Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Preliminary Revised Nomenclature For Plio-Pleistocene Formations In The Ventura Basin Portion Of The Los Angeles 30′×60′ Quadrangle

Abstract

The Plio-Pleistocene bedrock nomenclature of the onshore Ventura basin and associated San Fernando Valley embayment has been reevaluated for use on the Los Angeles 30′x60′ geologic map, which is currently being updated by the California Geological Survey as part of the State Map project. Strata of this age have a complex history of nomenclature in which formations have been defined by various workers based on lithologic characteristics, faunal assemblages, and environments of deposition. Conflicting terminologies are a result of a combination of the inconsistent criteria used to define the formations, extrapolation of formation names from other basins, and widespread occurrence of facies changes. The basic stratigraphic sequence consists of bathyal submarine fan and turbidite sequences overlain by dominantly granular shallow marine deposits, laterally variable transitional/paralic deposits, and capped by dominantly granular continental deposits. Lithologic unit boundaries are commonly time transgressive and generally young to the west as a result of westward migration of the paleoshoreline during basin infilling. For example, the base of the continental section decreases over 2 m.y. in age from the eastern to the western Ventura basin. The nomenclature adopted for the updated geologic map follows lithologic boundaries that generally correspond with changes in depositional environment. Most of the Plio-Pleistocene strata are assigned to the Towsley, Pico or Saugus Formations, following the definitions of Winterer and Durham (1962) for the type sections in the eastern Ventura basin. Tentative revisions to nomenclature used on the previous Los Angeles 30′x60′ map published by the USGS in 2005 include: 1) Dominantly fine-grained marine strata formerly assigned to the Santa Barbara Formation northwest of Fillmore are reassigned to the informal Mudpit shale member of the Pico Formation; 2) marine strata assigned to the Saugus Formation in the southern Santa Susana Mountains are reassigned as an informal member of the upper Pico Formation; 3) strata assigned to the San Pedro Formation on the south flank of South Mountain are reassigned to either the Saugus Formation if nonmarine or to the Las Posas Formation if marine. Mapping of the latter formation is tentatively adopted in general conformance with the Las Posas Sand shown on Dibblee Foundation maps. The terms Santa Barbara Formation and San Pedro Formation are excluded to areas outside of the Ventura basin.