Age of the lospe formation (early Miocene) and origin of the Santa Maria basin, California

The Lospe Formation is an 830-m-thick sequence of nonmarine and shallow-marine sedimentary rocks and interbedded rhyolitic tuffs at the base of the oil-bearing Neogene Santa Maria basin of south-central coastal California. New isotopic and biostratigraphic data demonstrate that the Lospe in its type area in the northwestern Casmalia Hills was deposited about 18-17 Ma during the late early Miocene. Samples of water-laid rhyolitic tuffs from 30 m and 210 m above the base of the Lospe gave single crystal laser fusion ⁴⁰Ar/³⁹Ar ages of 17.70±0.02 Ma (mean of seven determinations on sanidine) and 17.39±0.06 Ma (mean of six determinations on plagioclase), respectively. Samples of tuffs from 30 m and 40 m above the base of the Lospe analyzed by the ⁴⁰Ar/³⁹Ar incremental-heating method showed saddle-shaped spectra indicating maximum ages on sanidine of 18.46±0.06 Ma and 18.10±0.06 Ma, respectively. The exact location of the eruptive source of the Lospe tuffs is unknown, but it may have been in the vicinity of Tranquillon Mountain, about 30 km south of the Lospe outcrops. Currently available data are consistent with the hypothesis that tuffs in the Lospe were derived from the same eruptive source as (1) a sample of welded rhyolitic tuff from the Tranquillon Volcanics on Tranquillon Mountain, which yielded a single crystal laser fusion ⁴⁰Ar/³⁹Ar age of 17.80±0.05 Ma (mean of five determinations on sanidine); and (2) a sample of altered tuff from near the base of the Monterey Formation near Naples, about 60 km east of Tranquillon Mountain, which yielded a single crystal laser fusion ⁴⁰Ar/³⁹Ar age of 18.42±0.06 Ma (mean of four determinations on sanidine). Alluvial fan and fan-delta conglomerates, sandstones, and mudstones in the lower member of the Lospe Formation are unfossiliferous, but lacustrine and shallow-marine mudstones in the upper member of the Lospe contain palynomorphs of early to middle Miocene age and benthic foraminifers of probable Saucesian age. The Lospe is conformably and abruptly overlain by bathyal marine shale and sandstone of the Miocene Point Sal Formation. Samples from the lower 15 m of the Point Sal Formation yielded palynomorphs of early and (or) middle Miocene age, benthic foraminifers of Saucesian and Relizian age, planktic foraminifers of early Miocene zones N4-N6, and calcareous nannofossils of early and early middle Miocene zone CN3. The boundary between the Saucesian and Relizian Stages occurs about 1-2 m stratigraphically above the base of the Point Sal Formation, and about 205 m above the tuff within the Lospe Formation dated at about 17.4 Ma. The type Lospe Formation is younger than the middle Eocene to lower Miocene Sespe Formation of southern California, with which the Lospe ha been previously correlated. The Sespe is a regionally extensive fluvial and deltaic unit that was deposited in a subduction-related forearc basin by river systems flowing from Arizona and the Mojave Desert to the Santa Barbara-Ventura coastal area. In contrast, the Lospe is restricted to the central Santa Maria basin and records bathymetric deepening, volcanism, active faulting, and rapid tectonic subsidence that began about 18 Ma in concert with regional transtention and initial clockwise rotation of the western Transverse Ranges.