ABSTRACT This study provides new stratigraphic data and identifications for fossil marine mammals from the Monterey Formation in the Capistrano syncline, Orange County, California, showing that there are two distinct marine mammal assemblages. Until now, marine mammals from the Monterey Formation of Orange County have been considered to represent a single assemblage that is 13.0–10.0 Ma in age. By combining data from diatoms with the geographic positions of sites, faunal analysis, and data from the literature, we can assign 59 sites to three main levels: the lower part (ca. 16–13 Ma), the middle part (ca. 13–10 Ma), and the upper part (ca. 10–8 Ma). We assigned 308 marine mammal specimens to 38 taxa, resulting in 97 occurrences (unique record of a taxon for a given site). Of the 38 taxa we identified within the study area, 15 taxa are restricted to the lower part of the Monterey Formation, 15 are restricted to the upper part of the Monterey Formation, eight were found in both, and none has yet been reported from the middle (possibly condensed) section. Six of the eight taxa that occur in both the lower and upper parts of the Monterey Formation are higher-level taxa, which accounts for their broad temporal range. The recognition of two distinct marine mammal assemblages in the Monterey Formation of Orange County is an important step toward a better-calibrated sequence of faunal evolution in the region while improving the utility of marine mammals for regional biostratigraphy.

ABSTRACT The Monterey Formation of Central and Southern California has produced billions of barrels of oil since the early 1900s. The Monterey Formation in the Santa Maria Basin is a tectonically fractured reservoir, meaning that the fractures formed through natural geologic processes; they are not human-generated artifacts. Open natural fractures provide the effective porosity for oil storage and the permeability pathways through which oil flows from rocks to wells. Monterey strata are notable for a diverse range of lithologies characterized by contrasts in texture and composition. Not all Monterey rock types contain natural fractures. Structural geologists applied the concepts of mechanical stratigraphy to the Monterey Formation to explain fracture variability. Hard rocks, including chert, porcelanite, and dolostone, contain extensive open-fracture systems, while softer lithologies like siliceous mudstone and organic-rich mudstone have few or no open fractures. However, the words “hard and soft” or “strong and weak” are inexact and subject to interpretation. This report constrains these qualitative descriptions by using engineering-geology data to associate rock properties with quantitative measurements of rock mechanical strength.

ABSTRACT The late early to middle Miocene period (18–12.7 Ma) was marked by profound environmental change, as Earth entered into the warmest climate phase of the Neogene (Miocene climate optimum) and then transitioned to a much colder mode with development of permanent ice sheets on Antarctica. Integration of high-resolution benthic foraminiferal isotope records in well-preserved sedimentary successions from the Pacific, Southern, and Indian Oceans provides a long-term perspective with which to assess relationships among climate change, ocean circulation, and carbon cycle dynamics during these successive climate reversals. Fundamentally different modes of ocean circulation and carbon cycling prevailed on an almost ice-free Earth during the Miocene climate optimum (ca. 16.9–14.7 Ma). Comparison of δ 13 C profiles revealed a marked decrease in ocean stratification and in the strength of the meridional overturning circulation during the Miocene climate optimum. We speculate that labile polar ice sheets, weaker Southern Hemisphere westerlies, higher sea level, and more acidic, oxygen-depleted oceans promoted shelf-basin partitioning of carbonate deposition and a weaker meridional overturning circulation, reducing the sequestration efficiency of the biological pump. X-ray fluorescence scanning data additionally revealed that 100 k.y. eccentricity-paced transient hyperthermal events coincided with intense episodes of deep-water acidification and deoxygenation. The in-phase coherence of δ 18 O and δ 13 C at the eccentricity band further suggests that orbitally paced processes such as remineralization of organic carbon from the deep-ocean dissolved organic carbon pool and/or weathering-induced carbon and nutrient fluxes from tropical monsoonal regions to the ocean contributed to the high amplitude variability of the marine carbon cycle. Stepwise global cooling and ice-sheet expansion during the middle Miocene climate transition (ca. 14.7–13.8 Ma) were associated with dampening of astronomically driven climate cycles and progressive steepening of the δ 13 C gradient between intermediate and deep waters, indicating intensification and vertical expansion of ocean meridional overturning circulation following the end of the Miocene climate optimum. Together, these results underline the crucial role of the marine carbon cycle and low-latitude processes in driving climate dynamics on an almost ice-free Earth.

