CORRECTING FOR K-METASOMATISM REVEALS A LATE CRETACEOUS NON-STEADY-STATE WEATHERING REGIME, POINT LOMA FORMATION, SOUTHWESTERN CALIFORNIA, USA

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 late Campanian-early Maastrichtian (?) Point Loma Formation as being deposited in a forearc basin developed westward of a continental margin fringing magmatic arc system. In such accounts, during the subduction of an oceanic plateau (or aseismic ridge), rocks within the magmatic arc are uplifted and the volcanic cover is nearly stripped clean exposing its plutonic roots. Published detrital zircon data derived from samples of the Point Loma Formation indicate that exposed igneous rocks within the source area were derived primarily 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⁺, resulting mudstone and sandstone data plot in A-CN-K space within overlapping fields. Hence, recognizing and removing the effects of K-metasomatism revealed that during the late Campanian-early Maastrichtian (?), portions of the exposed roots of the Peninsular Ranges magmatic arc were undergoing non-steady-state weathering conditions.