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At least 1,117 records · Page 62Linked to original sources
USGS2011

Location and age of foraminifer samples examined by Chevron Petroleum Company paleontologists from more than 2,500 oil test wells in California

Chevron Petroleum Company in 2001 donated an estimated 50,000 foraminifer slides, 5,000 well logs, geologic and surface locality maps, and paleontologic reports to the California Academy of Sciences and Stanford University for safekeeping, because they stopped or cut back exploration for petroleum deposits in California. The material was loaned to Earl Brabb temporarily so that information useful to the U.S. Geological Survey could be extracted. Among the estimated 5,000 well logs, more than 2,500 were printed on fragile Ozalid paper that had deteriorated by turning brown and hardening so that they could be easily damaged. These 2,516 well logs were scanned to provide a digital copy of the information. The 2,516 wells extend over an area from Eureka in Humboldt County south to the Imperial Valley and from the Pacific Ocean east to the eastern side of the Great Valley and the Los Angeles Basin. The wells are located in 410 7.5-minute quadrangle maps in 42 counties. The digital information herein preserves the data, makes the logs easily distributed to others interested in subsurface geology, and makes previously proprietary information widely available to the public for the first time.

California↗
USGS2018

California State Waters Map Series — Offshore of Point Conception, California

Introduction In 2007, the California Ocean Protection Council initiated the California Seafloor Mapping Program (CSMP), designed to create a comprehensive seafloor map of high-resolution bathymetry, marine benthic habitats, and geology within the 3-nautical-mile limit of California’s State Waters. The CSMP approach is to create highly detailed seafloor maps through collection, integration, interpretation, and visualization of swath sonar data, acoustic backscatter, seafloor video, seafloor photography, high-resolution seismic-reflection profiles, and bottom-sediment sampling data. The map products display seafloor morphology and character, identify potential marine benthic habitats, and illustrate both the surficial seafloor geology and shallow subsurface geology. The Offshore of Point Conception map area is in the westernmost part of the Western Transverse Ranges geologic province, which is north of the California Continental Borderland. Significant clockwise rotation—at least 90°—since the early Miocene has been proposed for the Western Transverse Ranges province, and this region is presently undergoing north-south shortening. The offshore part of the map area lies south of the stee p south and west flanks of t he Santa Ynez Mountains . The crest of the range , which has a maximum elevation of about 340 m in the map area, lies about 5 km north and east of the arcuate shoreline. The onland part of the coastal zone is remote and sparsely populated. The road to Jalama Beach County Park provides the only public coastal access in the entire map area. North of this county park, the coastal zone is part of Vandenberg Air Force Base. South of Jalama Beach County Park, most of the coastal zone is part of the Cojo-Jalama Ranch, purchased by the Nature Conservancy in December 2017. A relatively small part of the coastal zone in the eastern part of the map area lies within the privately owned Hollister Ranch. The nearest significant commercial centers are Lompoc (population, about 42,000), about 10 km north of the map area, and Goleta (population, about 30,000), about 50 km east of the map area. The Union Pacific railroad tracks run west and northwest along the coast through the entire map area, within a few hundred meters of the shoreline. The map area has a long history of petroleum exploration, and the seafloor notably includes large asphalt mounds and pockmarks that result from petroleum seepage. Several offshore gas and oil fields were discovered, and some were developed, in and on the margin of California’s State Waters. Much of the shoreline in the Offshore of Point Conception map area is characterized by narrow beaches that have thin sediment cover above bedrock platforms, backed by low (10- to 20-m-high) cliffs that are capped by a coastal terrace. Beaches are subject to wave erosion during winter storms, followed by gradual sediment recovery or accretion in the late spring, summer, and fall months during the gentler wave climate. The map area lies in the west-central part of the Santa Barbara littoral cell, which is characterized by west-to-east transport of sediment from Point Arguello on the northwest to Hueneme and Mugu Canyons on the southeast. Sediment supply to the map area is mainly from relatively small coastal watersheds, including the Jalama Creek–Espada Creek drainage basin (about 63 km 2 ), as well as Cañada del Jolloru, Black Canyon, Wood Canyon, Cañada del Cojo, and Barranca Honda. Coastal-watershed discharge and sediment load are highly variable, characterized by brief large events during major winter storms and long periods of low (or no) flow and minimal sediment load between storms. In recent (recorded) history, the majority of high-discharge, high-sediment-flux events have been associated with El Niño phases of the El Niño–Southern Oscillation climatic pattern. Following the coastline, the shelf bends to the north and northwest around Point Conception, and the trend of the shelf break changes from about 298° to 241° azimuth. Shelf width ranges from about 5 km south of Point Conception to about 11 km northwest of it; the slope ranges from about 1.0° to 1.2° to about 0.7° south and northwest of Point Conception, respectively. Southwest of Point Conception, the shelf break and upper slope are incised by a 600-m-wide, 20- to 30-m-deep, south-facing trough, one of five heads of the informally named Arguello submarine canyon. The map area is located at a major biogeographic transition zone between the east-west-trending Santa Barbara Channel region of the Southern California Bight and the northwest-trending central California coast. North of Point Conception, the coast is subjected to high wave exposure from the north, west, and south, as well as consistently strong upwelling that brings cold, nutrient-rich waters to the surface. Southeast of Point Conception, the Santa Barbara Channel is largely protected from strong north swells by Point Conception and from south swells by the Channel Islands; surface waters are warmer, and upwelling is weak and seasonal. Seafloor habitats in the broad Santa Barbara Channel region consist of significant amounts of soft, unconsolidated sediment interspersed with isolated areas of rocky habitat that support kelp-forest communities in the nearshore and rocky-reef communities in deeper water. The potential marine benthic habitat types mapped in the Offshore of Point Conception map area are directly related to its Quaternary geologic history, geomorphology, and active sedimentary processes. These potential habitats lie primarily within the Shelf (continental shelf) but also partly within the Flank (basin flank or continental slope) megahabitats. The fairly homogeneous seafloor of sediment and low-relief bedrock provides characteristic habitat for rockfish, groundfish, crabs, shrimp, and other marine benthic organisms. Several areas of smooth sediment form nearshore terraces that have relatively steep, smooth fronts, which are attractive to groundfish. Below the steep shelf break, soft, unconsolidated sediment is interrupted by the heads of several submarine canyons, gullies, and rills, also good potential habitat for rockfish. The map area includes the large (58.3 km 2 ) Point Conception State Marine Reserve.

