MAGNETOSTRATIGRAPHY OF THE PALM SPRING FORMATION, MECCA HILLS, CA

We report results of an initial magnetostratigraphic study of the Plio-Pleistocene Palm Spring Formation in the Mecca Hills, CA. The age of strata in the Mecca Hills is important for understanding ages and rates of vertical crustal motions adjacent to the San Andreas fault in this area. Previous studies produced conflicting estimates of block rotation and age of the Palm Spring Fm, whose age is constrained by the 760-ka Bishop Ash high in the section. The predominantly reversed polarity of the thick section below the Bishop Ash suggests that the Palm Spring Fm is younger than the Matuyama-Gauss boundary (2.55 Ma), but the lack or poorly resolved nature of normal polarity zones produce large uncertainties in the age of the Palm Spring Fm (Chang et al, 1987; Boley et al., 1993). New mapping reveals a previously unrecognized conformable contact between the lower and upper members of the Palm Spring Fm in the Mecca Hills, 77 m above the base of a 772 m thick section. We tentatively correlate this section to a section that contains the Bishop Ash. This correlation, combined with preliminary new results of paleomagnetic analysis, yields a new estimate for the age of the Palm Spring Fm and underlying Mecca Fm. For this initial study, 66 paleomagnetic sites were collected at regular intervals in two measured sections through the Palm Spring and Mecca formations. Initial results of a.f. demagnetization from samples of the upper member of the Palm Spring Fm indicate that most sites have well-defined magnetizations, although a subset have difficult to remove normal overprints. These data allow us to define 4 polarity zones in the upper member of the Palm Spring Fm. Our preferred interpretation places the Bruhnes/Matuyama boundary (780 ka) roughly 145 m below the top of the Palm Spring Fm, which is overlain by Ocotillo gravel at 745 m in the section. Assuming this correlation, the top and base of the Jaramillo subchron are about 415 and 490 m below the top of the Palm Spring Fm, respectively, yielding accumulation rates of 0.9 and 1.3 mm/yr. At these rates, the conformable contact between the lower and upper Palm Spring Fm is estimated to be ca. 1.2-1.3 Ma. An alternate interpretation would place the base of the Jaramillo subchron 145 m below the top of the Palm Spring Fm, but available map data support our preferred correlation to the geomagnetic polarity timescale.