Abstract The Northern Coast Ranges (Fig. 61) are a composite geomorphic province consisting of structurally controlled, northwest trending, enechelon ridges and valleys that extend from the Klamath Mountains in the north to San Francisco Bay in the south. The core of the Coast Ranges consists of complexly deformed Upper Jurassic to Early Tertiary rocks of the Franciscan Complex. This core is structurally overlain to the east by less deformed, late Mesozoic marine, clastic rocks of the Great Valley Sequence. These two essentially coeval terranes are juxtaposed along a major low-angle fault, the Coast Range Thrust (Bailey et al., 1964), whose present geometry is extensively modified by Tertiary deformation of the Coast Ranges and extensive strike slip displacement along reactivated Mesozoic structural discontinuities (McLaughlin, 1974, 1981; Suppe and Foland, 1978; Suppe, 1979). The Franciscan Complex is a highly deformed, lithologically heterogeneous assemblage. It consists predominately of graywacke and metagraywacke with subordinate shale, altered mafic volcanic rocks, radiolarian chert and minor limestones, which range from unmetamorphosed through zeolite, prehnite-pumpellyite, greenschist, and blueschist facies. It also includes minor blueschist and eclogite facies exotic blocks. The distribution and relationships of these rocks is complicated by numerous thrusts and chaotic melange zones. Vestiges of original stratigraphic relations are only locally preserved in large tectonic slabs and thrust sheets. Thrust sequences “top” eastward and “young” to the west and the regional tectonic fabric dips eastward at high angles. Overall, the high P, low T metamorphic grade increases west to east but the metamorphic facies progression is more complicated in detail.

Abstract The local appearance of voluminous, monomineralic, foliate serpentinite breccias in the Wilbur Springs section of the Great Valley Sequence record a direct sedimentological response to a late Neocomian accretion event in the Coast Ranges. Field and petrologic studies establish the following events: 1) Early Cretaceous emplacement of lower plate Franciscan rocks (Indian Valley terrane) beneath the proto-Coast Range thrust, 2) deformation of both upper and lower plate rocks into the southeast plunging Wilbur Springs anticline, and 3) contemporaneous protrusion of serpentinite breccias. Recently discovered modern examples of diapiric serpentinite protrusions of similar magnitude from the Mariana arc-trench system suggest many parallel analogies between the tectonics of active forearcs and the late Mesozoic of the California margin. Two modes of occurrence of serpentinite have been recognized in the Wilbur Springs area. The first type, serpentinite derived from the alteration of ultramafic tectonite, is widely distributed throughout the Coast Ranges (Bailey et al., 1964). Once considered to be altered ultramafic intrusions and included as part of the Franciscan Complex, these serpentinite masses occur as regionally extensive, allochthonous fault slices or fault-bounded mèlange belts, and tend to be preferentially concentrated along or near contacts between the Franciscan complex and the Great Valley Sequence (Fig. 61). Based on the characteristic spatial association of the serpentinite with layered mafic plutonic rocks, submarine basaltic lavas, and pelagic sedimentary sequences, these serpentinized ultramafic rocks are now generally accepted to represent the basal, mantle tectonite of an ophiolite sequence, the Coast Range Ophiolite, upon which the sediments of the Great Valley Sequence were deposited (Bezore, 1969; Bailey et al., 1970; McLaughlin, 1974; Evarts, 1977; Hopson and Frano, 1977; Hopson et al., 1981).

