Abstract The Yellow Bank creek complex (YBCC) is a large, upper Miocene injectite complex, one of numerous injectites northwest of Santa Cruz, California. The feeder for these injectites is the Santa Margarita Sandstone, a shelfal sandstone unit that is also the reservoir rock in several exhumed oil fields. The impermeable cap rock for these oil fields, the Santa Cruz Mudstone, was breached by sand injectites, some of which reached the sea floor. Located near the edge of one of these oil fields, the YBCC is a dike-sill complex that shows evidence for multiple phases of injection by fluidized sand that was initially gas or water saturated and later possibly oil bearing. Vertical injection of a large sand dike along a fracture was followed by lateral injection of a sill from the dike along bedding planes in the Santa Cruz Mudstone. Flow differentiation during injection of fluidized sand into the sill formed centimeter-scale layering in its lower part. Subsequent emplacement of oil into this sand may have occurred by injection and by seepage that displaced pore water, producing sand masses that became preferentially cemented by dolomite. Some evidence suggests that the injection and cementation occurred at relatively shallow burial depths beneath the sea floor, with the injection resulting from a combination of possible seismic shaking and migration of overpressured fluids from more deeply buried parts of the Santa Margarita Sandstone. A pervasive lamination marked by limonite staining developed following uplift and subaerial exposure of the complex, possibly in a groundwater environment.

ABSTRACT Fluid flow out of the seafloor offshore Monterey Bay region is extensive. To date 16 major active and ancient, or dormant, seep sites have been identified and many of these sites are composed of smaller sites too numerous to map at a regional scale. These seeps have been identified by the presence of chemosynthetic communities that are primarily composed of chemoautotrophic organisms or by carbonate deposition and buildups. Of the 17 identified sites, 9 active cold seep sites support living chemosynthethic communities. Seven major dormant seep sites have been identified based upon the presence of carbonate deposits or buildups. Identified seep sites are primarily concentrated along fault trends associated with the boundary of the Salinian block or Palo Colorado-San Gregorio fault zone, and along the lower flanks and crests of tectonically uplifting slopes. A combination of transpressional squeezing and overburden pressures, vertical advection through hydrocarbon and organic-rich sediment, and seaward flow of meteoric waters supply fluids to the seep sites.

ABSTRACT An exceptional exposure of the San Gregorio Fault provides the opportunity for detailed observations of structural fabrics within an active fault zone. Where it is exposed in the intertidal zone in Moss Beach, California, the San Gregorio Fault juxtaposes different sedimentary lithologies within the Pliocene Purisima Fm. An approximately 10 meter-wide zone of clay-rich foliated gouge marks the fault. The damage zone is approximately 100 meters wide, and the distribution of deformation is heterogeneous across the fault zone. Structural fabrics in the northeast fault block include breccias and both microscopic and outcrop-scale shear zones; these record the effects of cataclasis on porous sandstones and conglomerates. Deformation in the mudstones of the southwest fault block is accommodated by an incipient scaly foliation as well as by numerous fractures and faults. Microstructural analyses indicate that the San Gregorio accommodates dextral strike-slip offset as well as a component of west-side up reverse motion. Evidence for the role of fluids in this fault zone includes field relations and geochemical data. Anomolous hydrocarbon content within the foliated fault gouge indicate that the fault is a migration conduit. Fluctuations in fluid pressure within this fault zone may help elucidate the mechanics and seismogenic potential of the San Gregorio Fault.

ABSTRACT Authigenic carbonate structures in Miocene biosiliceous sediments are well exposed near a late Miocene angular unconformity in coastal cliffs at Santa Cruz, California and closely resemble carbonate structures formed at modern seep sites on the seafloor, including the adjacent Monterey Bay. The Miocene vent structures show varied morphologies, including pipes (“chimneys”) and bedding-parallel slabs., but, unlike many modern seep carbonates, they lack an associated vent macrofauna. They are composed of low magnesium calcite which cements and partly replaces the host sediment, indicating that the structures formed below the sediment-water interface and not above the seafloor. Carbon and oxygen isotopic compositions suggest carbonate precipitation occurred in a low temperature pore fluid environment fairly near the seafloor, within the zone of bacterial sulfate reduction. The host rock, the Santa Cruz Mudstone, comprises interbedded siliceous mudstones and thin, brittle opal-CT porcelanite layers which show two dominant fracture sets, one striking N30°E, the other N60°W. Most of the carbonate vent structures occur within the porcelanite layers, and the orientations of many pipes and slabs parallel the strikes of the two fracture sets. This suggests that the fluids which precipitated the carbonates were channeled along fractures. This structural control and the proximity of the vent structures to an angular unconformity indicates that deformation was a major factor, creating fracture permeability and probably also causing tectonic compaction of sediments as well as expulsion of fluids. The main Miocene vent locality lies near three major fault zones (San Gregorio, Monterey Bay, and Ben Lomond), and we speculate that the deformation was related to tectonism on one or more of these faults. Among the unresolved issues is the time and burial depths at which the carbonate vent structures formed. Some evidence (e.g. preservation of opal-A diatoms in the calcite structures) favors carbonate precipitation prior to the opal-A to opal-CT phase transformation, while other evidence (e.g. lack of compaction around the carbonate structures) suggests precipitation occurred after or contemporaneously with the silica phase change. These conflicting scenarios might be reconciled if the silica phase transformation occurred relatively early at shallow burial depths in an environment of advecting fluids with low silica concentrations.

