Upper Cretaceous sandstone, mudstone, and conglomerate are exposed in an eastward-dipping homoclinal sequence at the western end of San Miguel Island. These strata were deposited in the inner, middle, and middle-fringe regions of a submarine fan during the Campanian and Maestrichtian Stages. Outer fan and basin-plain sedimentary rocks are absent, whereas thick sections of mudstone-dominated strata deposited in fan-fringe areas abound. The great thickness of the middle-fan fringe facies suggests that these rocks were deposited in an elongate basin, the long axis of which was oriented normal to the direction of sediment transport.
The middle-fan fringe strata on San Miguel Island are characterized by rhythmically interbedded mudstone, siltstone, and sandstone commonly found in thickening-upward sequences. The mid-fan strata are distinguished from outer fan and basin-plain deposits by the facies occurrence along strike from mid-fan channelized sequences, by paleocurrent indicators which trend subnormal to channel orientations, and by large- and small-scale slumping subnormal to channels.
The conglomerate clast suite on San Miguel Island is dominated by durable, but brittle, black dacites and plutonic rocks. Some black dacite clasts were shattered during transport to produce black sand grains that have given the sandstones a salt and pepper appearance.
Upper Cretaceous submarine fan strata in San Diego County contain a much smaller percentage of black dacite clasts than on San Miguel Island, but both are overlain by the same Eocene submarine fan strata dominated by Poway rhyolite clasts. This relationship suggests that these rocks were deposited as two separate, but closely adjacent, submarine fans which received coarse clastic detritus from similar and overlapping volcano-plutonic source terranes.
The Upper Cretaceous strata on San Miguel Island have been referred to the Jalama Formation as established on the mainland. However, the clast suites of the Jalama conglomerates in Santa Barbara County are dominated by metamorphic rocks which indicate a source terrane very different and separate from the volcano-plutonic dominated highlands that supplied detritus to the San Miguel Island and San Diego fans. Thus, the name Jalama Formation should not be used on San Miguel Island.