Multiple generations or genetic units of dunes characterized by different morphologies, sand grain size and sorting, and degree of post-depositional alteration of sediments occur in the Gran Desierto sand sea of Mexico. Dune generations are separated by super bounding surfaces. Episodic accumulation of dunes is a result of a complex response of different sediment sources to regional climatic and eustatic changes. Based on examples from three parts of the sand sea, relations between different generations of dunes suggest that there are two end-member modes of sand sea accumulation: (1) stacking or superposition of dune generations, and (2) accumulating mosaics of dunes of different morphological units. The mode of accumulation is determined by sand supply and accumulation rate: stacking occurs where sand supply is high and/or the sand sea area is restricted by topography; accumulation of mosaics is the preferred mode of accumulation where sand supply is low.