ABSTRACT Application of updated diatom biochronology to the Monterey Formation and related biosiliceous rocks reveals the imprint of both global paleoclimatic/paleoceanographic and regional tectonic events. A rise in global sea level combined with regional tectonic deepening associated with the development of the transform California margin resulted in the abrupt onset of deposition of fine-grained Monterey sediments that were relatively free from clastic debris between 18 and 16 Ma. The base of the Monterey Formation does not mark a silica shift in diatom deposition from the North Atlantic to the North Pacific Ocean. Rather, a North Atlantic Ocean decline of diatoms after ca. 13 Ma and increasing divergence in nutrient levels between the North Atlantic and North Pacific Oceans between ca. 13 and 11 Ma coincided with a major enhancement of diatom deposition in the Monterey Formation. A stratigraphically condensed interval of phosphate-rich sediments between 13 and 10 Ma in coastal southern California appears to have resulted from sediment starvation in offshore basins during a period of higher sea level, as inland sections such as those in the San Joaquin Valley commonly contain thick sequences of diatomaceous sediment. Increasing latitudinal thermal gradients in the latest Miocene, which triggered a biogenic bloom in the equatorial Pacific Ocean at 8 Ma, also led to enhanced diatom deposition in the uppermost Monterey Formation and overlying biosiliceous rocks. Uplift of the California coastal ranges after ca. 5.2 Ma resulted in an increasing detrital contribution that obscured the presence of diatoms in onshore sediments. Major reduction in coastal upwelling in the early Pliocene ca. 4.6 Ma then caused a drastic reduction of diatoms in sediments offshore southern California.

ABSTRACT This study presents three regional cross sections, a structural map analysis, and a schematic map restoration. The sections are constrained by surface geology and petroleum wells and were developed using model-based methods to be consistent with the regional tectonic context and balancing concepts. Together, these products depict the geometry and kinematics of the major fault systems. Insights from this research include the following. Franciscan complex blueschist-facies rocks in the Mount Diablo region were unroofed west of their current location and subsequently thrust beneath the Great Valley sequence in the mid-Eocene. East Bay structures are complicated by overprinting of Neogene compression and dextral strike-slip motion on a Paleogene graben system. Net lateral displacement between the Hayward fault and the Central Valley varies from 26 km toward 341° to 29 km toward 010° in the southern and northern East Bay Hills, respectively. Uplift above a wedge thrust generates the principal Neogene structural high, which extends from Vallejo through Mount Diablo to the Altamont Ridge. Anomalous structural relief at Mount Diablo is due to strike-parallel thrusting on the crest of a fault-propagation fold formed on the west-verging roof thrust. Uplift that exposes the Coast Range ophiolite in the East Bay Hills is formed by oblique thrusting generated by slip transfer at the northern termination of the Calaveras fault. The Paleogene extensional fault system likely extends farther west than previously documented. An east-dipping branch of that system may underlie the Walnut Creek Valley. Three-dimensional restoration should be applied to constrain geologic frameworks to be used for seismic velocity modeling.

ABSTRACT Two spatially separated areas of Neogene volcanic rocks are located on the northeast limb of the Mount Diablo anticline. The southernmost outcrops of volcanics are 6 km east of the summit of Mount Diablo in the Marsh Creek area and consist of ~12 hypabyssal dacite intrusions dated at ca. 7.8–7.5 Ma, which were intruded into the Great Valley Group of Late Cretaceous age. The intrusions occur in the vicinity of the Clayton and Diablo faults. The rocks are predominantly calc-alkaline plagioclase biotite dacites, but one is a tholeiitic plagioclase andesite. Mercury mineralization was likely concomitant with emplacement of these late Miocene intrusions. The northernmost outcrops of Neogene volcanic rocks occur ~15 km to the north of Mount Diablo in the Concord Naval Weapons Station and the Los Medanos Hills and are probably parts of a single andesite flow. A magnetometer survey indicates that the flow originated from a feeder dike along the Clayton fault. The lava flow is flat-lying and occupies ancient stream channels across an erosional surface of tilted Markley Sandstone of middle Eocene age. New radiometric dates of the flow yield an age of 5.8–5.5 Ma, but due to alteration the age should be used with caution. The flow is a calc-alkaline andesite rich in clinopyroxene and plagioclase. What appear to be uplifted erosional remnants of the flow can be traced northeastward in the Los Medanos Hills across a surface of tilted Cenozoic rocks that eventually rest on formations as young as the Lawlor Tuff dated at 4.865 ± 0.011 Ma. This stratigraphic relationship suggests that the andesite flow is probably late Pliocene in age and was impacted by the more recent uplift of the Los Medanos Hills but postdates the regional folding and faulting of the rocks of Mount Diablo. In terms of timing, location, and composition, the evidence suggests these two areas of dacitic and andesitic volcanics fit into a series of migrating volcanic centers in the California Coast Ranges that erupted following the northward passage of the Mendocino Triple Junction.

