MAGNETIC FABRICS AND PALEOMAGNETISM OF SEDIMENTS DEPOSITED IN ARROYO SECO DEL DIABLO DURING JANUARY 2010: IMPLICATIONS FOR PALEOCURRENT AND PALEOINTENSITY STUDIES OF MORE ANCIENT DEPOSITS

We collected oriented samples of sediments deposited in Arroyo Seco del Diablo, located in the Anza Borrego Desert State Park, CA. These sediments were deposited January, 2010, during heavy rainfall events. 30 oriented samples were collected from 8 sample sites from a variety of depositional environments.

Results of magnetic fabric (AMS) study found a close association between depositional environment and magnetic fabrics. Samples from quiet-water sites had weak (P=1.03), uniaxial oblate AMS fabrics with vertical kmin directions. Samples from channels or overbank deposits have larger degree of anisotropy - ellipsoid shapes tended to be triaxial in mid-channel sites, and more oblate in overbank sites. In all cases, the AMS orientation indicated that grain imbrication occurred, and that the AMS orientations closely matched the paleocurrent azimuth in these modern sediments.

Results of the paleomagnetic study found that only 5/8 of the sites yielded useful data. For the 5 sites, alternating field demagnetization isolated a well-defined characteristic component of magnetization between the 20 to 200 mT demagnetization steps. A mean direction of D = 22, I = 43, k = 43, a95 = 5 was obtained. The expected field direction for the site location (lat 32.90 N, lon 116.15 W) using IGRF 10 for Jan 2010 is Dx = 12, Ix = 58. The difference between the observed and expected direction is significant- we attribute this to a combination of imprecise sample orientations, and inclination error (~15 degrees). Relative paleointensity experiments (pseudo-Thellier- Tauxe et al 1995) on these samples found that the ARM gain vs NRM loss produced well-defined linear segments on Arai-type plots- between sample and between site variation in the ARM vs NRM slope were minimal (less than 5% variation in the relative paleointensity between sites).

Combined, these data indicate that AMS studies of these sediments can successfully discriminate between different fluvial depositional environments, and can be used to determine paleocurrent directions. The paleomagnetic studies indicate that DRM in these very young deposits is an accurate measure of the geomagnetic field- most interestingly that relative paleointensity techniques successfully account for variations in magnetic mineral concentration and yield accurate relative paleointensity values.