California↗
USGS2025

Contributions of erosion, deposition, and human activities to a change in sand storage in the bed of San Francisco Bay, California, 1980s to 2010s

This study by the U.S. Geological Survey (USGS) provides estimates of the change in sand storage in bed sediments from the 1980s to 2010s in the San Francisco Bay area, California. The study is part of a larger project called “Research to Understand Impacts of Bay Sand Mining on Sand Transport in San Francisco Bay and the Outer Coast” that has the goal of providing information for the California Coastal Conservancy to inform decision making regarding sand mining activities. Information from this study will contribute to the sand budget for the San Francisco Bay system by accounting for sand made available by erosion of bay sediment and sequestered by deposition in the bay. Sediment budgets for estuaries typically account for change in sediment storage in the bed without discriminating for sediment size. However, the physics of mud and sand erosion, deposition, and transport differ. Sediment budgets that treat mud and sand separately give a more complete understanding of the system, including how human activities related to sediment size, such as sand mining, affect the system. We used bathymetric change analysis in combination with a three-dimensional model to generate estimates of net change in sand storage within the San Francisco Bay floor. We document sediment volume change from a 1980s bathymetric surface to a 2010s bathymetric surface, in combination with information on the sand content of the bed sediment derived from sediment cores and surface samples from six different sediment studies, to estimate the net change in sand volume in the bed of San Francisco Bay. This analysis includes areas heavily affected by human activities (such as sand mining, dredging, and sediment disposal) as well as regions more representative of natural transport processes. Overall, the sediment bed of San Francisco Bay is losing sand. Across the total area surveyed in San Francisco Bay, including areas affected by natural processes, oyster shell beds, and human activities, a net loss of about 17 million cubic meters (Mm 3 ) of sand from the sediment bed occurred from the 1980s to 2010s, at a rate of about 0.8 Mm 3 per year. For the period of this study, sand loss from bed level changes in permitted sand-lease mining areas (about 11 Mm 3 ) accounts for about two-thirds of the total sand loss throughout the study area. It is important to consider potential uncertainty bounds when interpreting these findings. A key part of the report is an assessment of the uncertainties in our estimates of sand volumes. We estimate that variability in modeled sand content values of Bay floor sediments could result in an uncertainty of approximately 25 percent of the net sand volume change. Even larger uncertainty amounts may be associated with uncertainty in the systematic errors in the bathymetric surveys. Further refining estimates of uncertainty in bathymetric change is important in guiding the use of this study. The results presented here can fill a critical gap that may enable the creation of the first comprehensive sand budget of San Francisco Bay.

