The Pismo Formation and evidence for Pliocene tectonic evolution of the San Luis Range, central coast California

Our study of the stratigraphic framework, facies architecture, and structural relationships within the Pliocene Pismo Formation reveals at least two temporally distinct phases of deformation within the San Luis Range and provides important context for on-going analyses of late Quaternary deformation and seismic hazard in coastal San Luis Obispo County. The Pismo Formation is composed of roughly 2 km of sediment deposited during the late Miocene to late Pliocene along the inner and outer continental shelf. The lower Pismo Formation is the Miguelito and Edna Members-lateral equivalents that compose the majority of Pismo Formation by volume-and the upper Pismo Formation is the Gragg Member, the Belleview Member, and the Squire Member. The Gragg Member and the Squire Member both overlie basal unconformities that record separate episodes of deformation in the San Luis Range. The first phase of deformation is characterized by laterally continuous short wavelength WNW-ESE-oriented folds developed in the lower Pismo Formation that collectively form the basin-scale Pismo syncline. This NNE-SSW-directed shortening initiated sometime after the completion of lower Pismo deposition at about 6 Ma. These laterally continuous short wavelength folds are only recognized in the Irish Hills block, the western and topographically highest part of the San Luis Range, and San Luis Obispo Creek marks the position of a roughly NNE-SSW-oriented structural boundary that separates intensely folded lower Pismo Formation in the Irish Hills block from the broadly folded lower Pismo Formation in the Edna block and the eastern San Luis Range. On the west side of this structural boundary the upper Pismo Formation overlies lower Pismo Formation along an angular unconformity and on the east side this contact is disconformable, indicating differential uplift and early Pliocene emergence of the Irish Hills block and western San Luis Range. The second phase of deformation is represented by a broad E-W-oriented fold developed in the upper Pismo Formation. This fold is about 5 km long and trends 20 to 30 degrees oblique to the dominant structural grain of underlying lower Pismo and Monterey Formation strata (WNW-ESE). There is no clear evidence for syn-depositional deformation within the Gragg or Belleview Members of the upper Pismo, suggesting that N-S-directed compression probably initiated after deposition of the Belleview Member in the mid-Pliocene. Although the minimum age of this folding is not presently well-constrained, it may be coincident with vertical displacement along the San Miguelito fault recorded by the late Pliocene Squire Member of the upper Pismo Formation. Within the Pismo syncline the Squire Member overlies the Belleview Member shale along a disconformable contact, and between San Luis Bay and Pecho Creek the Squire Member overlies pre-Tertiary rock along wave-cut erosional surfaces. These relationships indicate that the San Miguelito fault was active by the time of Squire Member deposition, producing permissible post-Pliocene rates of differential uplift that broadly mimic published late Quaternary uplift rates recorded by marine terraces.