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GEOLOGY OF SOUTHEASTERN VENTURA BASIN 287

stone member of the Modelo are included in the shale. Elliposidal limy concretions, some more than
Towsley formation. 6 feet long, are scattered through the shale. At most
Rocks of the Modelo formation crop out in a band places concretions are alined along definite horizons.
along the crest of the Santa Susana Mountains and Thin lenses and small nodules of nearly white phos
along the axis of the Pico anticline. Although the base phatic material occur in the shale at several strati
of the formation is not exposed within the area shown graphic levels. The nodules commonly contain fish
on the geologic map (pl. 44), the Modelo formation bones and scales.
rests unconformably on the Topanga(?) formation a Layers of light-colored silty to coarse-grained sand
short distance south of the mapped area in the Aliso stone are interbedded in the shale at many places.
Canyon oil field (White and others, 1952). The The layers range from laminae to beds several feet
Modelo is present in the subsurface in all parts of the thick. The sandstone beds are generally graded from
eastern Ventura basin except for the area northeast of coarse at the bottom to fine grained at the top and are
the San Gabriel fault and the area east of a line ap like the sandstone beds in the overlying Towsley for
proximately through the towns of Newhall and Saugus. mation in every important respect.
In the subsurface near Newhall the Modelo formation The upper part of the Modelo formation is exposed in
wedges out and is overlapped by the Towsley formation. a belt along the axis of the Pico anticline from East
Westward from Newhall the thickness of the Modelo Canyon to Big Moore Canyon. In the vicinity of
increases to at least 5,000 feet, but because no wells Rice Canyon a thickness of about 1,000 feet of Modelo
in the western part of the area have reached the base formation is exposed below the Towsley formation.
of the formation, the total thickness is not known. In The lithology is similar to that of the upper part of the
the subsurface throughout most of the region the formation described above. Several deep exploratory
Modelo formation cannot consistently be differentiated wells that were drilled along the Pico anticline show that
from the overlying Towsley formation because of the the thickness of the Modelo formation increases from
lithologic similarity and lenticular nature of the sand about 3,000 feet at the crest of the Santa Susana
stone and conglomerate lenses in both formations. Mountains to at least 5,000 feet near the axis of the
anticline.
STRATIGRAPHY AND LITHOLOGY
FOSSILS AND AGE
Near the crest of the Santa Susana Mountains, in
The only mollusks found in the Modelo formation are
the area of Aliso and Rice Canyons, a complete section valves of a small Delectopecten. Fish bones and scales
of the Modelo formation is exposed. In Aliso Canyon are common, especially in the more platy shales, and
(outside the mapped area), the Topanga.(?) formation foraminifers are plentiful in some layers. The lower
is overlain by a unit about 300 feet thick of gray to part of the Modelo formation in Aliso Canyon contains
grayish-orange medium- to coarse-grained well-sorted a foraminiferal fauna representative of the Luisian
sandstone with lenses of pebble- to cobble-conglom stage of Kleinpell (White and others, 1952). Forami
erate. Above the basal sandstone of the Modelo a
niferal faunas from several localities in the upper part
sequence of grayish-brown, brownish-gray, and grayish of the Modelo formation in the Santa Susana Mountains
black, poorly indurated, thinly laminated to thin-bedded region are given in table 2. The faunas that have any
silty and sandy shale about 400 feet thick is overlain age significance seem to represent the Mohnian stage
by about 800 feet of pale yellowish-orange hard platy, of Kleinpell, including the Bolivina hughest zone, except
thinly laminated to thin-bedded cherty shale, clay shale, for the faunas from localities f10 and f11, between Rice
and porcellaneous shale. The upper part of the forma and East Canyons, which may represent the Delmon
tion, about 1,500 feet thick near Rice Canyon, con tian stage (Patsy B. Smith, oral communication, 1954).
sists chiefly of softer fine-grained rock. The siltstone,
UPPER MIOCENE AND LOWER PLIOCENE
claystone, and mudstone are laminated to thick bedded,
light brownish gray or grayish orange when fresh, TOWSLEY FORMATIOn
medium brown when weathered. The thinly lami
DISTRIBUTION
nated, platy to punky silty shale is grayish brown to
light brownish gray, grayish orange, pale red, or pale Along the north slopes of the Santa Susana Mountains
red purple. At many places fractures and bedding the fine-grained sedimentary rocks of the Modelo for
planes are encrusted with gypsum and coated with a mation are overlain by and interfinger with a sequence
yellow powdery mineral that is probably jarosite. of light-colored sandstone, conglomerate, and inter
Occasional beds of gray impure limestone, which bedded brown-weathering mudstone beds. This se
commonly weathers pale yellowish orange and is prob quence was named the Towsley formation by the
ably of concretionary origin, are interstratified in the authors (1954) for its good exposures in the vicinity
681734 O-62—2