MAGNETOSTRATIGRAPHY AND PALEOMAGNETIC ANALYSIS OF THE TULARE AND TURLOCK LAKE FORMATIONS, SAN JOAQUIN VALLEY, CALIFORNIA

The Tulare and Turlock Lake Formations were analyzed using rock magnetism and paleomagnetic techniques and a magnetostratigraphy was produced in order to provide better age constraints for the two Formations. Magnetostratigraphy from the Turlock Lake type locality supports an older age than what was previously determined by Packer et al. (1977), possibly the Jaramillo, the Cobb Mountain, or the Olduvai. Samples from the Tulare Formation behaved poorly. Most sections were too short to capture reversals; however, one tilted section may display a reversal. Analysis of the Turlock Lake Formation yielded well-behaved demagnetization of normal polarity. The Turlock Lake type locality data, however, revealed two major reversals and a 60 cm thick section of samples that contain multiple components with inflection points at approximately 150° and 300°C. These inflection points correspond to a change in the susceptibility along the high temperature Kappabridge plots, indicating magnetic material being created and destroyed. Rock magnetism experiments revealed that these samples contained mostly single- or pseudo-single-domain magnetite. Some experiments suggest additional magnetic minerals, such as goethite, maghemite, or hematite, which could be responsible for the unusual demagnetization at the Turlock Lake type locality.