Depositional facies and interpretation of salt-sediment interaction in the lower Triassic Moenkopi formation adjacent to the Castle Valley Salt Wall, Paradox Basin, Utah

Stratigraphic, sedimentologic and compositional data collected from the Triassic Moenkopi Formation adjacent to the Castle Valley salt anticline, Paradox Basin, Utah indicate passive diapirism of the salt wall with well-developed topography and exposed salt during the Early Triassic. Moenkopi strata thin to 150m on to the Castle Valley salt anticline and thicken to an estimated 275m away from it, indicating control of sedimentation by salt-generated topography. The distribution of nine terrestrial facies assemblages within Moenkopi Formation strata on the margins of Castle Valley Salt Wall, in combination with identification of three halokinetic sequences interpreted from stratal architecture adjacent to the salt wall, further support this assertion. Nine terrestrial facies were recognized within the Moenkopi Formation in the study area: 1) channel-form sandstone and organized conglomerate (axial fluvial), 2) thinly-interbedded siltstone and fine-grained sandstone (fluvial overbank), 3) tabular-lenticular siltstone and sandstone sheets (unconfined channels and sheet floods), 4) structureless to weakly-bedded sandstone (distal wadi fan), 5) disorganized conglomerate (debris flow from exposed diapir), 6) well-sorted, well-rounded sandstone (eolian sand sheets), 7) gypsic sandstone (eolian sand), 8) gypsic paleosols and 9) calcic paleosols. The location and distribution of axial channel facies, in addition to paleocurrents that are generally parallel to the northwest trending long axis of the salt wall, suggest that large drainage systems were controlled by salt diapir topography. At the northwest termination of Castle Valley, the salt wall bends abruptly to the north, where paleocurrents also shifted to the north. Local gypsum sheet sandstones and thin debris-flow beds containing reworked clasts of gypsum and carbonate “caprock” derived from the diapiric Paradox Formation of Pennsylvanian age provide evidence for significant topography and exposed evaporite during Moenkopi deposition. The salt diapir itself is therefore interpreted to have provided the sulfate source for both gypsic paleosols and gypsic sandstone sheets in the Moenkopi. The world\u27s oldest documented gypsic paleosols are found in overbank strata interbedded with high width/depth ratio sandstone bodies deposited by ephemeral rivers. In combination, these features indicate arid to semi-arid, wadi environments for the lower three Moenkopi Formation members (Tenderfoot, Ali Baba and Sewemup). The uppermost Pariott Member lacks gypsum and contains calcic paleosols, indicating a change to a semi-arid climate. This data in combination with facies differences between the uppermost member and the lower three members suggest that the Pariott Member should not be assigned to the Moenkopi Formation and should either be included in the Chinle Formation strata or reassigned to formation status. Each member is characterized by distinct facies associations, facies-association distributions and stratal geometries. As such, each of the four members represents a stage of salt-sediment deposition with corresponding wedge and hook halokinetic sequences (HS) identified. The HS comprise three composite halokinetic sequences (CHS): a 1) northeast flank of wedge HS that build into a tapered CHS, 2) a western Red Hills series of wedge HS that build into a tapered CHS and 3) an eastern Red Hills series of hook HS that build into tabular CHS. Wedge HS indicate that sediment accumulation rates were greater than diapiric rise rates to the northeast and to the west-southwest of the salt wall. The hook HS on the east side of the weld, where abrupt facies changes occur and strata thickens away significantly, indicates the area experienced higher diapiric rise rate than sediment accumulation, most likely due to deflation related to a sharp bend in the salt wall. Supplemental: Plate 1: Geologic map of the Moenkopi Formation members, northwest Castle Valley, Utah Plate 2: Correlated logged sections of the Triassic Moenkopi Formation, northwest Castle Valley, Uta