ABSTRACT

The stratigraphy of the Friars Formation and Poway Group in southwestern San Diego County is informally revised. The Friars Formation is locally divisible into three units: a lower sandstone and mudstone tongue, a middle conglomerate tongue, and an upper sandstone and mudstone tongue. As far as can be determined, the Friars Formation is entirely of early Uintan age. The Stadium Conglomerate is stratigraphically and geographically restricted, but is nevertheless locally divisible into a lower member of late early Uintan and possibly late Uintan age, and an upper member of late Uintan age. The Mission Valley Formation of late Uintan age is stratigraphically and geographically restricted only to those strata that are correlative with the type section.

We corroborate previous reports that the lower and upper parts of the type outcrops of the Friars Formation are of normal and reversed polarity, respectively, and concur in the assignment of these polarity intervals to Chrons C21n and C20r. However, the polarity of the lower, conglomerate, and upper tongues of the Friars as recognized here is variable, and may reflect significant time-transgression of these units. The lower member of the Stadium Conglomerate is tentatively assigned to C20r. The upper member of the Stadium may straddle the C20r/C20n boundary. The lower normal and upper reversed magnetozones of the Mission Valley Formation in the type section of this formation are tentatively correlated with C20n and C19r, based on the 40Ar39 Ar date of 42.83 ± 0.24 Ma.