A Late Mississippian synsedimentary fault system in an incised paleovalley: A re-examination of Thomas’ early work on the Hurricane Ridge syncline, southern West Virginia

ABSTRACT Upper Mississippian strata in southern West Virginia and southwestern Virginia include several previously reported examples of soft-sediment deformation. New highway construction has exposed a >190-m-long, 10-m-high incised paleovalley in the red member of the Upper Mississippian Bluestone Formation that is filled with fluvial-interfluvial strata displaced by a synsedimentary system of down-to-the-north listric normal faults. Hanging-wall sandstones are folded into a rollover geometry that shows stratal growth in overlying interfluvial red mudstones and interbedded sandstones. Fault-system slip was interpreted using a soft-domino model where faults were broadly synchronous and kinematically linked through ductile strain in each hanging wall. The downdip extent of the fault system is limited by a multistory stack of channel-form, quartzose sandstones. Fault displacement dmax versus estimated fault length L data were used to model the channel sandstones as prelithified sediment; results show the channel sands had higher elastic moduli μ and fault frictional strength (σ0 – σf), and were more resistant to brittle faulting and ductile soft-sediment folding in listric hanging walls than more mud-rich sands that dominated sedimentary fill in the incised valley. Therefore, local depositional conditions such as paleovalley geometry and internal sedimentary facies distribution appear to have controlled the geometry of the fault system. Exogenic versus endogenic triggers were evaluated within the context of a widely interpreted Late Mississippian semiarid climate, which limits conditions favorable for liquefaction in comparison to humid, ever-wet climates. An endogenic, climate-moderated model attributes fault initiation to seasonal flood-induced sediment loading and explains continued stratal growth through time. The listric fault system was compared to previously reported soft-sediment deformation within the Bluestone Formation, including a 75-m-long, 6-m-high partial exposure of a paleoslump at a similar stratigraphic position 2 km to the west. There, deformation exhibited semilocalized mass flow rather than localization along faults and fault-related folds.