LATE DEVONIAN SEDIMENTARY RECORD OF APPALACHIAN TECTONICS AND EROSION: GEOCHRONOLOGY AND GEOCHEMISTRY OF DETRITAL MUSCOVITE AND ZIRCON

Foreland sediment deposition in North-Central Pennsylvania during the Late Devonian records active collisional tectonics of the Acadian orogeny. The Catskill clastic wedge preserves a coarsening-upward stratigraphic succession from marine to fluvial environments associated with sediment progradation and sea-level regression, yet depositional timescales and geochemical affinity of detritus are currently unknown.

New U/Pb detrital zircon ages (n=737) from six sandstones spanning >2 km of stratigraphy overlap with ages reported from Appalachian bedrock sources exposed in the hinterland. Taconic (440–490 Ma, 13%), Grenville (900–1200 Ma, 46%), and Pre-Grenville (1201–2950 Ma, 26%) age populations and synorogenic Acadian (350–420 Ma, 4%) ages make up the zircon age spectra. New ⁴⁰Ar/³⁹Ar detrital muscovite ages (358–475 Ma, n=341) from three sandstones are chiefly Acadian (93%). The youngest population of detrital zircon ages from the lowermost sample indicates a maximum depositional age of ~369 Ma. The youngest population of detrital muscovite ages from the uppermost sample yields a maximum depositional age of ~361 Ma. These new maximum depositional ages are consistent with Late Devonian fossil vertebrates and palynomorphs reported from the sampled strata.

LA-ICP-MS geochemistry of detrital muscovite and muscovite within schistose lithic grains from eight sandstones implies mixed igneous and metamorphic sources. Detrital muscovite compositions with higher Ti, Ga, Tl, Sn, Cs, and negatively-sloped REEs (relative to chondrite) indicate igneous provenance and include Li-enriched pegmatitic sources. Schistose muscovite grains have lower K, higher Mg, lower Ga, Tl, Sn, Cs and Li, and flat REE profiles.

Collectively, the new geochronologic and geochemical data from detrital minerals indicate erosion of igneous and metamorphic hinterland sources to the east and transport westward into the foreland basin, consistent with previously reported paleocurrent and lithofacies trends. The <5 m.y. lag time between cooling of Late Devonian detrital muscovite and deposition in Late Devonian sediments implies rapid exhumation/erosion, consistent with Late Devonian muscovite cooling ages reported from hinterland bedrock sources and sedimentologic evidence for regional glaciation.