STRATIGRAPHY AND SEDIMENTOLOGY OF THE UPPER MISSISSIPPIAN PRIDE SHALE IN THE APPALACHIAN BASIN

The Upper Mississippian Pride Shale developed following a regional transgression and is an extensive prodeltaic deposit which, in places, records hundreds of years of nearly continuous sedimentation. It represents the most extensive tidal rhythmite succession identified in the geologic record. Submillimeter- to meter-scale cycles in the Pride Shale record a hierarchical bundling of tidal- and climate-forced processes. Sandstone/siltstone-shale couplets between 0.01 and 1 mm thick are interpreted as suspension fallout deposits related to semi-diurnal, ebb-tidal flows associated with the dominant tide of the day. Rare thick-thin pairs record diurnal dominant and subordinate tides. Up to 17 couplets display a progressive upward thickening and thinning in 0.1 to 3 cm thick neap-spring (semimonthly) tidal cycles. Successive neap-spring cycles are thicker and thinner, and are interpreted to reflect unequal perigean and apogean tides. Up to 18 neap-spring cycles are arranged in upward thickening and thinning bundles that range from 2 to 50 cm thick and reflect annual cyclicity. Annual cyclicity is interpreted to reflect climatic (seasonal) driven cycles related to monsoonal and intermonsoonal periods. Thicker, coarser-grained neap-spring cycles are interpreted to reflect the monsoonal season and thinner, finer-grained cycles record the inter-monsoonal period. Previously recognized and recently mapped, concave-up discontinuities in laterally extensive (100s of meters) outcrops along Interstate 77 at Camp Creek, West Virginia are interpreted as slump scar features related to over steepening of the depositional interface. Decompacted thicknesses of annual cycles indicate that accumulation rates for the Pride Shale typically ranged between 3 and 20 cm per year, but reached over 60 cm per year where sandy rhythmites developed as marginal infills of slump scars. Stable isotope data (δ¹³C and δ¹⁴N) reveal a strong marine signal at the base and a prominent terrestrial signal through the remainder of the Pride Shale. Ongoing research aims to investigate whether stable isotope variations can be used as a proxy for annual cyclicity.