Magnetic Stratigraphy of the Miocene Round Mountain Silt and Olcese Sand, Bakersfield, California

Abstract

The Round Mountain Silt and the underlying Olcese Sand, northeast of Bakersfield, California, are famous for their large collection of marine vertebrates from the Sharktooth Hill Bonebed and fossil mollusks from both units. Their ages are poorly constrained, although they are thought to be early to middle Miocene. Magnetic samples were taken from two sections spanning 180 feet of the Round Mountain Siltstone: one at Ernst Ranch and the other along Poso Creek; and from the 220-foot thick Nickel Cliff section of the Olcese Sand. The samples were demagnetized with both alternating filed and thermal demagnetization, and produced stable remanence that passed a reversal test. Most of the Round Mountain Siltstone is reversed in polarity except for the very base of the section, and a short normal magnetozone in the upper third. Based on a Sr isotope ages of 15.9 Ma for the base of section, the best correlation is with Chrons C5Bn1r to C5Cn1, or 15.0 to 16.2 Ma. The entire middle part of the section, including the Sharktooth Hill Bonebed, correlates with Chron C5Br (15.2-16.0 Ma). This confirms the middle Miocene (Barstovian) age of the unit, as has been suspected from the few terrestrial mammals recovered. The Olcese Sand is mostly reversed in polarity, except for short normal magnetozones at the base and top of the section. Based on Sr ages ranging from 16.9-17.6 Ma, we correlate the Olcese Sand with Chrons C5Cn3n-C5Dn (16.6-17.5 Ma), or latest early Miocene.