California↗
USGS2018

California State Waters Map Series — Offshore of Gaviota, California

Introduction In 2007, the California Ocean Protection Council initiated the California Seafloor Mapping Program (CSMP), designed to create a comprehensive seafloor map of high-resolution bathymetry, marine benthic habitats, and geology within the 3-nautical-mile limit of California’s State Waters. The CSMP approach is to create highly detailed seafloor maps through collection, integration, interpretation, and visualization of swath sonar data, acoustic backscatter, seafloor video, seafloor photography, high-resolution seismic-reflection profiles, and bottom-sediment sampling data. The map products display seafloor morphology and character, identify potential marine benthic habitats, and illustrate both the surficial seafloor geology and shallow subsurface geology. The map area is in the southern part of the Western Transverse Ranges geologic province, which is north of the California Continental Borderland. Significant clockwise rotation—at least 90°—since the early Miocene has been proposed for the Western Transverse Ranges province, and the region is presently undergoing north-south shortening. The offshore part of the map area lies south of the steep south flank of the Santa Ynez M ountains . The crest of the range , which has a maximum elevation of about 760 m in the map area , lies about 4 km north of the shoreline. Gaviota is an unincorporated community that has a sparse population (less than 100), and the coastal zone is largely open space that is locally used for cattle grazing. The Union Pacific railroad tracks extend westward along the coast through the entire map area, within a few hundred meters of the shoreline. Highway 101 crosses the eastern part of the map area, also along the coast, then turns north (inland) and travels through Cañada de la Gaviota and Gaviota Pass en route to Buellton. Gaviota State Park lies at the mouth of Cañada de la Gaviota. West of Gaviota, the onland coastal zone is occupied by the Hollister Ranch, a privately owned, gated community that has no public access. The map area has a long history of petroleum exploration and development. Several offshore gas fields were discovered and were developed by onshore directional drilling in the 1950s and 1960s. Three offshore petroleum platforms were installed in adjacent federal waters in 1976 (platform “Honda”) and 1989 (platforms “Heritage” and “Harmony”). Local offshore and onshore operations were serviced for more than a century by the Gaviota marine terminal, which is currently being decommissioned and will be abandoned in an intended transition to public open space. The Offshore of Gaviota map area lies within the western Santa Barbara Channel region of the Southern California Bight, and it is somewhat protected from large Pacific swells from the north and northwest by Point Conception and from south and southwest swells by offshore islands and banks. Much of the shoreline in the map area is characterized by narrow beaches that have thin sediment cover, backed by low (10- to 20-m-high) cliffs that are capped by a narrow coastal terrace. Beaches are subject to wave erosion during winter storms, followed by gradual sediment recovery or accretion in the late spring, summer, and fall months during the gentler wave climate. The map area lies in the western-central part of the Santa Barbara littoral cell, which is characterized by west-to-east transport of sediment from Point Arguello on the northwest to Hueneme and Mugu Canyons on the southeast. Sediment supply to the western and central part of the littoral cell is mainly from relatively small coastal watersheds. In the map area, sediment sources include Cañada de la Gaviota (52 km 2 ), as well as Cañada de la Llegua, Arroyo el Bulito, Cañada de Santa Anita, Cañada de Alegria, Cañada del Agua Caliente, Cañada del Barro, Cañada del Leon, Cañada San Onofre, and many others. Coastal-watershed discharge and sediment load are highly variable, characterized by brief large events during major winter storms and long periods of low (or no) flow and minimal sediment load between storms. In recent (recorded) history, the majority of high-discharge, high-sediment-flux events have been associated with El Niño phases of the El Niño–Southern Oscillation climatic pattern. Shelf width in the Offshore of Gaviota map area ranges from about 4.3 to 4.7 km, and shelf slopes average about 1.0° to 1.2° but are highly variable because of the presence of the large Gaviota sediment bar. This bar extends southwestward for about 9 km from the mouth of Cañada de la Gaviota to the shelf break, is as wide as 2 km, and is by far the largest shore-attached sediment bar in the Santa Barbara Channel. The shelf is underlain by bedrock and variable amounts (0 to as much as 36 m in the Gaviota bar) of upper Quaternary sediments deposited as sea level fluctuated in the late Pleistocene. The trend of the shelf break changes from about 276° to 236° azimuth over a distance of about 12 km, and it ranges in depth from about 91 m to as shallow as 62 to 73 m where significant shelf-break and upper-slope failure and landsliding has apparently occurred. The shelf break in the western part of the map area is notably embayed by the heads of three large (150- to 300-m-wide) channels that have been referred to as “the Gaviota Canyons” or as “Drake Canyon,” “Sacate Canyon,” and “Alegria Canyon.” Seafloor habitats in the broad Santa Barbara Channel region consist of significant amounts of soft, unconsolidated sediment interspersed with isolated areas of rocky habitat that support kelp-forest communities in the nearshore and rocky-reef communities in deeper water. The potential marine benthic habitat types mapped in the Offshore of Gaviota map area are directly related to its Quaternary geologic history, geomorphology, and active sedimentary processes. These potential habitats lie primarily within the Shelf (continental shelf) but also partly within the Flank (basin flank or continental slope) megahabitats. The fairly homogeneous seafloor of sediment and low-relief bedrock provides characteristic habitat for rockfish, groundfish, crabs, shrimp, and other marine benthic organisms. Several areas of smooth sediment form nearshore terraces that have relatively steep, smooth fronts, which may be attractive to groundfish. Below the steep shelf break, soft, unconsolidated sediment is interrupted by the heads of several submarine canyons and rills, some bedrock exposures, and small carbonate mounds associated with asphalt mounds and pockmarks, also good potential habitat for rockfish. The map area includes the relatively small (5.2 km 2 ) Kashtayit State Marine Conservation Area, which largely occupies the inner part of the Gaviota sediment bar.

