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Description
Provenance analyses of detrital minerals in sedimentary basins allow geologists to determine the characteristics of ancient highlands that are now eroded away and inaccessible to direct study. The purpose of this study is to apply techniques of provenance analysis to Upper Cretaceous forearc basin strata in San Diego and the Santa Ana Mountains. These strata were deposited adjacent to and locally onlap crystalline basement of the deeply denuded Peninsular Ranges batholith (PRB), the presumed source of forearc basin sediment. The PRB is divided into western and eastern zones based on distinct asymmetrical characteristics in the plutonic rocks that reflect transverse spatial variation across the batholith. The Cretaceous sediment characteristics of the fore-arc basin can be compared to the plutonic rocks to spatially resolve sources within the PRB. Most detrital sediment provenance studies utilize the framework mineral grains of sandstone such as feldspar and quartz. This study focuses on the heavy minerals to support previous studies that have shown the Upper Cretaceous sediments in the fore-arc basin were derived from the PRB as well as to narrow the provenance for most of the Upper Cretaceous sediments to the eastern zone of the batholith. Analytical methods selected for this detrital sandstone heavy mineral investigation include: in situ measurements of magnetic susceptibility, point counting of heavy mineral separates, X-ray diffraction (XRD) analysis of Fe-Ti oxide minerals, 87Sr/86Sr isotopic analysis of detrital apatite grains, and scanning electron microscope examination of selected mineral grains. Analysis was performed on sandstone samples from the Trabuco, Ladd, and Williams Formations of the Santa Ana Mountains and Lusardi, Point Loma, and Cabrillo Formations of the San Diego Embayment. The mineralogy of the Trabuco and Lusardi Formations at the base of the sections studied indicates a western zone PRB source for detritus. These formations are at least in part red-beds deposited in subaerial environments. The provenance of overlying formations demonstrate a predominantly eastern zone PRB source with sediments deposited in marine diagenetic conditions that allowed unstable minerals to survive. The fore-arc basin minerals and their condition indicate the distance from source to the sedimentary basin was relatively short. Batholithic source rocks had a partial cratonic signature based on 87Sr/86Sr apatite ratios, and were of an acidic igneous composition. Upsection shifts in provenance characteristics indicate progressive headward erosion of drainage basins into the denuding batholith during the deposition of the Point Loma and Williams Formations.