Abstract Voluminous serpentinite strata interfinger with Lower Cretaceous terrigenous clastics of the Great Valley sequence along the western edge of the Sacramento Valley. These deposits are exposed in the core of a regional, southeast-plunging structure, the Wilbur Springs anticline. The sedimentary serpenti-nites form a distinctive lithologic unit of foliate serpentinite breccias in road cuts along Highway 20 just west of its intersection with Highway 16. Extensive serpentinite masses of sedimentary origin were first reported in the area of Wilbur Springs by Taliaferro (1943). He recognized their association with fault-bounded serpentinite of the ophiolite belt but was unable to establish sufficient criteria to distinguish between the two types in many cases. Thus the origin of much of the exposed serpentinite remained unresolved. Lawton (1956), citing numerous fossil discoveries within serpentinite breccia and gradational contacts with surrounding Great Valley strata, suggested that all of the serpentinite bodies south of Wilbur Springs were the products of sedimentary processes. Although he noted textural variation between chaotic and stratified units, he did not attach genetic significance to these facies changes. He considered all the deposits detrital accumulations despite the poor stratification, chaotic nature, schistose fabric, and the admitted lack of internal clastic textures in many of the bodies. The foliation was attributed to later tectonism, which he thought had obliterated original depositional features. Moiseyev (1966), in his study of the mercury mineralization of the Wilbur Springs District, concurred with Lawton's findings and provided further documentation of the detrital nature of some of the ultramafic rocks. Stressing the inherent difficulty in discriminating sedimentary serpentinites, he concluded that lateral gradation of foliate breccia into coarse elastics of ultramafic composition and the presence of fossils in the serpentinite are the only unambiguous criteria of sedimentary origin (Moiseyev, 1970).

Abstract Hot springs along Sulphur Creek in Colusa County, California, have been recognized for about 130 years. Several researchers have proposed that the hot spring fluid there is derived from mixing of “connate” or “evolved connate” water which is derived from ancient seawater deposited in the Mesozoic sedimentary rocks. This water, which is similar in composition to Complexion Spring, mixes with meteoric water to form Wilbur Springs and other hot spring waters along Sulphur Creek. A δD - δ 18 O plot shows that Complexion Spring really does not plot along this trend; it must be isotopically modified to plot along the trend. Tuscan Springs, which is located 140 km NNE of Wilbur Springs, just NE of Red Bluff, has chemical and isotopic characteristics which are similar to the Sulphur Creek hot springs. Tuscan Springs vent from the Chico Formation of the Great Valley sequence and indicate that Tuscan Springs and Wilbur Springs are both derived from waters originating in the Great Valley sequence. Also δ 11 B correlates well with Cl, δD and δ 18 O, which originate in the Great Valley sequence, suggesting a similar source for the higher d 11 B values. Chemical geothermometry of the Sulphur Creek hot springs indicates a reservoir temperature of ~ 180 °C. This temperature agrees with measured homogenization temperatures from fluid inclusion which range from 150 to 180 °C. The calculated cation geothermometer temperatures are affected by the presence of dissolved Mg, even though the concentrations appear low.

Abstract The sedimentary serpentinites we will visit are exposed in the southeast plunging Wilbur Springs anticline, where they interfinger with lower Cretaceous turbidites of the Great Valley Sequence. The Wilbur Springs area (Fig. 65) lies astride this anticlinal structure on the western edge of the Sacramento Valley approximately 175 km north of San Francisco (Fig. 61). The area is named for a hot springs resort on Sulfur Creek, which is located upstream from the Lodoga Road connecting Bear Valley with Highway 20. Barren, block-strewn, grass-free slopes or dense brush characterize areas underlain by serpentinite. Outcrops are sparse, and mainly confined to road and stream cuts. However, even in areas of poor exposure, contacts are easily traced in soils and subcrop. Because of the serpentinite, the area is highly suceptible to landslides and soil creep which locally bury or displace contacts. The main outcrops of sedimentary serpentinite occur near Sulphur and Bear Creeks, south of Wilbur Springs. Here, thick bedded to massive, crudely stratified, foliate serpentinite breccia is the dominant lithologic facies. A quarry on Highway 20 and adjacent roadcuts provide excellent exposures of foliate serpentinite breccia and its stratigraphic relation with Great Valley strata. Turn out at the large vista point and walk back across the road to examine the sandstone-shale sequence and the foliate serpentinite breccia exposures in the quarry and in roadcuts at either side. Relocation of the highway and backfill of the original quarry cut has removed some of the section and considerably degraded what remains. Nevertheless, the relationships seen here, though now with some difficulty, are significant enough to justify a look.