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USGS1978

The marine geology of the eastern Santa Barbara Channel, with particular emphasis on the ground water basins offshore from the Oxnard Plain, Southern California

Marine geophysical investigations provide new data concerning the stratigraphy, tectonic and sedimentary history, and the ground water geology of the southeastern Santa Barbara Channel region. The offshore stratigraphy identified in seismic reflection profiles includes a succession of Neogene to Quaternary strata. The middle Miocene Conejo volcanics form an acoustical basement and the overlying late Cenozoic sedimentary rocks attain a thickness greater than 2,500 m. These sedimentary deposits fill a structurally controlled, physiographic and depositional depression called the Ventura Basin. Structure consists generally of a gently folded, east-trending Tertiary synclinorium bordered on the north by a regional thrust fault and on the south by a steep asymmetrical anticlinal ridge. Most structures show evidence of north-south compression that occurred during early Pleistocene time. Three well-defined unconformities represent widespread erosion in late Miocene, early to middle Pleistocene, and late Pleistocene time. The boundaries of Miocene, Pliocene, and lower Pleistocene strata continue uninterrupted eastward along the southern part of Santa Barbara basin to Hueneme Canyon, where they turn northeast and can be traced to the coast near Port Hueneme. These limits probably represent the south edge of the Santa Barbara basin during Pliocene and Pleistocene time. Fresh water-bearing materials of the Oxnard plain are unconsolidated Quaternary sediment laid down on more consolidated Tertiary rocks. Offshore, the total fresh water-bearing materials distinguished in the seismic reflection profiles attain a thickness of about 356 m and have an areal extent of over 760 km2. Strata that contain the offshore continuation of the five major on-land aquifers (Grimes Canyon, Fox Canyon, Hueneme, Mugu, and Oxnard aquifers) are identified in the seismic reflection profiles. These strata make up the two offshore ground-water basins, the Mound and Oxnard plain ground-water basins, which are separated by the east-west trending Oak Ridge fault. Possible entrance areas for salt water intrusion into fresh water aquifers are found along the walls of the submarine canyons and along the northern slopes of Santa Barbara and Santa Monica basins. Hueneme and Mugu aquifers are probably exposed locally in all five submarine canyons of the Oxnard offshore area and may also crop out along the upper northern slope of Santa Monica basin. In all of these areas, salt water readily intrudes the aquifers. A salinity-temperature-depth study made in April, 1971, does not indicate any great dilution of surface ocean water by fresh water that could be 'leaking' from the exposed aquifers along the walls of Hueneme Canyon and the landward slope of Santa Barbara Channel. Earthquakes in the vicinity of the Oxnard plain suggest that the region is seismically active. Epicenters are widely dispersed over the region. No distinct trend or alignment of earthquake epicenters occurs near the trace of any of the faults, although many epicenters are scattered around the Oak Ridge zone of deformation in the northern part of the region. The largest magnitude earthquake recorded in the area was a magnitude 5.7 that occurred on February 21, 1973, offshore of Point Mugu, south of the Oxnard plain.

