Recent field work in the Santa Cruz Mountains of California demonstrates that siliceous mudstone beds of Miocene age, previously mapped as "Monterey shale," are the products of two separate marine cycles of sedimentation. The older cycle was a middle Miocene (late Relizian--early Luisian) event; the younger was initiated in late Miocene (late Delmontian) time and lasted into Pliocene time.

Each cycle consists of a basal, transgressive sandstone unit followed by a thicker mudstone sequence. The more widespread, middle Miocene mudstone sequence is siltier and contains abundant calcareous Foraminifera, whereas the thicker, late Miocene mudstone sequence is more siliceous, locally diatomaceous, and contains abundant volcanic debris. Field relationships together with paleontologic data suggest that the basal sandstone unit of each sequence was deposited in a near-shore, shallow-water environment, while the overlying mudstone unit was deposited in an outer neritic to upper bathyal environment. The Purisima formation of Pliocene age represents a later, near-shore and partially regressive phase of the second cycle.

A landmass that separated the early Miocene ("Vaqueros") sea from the Pacific basin to the southwest disappeared prior to middle Miocene time, and the later Miocene basins were probably directly connected to the Pacific basin.

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