DECIPHERING THE CONTROLS ON SYNOROGENIC SEDIMENT COMPOSITION DURING THE PENNSYLVANIAN ALLEGHENY OROGENY

A variety of interpretations have been developed to explain the generation of quartz-rich synorogenic clastic sediment found across the Allegheny foreland. Some models implicate intense chemical weathering, whereas other models suggest sediment reworking by alluvial processes and still others implicate erosion of recycled sedimentary source rocks uplifted in the Allegheny orogen. The alluvial deposits in the Middle Pennsylvanian Allegheny Formation of central WV represent among the most proximal part of the preserved foreland basin system, and can be used to examine these competing models. Sandstone samples from a ~50 m section were subjected to modal framework grain compositional analysis. Lower sandstones were deposited by shallow, multi-thread channels, consistent with seasonally dry conditions. Sandstone composition through this interval reflects variable quartz abundance ranging between 55% to 80%. Upsection, sandstone sedimentology reflects deeper, single-thread channel morphologies, consistent with the development of wetter conditions. The lowermost sandstones in this interval contain markedly higher quartz abundances, with 94% of grains composed of monocrystalline quartz. However, younger single-thread channel sandstones contain much lower monocrystalline quartz abundances, with compositions ranging from 65-74%. The youngest sandstone sampled dropped to 55% monocrystalline quartz. Additionally, in all but 2 samples, feldspar content exceeded lithic content, indicating a lack of recycled sedimentary input and instead suggests derivation from continental block-type source rocks. These results may suggest weathered grus and soils mantling the Blue Ridge Thrust Complex that had accumulated during a period of stagnated uplift had become tapped by hinterland sediment transport systems. These systems delivered an initial influx of quartz-rich sediment that over time became less quartzose as unweathered bedrock became eroded by fluvial incision. Overall, results depict a high degree of compositional variability in proximal sandstone composition that suggests complex and uncertain interplay between the depositional system, climate regime and erosion of the synorogenic sediment source.