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USGS2013

Geologic map of the Valley Mountain 15’ quadrangle, San Bernardino and Riverside Counties, California

The Valley Mountain 15’ quadrangle straddles the Pinto Mountain Fault, which bounds the eastern Transverse Ranges in the south against the Mojave Desert province in the north. The Pinto Mountains, part of the eastern Transverse Ranges in the south part of the quadrangle expose a series of Paleoproterozoic gneisses and granite and the Proterozoic quartzite of Pinto Mountain. Early Triassic quartz monzonite intruded the gneisses and was ductiley deformed prior to voluminous Jurassic intrusion of diorite, granodiorite, quartz monzonite, and granite plutons. The Jurassic rocks include part of the Bullion Mountains Intrusive Suite, which crops out prominently at Valley Mountain and in the Bullion Mountains, as well as in the Pinto Mountains. Jurassic plutons in the southwest part of the quadrangle are deeply denuded from midcrustal emplacement levels in contrast to supracrustal Jurassic limestone and volcanic rocks exposed in the northeast. Dikes inferred to be part of the Jurassic Independence Dike Swarm intrude the Jurassic plutons and Proterozoic rocks. Late Cretaceous intrusion of the Cadiz Valley Batholith in the northeast caused contact metamorphism of adjacent Jurassic plutonic rocks. The Tertiary period saw emplacement of basanitoid basalt at about 23 Ma and deposition of Miocene and (or) Pliocene ridge-capping gravels. An undated east-dipping low-angle normal fault zone in the Pinto Mountains drops hanging-wall rocks eastward and may account for part of the contrast in uplift history across the quadrangle. The eastern Transverse Ranges are commonly interpreted as severely rotated clockwise tectonically in the Neogene relative to the Mojave Desert, but similar orientations of Jurassic dike swarms suggest that any differential rotation between the two provinces is small in this quadrangle. The late Cenozoic Pinto Mountain Fault and other strike-slip faults cut Quaternary deposits in the quadrangle, with two northwest-striking faults cutting Holocene deposits. Geographic Information System and metadata on most geologic features are available on the Geologic map of the Sheep Hole Mountains 30’ by 60’ quadrangle, U.S. Geological Survey map MF–2234, scale 1:100,000, available at http://pubs.usgs.gov/mf/2002/2344/.

California↗
USGS1992

The Conterminous United States Mineral Assessment Project; background information to accompany folio of geologic, geochemical, geophysical, and mineral resources maps of the Reno 1 by 2 degree Quadrangle, Nevada and California

The Reno 1 ? by 2 ? quadrangle in west-central Nevada was studied by an interdisciplinary research team to appraise its mineral resources. The assessment is based on geological, geochemical, and geophysical field and laboratory investigations, the results of which are published as a folio of maps, reports, figures, and tables, with accompanying discussions. This circular provides background information on the investigations and integrates the information presented in the folio. The selected bibliography lists references to the geology, geochemistry, geophysics, and mineral deposits of the Reno 1 ? by 2 ? quadrangle.