ABSTRACT Sandstone intrusions are widespread west of Santa Cruz, California and were emplaced during late Cenozoic tectonic deformation of this region. Among these is a very large and complex intrusion which is well exposed along the coastline at Yellow Bank Creek. Here, fluidized sands from the Miocene Santa Margarita Sandstone were injected upward into fractured biosiliceous rocks of the Santa Cruz Mudstone, probably due to faulting and seismic shaking. The complicated internal structure of this intrusion includes sedimentary xenoliths, fluidization structures, and secondary limonite staining. The latter likely occurred during oxidation by groundwater and produced conspicuous, complicated layering which serves to mask and confuse interpretations of the earlier-formed features. Among the earlier formed features are fluidization structures, comprising (1) flow banding which records injection of sands horizontally in silllike areas of the intrusion, and (2) heave structures which reflect mainly vertical injection of hydrocarbons and sands partially saturated with hydrocarbons into water-saturated sands. This latter type of injection appears to have occurred at a hydrocarbon front that was derived from either a localized petroleum accumulation or else from remnants of hydrocarbons that had mostly migrated updip prior to the clastic intrusion event. Dolomitic cementation occurred preferentially in the hydrocarbon-saturated sands due to degradation of the hydrocarbons. Paleotemperature estimates of the intrusive sandstones (by apatite fission track analysis) and of the host Santa Cruz Mudstone (by vitrinite reflectance) indicate maximum temperatures of about 60°C for the former, 50°C for the latter. Our data suggests that initial fluidization began in water-saturated sands of the bioturbated facies in the Santa Margarita Sandstone; following upward intrusion of these sands, fluidization and injection expanded into hydrocarbon-bearing sands within the cross-bedded facies of the same unit.

ABSTRACT Samples collected from the northern meander of Monterey Canyon, California, and the adjacent Soquel Canyon include Cretaceous granodiorites and middle Tertiary basaltic andesites and sandstones. Plagioclase separated from the granodiorite basement rocks from Soquel Canyon yielded an age of 79 ± 0.8 Ma and are isotopically similar to Salinia-terrane granitoids exposed on the Monterey Peninsula. The Soquel Canyon granodiorite is crosscut by mafic dikes that are basaltic andesite in composition. Plagioclase separated from one mafic rock has been dated at 23.7 ± 0.5 Ma, consistent with the middle Tertiary pulse of volcanism characteristic of this region. This mafic unit also intrudes an overlying sandstone unit forming a “peperite” texture resulting from contemporaneous volcanism and sedimentation. The peperite constrains the lithic-rich sandstone, which we propose to have been deposited in a sedimentary basin associated with local tectonic extension, to a late Oligocene and (or) early Miocene age. The artifacts of the sedimentary basin are truncated (and deformed) on the south by the Monterey Bay fault zone, and exposed within the northern meander of the Monterey Canyon. These new lithologies require a revision of the Neogene lithostratigraphy of Monterey Bay and may also be useful in linking the local volcanic, tectonic and sedimentary history to the complex tectonic development of central California during the middle Tertiary. We suggest that strike-slip or transtensional movement along the Monterey Bay Fault Zone opened a basin in late Oligocene and (or) early Miocene into which was deposited a coarse, lithic-rich (Vaqueros?) sandstone. The contemporaneous volcanism of basaltic andesite is alkalic in character and may have been a result of mantle upwelling within a slab window or local transtension along the major faults active during this period.

