DETRITAL CHRONOSTRATIGRAPHIC CONSTRAINTS ON UPPER DEVONIAN TETRAPOD AND FISH ASSEMBLAGES IN CATSKILL FORMATION SITES OF NORTH-CENTRAL PENNSYLVANIA: RECONSTRUCTING LATE DEVONIAN ECOSYSTEMS AND ENVIRONMENTS

Outcrops of the Upper Devonian Catskill Formation in north-central Pennsylvania, including the well-studied Red Hill site, yield diverse fossil fauna and flora that provide insight on the radiation of animals in freshwater and terrestrial settings, including the fin to limb transition in vertebrates. Fossil zones discovered recently in the upper Catskill Formation in the Steam Valley and Trout Run areas of Lycoming County yield diverse fossil remains of placoderms (Bothriolepis, Turrisaspis), acanthodians (Gyracanthus), sarcopterygian fishes (Holoptychius, Eusthenodon, megalicthyids), a whatcheerid-like tetrapod, and Archanodon bivalves. Fossil remains occur in upward-fining successions, with lenticular sandstones interpreted as the deposits of channel-bar complexes and mudrocks with evidence for pedogenic overprinting and ponded water interpreted as floodplain/overbank deposits. Transported fossil fragments accumulated as bedload lags on point bars and channel bottoms. Less fragmented fossils occur in flat-bottomed sandstones capped by mudcracks and rootlets interpreted as temporary, aqueous habitats (crevasse channels/splays). Abrupt facies changes and gaps in bedrock exposure have hampered correlation of vertebrate assemblages between sites. ⁴⁰Ar/³⁹Ar ages from detrital muscovites provide improved chronostratigraphic relations. Samples from Red Hill, Steam Valley, and Trout Run North yield chiefly Devonian muscovite ages (92% of ~338 grains) and youngest age clusters of ~365 Ma, ~367 Ma, and ~378 Ma, respectively. Detrital populations differ significantly between samples, indicating that fossiliferous horizons likely do not represent coeval, depositionally linked rivers. Faunal variations among Catskill sites may be explained by differences in habitats, taphonomy, or migration/evolution of different vertebrate taxa among sites. The brief (<5 m.y.) lag time between cooling of the youngest detrital muscovites (~365-367 Ma) below the muscovite ⁴⁰Ar/³⁹Ar closure temperature (~400°C) and deposition by ~361 Ma based on biostratigraphy, indicates rapid exhumation/erosion of bedrock sources. Rapid exhumation/erosion is consistent with alluvial progradation that resulted in a >3-km-thick coarsening upward sedimentary succession.