Circular↗
USGS2006

A stochastic estimate of ground motion at Oceano, California, for the M 6.5 22 December 2003 San Simeon earthquake, derived from aftershock recordings

The U.S. Geological Survey deployed a digital seismic station in Oceano, California, in February 2004, to investigate the cause of damage and liquefaction from the 22 December 2003 M 6.5 San Simeon earthquake. This station recorded 11 M > 2.8 aftershocks in almost 8 weeks. We analyze these recordings, together with recordings of the mainshock and the same aftershocks obtained from nearby stations in Park Hill and San Luis Obispo, to estimate the mainshock ground motion in Oceano. We estimate the Fourier amplitude spectrum using generalized spectral ratio analysis. We test a set of aftershocks as Green's functions by comparing simulated and recorded acceleration amplitude spectra for the mainshock at San Luis Obispo and Park Hill. We convolve the aftershock accelerograms with a stochastic operator to simulate the duration and phase of the mainshock accelerograms. This approximation allows us to extend the range of aftershocks that can be used as Green's functions to events nearly three magnitude units smaller than the mainshock. Our realizations for the mainshock accelerogram at Oceano yield peak ground accelerations distributed as 28% ?? 4%g. We interpret these realizations as upper bounds for the actual ground motion, because our analysis assumes a linear response, whereas the presence of liquefaction indicates that the ground behaved nonlinearly in Oceano.

Bulletin of the Seismological Society of America↗
USGS1986

The Conterminous United States Mineral Appraisal Program; background information to accompany folio of geologic, geochemical, geophysical, and mineral resources maps of the Medford 1 degree x 2 degrees Quadrangle, Oregon and California

The Medford 1 ? by 2 ? quadrangle in southern Oregon and northern California was studied by an interdisciplinary research team to appraise its mineral resources. The appraisal is based on geological, geochemical, and geophysical field and laboratory investigations, the results of which are published as a folio of maps, figures, and tables, with accompanying discussions. This circular provides background information on the investigations and integrates the information presented in the folio. The bibliography lists selected references to the geology, geochemistry, geophysics, and mineral deposits of the Medford 1 ? by 2 ? quadrangle.

Circular↗
USGS2012

Bathymetry and acoustic backscatter-outer mainland shelf, eastern Santa Barbara Channel, California

In 2010 and 2011, scientists from the U.S. Geological Survey (USGS), Pacific Coastal and Marine Science Center (PCMSC), acquired bathymetry and acoustic-backscatter data from the outer shelf region of the eastern Santa Barbara Channel, California. These surveys were conducted in cooperation with the Bureau of Ocean Energy Management (BOEM). BOEM is interested in maps of hard-bottom substrates, particularly natural outcrops that support reef communities in areas near oil and gas extraction activity. The surveys were conducted using the USGS R/V Parke Snavely, outfitted with an interferometric sidescan sonar for swath mapping and real-time kinematic navigation equipment. This report provides the bathymetry and backscatter data acquired during these surveys in several formats, a summary of the mapping mission, maps of bathymetry and backscatter, and Federal Geographic Data Committee (FGDC) metadata.

California↗
USGS2023

The 3D Elevation Program—Supporting Washington's economy

Washington State has a geographically diverse and spectacular landscape that is divided to the east and west by the largely volcanic mountains of the Cascade Range. Approximately 88 percent of the population lives in western Washington, mostly in urban areas. The climate is varied, with high precipitation and seasonal flooding in the western part of the State, while drier conditions are found east of the Cascades. Where the terrain is mountainous, the dominant vegetation is coniferous forests, which are prone to frequent seasonal fires. The climate and land use in combination with a dynamic geology result in frequent landslides. Washington has the second highest risk, after California, of large and damaging earthquakes because of its geologic setting. Critical applications that meet the State’s management needs depend on light detection and ranging (lidar) data that provide a highly detailed three-dimensional (3D) model of the Earth’s surface and aboveground features.