ABSTRACT The Monterey Formation in the Naples Beach section contains an exceptionally well developed array of phosphatic rocks, particularly in the carbonaceous marl member. Similar deposits occur in Neogene sediments along the continental margin (shelf-upper slope) of Peru. Phosphogenesis (= precipitation of authigenic carbonate fluorapatite or CFA) in both areas occurred in organic-rich host sediments deposited beneath zones of strong coastal upwelling. Both areas contain distinctive F-phosphates and D-phosphates. F-phosphates are friable and typically have a light color; they represent in situ precipitation of CFA in organic-rich sediments during early diagenesis just beneath the sea floor. D-phosphates are dense, well lithified nodules, conglomerates and hardgrounds of CFA which commonly have a dark color or dark external coatings. D-phosphates record cycles of phosphogenesis near the sea floor, exhumation by bottom currents, reworking, reburial and renewed phosphogenesis; evidence from the Naples Beach section indicates F-phosphates converted to D-phosphates during these cycles by repeated episodes of phosphogenesis. Some D-phosphate layers probably indicate extended periods of low net sediment accumulation, others are clearly products of transportation and redeposition. Associated with the phosphatic rocks in both the Naples Beach section and offshore Peru are thin (5-40 cm) layers and concretionary horizons of authigenic dolomites. These also probably formed during intervals of sharply reduced sedimentation and likewise represent a form of condensation. Phosphate-dolomite bedding patterns are different in the Naples Beach and Peruvian sections. At Naples Beach, dolomitic intervals tend to lie above phosphatic horizons, whereas the reverse occurs along the Peru margin where graded D-phosphate gravels commonly lie above eroded or burrowed dolomitic layers. In the Naples Beach pattern, the D-phosphate conglomerates represent winnowed and reworked horizons, and the phosphate-dolomite pairs record progressive condensation during relative sea level rises. In the Peru margin pattern (typical also of other Neogene shelfal regions), condensed dolomitic intervals formed during relative sea level rises commonly became exposed as hardgrounds or firmgrounds which are overlain by transported and redeposited D-phosphate conglomerates deposited during the succeeding relative sea level fall. A further speculative interpretation of the carbonaceous marl member at Naples Beach is that the D-phosphate layers represent middle Miocene glacial to interglacial transitions characterized by substantial winnowing and reworking, whereas F-phosphates formed during low energy highstand conditions of interglacial periods.

ABSTRACT The Half Moon Bay oil field was first developed in the 1880’s and further drilling has been done in each “oil boom” of the past 100 years. The field has produced an estimated 58,000 barrels of oil from about 19 wells within a maximum area of 155 acres. Recent peak field production was 11 B0PD in 1985. Efforts to develop new production have been severely limited by Coastal Zone restrictions. The main reservoir in the Half Moon Bay oil field consists of very thin sandstone layers within lower Pliocene (and upper Miocene?) mudstones of the Purisima Formation, at depths of 240 to 3,085 ft (73 to 940 m). Oil is high gravity: 43°-55° API. Petroleum is trapped on two separate structural features. Recent drilling has been concentrated in the Verde area on the northwest-trending Purisima anticline, where the Purisima Formation overlies a thick upper Miocene sequence. The potential for Monterey Shale production on that structure has not been adequately evaluated. Most of the earlier wells and production were located to the northeast in the Purisima Creek area in a fault block on which the Purisima Formation lies unconformably on lower Miocene and Eocene beds. The Eocene Butano Sandstone in that block has produced a very minor amount of oil. The northwest-striking faults which bound that block had significant pre-Pliocene offset. They appear to extend north to join the San Gregorio fault, and south to merge with a previously mapped major fault in the La Honda area. Similarities between the stratigraphic sections in the Half Moon Bay and Point Reyes areas support earlier estimates of about 44 mi (70 km) of right slip along the San Gregorio fault since the end of Miocene time. [Note: this paper is partially excerpted from A. J. Horn ( 1983 ), “The Resurrection of the Half Moon Bay Oil Field, San Mateo County, California”. Because that article emphasized the Purisima Creek area of the oil field, additional sources have been utilized to extend and update its coverage.]

ABSTRACT Well developed flights of marine terraces along the central California coastline, provide horizontal datums that record the neotectonics of the coastal area. The San Gregorio fault zone, a major branch off of the San Andreas fault deforms and offsets these marine terraces in coastal San Mateo County, and unambiguously offsets the marine terrace shoreline angles horizontally at Point Ano Nuevo. The fault zone is undergoing right lateral strike slip movement with 10-15 degrees of convergence, with the majority of the movement occurring along the two primary faults within the zone, the Frijoles and Coastways faults. Analysis of the offset of the two lowest marine terraces (Santa Cruz and Western terraces) indicates the rate of movement has been essentially constant throughout most of the late Pleistocene (past 230,000 years) at 6-10 mm/yr. Analysis of deranged stream patterns of two small creeks at Point Ano Nuevo suggests that horizontal slip has ranged between 4-8 mm/yr over the past 105,000 years.

ABSTRACT The central Monterey Bay region (Figure 1) has been a stable to subsiding depocenter and the locus of fluvial, alluvialfan, and eolian activity throughout the Quaternary. It and adjacent tectonically uplifting areas provide an opportunity to study the development of marine and non-marine depositional systems as a function of fluctuating sea level and tectonic setting.