Sediment composition variation in the two southernmost major Carboniferous basins of the eastern United States, the Black Warrior and the Pocahontas, permits location of their source terranes and reconstruction of the Carboniferous crust. These basins were involved in the tectonics which led to the rifting of Pangaea; thus, understanding of basin tectonics should limit the range of geophysical models for the rifting event. Examination of the immature sandstones of the Black Warrior basin indicates denudation of an uplifted greenschist terrane laced with extrusive and intrusive volcanics. Location of this southern Alabama source terrane, probably an extension of the Ouachita trend, was determined from compositional variation within the barrier island facies. Compositional variation within the Pocahontas basin suggests erosion of a granodioritic batholithic complex overlain by sediments, volcanics, and low-grade metamorphics, and underlain by migmatites. The source area for sediments in the Black Warrior terrane apparently subsided or was buried before erosion had penetrated the greenschist cover, whereas erosion reduced the Pocahontas source terrane to the sub-batholithic terrane now exposed in the Piedmont. The “premature death” of the southern terrane has been linked to formation of the early Gulf of Mexico.