NEW STRONTIUM ISOTOPE AGES FROM INVERTEBRATE MACROFOSSILS IN THE SAN JOAQUIN FORMATION, ELK HILLS, CALIFORNIA

Recent strontium isotope data suggest that the San Joaquin Formation at Elk Hills, California is older than previously assumed. Four samples were taken from two cores of the San Joaquin Formation in the Dry Gas Zone at Elk Hills. One sample contained an oyster shell from the First Mya sand at a depth of 935 feet. The other three samples were thin-shelled pelecypods (Mya sp.) taken from the Third Mya sand at depths of 1717, 1875, and 2149 feet. Carbon and oxygen isotope analyses indicated very little post-depositional alteration of the samples however, thin section analysis indicated some diagenetic alteration in the Mya shells.

87Sr/86Sr ages of the Mya shells from the Third Mya Sand range from 5.1-13.9 Ma. This wide range of ages and the relatively old dates from these samples suggest that the thinner-walled Mya shells may be more susceptible to alteration. The 87Sr/86Sr age for the oyster shell from the First Mya sand is approximately 3.75 Ma with an uncertainty of +/- 1.55 Ma using the strontium sea water curve from Farrell et al. (1995). This sample showed no alteration in thin section, possibly due to the relative stability of the oyster shell material. The date for the First Mya sand suggests that the Tulare-San Joaquin Formation boundary in the subsurface of the southern San Joaquin Basin is older than early Pleistocene as previously estimated and may, in fact, be early Pliocene. This older date compares well with ash dates of 6.5 Ma from the correlative, to slightly older, Kern River Formation to the east (Miller et al., 1998). The older strontium age also compares well with ash dates of 5.0 Ma from a tuff at the base of the San Joaquin Formation farther north near Coalinga (Loomis, 1992).