SANDSTONE PETROGRAPHY OF MIDDLE TO UPPER DEVONIAN APPALACHIAN FORELAND BASIN FILL IN THE CATSKILL FRONT, NEW YORK

The Middle to Upper Devonian strata of the Catskill Front in southeastern New York are deltaic plain sediments deposited within the Acadian foreland basin system. This study focuses on a vertical succession within the Catskill Front that begins with the oldest Plattekill Formation, into the overlying Manorkill Formation, and finishes in the youngest Oneonta Formation. Petrographic analysis with a minimum of 400 grains counted for 17 samples was conducted on medium to coarse-grained litharenite sandstones in these units. The Plattekill Formation has an average total quartz (Q), feldspar (F), and lithic grain (L) percentage of 44:2:54. The Manorkill Formation has an average Q:F:L of 49:1:50. The Oneonta Formation has an average Q:F:L of 54:1:45. Quartz includes monocrystalline, polycrystalline, and microcrystalline varieties. Feldspar includes plagioclase, orthoclase, and perthite. Lithic grains include foliated quartz-mica and fine mica-clay (low grade metamorphic), siltstone, carbonate, shale, and chert. Silica and hematite cement is present.

Petrographic analysis reveals the following trends in detrital framework grain composition from the Plattekill through the Oneonta Formations: an increase in monocrystalline and polycrystalline quartz, a decrease in fine-grained foliated metamorphic lithic grains (relatively low-grade varieties), and a small decrease in sedimentary lithic grains. These compositional changes suggest dissection through sedimentary strata (sandstones, siltstones, shales, carbonates, and cherts) and lower-grade metamorphic rocks (slates and phyllites) into higher-grade muscovite-quartz-chlorite schists and quartz-rich gneisses (represented by the increase in polycrystalline quartz). The source area for these litharenite sandstones is likely to have been within the thrust sheets of the advancing Acadian orogen to the east of the foreland basin system. These thrust sheets would have been comprised of sedimentary strata, underlain by regionally metamorphosed lower-grade micaceous rocks and higher-grade polycrystalline-quartz-rich rocks. The observed trends in the Catskill sandstones suggest that the higher-grade rocks became increasingly prevalent within the source area through time.