Abstract In a brief summary of California geology it is possible to avoid complications by treating most of the Tertiary series as units. Because of the complexity of the Miocene, however, some attention must be given to the subject of its classification into stages. A common classification recognizes the Monterey and the San Pablo groups. The former is divided into Vaqueros, Temblor, and Monterey formations or stages; the latter into Briones, Cierbo, and Neroly (or “Nerola”) formations. These groups are not readily usable, however, except locally in the district east of San Francisco Bay. In most districts where Vaqueros, Temblor, and Monterey formations can be recognized, the Briones and Cierbo are missing or unrecognizable and the Neroly may or may not be represented by the Santa Margarita or some other formation. In order to introduce a little system into the classification, a desirable first step is to abolish the term “Monterey group.” In the broad sense, it is now practically synonymous with Miocene, because, as will be shown later, the type section of the Monterey probably includes beds equivalent to those elsewhere considered to be Santa Margarita; that is, upper San Pablo. As a stage name, Monterey may be used for beds equivalent to those found in the type section of the Monterey shale on the Monterey Peninsula. The base of this section is represented by the post-Temblor Valvulineria californica zone, one of the most widespread microfaunal zones known in the Coast Ranges. It is recognizable in the San Joaquin Valley

ABSTRACT The structure and stratigraphy of the Miocene formations east of San Francisco Bay have been described in multiple studies for over a century. We integrated the results of past investigations and provide new data that improve understanding of formation age, the timing of deformation, and the amount of dextral displacement on selected faults. New geologic mapping and better age control show that formations previously inferred to be separate units of different ages are correlative, and new names are proposed for these units. Miocene structures associated with the development of the San Andreas transform system exerted significant control on Miocene deposition in the East Bay area. The developing structure created five distinct stratigraphic sections that are differentiated on the basis of differences in the stratigraphic sequence, lithology, and age. The stratigraphic changes are attributed to significant dextral displacement, syndepositional faulting, and distal interfingering of sediment from tectonically elevated source areas. New stratigraphic evaluations and age control show that prior to ca. 6 Ma, the developing fault system created local tectonically induced uplift as well as spatially restricted subbasins. Regional folding did not occur until after 6 Ma. Past evaluations have inferred significant dextral displacement on some of the faults in the East Bay. The spatial relationships between unique conglomerate clasts and known source areas, as well as the distribution of well-dated and unique tuffs, suggest that dextral displacement on some faults in the East Bay is less than previously reported.

Abstract Only subcommercial accumulations of heavy oil have been discovered in the Huasna Basin, which contains numerous active oil seeps and several source rocks. The petroleum geochemistry of a heavy oil sample from the only field in the basin and of five tar samples obtained at seeps has been determined and compared to the chemistry of core samples of Monterey and Rincon shales. The bulk chemistry and mineralogy of these Miocene shales can be used to identify several lithofacies, including phosphatic and siliceous shales containing variable amounts of detrital clay and carbonate minerals. Pyrolysis-FID and visual kerogen analysis demonstrate that Monterey and Rincon shales are good quality, oil-prone source rocks in the Huasna Basin. Thermal maturity modeling calibrated using bottom-hole temperature data and vitrinite reflectance measurements in several wells indicate that Monterey and Rincon source rocks are thermally mature in the axial syncline of the Huasna Basin. The tarry petroleum obtained at seeps is more severely biodegraded than a heavy oil sample from the Huasna field. The chemistry of these petroleum samples, especially their V/Ni ratios and sulfur concentrations, can be used to classify them into distinct groups. High-sulfur samples contain >4 wt. % sulfur and are enriched in V relative to Ni. Low-sulfur samples contain <2 wt. % sulfur and are enriched in Ni relative to V. One tar sample exhibits an intermediate chemistry. Sour oil and tars from the western flank of the Huasna Basin were generated by phosphatic Monterey shales or clay-poor siliceous Monterey shales containing sulfur-rich (Type II-S) kerogen. The sweeter petroleum seeping along the eastern margin of the basin was probably generated by more proximal, clay-rich Miocene shales containing Type II kerogen. Monterey lithofacies variations in the Huasna Basin and the distribution of subsurface oil and gas support this interpretation, as does the organic geochemistry of bitumen samples extracted from mature shales. Thus, inferred differences in Miocene oil quality within the Huasna Basin seem to have been influenced primarily by source effects (with a significant overprint of biodegradation) rather than by thermal maturity effects alone.

