A RECORD OF ACADIAN EXHUMATION: DEVONIAN CONGLOMERATES AND FORELAND BASIN MIGRATION, NEW YORK STATE

Conglomerates are scattered through New York’s Devonian strata, deposited during the Acadian Orogeny from ca. 410 to at least 370 Ma (Ogg et al. 2012 time scale). Changes in pebble composition over time reflect several factors, including exhumation history of the orogen, flexure and cratonward migration of the orogen and foreland basin over time, and possibly varied geographic sources along the orogen. Research is largely focused in eastern New York (Hudson Valley, Catskills), between ca. 410 to 372 Ma. Additional Devonian conglomerates (post-ca. 372 Ma) occur in western New York. The most common siliciclastic pebbles and granules are milky quartz, sandstone/metasandstone and in one interval, chert. Uncommon to rare clasts include jasper, low grade metamorphic rocks, rare igneous rocks, and others.

In part, shifts in conglomerate composition coming into New York during the Acadian orogeny reflect changing source areas. Exclusively milky quartz conglomerates and isolated pebbles between ca. 410 and 390 Ma appear to represent reworking of older intrabasinal sedimentary deposits as the back-bulge basin, a low relief bulge, and the distal margin of the foredeep progressively migrated into the state from the east, through successive quiescent to tectonically active to quiescent stages in the orogen (Oriskany to Onondaga Fms.).

With onset of renewed orogenic activity after 390 Ma, the foredeep migrates further into New York, bringing orogen-derived sediments from western New England (Hamilton Grp. and younger strata). Initial conglomerates consist of dominantly pure milky quartz and chert (ca. 388 Ma). By ca. 385 Ma, chert has essentially disappeared; milky quartz and sandstone clasts dominate. Similar gravels occur across the Middle-Late Devonian boundary in the Catskills (ca. 383 Ma); however, increased metamorphic influences are visible in both uncommon to common clasts (i.e., phyllite +/- slate, and some chlorite as inclusions in milky quartz or matrix in sandstone pebbles). Greenschist-grade metamorphic indicators increase upsection to highest strata in the Catskills (Slide Mountain Fm.).

Western New York conglomerates (post ca. 372 Ma) are again dominated by milky quartz. It is unclear if these younger, more distal gravels are sourced from similar or different points of origin along the orogen.