Comment on 'New insights on Franciscan Complex geology, architecture, depositional age, and provenance for the western Mt. Tamalpais area, Marin County, California' by Bero et al. (2020)

Serious errors and inconsistencies in the article undermine many of its interpretations to the point that principal conclusions are not valid. Much dependence is placed on the maximum depositional age (Dmax) of sandstone units based on zircon analysis of 10 samples, but calculation of those Dmax values is flawed, and their use confuses maximum with actual depositional ages and makes age distinctions finer than the resolution of the data. Conclusions that are compromised include the concept of a westward/downward-younging tectonostratigraphic stack of accretionary units, comparison of mapped sandstones with other California Coast Ranges sandstones, comparison of timing of events recorded in the map area with regional events, identification of melange only in a narrow band in the study area, and detrital provenance of some of the sandstone units.

Automated summary

Serious errors and inconsistencies in the article have undermined many interpretations and rendered the principal conclusions invalid. The reliance on flawed calculations of maximum depositional ages based on zircon analysis of sandstone units has resulted in confusion between maximum and actual depositional ages, making age distinctions finer than the resolution of the data.