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 The Point Arguello field is located in federal waters at the western end of the Santa Barbara Channel, offshore California. The reservoir consists of highly fractured fine-grained siliceous rocks of the Miocene Monterey Formation. Nine wells were examined, using cores and Formation MicroScanner (FMS) images, to determine the relationship of fractures to lithology and structure. The Point Arguello structure is a doubly plunging, slightly asymmetric anticline created by horizontal compressive stresses oriented N36°E. Monterey Formation rock types in the Point Arguello field consist of interbedded shale/mudstone, marl, porcelanite, chert, and dolostone. The siliceous rocks are in the quartz phase of diagenesis. Core and outcrop studies show that fracture development is highest in the more siliceous rocks (chert and porcelanite) and is lower in more argillaceous rocks. Fracture development is also higher in thin beds. The Monterey Formation is fractured into three distinct sets of joints oriented relative to the principal horizontal compressive stress direction: extensional, longitudinal, and shear. Extensional fractures have dips ranging from 70° to 90° and strike directions ranging + or -15° perpendicular to the fold axis. Longitudinal fractures have dips that are variable but are often normal to bedding and oriented subparallel (+ or - 15°) to the fold axis. Shear fractures cut the other fractures at an angle and occur as conjugate pairs, with the development of one half of the pair dominating over the other. The orientations of fracture sets are similar to those described in outcrop in the western Santa Ynez Mountains. Fractures are grouped according to their order of frequency for each well, with first order fractures having the highest frequency, and second, third, and fourth order fractures having lesser frequencies. All the wells have one dominant set of fractures (first order) with second and third order fractures occurring at comparative frequency ratios of 2:1 to 25:1. No one fracture type dominates in the order of development, and the development appears to be random across the structure. Extensional fractures are common to all the wells, whereas longitudinal and shear fractures are not. Extensional fractures probably provide a mechanism for connecting other fractures that have developed at all scales, from the microscopic to the megascopic. The result is that extensional fractures may be responsible for draining the reservoir. Fracture analyses were conducted on both core and Formation MicroScanner (FMS) images. Two wells had both types of data recorded from the same intervals. The fracture analyses and bed orientations derived from FMS images compare very well with cores and dipmeter data.

ABSTRACT The Mount Diablo region has been located within a hypothesized persistent corridor for clastic sediment delivery to the central California continental margin over the past ~100 m.y. In this paper, we present new detrital zircon U-Pb geochronology and integrate it with previously established geologic and sedimentologic relationships to document how Late Cretaceous through Cenozoic trends in sandstone composition varied through time in response to changing tectonic environments and paleogeography. Petrographic composition and detrital zircon age distributions of Great Valley forearc stratigraphy demonstrate a transition from axial drainage of the Klamath Mountains to a dominantly transverse Sierra Nevada plutonic source throughout Late Cretaceous–early Paleogene time. The abrupt presence of significant pre-Permian and Late Cretaceous–early Paleogene zircon age components suggests an addition of extraregional sediment derived from the Idaho batholith region and Challis volcanic field into the northern forearc basin by early–middle Eocene time as a result of continental extension and unroofing. New data from the Upper Cenozoic strata in the East Bay region show a punctuated voluminous influx (>30%) of middle Eocene–Miocene detrital zircon age populations that corresponds with westward migration and cessation of silicic ignimbrite eruptions in the Nevada caldera belt (ca. 43–40, 26–23 Ma). Delivery of extraregional sediment to central California diminished by early Miocene time as renewed erosion of the Sierra Nevada batholith and recycling of forearc strata were increasingly replaced by middle–late Miocene andesitic arc–derived sediment that was sourced from Ancestral Cascade volcanism (ca. 15–10 Ma) in the northern Sierra Nevada. Conversely, Cenozoic detrital zircon age distributions representative of the Mesozoic Sierra Nevada batholith and radiolarian chert and blueschist-facies lithics reflect sediment eroded from locally exhumed Mesozoic subduction complex and forearc basin strata. Intermingling of eastern- and western-derived provenance sources is consistent with uplift of the Coast Ranges and reversal of sediment transport associated with the late Miocene transpressive deformation along the Hayward and Calaveras faults. These provenance trends demonstrate a reorganization and expansion of the western continental drainage catchment in the California forearc during the late transition to flat-slab subduction of the Farallon plate, subsequent volcanism, and southwestward migration of the paleodrainage divide during slab roll-back, and ultimately the cessation of convergent margin tectonics and initiation of the continental transform margin in north-central California.

ABSTRACT This atlas of foraminifera is designed to update and enhance biostratigraphic correlations in the California Miocene. The study focuses primarily on the Monterey Formation, because it is a major source and reservoir of petroleum, and exclusively on the time interval during which it was deposited, 17.9 to 6.0 million years ago. Most of the recovered fauna derives from nine outcrop sections or areas in central and southern California, representing four Neogene coastal basins: Graves Creek (Salinas Basin), Indian Creek (Salinas Basin), Laguna Hills (Los Angeles Basin), Manville Quarry access road (Santa Maria Basin), Monterey County roadcuts (Salinas Basin), Naples Beach (Ventura Basin), Upper Newport Bay (Los Angeles Basin), San Clemente Island (Los Angeles Basin), and Topanga Canyon (Los Angeles Basin). The collections yield 112 genera and 391 (350 benthic, 41 planktic) species-group taxa. Eleven new species are described: Astacolus naplesensis, Bolivina exilicostata, Bolivina isaacsi, Bolivina woodruffi, Cancris lippsi, Evolutononion dumonti, Lenticulina barroni, Lenticulina douglasi, Lenticulina indianensis, Neoeponides navarrettei, and Valvulineria mcdougalli. Sequence checklists for each section tabulate the occurrences and relative abundances of species, the data from which are composited in faunal checklists that reveal provincial trends in the spatial and temporal distribution of each species. Illustrations of all recorded species should enable workers to compare taxa in order to evaluate and incorporate new information into preexisting data banks. They also provide the means for conformity among workers and serve as a training guide for new students. Future applications in geologic correlation should benefit from the greater degrees of precision and confidence afforded by the revised foraminiferal taxonomy and biostratigraphy presented in this compendium.