ABSTRACT Sequence stratigraphy has proven to be an invaluable tool for the analysis of coarse-clastic depositional systems and the integration of observations across scales from reflection seismic to scanning electron microscope. Applications to mudstone-dominated depositional sequences have been more limited, despite the fact that mudstones make up more than 60% of the global sedimentary volume and generally provide the most complete record of sedimentation in a basin. During the late 1970s and through the 1980s, Bob Garrison and his students at the University of California–Santa Cruz conducted numerous studies that revealed the basic sedimentary and stratigraphic framework of the Monterey Formation in California, advancing our understanding of the sedimentary processes at work in these deep-margin basins. We expanded on that framework using direct observations from outcrops and cores that have been integrated with other subsurface data, as well as a wide variety of information derived from paleontologic, chronostratigraphic, geochemical, and compositional analyses to illustrate a sequence-stratigraphic approach to interpreting fine-grained rocks and their associated depositional systems in these settings. These were some of the earliest investigations of mudstone sequence stratigraphy focused on slope and basinal environments. In this study, observations from outcrops in the Pismo Basin, California, provided the basis for developing a detailed sequence-stratigraphic framework for the Monterey Formation, expanding on the broad-scale characterization of Garrison and his colleagues. These outcrops represent deposition during different phases of basin evolution and in different borderland-type basin settings (slope and basin depocenters). Comparison of coeval strata from different depositional settings and locations documented variation at both the sequence and parasequence scale. Variation of parasequence character, in particular, provided a valuable tool for enhanced understanding of deposition and diagenesis in these margin basins. Extrapolation to the subsurface using gamma-ray logs greatly enhanced basinwide application compared to limited, partial-stratigraphic-section outcrops, and it facilitated the lateral characterization of mudstone depositional sequences. These elements served as the building blocks for improved models of deposition in margin-basin settings.

ABSTRACT Application of updated diatom biochronology to the Monterey Formation and related biosiliceous rocks reveals the imprint of both global paleoclimatic/paleoceanographic and regional tectonic events. A rise in global sea level combined with regional tectonic deepening associated with the development of the transform California margin resulted in the abrupt onset of deposition of fine-grained Monterey sediments that were relatively free from clastic debris between 18 and 16 Ma. The base of the Monterey Formation does not mark a silica shift in diatom deposition from the North Atlantic to the North Pacific Ocean. Rather, a North Atlantic Ocean decline of diatoms after ca. 13 Ma and increasing divergence in nutrient levels between the North Atlantic and North Pacific Oceans between ca. 13 and 11 Ma coincided with a major enhancement of diatom deposition in the Monterey Formation. A stratigraphically condensed interval of phosphate-rich sediments between 13 and 10 Ma in coastal southern California appears to have resulted from sediment starvation in offshore basins during a period of higher sea level, as inland sections such as those in the San Joaquin Valley commonly contain thick sequences of diatomaceous sediment. Increasing latitudinal thermal gradients in the latest Miocene, which triggered a biogenic bloom in the equatorial Pacific Ocean at 8 Ma, also led to enhanced diatom deposition in the uppermost Monterey Formation and overlying biosiliceous rocks. Uplift of the California coastal ranges after ca. 5.2 Ma resulted in an increasing detrital contribution that obscured the presence of diatoms in onshore sediments. Major reduction in coastal upwelling in the early Pliocene ca. 4.6 Ma then caused a drastic reduction of diatoms in sediments offshore southern California.

ABSTRACT We present here a comprehensive record of Miocene terrestrial ecosystems from exposures of the Monterey Formation along the Naples coastal bluffs, west of Santa Barbara, California. Constrained by an updated chronology, pollen analyses of 28 samples deposited between 18 and 6 Ma reflect the demise of mesophytic taxa that grew in a warm, wet environment during the late early and early middle Miocene and the development of a summer-dry/winter-wet Mediterranean climate during the late Miocene. Broadleaf tree pollen from mesophytic woodlands and forests now found in the southeastern United States and China ( Liquidambar , Tilia , Ulmus , Carya ) characterized the Miocene climatic optimum (16.9–14.7 Ma), the middle Miocene climate transition (14.7–13.8 Ma), and the interval up to ca. 13.0 Ma. Subsequently, during the late middle to early late Miocene, between 13.3 and 9.0 Ma, oak woodlands and herbs (Asteraceae, Amaranthaceae, Poaceae) from beach scrub and chaparral increased as ocean temperatures cooled and the climate became drier. Between ca. 8.9 and 7.6 Ma, pine increased mostly at the expense of oak ( Quercus ) and herbs, suggesting a period of increasing precipitation. During the latest Miocene (7.5–6.0 Ma), an increase of herb-dominated ecosystems (chaparral, coastal scrub) at the expense of pine reflects the full development of a summer-dry/winter-wet climate in coastal southern California.

