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 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 Rock properties play a critical role in dictating styles of deformation at all spatial scales, yet the effect of changes across and within diagenetic transition zones has been little studied, despite profound impact on resulting mechanical stratigraphy. Our analysis of the variation of fold strain at map scale and outcrop scale of the highly siliceous Monterey and Sisquoc Formations in the southern Santa Maria Basin, California, provides insight into the interplay among deformation, diagenesis, and rock composition. Diagenetic modification of these rocks has created intervals with high interstratal and interformational contrasts in competence. Map-scale analysis showed large variation in fold strain within the same area, with shortening values ranging from 5.5% to 21.1% between siliceous formations of different diagenetic grade and competence. Apparent shortening in the competent, diagenetically altered, thinly bedded Monterey Formation is twice as high as that in the overlying highly porous, diatomaceous, more massive Sisquoc Formation. The large difference in measurable apparent shortening suggests that the same amount of actual strain was chiefly accommodated by folding in the Monterey Formation versus horizontal compaction in the Sisquoc Formation, since there is no evidence of a detachment between the units. Strain analysis at outcrop scale provided insight into the ways in which both units express such different shortening ratios without having an unconformity or detachment fault between them.

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 Carrizo Plain, the only closed basin in California’s Southern Coast Ranges, preserves landforms and deposits that record both climate change and tectonic activity. An extensive system of clay dunes documents the elevations of late Pleistocene and Holocene pans. Clay dune elevations, drowned shorelines, eroded anticlinal ridges, and zones of perturbed soil chemistry provide evidence of two lake levels higher than today’s (currently 581 m above sea level [masl]), one at ~591 masl at ca. 20 ka and another at ~585 masl that existed at ca. 10 ka, based on optically stimulated luminescence (OSL) dates on clay dune sediment. Two cores from the abandoned floor of the lake provide additional evidence of a long-lived lake in the Carrizo Plain during the late Pleistocene. The longer of the two cores (~42 m) was sampled for palynology, environmental magnetism, and scanning electron microscope–petrography. The magnetic susceptibility signal contains two notable features corresponding to sedimentary materials consistent with reducing conditions. The higher of these features occurs near the surface, and the lower occurs at ~18 m depth. A 14 C date on charcoal from the upper reduced zone places the top of this zone at no older than 22.6–20.9 cal ka. This date is consistent with the OSL date on geomorphic features associated with a highstand above ~591 masl. Assuming that reducing conditions correspond to at least a few meters’ depth of relatively fresh water, the new 14 C date suggests that the upper reduced zone represents a marine isotope stage (MIS) 2 pluvial maximum lake in the Carrizo Plain. Pollen and ostracodes from the reduced sediments indicate a wetter and cooler climate than today. These conditions would have been capable of sustaining a lake with water much less saline than that of the modern lake. The timing of the oldest documented highstand (no later than 20 ka) is consistent with a modified jet stream migration model and is not consistent with a tropical incursion model. Northeast-to-southwest asymmetry across the lake floor may be consistent with southwestward tilting driven by Coast Range shortening normal to the San Andreas fault, as is seen throughout the region.

ABSTRACT Montecito, California, has a complicated Quaternary history of debris flows, the most recent being the Montecito debris flows of 9 January 2018, which were wildfire-debris flow–linked events that took 23 lives and damaged or destroyed several hundred homes. Relative flow chronology, based on boulder weathering, incision rates, and soil dates with limited numerical (radiocarbon and exposure) dating, is used to identify paths of prehistoric debris flows. Topography of debris flow fans on the piedmont is significantly affected by the south-side-up reverse Mission Ridge fault system. Examination of weathering rinds from Pleistocene debris flows confirms that the Rattlesnake Creek–Mission ridge debris flows are folded over the ridge, and that lateral propagation linked to uplift of marine terraces (uplift rate of ~0.5–1 m/k.y.) significantly altered debris flow paths. As communities continue to rebuild and live in these hazard-prone areas, disaster risk reduction measures must take into account both spatial and temporal components of vulnerability. This field guide includes four stops from Montecito to Santa Barbara. The first stop will be to observe debris flow stratigraphy over the past ~30 ka beneath an earthquake terrace and a prehistoric Chumash site on the beach near the Biltmore Hotel in Montecito. The second stop will be at San Ysidro Creek in San Ysidro Canyon, the site of the largest Montecito debris flow that occurred on 9 January 2018. We will discuss source area and processes of the debris flow, and take a short hike up the canyon to visit the debris flow basin and a ring net designed to reduce the future hazard. The final two stops will explore the debris flow chronology of Santa Barbara over the past ~100 ka. Figure 1 shows the location of the field-trip stops. There is no road log as field sites can be found with a search on a smartphone.

ABSTRACT Forearc basins are first-order products of convergent-margin tectonics, and their sedimentary deposits offer unique perspectives on coeval evolution of adjacent arcs and subduction complexes. New detrital zircon U-Pb geochronologic data from 23 sandstones and 11 individual conglomerate clasts sampled from forearc basin strata of the Nacimiento block, an enigmatic stretch of the Cordilleran forearc exposed along the central California coast, place constraints on models for forearc deformation during evolution of the archetypical Cordilleran Mesozoic margin. Deposition and provenance of the Nacimiento forearc developed in three stages: (1) Late Jurassic–Valanginian deposition of lower Nacimiento forearc strata with zircon derived from the Jurassic–Early Cretaceous arc mixed with zircon recycled from Neoproterozoic–Paleozoic and Mesozoic sedimentary sources typical of the continental interior; (2) erosion or depositional hiatus from ca. 135 to 110 Ma; and (3) Albian–Santonian deposition of upper Nacimiento forearc strata with zircon derived primarily from the Late Cretaceous arc, accompanied by Middle Jurassic zircon during the late Albian–Cenomanian. These data are most consistent with sedimentary source terranes and a paleogeographic origin for the Nacimiento block south of the southern San Joaquin Basin in southern California or northernmost Mexico. This interpreted paleogeographic and depositional history of the Nacimiento block has several implications for the tectonic evolution of the southern California Mesozoic margin. First, the Nacimiento forearc depositional history places new timing constraints on the Early Cretaceous unconformity found in forearc basin strata from the San Joaquin Valley to Baja California. This timing constraint suggests a model in which forearc basin accommodation space was controlled by accretionary growth of the adjacent subduction complex, and where tectonic events in the forearc and the arc were linked through sediment supply rather than through orogenic-scale wedge dynamics. Second, a paleogeographic origin for the Nacimiento forearc south of the southern San Joaquin Valley places new constraints on end-member models for the kinematic evolution of the Sur-Nacimiento fault. Although this new paleogeographic reconstruction cannot distinguish between sinistral strike-slip and thrust models, it requires revision of existing sinistral-slip models for the Sur-Nacimiento fault, and it highlights unresolved problems with the thrust model.