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ABSTRACT

The Maastrichtian age Brown Mountain Sandstone Member of the Panoche Formation is exposed in the southeastern Diablo Range along the west side of the San Joaquin Valley, California. The Brown Mountain Sandstone is one of many locally named, discontinuous sandstones that are stratigraphically near the top of the Mesozoic Great Valley Group, which crops out nearly continuously along the western edge of the San Joaquin and Sacramento Valleys. Sandstones with correlative foraminiferal assemblages and depositional facies include the gas-bearing upper Tracy submarine fan sandstones of the Sacramento Valley.

Outcrop studies, regional stratigraphic correlation, subsurface mapping, and the vertical stratigraphic succession indicate that the Brown Mountain Sandstone was deposited in sand-rich submarine fan systems with source areas to the east and northeast. Subsurface isopach maps from well data depict fan-like geometries, and paleocurrent orientations from outcrop are predominantly toward the southwest. Paleontological data, including foraminifera species Bolivina incrassata and Gyroidina quadrata, were used for age control and to infer water depths of 600-1,500 feet. Arkose to lithic arkose sandstone compositions derived from petrographic analysis indicate an immature, granitic, Sierran provenance. Two distinctive lithofacies are present areally within the Brown Mountain Sandstone outcrops and are distinguished by abundance of shale interbeds, composition, grain size, and sequences of sedimentary structures. The Anticline Ridge lithofacies typically contains extensively amalgamated sandstone beds, has abundant ripple and parallel laminated structures, and is a very fine-grained, well-sorted, arkosic sandstone. The Cantua Creek lithofacies typically has less extensively amalgamated sandstones, contains shale interbeds, is locally graded including complete Bouma sequences, and is a very fine- to coarse-grained, poorly sorted, arkosic sandstone.

The differences between the sedimentological characteristics of the Anticline Ridge and Cantua Creek lithofacies are related to grain size variations within the submarine fan systems. What actually controls the differences in grain size is speculative, but is probably related to the type of feeder system and/or coeval shelfal depositional systems. One model suggests that sand deposition occurred in at least two types of submarine fan systems that include: slope-fed fan systems with sediment supplied through ephemeral slope feeder channels (Anticline Ridge lithofacies); and canyon-fed fan systems with sediment supplied through established submarine gorges (Cantua Creek lithofacies). These Brown Mountain Sandstone submarine fan systems are interpreted as part of a southwesterly prograding Maastrichtian slope system within the late Cretaceous forearc basin.

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