Abstract Epithermal precious-metal and mercury deposits are present in the Sonoma and Clear Lake volcanic fields of central California and several hot springs in the Clear Lake volcanic field are presently depositing mercury and gold. The deposits and hot springs are associated with late Miocene to Holocene volcanic centers developed above a zone of thin crust and hot asthenosphere termed a slab window (Dickinson and Snyder, 1979, Benz and others, 1992) as the end of Pacific plate subduction was marked by the passage of the Mendocino triple junction along the California coast. Mercury deposition is actively occurring at the Sulphur Bank mercury mine, but no precious metals are present there because the geothermal system is vapor-dominated. In the water dominated geothermal systems at Wilbur Springs (Peters, 1990, Donnelly and others, 1993) and springs near the Cherry Hill gold deposit, both cinnabar and gold are being deposited (Pearcy and Petersen, 1990). Transport of mercury and gold is in a fluid which also contains high concentrations of petroleum and associated methane and CO2 derived from thermal degradation of organic matter in sedimentary rocks (Peabody, 1989). Chemical and isotopic analysis of oxygen and deuterium of the hot springs indicate that three types of fluid are present: moderate chloride, isotopically heavy, evolved formation fluid equilibrated with oceanic sedimentary rocks; evolved meteoric water; and isotopically light meteoric water (Peters, 1990,1991, Sherlock and Jowett, 1992, and Donnelly-Nolan and others, 1993). High concentrations of Hg, As, Sb, Au, and Ag occur in precipitates from hot springs composed dominantly of the isotopically heavy fluid, but not in the moderate-temperature, oxidized springs that are mixtures of these two fluid types (Peters, 1990, Donnelly-Nolan and others, 1993). The McLaughlin gold deposit (initial reserves of 2.9 million oz of gold) is economically the most important deposit in the Clear Lake and Sonoma volcanic fields. This precious metal-mercury hydrothermal system developed within and adjacent to andesitic vents and dikes emplaced along the Stony Creek fault zone (Lehrman, 1986). Gold occurs in opal, chalcedony, and quartz veins, and the highest gold values typically occur in amber to brown opal containing petroleum. Gold occurs in several sites within the petroleum-bearing opal: as a filling of 2050 micron- diameter oval voids representing large fluid inclusions; as 2-4 micron size crystals that coalesce to form dendrites of gold along primary vein banding; and in syneresis cracks which cut the vein banding. Oxide phases of Ga, In, Sn, and Ni are present within the petroleum-bearing opal. The isotopically heavy McLaughlin ore fluid plots in the field of andesitic magma volatiles (Hedenquist and Aoki, 1990, Giggenbach, 1987) and evolved formation waters (Sherlock and Jowett, 1992) suggesting that these two components are present Andesitic vents and dikes at the McLaughlin gold deposit suggest that a larger intrusion underlies the area and provided the heat source for the hydrothermal system. Andesitic vents along the Stony Creek fault provided a conduit for volatiles degassing from the intrusion to become entrained within the hydrothermal fluid composed of gas-oil-field water derived from the Great Valley sequence. The McLaughlin gold deposit reflects the complex interaction of three types of fluid each transporting a different elemental suite: evolved gas-oil field formation water transporting petroleum, Ga, In, Sn, Ni, and Hg; andesitic magmatic fluid transporting Au, Ag, Hg, Sb, and As; and near-surface meteoric water. Prospective areas for precious metal hot-spring deposits occur in the volcanic-structural environment above the thin crust and hot asthenosphere within the slab window in the Coast Ranges and parts of the Great Valley sequence where blind thrusts and associated faults are intruded by Pliocene to Holocene intrusive rocks. Mercury deposits with little or no gold content form along major structures from gas-oil field fluids with little or no magmatic component in the fluid and contain petroleum, Ni, Ga, In, Sn, and other transition elements. Epithermal gold deposits contain a significant magmatic component characterized by Au, Ag, As, Sb, and Hg as well as a gas-oil- field fluid component characterized by petroleum and transition metals. Both deposit types may occur along the same structures.

