U-PB DETRITAL ZIRCON (DZ) GEOCHRONOLOGY OF LOWER JURASSIC NEWARK BASIN PALEO-ALLUVIAL FANS, ATLANTIC SLOPE DRAINAGE DEVELOPMENT, AND DYNAMIC INTERACTIONS IN THE POST-OROGENIC LANDSCAPE EVOLUTION OF THE APPALACHIANS

Detrital zircon (DZ) geochronology is an emergent tool in the study of Earth surface and deep Earth process interactions. Here we report U-Pb ages on detrital zircons (DZ) from a Lower Jurassic paleo-alluvial fan exposed in the Newark Basin along the Delaware River in Holland Township, NJ to document the unroofing of the Appalachian foreland, reconstruct Atlantic slope syn-rift-flank drainage, and speculate on the unsteady expansion of that drainage westward. Paleo-alluvial fan facies are preserved in the hanging wall basin in contact with the normal fault that uplifts Grenville basement and lower Paleozoic sediments, including carbonates, in the footwall. Fan stratigraphy records an uroofed sequence of quartzite pebble conglomerate of unknown provenance superseded by a limestone fanglomerate, also containing basement clasts, sourced in the proximal footwall. We extracted zircons from sandstone beds in the quartzite pebble conglomerate and conducted U-Pb geochronology with the LA-MC-ICPMS at the Arizona LaserChron Center. Nearly all DZ ages fall on Concordia and the age spectra are a mix of Taconic (~450 Ma) and Grenville (~1100-1400 Ma) ages, results comparable to published DZ ages of Pennsylvanian and younger clastic rocks in the Appalachian foreland and most closely matching those of Permian-aged clastics preserved near Pittsburgh, PA. These results permit contrasting geodynamic-surface process interpretations based on the evolution of the Atlantic slope drainage. The Appalachian foreland may have been reduced by erosion in the Permo-Triassic to its current structural level of exposure, and lower than present relief, prior to rifting, with the Atlantic slope drainage rapidly pushing westward and superimposing, where it has remained nearly unchanged for ~180 Ma. The alternative explanation, and the one we currently favor, is that a late Paleozoic molassic cover of quartzite pebble conglomerate was unroofed syn-rift, a portion of which is now preserved in the paleo-alluvial fans. In this scenario, the Atlantic slope drainage remained pinned to the proximal footwall syn-rift, but has since been unsteadily jumping westward, driven by erosion, flexural, and dynamic topography interactions, underscoring the longevity of such processes in the post-orogenic phase of mountains and their forelands.