ABSTRACT Over the past 150 years, Mount Diablo has served as a window into the evolving understanding of California geology. In the 1800s, geologists mapped this easily accessible peak located less than 100 km (62 miles) from the rapidly growing city of San Francisco and the geology departments at the University of California at Berkeley and Stanford University. Later, the mountain served as a focal point for investigating San Francisco Bay area tectonics. The structural interpretation of the up-thrusting mechanisms has evolved from a simple compressional system involving a few local faults to a more complex multifault and multiphase mountain-building theory. The stratigraphic interpretation and understanding have been advanced from a general description of the lithologies and fossils to a detailed description using sequence stratigraphy to define paleogeographic settings and depositional regimes.

ABSTRACT Franciscan subduction complex rocks of Mount Diablo form a 8.5 by 4.5 km tectonic window, elongated E-W and fault-bounded to the north and south by rocks of the Coast Range ophiolite and Great Valley Group, respectively, which lack the burial metamorphism and deformation displayed by the Franciscan complex. Most of the Franciscan complex consists of a stack of lawsonite-albite–facies pillow basalt overlain successively by chert and clastic sedimentary rocks, repeated by faults at hundreds of meters to <1 m spacing. Widely distributed mélange zones from 0.5 to 300 m thick containing high-grade (including amphibolite and eclogite) assemblages and other exotic blocks, up to 120 m size, form a small fraction of exposures. Nearly all clastic rocks have a foliation, parallel to faults that repeat the various lithologies, whereas chert and basalt lack foliation. Lawsonite grew parallel to foliation and as later grains across foliation. The Franciscan-bounding faults, collectively called the Coast Range fault, strike ENE to WNW and dip northward at low to moderate average angles and collectively form a south-vergent overturned anticline. Splays of the Coast Range fault also cut into the Franciscan strata and Coast Range ophiolite and locally form the Coast Range ophiolite–Great Valley Group boundary. Dip discordance between the Coast Range fault and overlying Great Valley Group strata indicates that the northern and southern Coast Range fault segments were normal faults with opposite dip directions, forming a structural dome. These relationships suggest accretion and fault stacking of the Franciscan complex, followed by exhumation along the Coast Range fault and then folding of the Coast Range fault.

ABSTRACT The California Coast Ranges mercury deposits are part of the western North America mercury belt, in which mercury occurs most commonly as red cinnabar (α-HgS), sometimes associated with its high-temperature polymorph, metacinnabar (β-HgS). In the Coast Ranges, ores were deposited from hydrothermal solutions and range in age from Miocene to Holocene. Ore deposition at Mount Diablo generally occurred along active faults and associated extension fractures in the Franciscan complex, most often in serpentinite that had been hydrothermally altered to silica-carbonate rock. The Mount Diablo mine lies ~48 km (~30 miles) northeast of San Francisco in Contra Costa County and is mineralogically unique in California, because metacinnabar, the higher-temperature polymorph of mercury sulfide, is a major primary ore mineral in the deposit, while at all other mercury mines in California, it is quite rare. In addition, hydrothermal activity is so recent that sulfurous gases and methane continued to be released into the mine at least into the 1940s. Historically, long before active large-scale mining began in the 1800s, the Mount Diablo mercury deposits were known to the Indigenous people of the Ohlone tribes, who used the cinnabar in rituals as well as for red pigment to decorate their bodies, and as a prized trade item. The deposit was later rediscovered in 1863 and mined intermittently until 1958. The Mount Diablo mine and adjacent Rhyne (also variously spelled Ryne or Rhine) mine were the sites of most of the mercury operations in the region, and at both mines, mercury ore occurs in structurally controlled lenticular bodies of silica-carbonate rock and serpentinite. The total district production probably exceeded 12,300 flasks (at 76 pounds or ~34.5 kg per flask) at an estimated grade of 2711 g per metric ton. Low-grade ore reserves are believed to still exist, with 17,000 short tons of indicated and inferred ore. Other minor deposits of copper, silver, and gold occur on Mount Diablo, principally in and around Eagle Peak, but mercury is not associated with these deposits. The most serious environmental problem associated with the mercury mines is the transport of mercury via mine drainage from the Mount Diablo mines to Dunn Creek, a tributary to Marsh Creek, which ultimately drains into Suisun and San Francisco Bay. The concern is that elemental mercury (Hg 0 ) and the mercury ion Hg(II) can be biologically converted to toxic monomethylmercury (MeHg). To combat this problem, a long-term process of mine remediation has been undertaken.

