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Depositional Environments of Painted Rock Sandstone Member of Miocene Vaqueros Formation in Eastern Caliente Range, San Luis Obispo County, California

Michael W. Oldershaw

The Painted Rock Sandstone Member of the Miocene Vaqueros Formation in the southeastern Caliente Range, San Luis Obispo County, California, crops out in a narrow band along the anticlinal Caliente Range. The study area is on the eastern flanks of the northwest-trending range.

The Painted Rock Sandstone Member is conformably underlain by the Soda Lake Shale Member of the Vaqueros. In the eastern part of the study area, the member is conformably overlain or interfingers with the Oligocene-Miocene Caliente Formation. In the western study area, Painted Rock is conformably overlain by the Miocene Monterey Formation.

The Painted Rock Member thickens rapidly to the west with measured thickness ranging from 130 m in the easternmost section to 1,800 m in the westernmost and type section.

The member generally coarsens upward from interbedded mudstone and fossiliferous, laminated, fine-grained sandstones to cyclical couplets of fossiliferous, medium-grained, cross-bedded sandstones underlying laminated, medium-grained sandstones and local conglomerates. Interspersed within the member is a medium to coarse-grained, structureless sandstone. The Painted Rock generally coarsens to the east, except for almost completely conglomeratic outcrops in a central section. The Painted Rock sequence represents a gradual shoaling from lower-shoreface environments (fine sands and mudstones), to shoreface and foreshore deposits (sandy couplets). The structureless sandstone may fit in this sequence or it may record a subaqueous part of the nearby Caliente delta. The conglomerate section r presents a basin-feeding channel.

The anomalously thick type section coupled with the conformably overlying deep-marine Monterey Formation records rapid basin subsidence in the western study area.

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