The Upper Cretaceous Rosario Formation between Punta Banda and Punta San Jose was deposited along the eastern reaches of a forearc basin bounded to the east by the Alisitos arc highlands. Eustatic sea-level changes controlled sedimentation producing first a transgressive sequence and secondly a regressive sequence during the latest Cretaceous. Sedimentation began during the early Campanian (?) with the deposition of conglomerate and sandstone within the non-marine braidplain of a lobate fan-delta. Coarse sediment from the fan-delta poured from the eastern highlands westward toward the coast. As sea level rose these deposits interfingered with, and were eventually overlain by, late Campanian - early Maastrichtian marine mudstone and sandstone. In the northern areas the mud stone and sandstone was deposited on the shelf where sedimentation was largely affected by storm waves. To the south, fossiliferous mudstone was deposited in a relatively deep basin in which sedimentation occurred from slow suspension settle-out. During the early Maastrichtian sea level began to drop and a prograding submarine fan developed in the southern basin. The fan is dominated by mid-fan channel sandstones which are overlain by inner-fan conglomerates. A well exposed mid-fan channel at Punta San Jose is approximately 0.5 mile wide. In the northern areas shoreline and fan-delta sandstone and conglomerate prograded over the finer-grained shelf sediment. Sediment throughout the Campanian and Maastrichtian was derived primarily from the local volcanic-volcaniclastic Alisitos Formation and the plutonic Peninsular Ranges batholith. However, a small amount of sediment was also derived from the Jurassic-Cretaceous (?) "flysch" terrane to the east.