ABSTRACT The mid-Cenozoic succession in the northeast limb of the Mount Diablo anticline records the evolution of plate interactions at the leading edge of the North America plate. Subduction of the Kula plate and later Farallon plate beneath the North America plate created a marine forearc basin that existed from late Mesozoic to mid-Cenozoic times. In the early Cenozoic, extension on north-south faults formed a graben depocenter on the west side of the basin. Deposition of the Markley Formation of middle to late? Eocene age took place in the late stages of the marine forearc basin. In the Oligocene, the marine forearc basin changed to a primarily nonmarine basin, and the depocenter of the basin shifted eastward of the Midland fault to a south-central location for the remainder of the Cenozoic. The causes of these changes may have included slowing in the rate of subduction, resulting in slowing subsidence, and they might also have been related to the initiation of transform motion far to the south. Two unconformities in the mid-Cenozoic succession record the changing events on the plate boundary. The first hiatus is between the Markley Formation and the overlying Kirker Formation of Oligocene age. The succession above the unconformity records the widespread appearance of nonmarine rocks and the first abundant appearance of silicic volcanic detritus due to slab rollback, which reversed the northeastward migration of the volcanic arc to a more proximal location. A second regional unconformity separates the Kirker/Valley Springs formations from the overlying Cierbo/Mehrten formations of late Miocene age. This late Miocene unconformity may reflect readjustment of stresses in the North America plate that occurred when subduction was replaced by transform motion at the plate boundary. The Cierbo and Neroly formations above the unconformity contain abundant andesitic detritus due to proto-Cascade volcanism. In the late Cenozoic, the northward-migrating triple junction produced volcanic eruptive centers in the Coast Ranges. Tephra from these local sources produced time markers in the late Cenozoic succession.

ABSTRACT We present a tephrochronologic/chronostratigraphic database for the Mount Diablo area and greater San Francisco Bay region that provides a spatial and temporal framework for geologic studies in the region, including stratigraphy, paleogeography, tectonics, quantification of earth surface processes, recurrence of natural hazards, and climate change. We identified and correlated 34 tephra layers within this region using the chemical composition of their volcanic glasses, stratigraphic sequence, and isotopic and other dating techniques. Tephra layers range in age from ca. 65 ka to ca. 29 Ma, as determined by direct radiometric techniques or by correlation to sites where they have been dated. The tephra layers are of Quaternary or Neogene age except for two that are of Oligocene age. We correlated the tephra layers among numerous sites throughout northern California. Source areas of the tephra layers are the Snake River–Yellowstone hotspot trend of northern Nevada, southern Idaho, and western Wyoming; the Nevadaplano caldera complex of central Nevada; the Jemez Mountains–Valles Caldera in northwestern New Mexico; the Southern Nevada volcanic field and related source areas in eastern California and west-central Nevada; the Quien Sabe–Sonoma volcanic centers of the California Coast Ranges; and the young Cascade Range volcanic centers of northeastern California and Oregon.

ABSTRACT We extend a published 9000 yr fire history record from Little Lake, in the Oregon Coast Range, to 35,000 yr and compare it with the established pollen record from the site. The fire history is based on a high-resolution analysis of charcoal preserved in lake sediments, providing a fire history record that spans the Last Glacial Maximum in North America. The data enabled us to address questions regarding the interactions between large-scale climate changes associated with the shift from glacial to interglacial conditions and the accompanying changes in forest vegetation and fire regimes. The vegetation history indicates a change from open subalpine forests to closed western hemlock and Douglas fir forests as climate moved from cold and dry full glacial to warm and wet Holocene conditions. The fire history indicates that although there was more biomass burned in the Holocene, the frequency of fires between glacial and interglacial conditions was not significantly different, and the fire frequency did not change in concert with regional shifts in vegetation. This suggests that fire is a product of seasonal or multiyear variations in climate that may not cause significant shifts in vegetation. Also, as this short-term climate variability becomes more common in the near future, conditions for fires in these mesic forests may become more common as well.

ABSTRACT The Carrizo Plain, the only closed basin in California’s Southern Coast Ranges, preserves landforms and deposits that record both climate change and tectonic activity. An extensive system of clay dunes documents the elevations of late Pleistocene and Holocene pans. Clay dune elevations, drowned shorelines, eroded anticlinal ridges, and zones of perturbed soil chemistry provide evidence of two lake levels higher than today’s (currently 581 m above sea level [masl]), one at ~591 masl at ca. 20 ka and another at ~585 masl that existed at ca. 10 ka, based on optically stimulated luminescence (OSL) dates on clay dune sediment. Two cores from the abandoned floor of the lake provide additional evidence of a long-lived lake in the Carrizo Plain during the late Pleistocene. The longer of the two cores (~42 m) was sampled for palynology, environmental magnetism, and scanning electron microscope–petrography. The magnetic susceptibility signal contains two notable features corresponding to sedimentary materials consistent with reducing conditions. The higher of these features occurs near the surface, and the lower occurs at ~18 m depth. A 14 C date on charcoal from the upper reduced zone places the top of this zone at no older than 22.6–20.9 cal ka. This date is consistent with the OSL date on geomorphic features associated with a highstand above ~591 masl. Assuming that reducing conditions correspond to at least a few meters’ depth of relatively fresh water, the new 14 C date suggests that the upper reduced zone represents a marine isotope stage (MIS) 2 pluvial maximum lake in the Carrizo Plain. Pollen and ostracodes from the reduced sediments indicate a wetter and cooler climate than today. These conditions would have been capable of sustaining a lake with water much less saline than that of the modern lake. The timing of the oldest documented highstand (no later than 20 ka) is consistent with a modified jet stream migration model and is not consistent with a tropical incursion model. Northeast-to-southwest asymmetry across the lake floor may be consistent with southwestward tilting driven by Coast Range shortening normal to the San Andreas fault, as is seen throughout the region.