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.