- GeoRef, Copyright 2008, American Geological Institute.
The Pride Shale is a 60-m-thick succession of thinly interlaminated dark shales, siltstones, and fine-grained sandstones within the Bluestone Formation (Upper Mississippian) of southern West Virginia. A hierarchy of submillimeter- to meter-scale cycles in this succession preserves a spectrum of tidal and climatic periodicities. Sub-millimeter-thick, fine-grained sandstone/shale or siltstone/shale couplets are interpreted as the product of suspension settling associated with individual ebb-tidal flows. Up to 17 couplets systematically thicken and thin within millimeter- to centimeter-scale bundles interpreted as neap-spring tidal cycles. Successive neap-spring cycles display a thick-thin relationship interpreted to reflect unequal perigean and apogean tides. The fine scale and abbreviated character of these microlaminated rhythmites is suggestive of a distal, subtidal setting wherein subordinate daily and neap and apogean ebb flows were generally of insufficient strength to transport sand and silt. Decimeter-scale cycles in the Pride Shale are manifested by the progressive upward thickening and thinning of up to 18 millimeter- to centimeter-thick (neap-spring) cycles. This bundling is interpreted to reflect a climatic (monsoonal) signal which defines that part of the year when adequate fluvial-tidal discharge forced suspended sediment into the basin. Lastly, a multi-year cyclicity is recognized in thick exposures where 17-22 annual beds display a crude upward thickening and thinning within meter-scale bundles. This bundling is interpreted to reflect the 18.6-year lunar nodal cycle, with the thick annual beds representing years during which the inclination of the lunar orbital plane favored increased tidal amplitudes. Decompacted thicknesses of 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 the marginal infill of large slump scars. The stratigraphic relationships and sedimentation rates for the Pride Shale are consistent with paleontological evidence of rapid sedimentation in turbid waters seaward of multiple points of fluvial discharge. The Pride Shale is interpreted as an extensive prodeltaic deposit which, in places, records hundreds of years of nearly continuous tidal sedimentation along the eastern margin of the Appalachian Basin.