Taxonomic composition and stratigraphic distribution of megafossils in the Pico Formation south of Newhall, northern Los Angeles County, Southern California, are described in detail. Eighty-three taxa, from 15 localities, were found: one brachiopod, 36 bivalves, 40 gastropods, one scaphopod, one crab, one barnacle, one sea urchin, one shark, and one land plant. All are illustrated here. The pectinid bivalve Argopecten invalidus (Hanna, 1924) is put into synonymy with A. subdolus (Hertlein, 1925) and A. callidus (Hertlein, 1925). Rare specimens of the gastropods Calliostoma and Ocinebrina might be new species.
The mollusks, which are indicative of a late Pliocene age, lived in waters of inner sublittoral depths and normal marine salinity. Most of the 41 extant species indicate warm-temperate waters similar to those occurring today off the adjacent coast, although a few species, both extant and extinct, indicate a southerly warmer water component. The fauna lived predominantly in, or on, soft sands, but a few lived on other shells or possibly on large rock clasts.
Geologic field mapping done as part of this present study revealed that the Pico Formation south of Newhall was deposited at the site where a braided river entered the marine environment (i.e., braid delta). Initially, the river gravel and coarse sand interfingered with relatively deep offshore silts, barren of megafauna, in the lower and middle parts of the formation. Eventually, the delta built up, and the resulting shoaling conditions in the upper part of the formation were conducive for the megafauna to live in, or immediately adjacent to, the deltaic shoreface fine sands. Storm waves raked the delta and concentrated the shells of the megafauna, along with cobbles of igneous and metamorphic basement rocks, into channelized deposits. Postmortem transport distance was short, as evidenced by many paired-valved bivalve shells.
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