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SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGY OF SOUTHEASTERN VENTURA BASIN,
LOS ANGELES COUNTY, CALIFORNIA

By E. L. WINTERER and D. L. DURHAM

ABSTRACT
present in the subsurface section east of Newhall and Saugus
The late Cenozoic Ventura basin, Ventura and Los Angeles and is not recognized north of the San Gabriel fault. Westward
Counties, Calif., is an elongate sedimentary trough which, to from the town of Newhall, its thickness increases to at least
gether with its deformed structures, trends approximately east 5,000 feet at the Ventura County line. The formation is chiefly
west. The mapped area of this report lies entirely within Los siltstone, mudstone, and shale, but it also contains sandstone
Angeles County. and conglomerate.
Most of the southeastern part of the Ventura basin is in the The Towsley formation, of late Miocene and early Pliocene
Santa Clara River watershed. Old high-level erosion surfaces, age, overlies and in places interfingers with the Modelo formation.
some more than 3,000 feet in altitude, and younger, topo Near Newhall and San Fernando Pass it overlaps the Modelo
graphically lower river-terrace surfaces and deposits are con formation and lies directly on the Sespe(?) formation and on
spicuous. The river valley is now partly filled with alluvium Eocene and pre-Cretaceous basement rocks. The formation
which is being dissected during the present cycle of erosion. ranges in thickness from about 4,000 feet in the southwestern
Pre-Cretaceous basement rocks exposed in the San Gabriel part of the area to 0 where it is overlapped by younger rocks to
Mountains are the oldest rocks in the area. These rocks include the east. In the Santa Susana Mountains it grades upward
the fractured schist, gneiss, quartzite, and marble of the Placerita into the Pico formation. Beds assigned to the Towsley forma
formation of Miller (1934) and also the somewhat gneissic tion rest unconformably on the Mint Canyon formation north
Rubio diorite of Miller which intrudes them. Both of these of the San Gabriel fault.
rock units are intruded by quartz plutonites ranging in compo The Towsley formation consists chiefly of interfingering lentic
sition from granite to quartz diorite. Outside the mapped area ular beds of sandstone, mudstone, and conglomerate. Clasts
a large body of anorthosite and norite makes up part of the in the conglomerate beds are of rock types found to the east
basement rocks. and northeast in the San Gabriel Mountains. Sandstone and
Within the mapped area, the oldest sedimentary rocks are conglomerate beds in the Santa Susana Mountains contain
well-indurated sandstone and siltstone containing a marine fauna many sedimentary structures, including graded beds, load casts,
indicative of a middle Eocene or early late Eocene age. Some intraformational breccias, current ripples and lineations, slump
wells in the area have penetrated several thousand feet of Eocene structures, and convolute bedding. At several localities graded
rocks. sandy beds contain mixed assemblages of shallow- and deep
Some wells drilled near San Fernando Pass have penetrated water mollusks, the latter representing depths of more than 600
varicolored unfossiliferous beds that overlie Eocene rocks and feet. The sedimentary structures and fossils, taken together,
are overlain by upper Miocene marine strata. These vari indicate that marine turbidity currents were major factors in
colored rocks have a thickness of nearly 2,000 feet but do not the transportation and deposition of the sediments that now
crop out in the area. They do not appear to interfinger with the constitute the Towsley formation. A study of the molluscan
overlying marine beds and are tentatively correlated with the fauna of the Towsley formation indicates that the shallow-water
species are allied to Recent faunas now living south of the lati
Sespe formation of late Eocene to early Miocene age.
tude of the Ventura basin.
Near the Aliso Canyon oil field, wells have been drilled through
as much as 2,500 feet of strata; predominantly sandstone but The Pico formation, of Pliocene age, is distinguished from
the Towsley formation largely by its soft olive-gray siltstone
including a layer of amygdaloidal basalt, correlated tentatively
with the Topanga formation of middle Miocene age. Rocks that generally contains small limonitic concretions. In the
representing the Luisian (middle Miocene) stage of Kleinpell area near San Fernando Pass, the Towsley and Pico interfinger,
overlie these strata. but farther to the northeast there is an unconformity at the base
of the Pico. Although the Pico is marine, it interfingers with
The nonmarine Mint Canyon formation of late Miocene age
is present in the northeastern part of the area, north of the San brackish-water and nonmarine beds belonging to the basal part
of the overlying Saugus formation. Near the west edge of the
Gabriel fault. This formation consists of fluviatile sandstone
area it is at least 5,000 feet thick. Most of the sedimentary
* conglomerate and lacustrine siltstone and tuff and has a
structures suggestive of turbidity currents found in the Towsley
thickness of at least 4,000 feet north of the mapped area.
are also present in the lower part of the Pico in the Santa Susana
The marine Modelo formation, of late middle and late Miocene Mountains. Abrupt lateral gradations from siltstone to sand
** is exposed in the southwestern part of the area but is not
stone and conglomerate are common in the Pico.
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