Abstract Manifestations of a major thermal anomaly in the Geysers-Clear Lake area of northern California include the late Pliocene to Holocene Clear Lake Volcanics, The Geysers geothermal field, abundant thermal springs, and epithermal mercury and gold mineralization. The epithermal mineralization and thermal springs typically occur along high-angle faults within the broad San Andreas transform fault system that forms the western boundary of the North American plate in this area. The young volcanic rocks overlie Mesozoic marine rocks of the Great Valley sequence which have been thrust above the coeval Franciscan Complex and penecontemporaneously dropped back down along low-angle detachment faults. Many of the waters of the region are non-meteoric as defined by their isotopic signature. One type of isotppically shifted water emerges from or near Great Valley sequence rocks and is the most chloride rich. It is interpreted to be evolved connate in origin. A second type, evolved meteoric water has moderate chloride contents, high boron contents, and high B/Cl ratios and is found locally in Franciscan rocks, notably at the Sulphur Bank mercury mine where it probably results from near-closed-system, repeated boiling of meteoric water in host rocks that also contribute organic components to the water. At the Sulphur Bank mine fracturing of otherwise impermeable Franciscan rocks by faulting has created a localized zone of permeability in which thermal water boils repeatedly with limited venting to the surface. Boron-rich fluids were apparently present at depth in The Geysers when intrusion of silicic magma occurred because the concealed intrusion of felsite is surrounded by a halo of tourmaline-bearing hornfels. The volume of this poorly dated early to middle Quaternary intrusive body probably exceeds the 100 km 3 of erupted Clear Lake Volcanics. Similar intrusions may have occurred in the eastern part of the area at Wilbur Springs and the McLaughlin mine, where gold deposition and evidence of hydrothermal phenomena suggest more magmatic activity than is indicated by small exposed bodies of early Quaternary basaltic lava. The Clear Lake Volcanics are the present locus of volcanism in the northern Coast Ranges and other volcanic centers are progressively older to the south. Geophysical data suggest that a large silicic magma body may be centered north of The Geysers steam field providing the heat for the geothermal field. Geothermal power production has peaked at The Geysers and pressure declines indicate significant depletion of the fluid resource. The vapor-dominated field evolved from a pre-existing hydrothermal system within fractured, otherwise impermeable Franciscan metamorphic rocks. A deep water table of saline fluid has been postulated to be present under the steam field, but no chloride-rich water has been found at drillable depth. We propose that recently discovered, isotopically shifted steam in the northwest Geysers area indicates the presence not of deep connate water but rather of boiled-down, boron-rich Franciscan evolved meteoric water. This water is likely to be present in limited quantities and will not provide a significant hot water resource for geothermal power production at The Geysers or from the main Clear Lake volcanic field.

The basal Great Valley sequence in Napa and southern Lake Counties, California, is a mappable chaotic unit composed largely of ophiolitic debris. Serpentinite flows and breccias, mafic breccias and associated finer-grained clastic rocks, and blocks of extrusive greenstone, mafic breccia, chert, bedded and unbedded clastic sedimentary rocks, phyllites, actinolitic greenschists, and hornblende amphibolites are mixed with Great Valley sequence mudstone and serpentinous mudstone. The chaotic unit extends along strike for at least 50 km. Cross-sections indicate that it extends for at least 20 km across strike and is up to 1 km thick. It is involved in complex folds caused by imbricate thrust faulting. The unit lies directly above the serpentinite that represents the Coast Range Ophiolite within the study area and below the well-bedded Great Valley sequence of Upper Jurassic and Cretaceous age. Its lower contact is enigmatic but is probably depositional; the upper contact is sheared and gradational. Locally the unit represents the entire Tithonian Stage. Ophiolitic detritus in the lower Great Valley sequence is also found elsewhere in the Northern California Coast Ranges—near the Geysers, in Rice Valley, near Wilbur Springs, along the Bartlett Springs Road near Walker Ridge, near Cooks Springs, and at Crowfoot Point west of Paskenta. Other accumulations of ophiolitic debris are inter-layered in the Great Valley sequence at various stratigraphic levels in and near the study area. This detritus takes four forms, which may be mixed together: (1) sedimentary serpentinite debris