Sedimentology of the Upper Cretaceous Joaquin Ridge Sandstone member of the Panoche Formation near Coalinga, Fresno County, California

The Upper Cretaceous Joaquin Ridge Sandstone crops out along the flank of Joaquin Ridge and in the Alcalde Hills north and west of Coalinga, California. It has a maximum local thickness of 1,500 m (4,920 ft) and is conformably overlain by the deep-marine Ragged Valley Shale. The deep-marine Alcalde Shale conformably underlies the Joaquin Ridge Sandstone, and may be laterally equivalent to the basal Joaquin Ridge Sandstone in the Alcalde Hills area. The Joaquin Ridge Sandstone probably was deposited in the braided and channelled portions of submarine suprafan lobes. Mostly structureless, quartzofeld-spathic, pebbly sandstone beds with erosive bases and dish structures dominate the lithology and suggest that fluidized debris or grain flow was the dominant depositional process. Lesser amounts of subtly laminated sandstone with fluid escape structures typically overlie the mostly structureless sandstone, and where together, commonly form thinning and fining upward sequences. Some basal contacts are sharp and others are gradational. The subtly laminated lithesome is characteristic of turbidity flow and rapid, tractive flow. Where coupled with underlying mostly structureless sandstone, they probably represent deposition from events that began as a fluidized grain or debris flows and evolved into overlying turbidity flows and then into tractive flows. Conglomerate is volumetrically minor and is composed of matrix- and clast-supported, rounded igneous clasts that were deposited by grain or debris flows. Interchannel and overbank deposits are indicated by volumetrically minor amounts of laminated, very fine sandstone and silty and clayey shale. Carbonaceous fragments and mudstone rip-up clasts are found commonly in the sandstone beds. Ophiomorpha burrows and mottling occur near the tops of many beds. Foraminiferal data from interbedded shale suggest that the Joaquin Ridge Sandstone was deposited in middle to lower bathyal depths during the late Campanian. The lack of deformation in the sandstone deposits, along with the paleontologic paleo-water depths, suggest that the suprafan lobes were deposited beyond the slope. The Joaquin Ridge Sandstone was derived from a northern andesitic/granitic source terrane. The sudden appearance of huge volumes of sand into a muddy bathyal plain may be due to sudden tectonic events responsible for rapid elevation changes of the basin. Basin uplift would lower the effective sea level causing mobilization of near-shore, sand-rich environments.