PROVENANCE INVESTIGATIONS USING MAGNETIC-SUSCEPTIBILITY

In situ outcrop measurement of magnetic susceptibility with a portable hand-held meter is a quick and useful tool in provenance investigatio ns of siliciclastic strata. The volume percent of magnetite in sandsto ne and granitoid conglomerate clasts can be approximated and related t o potential source areas. Two provenance investigations in southern Ca lifornia illustrate this technique. Magnetic-susceptibility measuremen ts of sandstone and granitoid conglomerate clasts in Upper Cretaceous forearc strata of the Cabrillo Formation in San Diego indicate that th e source region(s) were dominated by ilmenite-series granitoid rocks s imilar to those that underlie the eastern part of the mid-Cretaceous P eninsular Ranges batholith (PRB), These data are consistent with recen t thermochronologic studies of the batholith and forearc sandstone tha t document rapid uplift, cooling, and erosion in the eastern part of t he batholith at approximately the same time the Cabrillo Formation was deposited. Magnetic susceptibility of sandstone through part of a sim ilar to 6 km-thick section of Neogene rift-related strata in the weste rn Salton Trough clearly record the abrupt arrival in earliest Pliocen e time of ancestral Colorado River sediments into the basin, Pre-Color ado River sediments are dominated by local ilmenite-only granitoid sou rce regions of the eastern Peninsular Ranges that contrast with region al magnetite-bearing Colorado River sands. The transition previously h as been recognized via painstaking petrographic and micropaleontologic studies, The change in provenance can be recognized in a few minutes in the field using in situ magnetic-susceptibility measurements.