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 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.