Shoaling of the Great Valley fore-arc basin, which began in the northern Sacramento basin during the Campanian Stage, shifted southward in the Maestrichtian to the southern Sacramento and northern San Joaquin basins, where a regressive sequence of submarine fan, slope, and deltaic deposits aggregates 6,500 ft (2 km) in thickness. Water depth decreased to sea level as ponded submarine fans aggraded the basin floor, and slope and deltaic systems prograded basinward over them. The central part of the fore-arc basin was filled by the end of the Cretaceous; the southern part was not filled until the middle Eocene.
Deltas prograded westward and southwestward from the Sierran magmatic arc. The Starkey delta system evolved from cuspate/lobate wave-dominated deltas with extensive upward-coarsening delta-front sand beds to highly lobate fluvial-dominated deltas with moderately developed delta-plain facies. This system was succeeded by elongate fluvial-dominated deltas with thick delta-plain/fluvial sequences in the Bethel Island fluvial-deltaic system. Thin, fine-grained, black, glauconitic shale beds were deposited during intervals of regional transgression and deltaic abandonment. Several progradational-transgressive cycles are recorded.
The Roberts Island slope system comprises several thick shale sections deposited during progradational events. Benthic foraminifera grade upward from abundant, varied, lower bathyal species to sparse upper bathyal or neritic species. Clinoform beds that offlap from east to west are evident in electric-log cross sections and seismic profiles. Lenticular, channeled sand beds, transverse to the slope, appear to be incised slope channels that funneled sand from the delta front to submarine fans on the basin floor. Shelf-edge slumps can locally be recognized by flow folds, mudstone blocks, and anomalous thicknesses and bedding attitudes, particularly near growth faults.
The Union Island submarine fan system consists of several fans; most are elongate parallel to the southeasterly trend of the basin. Fans consist of upper, middle, and lower segments. Thick-bedded, channelized, upward-fining, coarse-grained sand beds characterize the upper fan. Medium to thick-bedded, channelized, massive fine to medium-grained sand beds predominate on the midfan. Lower fan deposits are not extensive but consist of symmetrical or upward-coarsening sequences of shale and thin to medium-bedded fine to very fine sand. Channeling is less common on the lower fan. Most fans have multiple lobes, each lobe having been fed by a different delta.
Gas is produced from deltaic and submarine fan deposits. Delta-front sand beds form very small reservoirs where growth faults provide subtle structural traps. Distributary-channel sand beds are highly productive, owing mainly to stratigraphic pinch-outs but locally perhaps to structural traps. Submarine fan traps have so far been located mainly in midfan deposits, either in the structurally high suprafan mounds or along lateral fan margins near the base of the slope where fan deposits onlap and pinch out against slope shale. Exploration in lower and upper fan deposits has barely begun.