ABSTRACT

Previous petrologic study of sandstones from the Upper Cretaceous Point Loma Formation utilized methodology advocated by Folk (1974). In order to improve comparability with other provenance studies of Cretaceous sandstones in California, new modal data for sandstones from the Point Loma Formation were determined using the Gazzi-Dickinson point-counting method. In addition, varieties of quartz were studied following methods described by Basu et al. (1975). The new petrologic data indicate that sandstones in the Point Loma Formation are composed of detritus derived primarily from the exposed plutonic/metamorphic roots of a segment of the magmatic arc of the Peninsular Ranges. This conclusion is significant because it implies that parts of the magmatic arc of the Peninsular Ranges were u lifted, deeply dissected, and nearly stripped clean of their volcanic cover by Late Cretaceous time. The Late Cretaceous uplift of parts of the magmatic arc of the Peninsular Ranges occurred at the same time as the uplift of the magmatic arc of the Sierra Nevada, and both uplift events may have been associated with the early phase of an abnormally low angle of subduction during the Laramide orogeny.