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Contributions in Science, 520:73–93 21 December 2012

LATE PLIOCENE MEGAFOSSILS OF THE PICO FORMATION,NEWHALL AREA,

1
LOS ANGELES COUNTY,SOUTHERN CALIFORNIA

2
RICHARD L. SQUIRES

ABSTRACT. 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.

INTRODUCTION were done. It was necessary, therefore, to do my own geologic
mapping in order to understand the fundamental geologic
During the Pliocene, the Pico Formation was deposited for a relationships of the easternmost Pico Formation in the Ventura
distance of approximately 92 km along the axis of the Ventura Basin. The outcome is that the Pico Formation in the Newhall
Basin, which trends parallel to the present course of the modern area is recognized for the first time as having been deposited in a
Santa Clara River in Southern California (Fig. 1). The formation braid-delta environment. This study is important because it
has its broadest extent of outcrops in the Ventura area, and the affords the unusual opportunity to observe the complex inter-
outcrop pattern narrows significantly eastward toward the fingering between the fluvial and marine components of a
Newhall area. The Pico Formation represents the youngest Tertiary-age, predominantly marine formation in Southern
marine deposits in the eastern Ventura Basin. Throughout most California. The study area encompasses where the two environ-
of this basin, the Pico Formation is an offshore-marine sequence ments interfinger for a lateral distance of approximately 5 km,
consisting of siltstone, mudstone, and claystone with some minor and the lateral-fluvial component extends eastward for an
amounts of sandstone and conglomerate. Megafossils are sparse, additional 3 km (Fig. 2).
but relatively deep-water benthic foraminifera are common. To There might be a few small outcrops of the Pico formation just
the east, toward Val Verde and Valencia (Fig. 1), the formation south and southeast of the study area in the San Fernando Valley
becomes increasingly sandier and conglomeratic, and shallow- (e.g., Lopez Canyon) (Chen, 1988) and, possibly, a fault-
marine gastropods and bivalves are locally common in the upper bounded, small outcrop approximately 22 km southeast of
part (Grant and Gale, 1931; Squires et al., 2006). The purposes Newhall (Berry et al., 2009) in Gold Creek, a tributary of Big
of this present study are to 1) determine how far east the shallow- Tujunga Canyon.
marine megafossiliferous beds continue beyond the Valencia area
into the stratigraphically and structurally complex Newhall area,
2) tabulate and illustrate the taxonomic composition of the PREVIOUS WORK
megafauna, and 3) establish its age, depositional environment, The earliest work on fossils from the study area was by Gabb
and zoogeographic implications. (1869:49), who described a few species of Pliocene mollusks from
All preexisting geologic maps (e.g., Winterer and Durham, an area originally referred to as Fremont Pass, later known as San
1958, 1962; Dibblee, 1991a, 1992a) of the Newhall area are Fernando Pass, and now known as Newhall Pass, located just north
inconsistent in regard to 1) the differentiation of the Pico of the junction of U.S. Interstate 5 and California State Highway
Formation from the other Neogene stratigraphic units in the area 14. Ashley (1895:338) listed some mollusks from the same general
(i.e., Towsley Formation, Saugus Formation, and Sunshine Ranch area. None of his specimens were illustrated nor were they assigned
Member of the Saugus Formation), 2) the structural geology of a museum catalog number; they could not be located.
the area, and 3) the depositional environments the Pico Eldridge and Arnold (1907:22) used the name ‘‘Fernando’’ for
Formation. Also, no previous detailed megafossil investigations
an enormous section of siliciclastics, largely of Pliocene age, that
crops out over a considerable area of Southern California,
1 URL: www.nhm.org/scholarlypublications
2 Department of Geological Sciences, California State University, including the study area. Instead of basing the section on
18111 Nordhoff Street, Northridge, California, 91330-8266, USA; lithology, they improperly based it on three megafossil zones
Research Associate, Invertebrate Paleontology, Natural History Museum (collectively of Pliocene age). They erroneously lumped fossils
of Los Angeles County, 900 Exposition Boulevard, Los Angeles, found in Newhall Pass and Elsmere Canyon, but they listed only
California, 90007, USA. E-mail: richard.squires@csun.edu the fossils from Elsmere Canyon. The former beds belong to the

E Natural History Museum of Los Angeles County, 2012
ISSN 0459-8113 (Print); 2165-1868 (Online)