Abstract

The Miocene Monterey Formation in the Santa Barbara coastal area has a distinctive lithostrati-graphic sequence. The overlying Sisquoc Formation (late Miocene to Pliocene) consists of siliceous mudstone with some porcelanite (or, where silica is biogenous, diatomaceous mudstone with some diatomite). The Monterey Formation includes five informally designated members, from youngest to oldest, as follows.

1. The siliceous member consists of meter-thick units of laminated cherty porcelanite and porcelanite alternating with meter-thick units of massive siliceous mudstone (or, where silica is biogenous, diatomite interbedded with diatomaceous mudstone). The member is free of carbonate except in dolomite nodules. Strata are late Mohnian.

2. The upper calcareous shale member consists of thin-bedded calcareous shales, calcareous porcel-anites, and calcareous cherts (or, where silica is biogenous, calcareous diatomaceous shales and calcareous diatomites) which are locally dolomitic. Rocks are laminated and typically divided into 1-to 2-m units grading upward from shale to chert (or, where silica is biogenous, from shale to diatomite). Strata are late Mohnian.

3. The transition member is similar to the overlying member except for including beds of organic-rich phosphatic calcareous shale which decrease in abundance upward from about 40% to <5%. Strata are near the boundary between early and late Mohnian.

4. The organic shale member consists mainly of laminated organic-rich phosphatic calcareous shales that contain minor (5–25%) silica; also present are dolomites, calcareous cherts, calcareous porcelanites, and moderately siliceous calcareous shales (or, where silica is biogenous, calcareous diatomites and silica-rich calcareous diatomaceous shales). Strata are Luisian and early Mohnian.

5. The lower calcareous shale member consists of calcareous shales, calcareous porcelanites, and calcareous cherts (or, where silica is biogenous, calcareous diatomaceous shales and calcareous diatomites) which are locally dolomitic. Rocks are distinctly laminated in the upper part but thick bedded and massive in the lower part. Strata are uppermost Saucesian, Relizian, and Luisian.

Underlying the Monterey Formation is the early Miocene Rincon Shale which consists of massive carbonate-poor claystone. Locally between the two formations is a unit of bentonite or tuff correlated with the Tranquillon Volcanics of Dibblee (1950).

The lithostratigraphic sequence in the Santa Barbara coastal area closely resembles that in the Santa Maria basin as described by Canfield (1939) and Woodring and Bramlette (1950).