Depositional Sequence of Argillite, Diamictite, Hyaloclastite, and Lava Flows within the Franciscan Complex, Northern California

An originally conformable sequence of argillite-diamictite-hyaloclastite-pillow breccia-pillow basalt exists in a 3 km-long, up to 200 m-thick, fault-bounded slab among Franciscan rocks 6 km south of the Klamath River mouth. Argillite and diamictite grade laterally into one another over short distances. Diamictite contains clasts of chert and graywacke that exhibit detrital rounding. Greenstone clasts are irregularly shaped and compositionally similar to overlying volcanic rocks. Hyaloclastite consists of globules and granules of altered basalt and basaltic glass with plagioclase phenocrysts, and minor plagioclase crystals and altered glass. The hyaloclastite is complexly interstratified with diamictite, and features suggesting soft-sediment deformation exist. Locally, tongues of chloritized basaltic glass have intruded diamictite matrix. Pillow breccia has hyaloclastite matrix; breccia fragments and whole pillows are abundant at the top of the sequence. Trace element geochemical data suggest that the sequence originated by somewhat alkalic within-plate ocean volcanism. Volcanic rocks were mixed with and deposited on seafloor muds that locally were pebbly before the sequence was incorporated into the Franciscan Complex. Blocks containing complexly deformed mixtures of argillite and volcanic rocks, common in Franciscan and other Cordilleran melanges, may have had a similar origin.