North American Ordovician strata record a large shift in their neodymium isotopic composition (ΔεNd = ~ 7) at around 450 Ma. As part of a continuing effort to understand this phenomenon, we studied the provenance of Middle and Upper Ordovician clastic sedimentary rocks along a transect through the southern Appalachian and Ouachita Mountains using a combination of high-resolution graptolite-neodymium isotopic analysis and U-Pb dating of detrital zircon. Taconian (Blountian)-age clastic sediment (ca. 465 Ma) in the southern Appalachians (εNd = -8) carries a neodymium isotopic signature distinct from strata of the same age in the Ouachita region (εNd = -15). However, the Blountian signature (εNd = -8) becomes firmly established in the eastern Ouachita region by 455 Ma (Arkansas), and by 450 Ma in the westernmost part of the Ouachita fold belt (Oklahoma). In the Ouachitas, craton-dominated sources (εNd = -15) apparently mixed with orogenic sources (εNd = -8) in variable proportions over a several-million-year period before orogenic sources became dominant. We conclude from this that, superimposed on a general westward regional shift in sediment sources with time, there were also complex local effects involving multiple (unmixed) sediment sources that persisted long after the initial pulse of orogenic material arrived. The combined "simultaneous" nature of the isotopic shift, an Ordovician sea-level high stand, and the emergence of the Appalachian-Taconian-Caledonian orogenic belt as a primary sediment source, leads us to conclude that by 450 Ma, seafloor south of North America was being supplied by well-mixed, isotopically homogeneous sediment delivered from uplifted fold-thrust belts and foreland basins of the Appalachian Taconian highlands. U-Pb detrital zircon ages from bracketing sandstone units reinforce the Nd evidence for a complete changeover in provenance between 465 Ma (abundant Archean-age zircons) and 440 Ma (no Archean-age zircons) in the Ouachita region.
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