Washington↗
USGS2011

Database for potential hazards from future volcanic eruptions in California

More than 500 volcanic vents have been identified in the State of California. At least 76 of these vents have erupted, some repeatedly, during the past 10,000 yr. Past volcanic activity has ranged in scale and type from small rhyolitic and basaltic eruptions through large catastrophic rhyolitic eruptions. Sooner or later, volcanoes in California will erupt again, and they could have serious impacts on the health and safety of the State's citizens as well as on its economy. This report describes the nature and probable distribution of potentially hazardous volcanic phenomena and their threat to people and property. It includes hazard-zonation maps that show areas relatively likely to be affected by future eruptions in California. This digital release contains information from maps of potential hazards from future volcanic eruptions in the state of California, published as Plate 1 in U.S. Geological Survey Bulletin 1847. The main component of this digital release is a spatial database prepared using geographic information systems (GIS) applications. This release also contains links to files to view or print the map plate, main report text, and accompanying hazard tables from Bulletin 1847. It should be noted that much has been learned about the ages of eruptive events in the State of California since the publication of Bulletin 1847 in 1989. For the most up to date information on the status of California volcanoes, please refer to the U.S. Geological Survey Volcano Hazards Program website.

California↗
USGS2024

Mafic alkaline magmatism and rare earth element mineralization in the Mojave Desert, California: The Bobcat Hills connection to Mountain Pass

Occurrences of alkaline and carbonatite rocks with high concentrations of rare earth elements (REE) are a defining feature of Precambrian geology in the Mojave Desert of southeastern California. The most economically important occurrence is the carbonatite stock at Mountain Pass, which constitutes the largest REE deposit in the United States. A central scientific goal is to understand the genesis of the carbonatite ore body in the context of widespread REE-rich igneous activity. A swarm of mafic alkaline (shonkinite) dikes has been mapped and sampled at Bobcat Hills, 65 km southeast of the Mountain Pass mine. Whole-rock geochemistry and zircon geochronology demonstrate a clear affinity to the ca. 1.4 Ga Mountain Pass intrusive system. Bobcat Hills dikes have comparably high REE concentrations (La ∼1,000× chondritic) and an error-weighted mean 207 Pb/ 206 Pb zircon crystallization age of 1,426 ± 2 Ma (2 σ ). Unlike the alkaline intrusions at Mountain Pass, which have abundant inherited zircon from Paleoproterozoic basement rocks and crustally influenced oxygen isotope compositions (δ 18 O zircon = 6.5–7.5‰), the Bobcat Hills dikes lack any evidence of crustal assimilation and have oxygen isotope values that overlap a mantle range (Bobcat Hills average δ 18 O zircon = 5.6 ± 0.3‰). The dikes were a high-temperature, early center of mafic alkaline magmatism in the Mojave Desert that serve as a snapshot of melt generation from a spatially extensive, metasomatized mantle source. We propose that modification of the crust over many tens of Myr at Mountain Pass created an environment that favored crustal assimilation and enabled ascent of late-stage, REE-rich carbonatite magmas.

California↗
USGS1977

A compilation of data from the July, 1976 Oroville, California seismic-refraction experiment

The August 1, 1975, Oroville, California, earthquake (M b = 5.7) and its aftershocks focused considerable interest on current tectonic processes operating in the western foothills of the Sierra Nevada range. As part of an intensive study of this earthquake sequence, the United States Geological Survey with support from the California Department of Water Resources conducted a seismic refraction experiment during July 1976 in the Oroville region. The objective of the experiment is to resolve more clearly the P- and S-wave velocity structure of the crust in the region as a basis for both more accurate hypocenter locations and evidence on the physical properties at depth within the crust.

California↗
USGS1990

The Loma Prieta earthquake of October 17, 1989 : a brief geologic view of what caused the Loma Prieta earthquake and implications for future California earthquakes: What happened ... what is expected ... what can be done.

The San Andreas fault, in California, is the primary boundary between the North American plate and the Pacific plate. Land west of the fault has been moving northwestward relative to land on the east at an average rate of 2 inches per year for millions of years. This motion is not constant but occurs typically in sudden jumps during large earthquakes. This motion is relentless; therefore earthquakes in California are inevitable.

California↗