ABSTRACT The Monterey Formation and related formations in California have long been the subject of field and laboratory studies on silica diagenesis. Biogenic or amorphous silica (opal-A) alters to a more-ordered opal-CT and eventually to the crystalline end member, quartz, with increasing burial depth and temperature. Low-pressure nitrogen sorption serves as an indicator of silica alteration by detecting the nanometer-scale pore structures associated with opal-CT while excluding contributions from larger pores. To apply this method, calibrations with known compositions are not required, sample preparation and measurements are straightforward, hazardous waste is not generated (as with mercury porosimetry), and subtle changes in silica phase are readily detected. Nitrogen desorption isotherms, collected on mini cores (~0.8 cm diameter × 1 cm) after outgassing at 50 °C and processed using the Barrett-Joyner-Halenda method, provide nanometer-scale pore throat size distributions (nPSD), pore volumes (nPV), and surface areas (nSA). A scatter plot of nPV and nSA reveals two distinct trends. Samples with more nSA per unit volume contain opal-CT, either in transition from opal-A or completely converted. The other nSA trend consists of opal-A and quartz samples in the small nSA and nPV range, whereas samples with small nSA and large nPV also contain opal-CT and are in transition to quartz. These distinct trends are also apparent in the nPSD. Samples with more nSA exhibit a peak between 4 and 10 nm, whereas samples with less nSA have a broad peak between 10 and 100 nm if they contain opal-CT. Images collected via scanning electron microscopy reveal that opal-CT morphologies account for these differences.

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.

ABSTRACT Tuff beds (volcanic ash beds and tuffs) have been known in the Miocene Monterey and Modelo Formations since they were initially described nearly 100 yr ago. Yet, these tephra layers have remained largely ignored. The ages and correlation of the Monterey and Modelo Formations are predominantly based on associated biostratigraphy. Here, we combined tephrochronology and biostratigraphy to provide more precise numerical age control for eight sedimentary sequences of the Monterey and Modelo Formations from Monterey County to Orange County in California. We correlated 38 tephra beds in the Monterey and Modelo Formations to 26 different dated tephra layers found mainly in nonmarine sequences in Nevada, Idaho, and New Mexico. We also present geochemical data for an additional 19 tephra layers in the Monterey and Modelo Formations, for which there are no known correlative tephra layers, and geochemical data for another 11 previously uncharacterized tephra layers in other areas of western North America. Correlated tephra layers range in age from 16 to 7 Ma; 31 tephra layers erupted from volcanic centers of the Snake River Plain, northern Nevada to eastern Idaho; 13 other tephra layers erupted from the Southern Nevada volcanic field; and the eruptive source is unknown for 12 other tephra layers. These tephra layers provide new time-stratigraphic markers for the Monterey and Modelo Formations and for other marine and nonmarine sequences in western North America. We identified tephra deposits of four supereruptions as much as 1200 km from the eruptive sources: Rainier Mesa (Southern Nevada volcanic field) and Cougar Point Tuff XI, Cougar Point Tuff XIII, and McMullen Creek (all Snake River Plain).

Abstract Porosity and pore size distribution (PSD) are required to calculate reservoir quality and volume. Numerous inconsistencies have been reported in measurements of these properties in shales (mudrocks). We investigate these inconsistencies by evaluating the effects of fine grains, small pores, high clay content, swelling clay minerals and pores hosted in organic content. Using mudrocks from the Haynesville, Eastern European Silurian, Niobrara, and Monterey formations, we measured porosity and pore or throat size distribution using subcritical nitrogen (N 2 ) gas adsorption at 77.3 K, mercury intrusion, water immersion, and helium porosimetry based on Gas Research Institute standard methodology. We used scanning electron microscope (SEM) images to understand the pore structure at a microscopic scale. We separated the samples from each formation into groups based on their clay and total organic carbon (TOC) contents and further investigated the effects of geochemical and mineralogical variations on porosity and PSD. We find that differences in the porosity and PSD measurement techniques can be explained with thermal maturity, texture, and mineralogy, specifically clay content and type and TOC variations. We find that porosity and PSD measurement techniques can provide complementary information within each group provided the comparison is made between methods appropriate for that group. Our intent is to provide a better understanding of the inconsistencies in porosity measurements when different techniques are used.