ABSTRACT The Monterey Formation, consisting of siliceous and calcareous biogenic sediments, was deposited during the transition from a relatively warm greenhouse climate in the early Miocene to the cooler temperatures of icehouse climatic conditions during the early middle to late Miocene. This cooling event was associated with global paleoclimatic and oceanic changes assumed to be related to the deposition of organic carbon–rich sediments into the marginal basins of California. This chapter introduces an age model for the Miocene strata at Naples Beach based on a composite stratigraphic section and standardized data set, providing the framework for the integration of biostratigraphic zones with a series of astronomically tuned siliceous and calcareous microfossil bioevents, an updated strontium isotope stratigraphy, new tephrochronology ages, and ages from specific magnetostratigraphic units. This multidisciplinary approach, utilizing the integration of microfossil disciplines with independent age controls, is critical to obtaining an age resolution of ~200 k.y. for the majority of the Monterey Formation section. This chronostratigraphic framework improves the age control of the boundaries between the California benthic foraminiferal stages and provides more age refinement for the possible hiatus and condensed interval within the Carbonaceous Marl member of the Monterey Formation. The recalibrated ages for the tops of the Miocene benthic foraminiferal stages are Saucesian (ca. 17.4 Ma), Relizian (15.9 Ma), Luisian (13.1 Ma), and Mohnian (7.7 Ma). Also, the time missing in the hiatus between the Luisian and Mohnian is <200 k.y., and the duration of the condensed interval is from 13.0 to 11 Ma. This refined age model provides a correlation of the organic carbon–rich intervals occurring in the Luisian and lower Mohnian stages within the Naples Beach strata to the deep-sea δ 13 C maxima events CM5 (ca. 14.7 Ma) and CM6 (ca. 13.6 Ma), suggesting episodic increases in organic carbon deposition along the continental margins coincided with the Miocene carbon isotope excursion found in deep-sea cores. The transition from the Miocene climatic optimum to the icehouse world consisted of four climatic and oceanic phases (from ca. 17.5 to ca. 7 Ma), which are represented in the onshore section by variations in the organic carbon and phosphate contents, the occurrence of calcareous and siliceous lithologic facies, and the distribution of microfossils, especially changes in the benthic foraminiferal assemblages.

The Midway-Sunset Oil Field produces primarily from Neogene sands and gravels which are found within the upper Monterey Formation, the Reef Ridge Shale, the Etchegoin Formation, the San Joaquin Formation and the lower and middle Tulare Formation. Age dates based on radiometric and paleontological analyses suggest that these producing horizons range in age from about 8.9 Ma (late early Mohnian) to about 1.0 Ma (Pleistocene).

ABSTRACT Within coastal outcrop sections west of Santa Barbara, California, the upper part of the Middle Miocene is condensed into a narrow stratigraphic interval that is characterized by phosphatic hardgrounds. Foraminiferal biostratigraphy shows that the base of the condensed interval is diachronous, varying from a position within the Luisian Stage at Gaviota Beach, to a position just above the Luisian-Mohnian boundary at Naples Beach and probably at El Capitan Beach. Termination of the main episode of hardground deposition occurred within the Early Mohnian, and is synchronous within the resolution of the biostratigraphic correlations. Phosphatic hardground sections at Gaviota Beach, El Capitan Beach, and Naples Beach correlate to a biogenic silica-rich interval characterized by chert, porcelanite, and organic phosphatic shale at Arroyo Burro Beach. The Upper Luisian to Lower Mohnian siliceous sediments at Arroyo Burro Beach correlate in part or in whole to the massive chert member of the Monterey Formation, the main fractured chert reservoir interval of Monterey oil fields in Santa Barbara Channel.

