Well-lithified Tertiary sedimentary rocks crop out within the San Andreas fault zone south of the San Bernardino Mountains. At the east end of the outcrop is the pre-Pliocene Mill Creek Formation. The diverse compositional facies of the Mill Creek Formation can be explained in terms of a strike-slip basin model. The northern and southern flanks of the basin are characterized by sediments of quite different provevance: garnet- and muscovite-bearing granitoids from the north and Pelona grayschists from the south. Sediments transported into the basin from the southeast are characterized by clasts of volcanic rocks. Conglomeratic sandstone with hornblende- and biotite-bearing granitoid and gneiss clasts entered the basin from the northwest and dominate the axis of the basin. All of the Tertiary outcrop east of the Mill Creek Formation is assigned to the Potato Sandstone, which has much less compositional variety. The two units are separated by a fault that is probably a major strand of the San Andreas fault, the Wilson Creek strand. The composition and paleocurrents of the Potato Sandstone do resemble the axial deposits in the Mill Creek basin that were derived from the northwest, but the rapid facies changes in strike-slip basins make lithostratigraphic correlations rather unreliable. In order to account for the garnet- and muscovite-bearing granitoids on the northern flank of the Mill Creek Basin, we suggest that the basin formed in the active Clemens Well-Fenner-San Francisquito fault zone. This is consistent with the pre-Pliocene age of the basin. The Clemens Well fault formed the southern margin. The fault on the northern margin may have been a very early strand in the San Andreas fault zone. The basement clasts in the Potato Sandstone have affinities with the Little San Bernardino Mountains. This suggests that the Potato Sandstone was deposited to the northwest of the Mill Creek basin, perhaps at a later time.

Abstract The basal sandstone of the Oil Creek Formation is well exposed in a quarry located 5 mi (8 km) north northwest of the small town of Mill Creek in Johnston County, Oklahoma (Sec.14, T.1S., R4E.) (Figs. 1 and 2). The quarry is owned by the Pennsylvania Glass Sand Corporation and is about 0.5 mi (0.8 km) west of Oklahoma 7, which passes through Mill Creek and connects with I-35 about 20 mi (32 km) to the west.