 2, 5, 6, 7, 8, and 10 are present in the middle member as well, which together are interpreted as evidence of suspended-load channels. The upper member is sandstone-dominated and consists of FAs 1, 2, 3, 5, 7, and 8. FAs 5 and 7, which occur at the base of the upper member, are interpreted as tidally influenced channels and suggest a marine incursion during deposition of the upper member. The capping sandstone is characterized by FAs 3, 4, and 6, and is interpreted to represent a major change in depositional environment, from meandering river systems in the lower three members to a low-accommodation, braided river system. Combined results of facies and palaeosol analyses suggest that the overall climatic conditions in which the Wahweap Formation was deposited were generally wet but seasonally arid, and that iv conditions became increasingly moist from the time of lower member deposition up to the time of middle member deposition. Improved age constraints were obtained for the Wahweap Formation by radiometric dating of two devitrified ash beds (bentonites). This allowed for deposition to be bracketed between approximately 81 Ma and 76 Ma. This age bracket has two important implications: firstly, it shows that the Wahweap Formation is synchronous with fossiliferous deposits of the Judithian North American Land Mammal Age, despite subtle differences in faunal content. Secondly, it shows that the middle and upper members were deposited during the putatively eustatic Claggett transgression (T8 of Kauffman 1977) in the adjacent Western Interior Seaway. This is consistent with facies analysis which shows a marked increase in tidally-influenced sedimentary structures and trace fossils at the top of the middle and base of the upper members. Following recent alluvial sequence stratigraphic models, the middle member is interpreted as the isolated fluvial facies tract, while the upper member represents the tidally influenced and highstand facies tracts. Maximum transgression occurred during deposition of the lowest part of the upper member, synchronous with the Claggett highstand in other parts of the Western Interior Basin. The sequence boundary is placed at the base of the overlying capping sandstone member, diagnosed by a major shift in petrography and paleocurrent direction, as well as up to 4 m of fluvial incision into the underlying upper member. The capping sandstone member is interpreted as the amalgamated fluvial facies tract of an overlying sequence. Analysis of the western-most exposures of the Wahweap Formation on the Markagunt and Paunsaugunt plateaus shows facies variations in the proximal and distal parts of the central Western Interior Basin. The inconsistent thickness and variations in fluvial architecture, as well as the presence of unconformities and generally poor exposure in the west, hinder correlation attempts and also prevent the subdivision of the Wahweap Formation into members. Only the capping sandstone, which can be positively identified west of the Paunsaugunt fault, has a consistent thickness and fluvial architecture across the west-east extent of the Wahweap Formation. The capping sandstone also bears remarkable lithological similarity to the Tarantula Mesa Formation which is exposed to the east in the Henry Mountains Syncline, and it is suggested that these two units be equated under the name “Tarantula Mesa Formation”, which has precedence.
Ph.D., Faculty of Science, University of the Witwatersrand, 2011
Paleontology, cretaceous, Geology, stratigraphic, cretaceous