Abstract: Magnetic Stratigraphy and Biostratigraphy of the Middle Eocene Friars Formation and Poway Group, Southwestern San Diego County, California

PROTHERO, DONALD R., DAVID J. LUNDQUIST, and STEPHEN L. WALSH

The magnetic stratigraphy of the Friars Formation and Poway Group in, southwestern San Diego County has been studied previously (Flynn, 1986; Bottjer et al., 1991), with conflicting results. We sampled 11 key sections, including many new exposures produced by recent development and road building, and added much recently obtained mammalian biostratigraphic data. Over 500 magnetic samples were taken, and analyzed with both thermal and AF demagnetization. A stable remanence which passed a reversal test was obtained between 300-500 degrees C.

The early Uintan Friars Formation apparently correlates with late Chron C21n and early Chron C20r (47-46 Ma). However, the polarity of the lower, middle conglomeratic, and upper tongues of the Friars are variable, and may reflect significant time-transgression of these units. The lower member of the early Uintan Stadium Conglomerate apparently correlates with Chron C20r, and the upper member appears to straddle the Chron C20r/C20n boundary (43.8 Ma). The late Uintan Mission Valley Formation probably correlates with Chrons C19r and C20n, based on a {40}Ar/{39}Ar date of 42.83 plus or minus 0.24 Ma in rocks of normal polarity (contrary to Bottjer et al., 1991, who correlated it with Chron C18n based on questionable P13 Zone planktonic foraminifera). These correlations contradict several sequence stratigraphic correlations of these strata. May et al. (1984), May (1985) and May and Warme (1987) correlated the Friars/Stadium contact with nanno zone CP15b and foram zone P15 (about 36 Ma), but this clearly conflicts with the date on the overlying Mission Valley Formation by at least 6 million years.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California