Abstract

Nd isotopes for the overthrust deep-marine Ouachita-Marathon sedimentary assemblage of Arkansas-Oklahoma and west Texas, and associated Paleozoic shelf and foreland deposits, resolve into three distinct populations: (1) Lower to Middle Ordovician, ϵNd = −13 to −16 (average TDM = 2.0 Ga); (2) Upper Ordovician to Pennsylvanian, ϵNd = −6 to −10 (average TDM = 1.6 Ga); and (3) Mississippian tuffs, ϵNd = −1 to −3 (average TDM = 1.1 Ga). A rapid shift in ϵNd from −15 (passive margin shales) to −7 (orogenic turbidites) in the Ouachita assemblage at ca. 450 Ma implies termination of craton-dominated sources and the emergence of the Appalachian orogen as the primary source of sediment for sea floor lying south of North America. This connection is reinforced by Nd isotopes in Ordovician-Silurian turbidites from both the Ouachita assemblage and the southern Appalachian Sevier-Martinsburg (Taconic) foredeep, which are identical (ϵNd = −7 to −9). The post–450 Ma Ouachita assemblage falls along a single Nd isotopic trend that, significantly, is not deflected by onset of Carboniferous flysch (ϵNd = −7 to −10) sedimentation nor by associated regional volcanism. The less negative ϵNd (−2) of Mississippian ash-flow tuffs that erupted from arc(s) to the south probably resulted from isotopic mixing of old (Precambrian) crust with young, mantle-derived components within a continental margin arc. There is little isotopic, trace element, or petrographic evidence for any significant volcaniclastic detritus in the Carboniferous turbidites, indicating that volcanic arc 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 (ϵNd = −7 to −10), imply that continental margin pathways and interior basins received the same detritus as the Ouachita trough by Pennsylvanian time. These data are consistent with a composite Carboniferous Ouachita submarine fan complex built down the axis of a remnant ocean basin from varied mature/immature delivery systems tapping dominantly Appalachian fold-thrust belt sources to the east (Graham et al., 1975). Carboniferous turbidites from the Marathon fold belt (west Texas), which are isotopically similar (ϵNd = −8 to −11) to Ouachita turbidites, may have been ultimately derived from similar sources; however, they probably do not represent merely distal turbidites of a Ouachita fan complex. It is suggested that dominantly Appalachian-derived detritus, augmented by uplifted plutonic and fold-thrust belt sources south of the Marathon basin, was swept up into subduction complexes on the north side of the approaching arc and recycled along the collision zone.

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