ABSTRACT

Claystone paleosols with abundant vertic features occur in Upper Ordovician (Ashgillian) red beds of the Juniata Formation in the southern Appalachians. The Beans Gap Claystone paleosol formed during Late Ordovician pedogenesis of estuarine and upper tidal flat mud deposits. Pedogenic processes were dominated by shrinking and swelling of clay in a subtropical to warm-temperate (paleolatitude 30-35° south) paleoclimate characterized by a seasonal moisture deficit (dry 4-8 months/year). Large-scale pedogenic slickensides are abundant and locally define pseudo-anticline structures. The predominance of sepic paleosol micro-fabrics compares favorably with Holocene Vertisol micromorphology. Post-pedogenic marine transgression resulted in erosion of the upper parts of paleosols, followed by marine bioturbation and introduction of marine fluids. Changes in whole-rock chemistry associated with early marine diagenesis include elevated Na₂O, CaO, MgO, MnO, and P₂O₅ in the preserved top of each paleosol; reduction of Fe³⁺ to Fe²⁺ resulted in a decrease in Fe₂O₃ and formation of "drab halos" around burrows. The original smectitic mineralogy was converted to a well-ordered 2M₁ illite through progressive burial diagenesis.