ABSTRACT Mount Diablo is flanked on its northeast side by a thick section of Late Cretaceous and Tertiary sedimentary rocks, which produced small hydrocarbon accumulations in the Los Medanos, Willow Pass, Mulligan Hill, and Concord gas fields. The first well was drilled in 1864, and today most of the active wells on the northeast flank are used for gas storage by Pacific Gas and Electric Company. These fields, which also include the Brentwood oil field, lie to the northeast of Mount Diablo and have produced 6.4 million cubic meters (225 billion cubic feet) of natural gas and over 57 million cubic meters (9.1 million barrels) of oil. The main reservoirs for the Sacramento Basin are sandstones in the Late Cretaceous and Paleogene section. The source rock there is primarily from the Upper Cretaceous Dobbins Shale, which began generation 75 m.y. ago, and the Winters Shale, which began generation 35 m.y. ago. The Livermore Basin is located on the western and southwestern sides of the mountain. The only commercial field in that basin is the small Livermore oil field. This field produces primarily from Miocene sandstones. The Livermore Basin is a Neogene basin that was syntectonically formed in the last few million years and continues to grow today. Studies of the black oils found in the Livermore field show that the source rock is likely the Eocene Nortonville Shale, though the Upper Cretaceous Moreno shale is also considered to be a possible source. The Livermore field has produced 12 million cubic meters of oil (1.9 million barrels).

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 The Great Valley forearc basin records Jurassic(?)–Eocene sedimentation along the western margin of North America during eastward subduction of the Farallon plate and development of the Sierra Nevada magmatic arc. The four-dimensional (4-D) basin model of the northern Great Valley forearc presented here was designed to reconstruct its depositional history from Tithonian through Maastrichtian time. Based on >1200 boreholes, the tops of 13 formations produce isopach maps and cross sections that highlight the spatial and temporal variability of sediment accumulation along and across the basin. The model shows the southward migration of depocenters within the basin during the Cretaceous and eastward lapping of basin strata onto Sierra Nevada basement. In addition, the model presents the first basement map of the entire Sacramento subbasin, highlighting its topography at the onset of deposition of the Great Valley Group. Minimum volume estimates for sedimentary basin fill reveal variable periods of flux, with peak sedimentation corresponding to deposition of the Sites Sandstone during Turonian to Coniacian time. Comparison of these results with flux estimates from magmatic source regions shows a slight lag in the timing of peak sedimentation, likely reflecting the residence time from pluton emplacement to erosion. This model provides the foundation for the first three-dimensional subsidence analysis on an ancient forearc basin, which will yield insight into the mechanisms driving development of accommodation along convergent margins.