Lithostratigraphy and diagenesis of the middle Miocene Monterey Formation were studied at seven localities parallel with the axial trend of the Miocene Santa Barbara basin between Goleta and the North Sulphur Mountain area of the Ojai oil field in Santa Barbara and Ventura Counties, California. The lithologic sequence of the Monterey Formation does not change markedly within the area except for the addition of discrete sand layers, as much as 1.6 ft (0.5 m) thick, in the east near the Ojai oil field. The most complete, accessible, and representative section studied is 1.2 mi (2 km) south of Oakview, where the formation can be divided into five units, which correlate well to C. M. Isaac's five informal members along the Santa Barbara coast. At the base of the Monterey, dis inctive,

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resistant, thick beds of siliceous calcareous shale and porcelanite are interbedded with soft poorly exposed calcareous shale containing sparse silica. Above this basal unit is a sequence dominated by very poorly exposed, organic-rich calcareous shale, mostly containing sparse silica, and in some layers, abundant phosphate. Overlying is a transition unit within which the poorly exposed organic-rich calcareous shale is interbedded with resistant calcareous chert and porcelanite. The upper two units both consist of interbedded porcelanite, siliceous mudstone, and chert; however, the uppermost unit is noncalcareous and has interbedded tuff layers ranging in thickness from 1 cm to more than 30 cm.

The regional pattern of diagenesis was evaluated by determining the diagenetic maturity of a single stratigraphic unit (the transition unit) at each of the seven localities. At Goleta the section is completely diatomaceous (opal-A), whereas preliminary results indicate that diagenetic quartz as well as opal-CT are present in the North Sulphur Mountain area. Although this pattern is complicated by local structural deformation, diagenesis generally increases eastward from Goleta producing a trend opposite to that in the Santa Barbara coastal area, where diagenetic maturity increases westward from Goleta. This regional pattern of diagenesis, influenced mainly by Pliocene depositional trends, is consistent with paleogeographic reconstruction of the post-Miocene breakup of the ancestral Sa ta Barbara basin.

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