Description
Detailed modal data on the composition of Holocene sand derived from pre-Tertiary rocks of the Peninsular Ranges are used to independently test previously proposed provenance-discrimination models, and are also compared to data for sandstones from the Late Cretaceous Point Loma Formation. The results of this study indicate that detritus in sandstones from the Point Loma Formation was derived from the deeply dissected plutonic/metamorphic roots of the Peninsular Ranges magmatic arc. This conclusion is significant because it implies that parts of the Peninsular Ranges magmatic arc were uplifted, deeply dissected, and nearly stripped clean of their volcanic cover by Late Cretaceous time. There is little evidence in the San Diego area for the volcanic detritus that was presumably derived from the denudation of the Peninsular Ranges magmatic arc. This apparent paradox may imply that: (1) the Peninsular Ranges magmatic arc system was unlike modern arc systems and lacked an extensive volcanic cover; or, (2) the volcanic debris derived from the erosion of the volcanic cover is hidden beneath the Pacific Ocean, or was tectonically removed.
