ND ISOTOPIC CONSTRAINTS ON SEDIMENT SOURCES OF THE OUACHITA-MARATHON FOLD BELT

Nd isotopes for the overthrust deep-marine Ouachita-Marathon sedimenta ry assemblage of Arkansas-Oklahoma and west Texas, and associated Pale ozoic shelf and foreland deposits, resolve into three distinct populat ions: (1) Lower to Middle Ordovician, epsilon(Nd) = -13 to -16 (averag e T-DM = 2.0 Ga); (2) Upper Ordovician to Pennsylvanian, epsilon(Nd) = -6 to -10 (average T-DM = 1.6 Ga); and (3) Mississippian tuffs, epsil on(Nd) = -1 to -3 (average T-DM = 1.1 Ga). A rapid shift in epsilon(Nd ) from -15 (passive margin shales) to -7 (orogenic turbidites) in the Ouachita assemblage at ca. 450 Ma implies termination of craton-domina ted sources and the emergence of the Appalachian orogen as the primary source of sediment for sea floor lying south of North America. This c onnection is reinforced by Nd isotopes in Ordovician-Silurian turbidit es from both the Ouachita assemblage and the southern Appalachian Sevi er-Martinsburg (Taconic) foredeep, which are identical (epsilon(Nd) = -7 to -9). The post-450 Ma Ouachita assemblage falls along a single Nd isotopic trend that, significantly, is not deflected by onset of Carb oniferous flysch (epsilon(Nd) = -7 to -10) sedimentation nor by associ ated regional volcanism. The less negative epsilon(Nd) (-2) of Mississ ippian ash-flow tuffs that erupted from arc(s) to the south probably r esulted from isotopic mixing of old (Precambrian) crust with young, ma ntle-derived components within a continental margin are There is littl e isotopic, trace element, or petrographic evidence for any significan t volcaniclastic detritus in the Carboniferous turbidites, indicating that volcanic are sources were minimal. Nd isotopes in fluvio-deltaic strata of the Ouachita-Appalachian foreland and continental interior, that is, Arkoma, Illinois, and Black Warrior basins (epsilon(Nd) = -7 to -10), imply that continental margin pathways and interior basins re ceived the same detritus as the Ouachita trough by Pennsylvanian time. These data are consistent with a composite Carboniferous Ouachita sub marine fan complex built down the axis of a remnant ocean basin from v aried mature/immature delivery systems tapping dominantly Appalachian fold-thrust belt sources to the east (Graham et al., 1975). Carbonifer ous turbidites from the Marathon fold belt (west Texas), which are iso topically similar (epsilon(Nd) = -8 to -11) to Ouachita turbidites, ma y have been ultimately derived from similar sources; however, they pro bably do not represent merely distal turbidites of a Ouachita fan comp lex. It is suggested that dominantly Appalachian-derived detritus, aug mented by uplifted plutonic and fold-thrust belt sources south of the Marathon basin, was swept up into subduction complexes on the north si de of the approaching are and recycled along the collision zone.