flows; (2) mafic breccias; (3) basaltic sandstones; and (4) polymict, polymorphous chaotic units with blocks-in-matrix texture, like the rocks in Napa County described here. Widespread detrital textures and the common occurrence of spaced, rather than penetrative, shear foliation in its matrix demonstrate that the chaotic unit in Napa County is not a tectonic melange, and I interpret it to be an amalgam of olistostromes. These ophiolitic olistostromes are a facies distinct from the overlying turbidites. Thus, the basal Great Valley sequence in this area is composed of two different rock types: very proximal ophiolitic debris flows, and substantially more distal subsea-fan rocks derived from a volcanic arc. Ophiolites may form at mid-ocean ridges, in back-arc or forearc basins, or in island arcs. Ophiolitic detritus may be eroded and deposited on ophiolitic basement in any environment in which the oceanic crust is deformed. The geology of surrounding terranes and the petrologic features of the ophiolitic basement below the Great Valley sequence suggest that the basement was formed in a back-arc basin. The stratigraphy of the chaotic rocks that overlie the basement in Napa County suggests that they were deposited on deeply eroded basement in a technically active forearc basin. Large volumes of rock stuffed under the hanging-wall slab after the onset of subduction may have uplifted the forearc basin and subjected its basement to erosion. A wave of uplift may have passed across the basin, so that debris shed from eroding oceanic basement was deposited directly on freshly exposed harzburgite tectonite. Some blocks may have been carried completely across the forearc basin and into the trench, and incorporated into the Franciscan melange wedge, which is also rich in ophiolitic blocks. The change from back-arc to forearc basin was probably caused by collisional tectonics and the establishment of a new subduction zone off the western coast of California during the Late Jurassic Nevadan orogeny. Stratigraphic relationships in and above the Coast Range Ophiolite are unusual through much of the Northern Coast Ranges. Nearly complete ophiolites are the exception rather than the rule, and in many areas only serpentinite is present. In some areas, ophiolitic debris different from that described here overlies the serpentinite. In other areas, arc-derived submarine fan rocks of the Great Valley sequence directly overlie serpentinized harzburgite tectonite. The relationships described here suggest that many of these contacts are not tectonic, and that the Coast Range Ophiolite does not owe its fragmentary nature to tectonic dismemberment. Rather, it is likely that the ophiolitic basement below the Great Valley sequence was deeply eroded during Mesozoic time. Many of the contacts throughout the Coast Ranges along which sedimentary rocks overlie serpentinite—which must represent deep layers of the oceanic crust or the upper mantle—are in the main nonconformities.

Abstract This field trip examines spectacular exposures of sedimentary serpentinites that occur in contrasting tectonic regimes of the Neogene transform-dominated San Joaquin Basin and the late Mesozoic forearc of the Great Valley Sequence, Sacramento Valley (Fig. 1). Emphasis will be on depositional mechanisms which range from intrusive/extrusive protrusions to detrital accumulations, in subaerial to deep marine environments. The interplay between tectonic events and the deposition of sedimentary serpentinite will be stressed. For it is this interrelationship--the tectonic mobilization of serpentinized ultramafic masses from deep structural levels, their forceful protrusion to the surface, and the generation of active extrusive serpentinite flows into the sedimentary environment--that underscores the importance of these deposits in the stratigraphic record. Voluminous, monomineralic accumulations of serpentinous strata, such as the Big Blue Formation and the foliate breccias of the Wilbur Springs area, should be viewed not merely as compositional curiousities but rather as unique sedimentologic responses to tectonic events. The first day we will examine homoclinal exposures of the Big Blue Formation along the west side of the central San Joaquin Valley (Fig. 1). Six stops are planned in the Big Blue serpentinous strata. In the southern portion of the area between Anticline Ridge and Domengine Ranch, recent work by Bate describes interfingering of distal alluvial fan and tidal flat environment. Dickinson and Casey's (1976) descriptions and discussion of the Big Blue Formation in the area near Cantua Creek are recapitulated. North of Martinez Creek, they recognize a main body of subaerial protrusive serpentinite than can be traced along strike into fringing alluvial aprons that grade, with increasing distance from the source protrusion, into shallow marine facies.