UNROOFING THE ACADIAN OROGEN: COMPOSITION AND PROVENANCE OF DEVONIAN CLASTICS, NEW YORK STATE

The unroofing history of an orogen is recorded in the synorogenic detritus (conglomerate, sandstone and mudrocks) deposited in the adjacent foreland basin. Beginning in the Late Silurian, collision of eastern North America and another landmass initiated the Acadian orogeny.

New conglomerate data from New York provides perspectives on changing clastic composition and Acadian unroofing history/provenance over approximately 40 million years (ca. 410-370 million years ago). Initial milky quartz gravels (ca. 410, 395 Ma; upper Pragian-lower Eifelian) were succeeded compositionally by mixed chert- and quartz-rich gravels (ca. 388 Ma; lower Givetian), to slightly more quartz-rich gravels with abundant sandstone-/low grade metasandstone pebbles and rare volcanic and granulite-grade clasts (ca. 383-376 Ma; Frasnian). The youngest conglomerates from this study (ca. 376-370 Ma; lower to mid Famennian) continue to show increasing quartz content, culminating in nearly pure milky quartz compositions approaching those of the oldest conglomerates. Overall, Middle to lower Upper Devonian conglomerates indicate unroofing of dominantly sedimentary and low grade metamorphic rocks in the orogen.

These findings agree with sandstone compositional data from Middle to Devonian strata in the Catskills, interpreted to be largely derived from sedimentary and chlorite-rich, low grade metamorphic rocks in the Acadian orogen. Clastic rock compositions show that few magmatic or high-grade metamorphic rocks were exposed in the orogen, within the paleodrainage basins feeding into New York. No major changes in sandstone composition occurred between ca. 388-376 Ma (Mount Marion to Slide Mountain fms.), reflecting no major changes in Acadian provenance within the paleodrainage basins feeding into the Catskills. A hidden source of abundant smectitic clays (indicated by clay mineralogy and common vertisol-type paleosols) may have been sourced from weathering igneous rocks in the orogen, or airborne volcanic ash.

Recent geochronologic and geothermometry studies by various authors portray a complex picture of sediment provenance (e.g., detrital zircons and micas yield ages from 1230 to 387 Ma). These stratigraphically- and regionally-limited analyses need to be expanded, along with more traditional petrologic study.