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Age, Correlation, and Origin of the Type Lospe Formation (Lower Miocene), Santa Maria Basin, Central California

STANLEY, R. G., U.S. Geological Survey, Menlo Park, CA, S. Y. JOHNSON and M. L. TUTTLE, U.S. Geological Survey, Denver, CO, M. A. MASON, University of California, Berkeley, CA, C. C. SWISHER III, Berkeley Geochronology Center, Berkeley, CA, M. L. COTTON THORNTON, D. R. VORK, and M. V. FILEWICZ, Unocal Corporation, Ventura, CA, R. B. COLE, University of Rochester, Rochester, NY, and J. D. OBRADOVICH, U.S. Geological Survey, Denver, CO

The type Lospe Formation in the Casmalia Hills is an 800-m-thick sequence of sedimentary and minor volcanic rocks. The Lospe is entirely of early Miocene (Saucesian) age on the basis of palynomorphs, benthic foraminifers, and 40Ar/39Ar ages of 17.70 +/- 0.03 Ma (mean of seven determinations) and 17.39 +/- 0.12 Ma (mean of six determinations). The 40Ar/39Ar ages were measured on water-laid tuffs; these tuffs may have erupted from the same volcanic source as a welded tuff yielding an 40Ar/39Ar age of 17.79 +/- 0.10 Ma (mean of five determinations) from the Tranquillon volcanics on Tranquillon Mountain in the westernmost Transverse Ranges.

Alluvial fan and fan-delta facies within the basal part of the Lospe are as thick as 200 m and consist mainly of conglomerate and sandstone derived from nearby fault-bounded uplifts of Mesozoic rocks. These coarse-grained facies grade upward into a sequence of interbedded sandstone and mudstone that accumulated in a shallow lake. Gypsum layers in the lake deposits contain sulfate depleted in 34S (0 to +3(sigma)), suggesting that the sulfur had a hydrothermal origin. The uppermost 30 m of the Lospe consists of storm-deposited sandstone and mudstone containing shallow-marine microfossils. The shallow-marine deposits are abruptly overlain by bathyal marine shale of the Point Sal Formation. The Saucesian-Relizian benthic foraminiferal stage boundary is about 2 m above the base of the Poin Sal Formation.

The Lospe Formation records active faulting, volcanism, hydrothermal activity, and rapid subsidence during initial formation of the Neogene Santa Maria basin. These events may have resulted from crustal extension related to the beginning of clockwise rotation of the western Transverse Ranges about 18 to 17 Ma.


AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)