Abstract Most people, when they think about the geology of California, focus first on earthquakes. Indeed, as the mountains and valleys of California have been shaped, earthquakes have been the pounding of the carpenter’s hammer, a resounding echo from Earth-forming processes. Thus, even though most of us are horrified by the prospect of another devastating earthquake, the great beauty and abundant resources in California are a direct consequence of the very events that trigger these periodic Earth-shaking temblors. Geology plays an important role in controlling the quality and nuances of California’s wine. It affects the soils—their chemistry and texture, the topography—mountain slopes versus valley floors, and the climate—humidity, sun angle, and temperature. The three S’s—soil, slope, and sun, helps us understand the principle factors that control wine quality. How the vintner manipulates these controls determines the specific attributes of each bottle of wine. Understanding the geology of different California regions, and how it relates to wine growing, can be broken down into three basic categories: how the bedrock formed, evolution of the landscape, and relating these two factors to soil development and microclimatic conditions (Fig. 1 ). Starting points in descriptions of Earth history are generally arbitrary owing to the age of the planet. This history is read from rocks; in North America we have identified rocks that are nearly 4000 m.y. old. Fortunately for you, and those of us who are trying to reconstruct the geologic history of California, it is not necessary to reach that far back in

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 Since the discovery of gold and silver in the northern part of the Napa Valley in 1858, ore deposits and geothermal systems have drawn a variety of geologists to study one of the few areas in the United States where hot springs are actively depositing gold and mercury. The geothermal systems and very young precious-metal and mercury deposits occur in two adjacent volcanic fields, the older Sonoma volcanic field and the younger Clear Lake volcanic field. In the eastern foothills of the Napa Valley, precious metal deposits hosted by the Sonoma volcanic field produced only a small amount of gold and silver. The fertile soil and good drainage of the volcanic rocks in this area gave way to vineyards and wineries and the mines were closed and abandoned. The younger Clear Lake volcanic field has gone through several cycles of mineral and geothermal development The hot springs in the volcanic field were developed initially for their supposed medicinal benefits although many of the springs contained toxic levels of mercury. Mercury and sulfur were mined from several of the deposits present throughout the volcanic field and spectacular samples containing plumes of native gold within cobbles of cinnabar were discovered in the Sulphur Creek District. In spite of the known association of gold and mercury, mercury mining dominated the mineral development within the volcanic field until the mid-1940's. Development of The Geysers for geothermal power in 1960 began a new phase of economic development, and geothermal power production has continued to be important in the western part of the volcanic field. The most recent mineral development was the discovery of the McLaughlin gold deposit in 1978 at the site of the old Manhattan Mercury Mine. Since that time exploration has continued for auditional epithermal precious-metal deposits but without success. This guidebook provides an overview of the geothermal systems and ore deposits in the Sonoma and Clear Lake volcanic fields. Several research papers in this guidebook provide important new concepts and data on the ore deposits, geothermal systems, and volcanic rocks within the two volcanic fields from the perspective of geologists, geochemists, geophysicists, and petrologists. In addition, a paper by Fraser Goff and Cathy Janik provides the ftrst comprehensive field guide to the geothermal features within the Clear Lake volcanic field. This field conference and guidebook should provide the basis for new research and a better understanding of the processes that have contributed to the formation of the ore deposits and geothermal systems in the Clear Lake and Sonoma volcanic fields.

Abstract Great Basin and Sierra Nevada, the second volume of the Geological Society of America Field Guide Series, focuses on the dynamic and spectacular geology of this region, providing the inspiring backdrop for the 2000 GSA Annual Meeting in Reno. This volume gives complete coverage of field trips held in conjunction with that meeting, and contains 20 chapters organized into three sections. The first section consists of 16 chapters arranged in geochronological order, beginning with the active tectonics of Lake Tahoe and the historical surface faulting and paleoseismicity of the central Nevada seismic belt, and ending with the Neoproterozoic glacial record of Death Valley. In between are chapters dealing with Basin and Range extension, Eocene magmatism, Mesozoic plutonism in the Sierra Nevada, Paleozoic subduction, and Ordovician stratigraphy, to name a few. The second section covers the geology of the Nevada Test Site and the nuclear-waste repository at Yucca Mountain. The last section is an invited field guide from the 1999 GSA Cordilleran Section meeting that covers the wines and geology of Napa Valley, California. Overall, Great Basin and Sierra Nevada is a comprehensive compilation of new and exciting research on this amazingly diverse region, with well-crafted guides to field localities of special interest. Full-color plates in some chapters make this guide an especially appealing and useful volume.