PETROGRAPHIC SIMILARITIES BETWEEN FRAMEWORK GRAINS IN ORDOVICIAN ARENITES FROM THE BLOUNT AND QUEENSTON MOLASSE OF THE TACONIC OROGENY, VIRGINIA APPALACHIANS

Two pebbly to locally conglomeratic Ordovician sandstones in the Blount (older) and Queenston (younger) molasse sediments associated with deposition in the Taconic foredeep share some similarities in framework grain composition. Thin sections of the Walker Mountain Sandstone (Blount molasse) from 11 localities and of the Oswego Sandstone (Queenston molasse) from 4 localities have been studied and point counted. The Walker Mountain Sandstone is a quartz arenite, with some sublitharenite horizons and zones; the Q_tFL plot shows that these samples plot in the Recycled Orogenic field, and the Q_mFL_t plot shows that most samples plot in the Quartzose Recycled field. The Oswego Sandstone is a sublitharenite to lithic arenite; the Q_tFL plot shows that these samples likewise plot in the Recycled Orogenic field, but the Q_mFL_t plot shows that these samples plot in both the Quartzose Recycled and Transitional Recycled fields. In both sandstones, monocrystalline quartz is the most common framework grain, with grains of stretched and unstretched polycrystalline quartz, vein quartz with inclusions of vermicular chlorite, chert including silicified ooids, and sandstone and siltstone sedimentary rock fragments (SRFs) also present. Framework grains unique to the Walker Mountain Sandstone include a tourmaline schist metamorphic rock fragment (MRF), jasper, banded agate, carbonate SRFs of varying texture, and vein quartz with specular hematite inclusions. Framework grains unique to the Oswego Sandstone include many pelitic rock fragments that are either SRFs or very low grade MRFs. This assemblage suggests that both sandstones at times received sediment derived from an eroding terrane in the Taconic Highlands that included older sedimentary rock, low-grade to high-grade metamorphic rock, hydrothermally altered rock, and even perhaps pegmatite, and that this terrane persisted as a source of siliciclastic sediments along the tectonically active eastern or southeastern margin of the Taconic foredeep even though the time that elapsed between deposition of the Walker Mountain Sandstone and deposition of the Oswego Sandstone is perhaps as much as 6? to 8? m.y.