The stratigraphy of the South-Pole Aitken basin (SPA) interior is consistent with that of a massive impact melt sheet that differentiated to form cumulates. Spectroscopic and geophysical constraints on the stratigraphy of SPA suggest a ~12.5 km thick layer of norite above ultramafic pyroxenite and dunite layers. A similar stratigraphy is produced from differentiation by crystal settling of a ~50 km thick impact melt sheet (lunar impact melt sheets >10 km thick likely undergo differentiation by crystal settling) formed by an oblique impact (and thus containing ~20 vol. % crustal material) into a lunar mantle that has undergone cumulate overturn. Impact melt differentiation accounts for geophysical (nonzero crustal thickness) and geochemical (~2 ppm Th) anomalies in SPA and has implications for SPA sample return strategy. We explore the implications of these results for other lunar basins and the lunar sample suite.