The upper Miocene, late Mohnian age, Williams Sand crops out in the southeastern Temblor Range along the southwest margin of the southern San Joaquin Valley, California. The Williams is composed of lenses of generally coarse material within the siliceous Antelope Shale Member of the Monterey Formation and is stratigraphically equivalent to the Stevens Sand. The sands and shales were deposited in paleontologically defined waters as much as 4,000 ft (1,219 m) deep. Based on field and laboratory studies, the Williams is interpreted as a

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submarine fan deposit.

Continuous grain-size plots of the outcropping sands were drafted and distinct turbidite facies were identified using Mutti and Ricci-Lucchi's classification. At the surface, the Williams dips northeast into the valley and contains many rather thin-bedded, usually graded, sandstones and conglomerates, with high porosity and permeability, which are interbedded with siltstone and shale. Thick deposits of shale, commonly cherty, separate large bundles of turbidites that pinch out along strike. The sandstone bundles often form coarsening- and thickening- upward sequences typical of deposition on the smooth suprafan lobes of the midfan. Other sandstones form fining- and thinning-upward sequences diagnostic of deposition on the channelized suprafan, although lack coarse pebbly material. App rently, the Williams fan continued to prograde until at least midway through its depositional history, when significant lateral shifting of coarse-grained sandstone bodies occurred.

According to petrographic modal analyses, the sands are arkosic, having a clay matrix and occasional carbonate and opal cements. Lithologic descriptions plus petrographic and paleocurrent data suggest the source terrane was only a few miles west-southwest, and most likely was the northern Gabilan Range. The northern Gabilan Range is composed of granitic and metamorphic rock, and is now located 150 mi (240 km) northwest of the outcropping Williams owing to Miocene and post-Miocene strike-slip displacement along the San Andreas fault.

In the subsurface, the Williams is composed primarily of fine to medium-grained sands, which are rather thinly bedded, poorly sorted, and interbedded with thick shales and siltstones characteristic of a lower fan facies. Oil is produced from Williams sands less than 2 mi (3.2 km) north of the study area from the Spellacy and Midway anticlines of the Midway Sunset field. Shales, and occasionally bentonites, separate the producing Williams into different reservoirs.

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