Abstract

During steady-state weathering, the rate of removal of the exposed weathering zone is balanced by the rate of generation of mineralogically identical material through chemical weathering. As steady-state conditions prevail, the mineralogical and chemical composition of sediments derived from the weathering profile should be invariant over time. In A-CN-K space, a steady-state weathering condition is characterized by tightly grouped compositions, with dispersion between sands and muds a consequence of sorting during fluvial transportation. The purpose of this study was to assess whether such a condition was present during the development of the Upper Cretaceous Point Loma Formation.

Published paleotectonic reconstructions portray the upper Campanian–lower Maastrichtian(?) Point Loma Formation as being deposited in a forearc basin developed westward of a continental-margin magmatic arc. In such accounts, during the subduction of an oceanic plateau (or aseismic ridge) or during constant-dip subduction, rocks in the magmatic arc were uplifted and the volcanic cover nearly stripped clean, exposing its plutonic roots. Published detrital-zircon data derived from samples of the Point Loma Formation indicate that exposed igneous rocks in the source area were mainly from 135 Ma to 100 Ma plutons located in the western zone of the Peninsular Ranges batholith. New point-count, XRD, and chemical data show that detritus in mudstones and some fine-grained feldspathic sandstones of the Point Loma Formation following deposition and shallow burial were subjected to a K-metasomatic event. After correcting for introduction of K⁺, and consideration of the effects of hydraulic sorting, resulting mudstone and fine-grained sandstone data plot in A-CN-K space in overlapping fields, whereas samples of coarse-grained sandstone lie between rays extending through the most and least weathered mudstones. Hence, detritus in the Point Loma Formation was derived mainly from regolith during non-steady-state weathering conditions.