ABSTRACT The major and trace element composition of seventy whole-rock samples of volcanic ash, tuff, and bentonite from beach outcrops of the Monterey Formation west of Santa Barbara were analyzed to determine if these tephra layers could be used as a stratigraphic correlation tool. Five broad groups of ashes can be distinguished based on the abundance of seven elements that are immobile during alteration of the ashes to bentonites (La,Sm,Yb,Nb,Th,Ti,Zr). Lower to middle Miocene ash beds that are correlative with the Tranquillon, Obispo, and possibly the Santa Cruz Island Volcanics are found in the lower Monterey Formation. Upper Miocene ashes that are correlative with ash beds in the type section for the Monterey Formation in Monterey County, as well as ashes from a nonmarine sequence north of the Garlock fault, are found in the upper Monterey Formation at Naples Beach. An ash bed zonation was developed for the Monterey Formation which was used to correlate between stratigraphic sections along the Santa Barbara coastline. The concurrent range zonation developed for these ashes has a resolution that is comparable to the resolution achievable with benthic foraminifera in this part of the time scale.

ABSTRACT Natural gamma-ray spectrometry is a powerful addition to the traditional lithostratigraphic analysis of mudrocks. Gamma-ray data can reveal many aspects of mudrock deposition: lithologic variation, organic-matter content, hydrocarbon source quality, bedding thickness, stratal stacking patterns, depositional environments, and significant stratigraphic surfaces and packages and their regional distribution. It is especially valuable in studying the Monterey Formation with its complex composition and lithologic variations. Our studies of the Monterey Formation show that: 1) potassium and thorium are reliable indicators of detritus content (r 2 = 0.75 v. Al 2 O 3 ), 2) high levels of thorium occur coincident with volcanic ash beds, 3) uranium correlates with total-organic-carbon content (TOC, r 2 = 0.80), and 4) K/U correlates well with hydrocarbon source quality measured by Alumina/TOC. Gamma-ray spectra also portray the relative proportions of hemipelagic and pelagic components in these rocks and can reveal the depositional environment when considered along with the stratal stacking patterns (Bohacs, 1990). The data obtained from an outcrop gamma-ray survey is analogous to the data obtained from the NGT or spectral gamma-ray well log from boreholes. This enables a detailed calibration of well-log response with numerous outcrop samples and provides a powerful correlation and interpretation tool for integrating outcrop and subsurface data. Many important stratigraphic surfaces have distinct spectral gammaray signatures. This is a critical element for stratigraphic analysis and developing depositional models: the surfaces approximate time lines whilst lithologic packages often range widely in age. For example, the very organic-rich phosphatic shales at Naples Beach span several million more years than the most enriched rocks at Point Pedernales (Bohacs, 1993); see also Hornafius (1991) for other examples.

ABSTRACT The purpose of this road log is to provide information on the location of significant stratigraphic horizons within the Monterey Formation along the coastline west of the city of Santa Barbara. Particular emphasis is placed on the location of the Luisian-Mohnian stage boundary, and the lithostratigraphy of the Luisian and Mohnian Stages. The Luisian-Mohnian boundary is the most abrupt faunal change which occurs in the benthic foraminiferal zonation of the Miocene in California, and it has been located by Echols (this volume) in four of the stratigraphic sections described in this road log. The upper Luisian and lower Mohnian is also of interest, because this is the interval that shows the most dramatic lateral facies changes along the Santa Barbara coastline. Between Gaviota and Naples Beach this interval was deposited on a submarine bank top, and consequently is characterized by an extremely condensed interval or a depositional hiatus represented by a series of phosphatic hardgrounds. In contrast, this interval is represented by a 100-ftthick chert and porcelanite interval in the city of Santa Barbara that was deposited near the axis of a depositional basin (see the overview at the beginning of this volume). The road log starts on Highway 101 at the turnoff for Gaviota Beach State Park. Gaviota is located 30 miles west of the city of Santa Barbara and 43 miles south of the city of Santa Maria. The mileage that is listed for each stop is for southbound traffic on Highway 101, starting at the turnoff for Gaviota Beach State Park. References cited in the road log are found in the overview at the beginning of this volume.