Chico Formation
Chico Formation
Late Cretaceous
Variably fossiliferous nonmarine to marine sedimentary sequence occurring along the western margin of the central and northern Sierra. Haggart and Ward (1984) divide the Chico Formation into four members, with dominant lithologies (from stratigraphically lowest to highest) consisting of coarse fluvial conglomerate, coarse conglomeratic sandstone, fine-grained silty sandstone, and muddy siltstone to silty mudstone, which they interpret to represent a transgressive sequence. Clast lithologies consist of chert, metavolcanics, quartzite, schist, and slate (Haggart and Ward, 1984). Depositional environments likely range from fluvial, to deltaic, brackish nearshore (or littoral) to shallow marine. Strata interpreted to be nonmarine to littoral contain woody debris and lack normal marine fauna. However, much of the other strata host numerous species of Santonian to Campanian (~86 to ~72 Ma) ammonites, inoceramids, and other mollusks, as well as bivalves, gastropods, and cephalopods (e.g. Stanton, 1893; Diller and Stanton, 1894; Matsumoto, 1959; Popenoe and others, 1960; Haggart and Ward, 1984; Ward and others, 2012). Detrital zircon age spectra indicate a depositional age of ~88-77 Ma (deGraaf-Surpless and others, 2002). Magnetostratigraphy from Verosub and others (1989) identified an interval of reversed polarity consistently associated with the Baculites chicoensis Zone and correlated with the first reversed interval following the Cretaceous long normal interval, which indicates that this zone is not significantly time-transgressive. Strata dip gently to southwest and unconformably overlie metamorphic basement.