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 Late Cenozoic growth of the Mount Diablo anticline in the eastern San Francisco Bay area, California, USA, has produced unique 3D exposures of stratigraphic relationships and normal faults that record Late Cretaceous uplift and early Tertiary extension in the ancestral California forearc basin. Several early Tertiary normal faults on the northeast flank of Mount Diablo have been correlated with structures that accommodated Paleogene subsidence of the now-buried Rio Vista basin north of Mount Diablo. Stepwise restoration of deformation at Mount Diablo reveals that the normal faults probably root into the “Mount Diablo fault,” a structure that juxtaposes blueschist-facies rocks of the Franciscan accretionary complex with attenuated remnants of the ophiolitic forearc basement and relatively unmetamorphosed marine forearc sediments. This structure is the local equivalent of the Coast Range fault, which is the regional contact between high-pressure Franciscan rocks and structurally overlying forearc basement in the northern Coast Ranges and Diablo Range, and it is folded about the axis of the Mount Diablo anticline. Apatite fission-track analyses indicate that the Franciscan rocks at Mount Diablo were exhumed and cooled from depths of 20+ km in the subduction zone between ca. 70−50 Ma. Angular unconformities and growth relations in the Cretaceous and Paleogene stratigraphic sections on the northeast side of Mount Diablo, and in the Rio Vista basin to the north, indicate that wholesale uplift, eastward tilting, and extension of the western forearc basin were coeval with blueschist exhumation. Previous workers have interpreted the structural relief associated with this uplift and tilting, as well as the appearance of Franciscan blueschist detritus in Late Cretaceous and early Tertiary forearc strata, as evidence for an “ancestral Mount Diablo high,” an emergent Franciscan highland bordering the forearc basin to the west. This outer-arc high is here interpreted to be the uplifted footwall of Coast Range fault. The stratigraphic and structural relations exposed at Mount Diablo support models for exposure of Franciscan blueschists primarily through syn-subduction extension and attenuation of the overlying forearc crust in the hanging wall of the Coast Range fault, accompanied by (local?) uplift and erosion of the exhumed accretionary prism in the footwall.

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 Mount Diablo Coalfield was the largest producer of coal in California from the 1860s to 1906. The now-depleted coalfield is located on the northeast limb of the Mount Diablo anticline. The mineable coal seams occur in the Middle Eocene Domengine Formation, which is predominantly composed of quartz-rich sandstone with several thin coal seams. As many as 26 mine operations were established to mine the coal, and it has been estimated that the total production exceeded 4 million tons. The coal fueled the industrial growth of the major cities of northern California. The mines closed at the turn of the nineteenth century as competition from better coals from Washington Territory and overseas entered the market. After coal mining was abandoned, sand operations were established in the early and mid-twentieth century to mine the silica-rich sandstone. The extraction methods used for sand were underground room-and-pillar mining and surface open-pit mining. The high-quality sand was used widely in the production of pottery and glass, and in foundries. Previous studies have interpreted the environment of deposition of these quartz-rich sandstone and coal deposits as barrier island with tidal channels or delta, tidal shelf, and marsh complexes along a north-south–trending shoreline. However, the excellent exposures in the sand mines display abundant evidence for their deposition in a fluvial/estuarine system. Their regional distribution indicates that they were deposited in a northeast-southwest–trending incised-valley system formed by fluvial incision during a lowstand. The incised valley was filled with fluvial and estuarine deposits made up of quartz-rich sand brought in by streams that flowed westward from the Sierra Nevada.

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 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 The Concord area is a 65-70± square mile area around the City of Concord, in the Suisun Bay portion of the Sacramento Basin. The stratigraphic section present includes predominately marine shale and sandstone, late Cretaceous through Miocene age, and predominately non-marine sediments of Pliocene through Recent age. The structure of the area has been formed by compressive forces and is extremely complex. Three major anticlinal trends, running northwest-southeast across the area, are recognized. From northeast to southwest these are, respectively, the Los Medanos, Concord and City of Concord trends. Major and minor thrust, reverse and normal faults are ubiquitous along and across the anticlinal trends. The area is bounded on the southwest by the Concord fault, a strike-slip fault with right lateral movement, and on the southeast by the major Mount Diablo uplift. Drilling of wells within the area has resulted in the discovery of approximately 46 BCF of dry gas. These wells have provided valuable geological data.

ABSTRACT The Meganos Gorge is a large fossil canyon of late Paleocene-early Eocene age present in the subsurface in portions of Contra Costa, San Joaquin and Sacramento Counties, California. The Meganos Gorge Fill crops out on the north flank of Mount Diablo as Divisions, A, B and C of the Meganos Formation as defined by Clark and Woodford in 1927. Axial length of the preserved portion is 71 kilometers (44 miles) covering an area of approximately 518 square kilometers (200 square miles). Maximum thickness of Gorge Fill is about 762 meters (2500 feet). Volume of the preserved section is approximately 104 cubic kilometers (25 cubic miles). Average slope of the gorge sides varies from 5 to 16 degrees. The Gorge Fill is dominantly shale, entirely marine and contains a foraminiferal fauna of Laiming’s D Zone (early Eocene). An extensive thickness of sand fill at the surface outcrop may represent a lower fan deposit portion of the Gorge Fill. Sediments beneath the Capay Shale and above the Starkey Sands east of the Midland Fault are predominantly Upper Cretaceous. These sediments have in the past been erroneously designated “Meganos-Martinez undifferentiated” because of paleontological determinations from wells which had penetrated Gorge Fill shale prior to recognition that a gorge was present. The eastern (inland) extremity of the gorge at Walnut Grove has considerable sand concentrated in the central portion of the Gorge Fill with shale at both sides. Erosion was subaerial east of the Midland Fault which approximated the eastern limit of the depositional basin at the time of gorge cut. Erosion west of the Midland Fault was submarine. Although the Midland Fault was active before and after the cut and fill of the gorge there is no indication of measurable offset during the time interval of the cut and fill. The dominant portion of the Gorge Fill is marine shale that has undergone considerable compaction relative to the surrounding sediments into which the gorge was cut. This compaction exerts a major effect on structure and the thickness of overlying formations. Failure to recognize the geologic effects of this compaction leads to serious errors in interpretation, including: 1. False interpretation of seismic closure in areas near the edge of the gorge. 2. Mapping faults that do not exist. 3. Anticipation of anticlinal closure at depth beneath compaction anticlines. 4. Failure to recognize the correct productive limits of fields. 5. Incorrect subsurface structural interpretations. The Gorge Fill shale provides entrapment of gas in several fields from different types of stratigraphic traps. These fields include many of the best accumulations in the southern Sacramento Valley.