While most interpretations of the origin of the Gulf of Mexico basin involve rifting by counter-clockwise rotation of the Yucatan block, the exact placement of the ocean-continent boundary has remained controversial. Published interpretations (Bird et al., 2005) usually place the boundary near the basinward limit of autochthonous salt. Our interpretation, outboard of salt and southeast of the Mississippi Fan Fold Belt, suggests a more complex assemblage of crustal elements. Seismic interpretation and potential field modeling indicate a zone of highly extended transitional crust dominated by oceanic material although containing large, discreet fragments of continental crust. This domain of anomalous, crustal material displays evidence that emergent to shallow marine conditions lasted longer than currently published interpretations. We believe this area supports a multitude of potential hydrocarbon play types.