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.