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Depositional Environments of Quail Canyon Sandstone and Soda Lake Shale Members of Miocene Vaqueros Formation in Southeastern Caliente Range, San Luis Obispo County, California

Deborah C. Goaldman

The Quail Canyon Sandstone and the Soda Lake Shale Members are the lower members of the Miocene Vaqueros Formation in the southeastern Caliente Range, San Luis Obispo County, California. The Quail Canyon Sandstone Member is conformably underlain by the nonmarine Oligocene Simmler Formation. The Soda Lake Shale Member is conformably overlain by the Painted Rock Sandstone Member of the Vaqueros Formation.

The rarely fossiliferous Quail Canyon Sandstone Member is medium to coarse grained and well sorted in its lower half, becoming medium to fine grained upsection. The lower rocks consist of parallel-laminated and large-scale cross-bedded sandstones, representing an upper-shoreface environment. The upper rocks are primarily structureless sandstone and represent a lower-shoreface environment. The entire section ranges in thickness from 50 to 150 m, thinning westward.

The Soda Lake Shale Member consists primarily of gray sandy siltstone, brown siltstone, and structureless sandstone. The sandy siltstone and structureless sandstone are interbedded at the bottom of the unit and indicate lower-shoreface to transitional-marine environments. The rocks become progressively finer into the characteristic, usually structureless, brown siltstone of the Soda Lake Shale Member. The brown siltstone represents an offshore environment, perhaps a restricted bay. Farther upsection, the brown siltstone is interbedded with lenticular structureless sandstone, either of which is locally the dominant lithology. Locally interbedded with the upper rocks is organic-rich, clay-rich sandstone. Above the organic sandstone, the other rocks may contain laminations, grading, and hannels that are absent to sparse lower in the section. The upper rocks indicate a shallower and more restricted bay alongside a delta.

The Soda Lake Shale Member ranges in thickness from 30 m in the east to 350 m in the west. It interfingers with the Quail Canyon and the Painted Rock Members.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.