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276 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

Molluscan faunas of mixed depth assemblage in the Pico INTRODUCTION
formation suggest turbidity currents as a mode of deposition. PURPOSE AND SCOPE
The southern affinities of the shallow-water species suggest that
the surface water temperature in the Ventura basin during the The Ventura basin (fig. 49) has long been one of the
Pliocene was somewhat warmer than that prevailing now in the important oil-producing districts of California. The
Pacific Ocean near Ventura. The northern affinities of the
eastern part of the basin includes some of the oldest oil
deeper water species indicate that the water was cold enough at
fields in California and also several of those most
depth to accommodate them. The vertical sequence of fora
recently discovered. Since 1937 several new fields have
miniferal faunas suggests a gradual shoaling from water depths
of about 2,500 feet in the deeper parts of the basin at the be been discovered in the area and undoubtedly more oil
ginning to very shallow water depths at the close of Pico deposi remains to be found.
tion. Lateral changes in foraminiferal faunas, shown by the
The first work of the Geological Survey in this region
crossing of foraminiferal correlation lines (water-depth lines)
was by Eldridge and Arnold (1907); Kew (1924) de
by lithologic units deposited by turbidity currents (time lines),
indicate that at any one time the water was shallowest near scribed part of the region studied by Eldridge and
Newhall and became deeper toward the west. Turbidity cur Arnold as well as much of the remainder of the Ventura
rents deposited coarse sand and gravel on this west-sloping basin. Since the publication of Kew's report, and
bottom.
especially since 1937 when the Newhall-Potrero oil
The Pico formation grades upward and laterally into the
field was discovered, interest in the eastern part of the
Saugus formation, of Pliocene and early Pleistocene age, which
consists of interfingering shallow-water marine, brackish-water, Ventura basin has steadily increased.
and nonmarine beds that in turn grade into exclusively non The present report includes a small part of the area
marine beds. Sandstone, conglomerate, and reddish- and of Kew's report but on the larger scale of 1:24,000
greenish-gray siltstone are characteristic of the formation.
(pl. 44; fig. 49). Attention is focused particularly on
South of San Fernando Pass it is practicable to divide the Saugus
the surface stratigraphy. Subsurface data were col
formation into a lower member, the Sunshine Ranch member,
characterized by greenish-gray siltstone beds, and an upper, lected where available, but they are necessarily
coarser grained member. In this area the marine and brackish incomplete.
water Sunshine Ranch member is separated from the upper
nonmarine member by an unconformity. North and west of FIELDWORK
San Fernando Pass, the division of the Saugus formation into
members is difficult, if not impossible. The thickness of the Fieldwork was begun by Winterer in the summer of
Saugus formation probably exceeds 7,000 feet in the west-central 1949 and carried on a few days each month until the
part of the area.
summer of 1950, after which work was more continuous.
Stream-terrace deposits of late Pleistocene age are very similar
Durham joined the project in the summer of 1951 and
lithologically to parts of the Saugus formation. East of Saugus
fieldwork was completed in September 1952. The areas
these deposits are as much as 200 feet thick and lie with marked
unconformity on the Saugus formation. mapped by each author are shown on an index map on
The Ventura basin is a narrow trough, filled with sedimentary plate 44. T. R. Fahy assisted in the collection of most
rocks, whose axis approximately coincides with the Santa Clara of the foraminiferal samples. -
River valley and the Santa Barbara Channel. The narrow
The geology was mapped on aerial photographs, a
troughlike form did not begin to develop until near the begin
few of which were 1:24,000 contact prints but most of
ning of the Miocene epoch. Near the south margin of the
Ventura basin the thick section of upper Cenozoic rocks has which were 1: 10,000 enlargements. The data on the
been thrust southward along the Santa Susana fault toward the photographs were transferred to the topographic base
older rocks of the Simi Hills. The southeastward-trending San map, which consists of enlargements of the following
Gabriel fault transects the northeastern part of the basin. Dis
Geological Survey 6-minute quadrangle maps: Pico and
similar facies in the pre-Pliocene rocks on opposite sides of this
Newhall and parts of Santa Felicia Canyon, Castaic,
fault indicate a long period of continued movement along it.
The major folds and faults between the San Gabriel and Santa Saugus, Humphreys, and Sylmar, all in Los Angeles
Susana faults trend northwestward; most of the faults are County.
southward-dipping reverse faults. The stratigraphic sections of the Towsley formation
The first successful oil well in the area was completed in 1875.
(pl. 46) were measured with plane table and alidade;
Most of the early development was in the Newhall oil field along
the stratigraphic sections of the Pico formation (pl. 47)
the Pico anticline and near the southeast margin of the basin
were measured with tape and Brunton compass.
near the San Gabriel Mountains. Interest in the petroleum
possibilities of the region was renewed with the discovery of the
ACENOWLEDGMENTS
Newhall-Potrero oil field in 1937. Discovery of the Del Valle
oil field (1940), Ramona oil field (1945), Castaic Junction oil
Geologists on the staffs of oil companies operating
field (1950), and a second period of development in the Placerita
in the region made much information available. Par
oil field (1948) followed. More than 80 million barrels of oil
were produced from wells in the area, and at least 236 nonpro ticular acknowledgment is due to E. J. Bartosh of the
ductive exploratory wells were drilled prior to June 30, 1953. Bankline Oil Co.; T. L. Macleod of the Bell Petroleum