Abstract
Medium to coarse-grained lithic sandstones and lithic-pebble conglomerates of the Bald Eagle and Juniata Formations contain recycled sedimentary, low-grade metamorphic, and volcaniclastic detritus that was shed northwestward in an alluvial and/or fluvial system from an uplift produced during the Taconian orogeny.
Intermediate to felsic volcaniclastic rock fragments comprise approximately 3% of the framework grains. They consist of devitrified shards, partially kaolinized plagioclase laths, and biotite in a cryptocrystalline groundmass. Siltstone, mudstone, chert, radiolarian chert, and recycled grains are indicative of a sedimentary source. Chlorite-quartz, epidotequartz, and muscovite-chlorite phyllite clasts indicate a low-grade metamorphic provenance. Quartz varieties plotted on a diagram are consistent with a low-grade metamorphic provenance. Metaquartzite, slate, and cataclastic lithic fragments indicate tectonic deformation in the source area.
Provenance criteria support a plate tectonic model involving an eastward-dipping subduction zone in the central Appalachians during the Middle to Late Ordovician. The climax of the Taconian orogeny has been described as a collision of an island arc with the North American continental margin. Petrologic data from the Bald Eagle and Juniata Formations suggest that this collision was accompanied by uplift of accretionary prism and continental margin or forearc basin sediments prior to the Richmondian. The presence of sedimentary, low-grade metamorphic, cataclastic, and volcaniclastic lithic fragments, and the notable absence of higher grade metamorphic and plutonic grain-types, indicate unroofing at relatively shallow levels of the orogene.
