SEDIMENTS DERIVED FROM MOUNTAIN CHAINS DO NOT ALWAYS LAND IN FOREDEEPS: SOUTHERN APPALACHIAN SEVIER-BLOUNTIAN BASIN DYNAMICS

The Middle Ordovician southern Appalachian Sevier–Blountian clastic wedge filled a basin that extended from GA to VA. From TN into VA the Late Ordovician Martinsburg basin, likely the central Appalachian foredeep, is a successor atop the Sevier–Blountian basin. Clastics in both were derived from the southern Appalachian-central Appalachian internides, earlier termed “tectonic land,” Taconian highlands, or obducted arc; both basins were called foredeeps. Clasts in conglomerates at the same stratigraphic level on the eastern flank of the basin are composed predominantly of Cambro-Ordovician platform carbonates, with some 1.1 Ga basement, and Lower Cambrian rifted-margin clastics. Twenty-four Middle Ordovician Sevier Shale detrital zircon ion microprobe analyses contain only pre-Paleozoic cores, duplicating ages from nearby rifted margin detrital zircon analyses. Detrital zircon analyses from Ordovician to Pennsylvanian clastics derived from the southern Appalachian and central Appalachian internides (Gray and Zeitler; Eriksson et al.; Becker et al.) reveal few Paleozoic cores until the Carboniferous, indicating late Paleozoic unroofing of the southern and central Appalachians. The easterly source must be an uplifted part of the Laurentian rifted margin—probably the peripheral bulge in front of the obducting Early to Middle Ordovician (480-460 Ma) volcanic arc-ocean crust. Palinspastic reconstructions of the Alleghanian foreland fold-thrust belt, Blue Ridge-Piedmont thrust sheet, and pre-Alleghanian terranes, require that the Sevier–Blountian basin formed as much as 300 km from the Middle Ordovician Laurentian margin, not adjacent to the obducted oceanic crust. The Sevier–Blountian basin is more likely a modified back-bulge basin, more similar to that NW of the Taranaki retroarc foredeep NW of New Zealand than to a classic foredeep. A better candidate for the southern Appalachian Middle Ordovician foredeep basin is the Mineral Bluff sequence preserved above an unconformity in the Murphy syncline in the Blue Ridge-Piedmont sheet in NC and GA. Paleozoic uplift of the Cincinnati arch peripheral bulge may have begun with Late Middle Ordovician to Late Ordovician uplift in response to formation of the Martinsburg foredeep.