ABSTRACT Heavy petroleum is found in limited deposits as tar-saturated sandstone within the western Santa Cruz Mountains of central California. The largest accumulation of petroleum is in the middle and upper Miocene Santa Margarita Sandstone. The Santa Margarita Sandstone rests disconformably on pre-Tertiary Salinian basement rocks and unconformably on the Monterey Formation (middle Miocene) and older Tertiary rocks. The Santa Cruz Mudstone (upper Miocene), which conformably overlies the Santa Margarita Sandstone, provided an initial seal for petroleum entrapment. The present distribution of petroleum apparently is related to the depositional environment of the Santa Margarita Sandstone and to later structural development. The Santa Margarita Sandstone is a tidally-dominated marine shelf deposit. An 8- to 10-km (5-6 mi) wide, northeast-trending facies of unidirectional, large-scale cross-strata of uncemented sand and gravel represents a zone of intense tidal currents; this facies provided a conduit for initial petroleum migration. West-dipping homoclinal folding, including northeasttrending and southwest-plunging compaction anticlines, formed structures for initial petroleum entrapment within the cross-bedded facies. Strike-slip movement on the San Gregorio fault system formed a postdepositional conjugate fault-and-fracture system in the western Santa Cruz Mountains. Where petroleum had previously accumulated on structures, faulting and fracturing caused intrusion of the petroleum-saturated sand into the overlying siliceous mudstone; these intrusions range in width from a few centimeters (inches) wide to complex injection zones more than 200 m wide (650 ft). Faulting and associated clastic intrusions also formed partial seals across structures within the Santa Margarita Sandstone. The maximum thickness, 30 m (100 ft), of tar-saturated sandstone occurs in fault traps downdip on southwest-plunging anticlines within the cross-bedded facies. Petroleum accumulations also occur in stratigraphic traps where sand pinches out on paleotopographic highs, in structural traps within fault-bounded blocks on homoclinal folds, and in possible diagenetic traps where sandstone lies on marble.

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 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 Between Anticline Ridge on the south and the Ciervo Hills on the north, serpentinous strata of the Big Blue Formation lie strati graphically between Middle Miocene Temblor Formation and Upper Miocene Santa Margarita Formation in homoclinal exposures of Tertiary strata that flank the western margin of the Great Valley. Mapping of three distinctive lithologic units within the Big Blue Formation near Cantua Creek defines key aspects of its unusual origin (see Fig. 30). Foliate but unstratified serpentinite breccia, thickest on the outcrop between Salt and Martinez Creeks, forms the body of a protrusive serpentinite extrusion that flowed as a sheared mass across an unconsolidated substratum of Temblor sand, which was partly scraped away beneath the mass-flow and was locally plowed into chaotically rumpled folds that piled up at the advancing margin of the protrusive mass. Bedded serpentinite-clast conglomerates and breccias, with intercalated serpentine-grain sandstones and associated serpentinous debris-flow deposits, were formed by fluvial reworking of the protrusive serpentinite. Detrital serpentinite locally underlies but more commonly overlies and most typically flanks the latter as a facies equivalent. Interbedded serpentine-grain sandstones and serpentinous claystones of probable marine origin exposed laterally along strike apparently formed as an extensive facies fringe of serpentinite detritus dispersed widely from a central core of protrusive and coarser detrital serpentinite. Santa Margarita beds rest conformably on the finer detrital serpentinite but uncon-formably on protrusive serpentinite and associated deposits. Easterly paleocurrents from clast imbrications in detrital serpentinite suggest that the serpentinite source lay west of the present outcrop belt. Dispersal of serpentinite debris perhaps was fed by protrusive diapiric movement of Mesozoic serpentinite mobilized in the core of an ancestral Joaquin Ridge anticline, whose initial growth thus may have coincided with the Miocene onset of rapid motion along the late Cenzoic San Andreas fault system.

Abstract The expression of stratigraphic and combination traps in upper Miocene Stevens turbidite sandstones of the Bakersfield arch, southern San Joaquin Valley, California, can be placed within a depositional sequence framework by integrating seismic, wireline-log, and core data. Insights from this study should reduce exploration risks in this mature basin and serve as exploration analogs in similar sand-rich turbidite systems globally. Stevens sandstones occur as turbidites within the lowstand systems tracts of three depositional sequences (Coulter, Gosford, and Bellevue) that were deposited in a narrow, deep-water forearc basin. Regional marine-condensed sections containing siliceous shale and cherts separate the lowstand turbidite systems. Turbidites comprising the oldest (Coulter) system were transported by sediment gravity flows through distinct canyons on a sloping deep-water ramp, whereas the stratigraphically younger Gosford and Bellevue turbidite systems were deposited by gravity flows originating on high-relief deltaic slopes. The turbidite systems contain sandy, high-density (primarily) and low-density turbidite and debris-flow deposits in confined and unconfined complexes. Preexisting structural highs and depositional topography influenced the stacking geometries of the turbidite systems. Four principal stratigraphic and combination stratigraphic/structural trap types in the Stevens turbidites are recognized: (1) permeability (facies) changes from proximal, thick, coarse-grained sandstones to distal, thin, fine-grained, interbedded sandstones and shales that occur on anticlinal flanks and account for the largest accumulations (up to 60 MMBOE); (2) channelized turbidite sandstones that pinch out against structural highs and contain reserves up to tens of MMBOE; (3) numerous small traps (<10 MMBOE) formed by compactional, low-relief, four-way structural closures; and (4) pinch-out of channelized, gully-fill turbidite sandstones within the less-explored middle and lower deltaic slopes of the Santa Margarita Formation.