Page 13 - wintererdurham1962

GEOLOGY OF SOUTHEASTERN VENTURA BASIN 281

Remnants of several terrace and erosion surfaces further evidence of the former existence of an old
are present east of Saugus and Newhall, in La Placerita surface of erosion.
Canyon and along Newhall Creek. The most exten
PRESENT EROSION CYCLE
sive of these surfaces is on the hills north and south of
the lower part of La Placerita Canyon, where its lowest
The Santa Clara River is a graded stream. The
altitude is about 150 feet higher than the altitude of river normally occupies only a comparatively narrow,
the adjacent canyon bottom. This surface increases
sinuous channel, but during floods it may cover its
in altitude to merge with an old erosion surface on
entire flood plain.
the hills in the vicinity of the Placerita oil field. The
The Santa Clara River valley in this area and the val
highest part of this erosion surface has an altitude of
leys that are tributary to it were alluviated just prior to
over 1,900 feet, or more than 500 feet above the lowest
the present cycle of erosion. The river and its tribu
part of the extensive surface. At its western end, the taries now flow in channels that have been cut as much
terrace surface is formed on a thick accumulation of
as 25 feet into the older alluvial deposits. At a point
river-terrace material but toward its eastern end it
about 1 mile west of the Los Angeles-Ventura County
is underlain by only a thin mantle of terrace material line the Santa Clara River valley floor narrows abruptly.
covering rocks of the Saugus formation. This same According to local residents, bedrock was exposed con
surface is correlated with the extensive surface on the
tinuously across the river channel at this point immedi
west side of the wide valley west of Newhall and Saugus ately following a flood in 1938. Along the river between
and with surfaces of less extent near the Castaic
Bouquet Canyon and Castaic Creek, broad paired
Junction and Del Valle oil fields.
alluvial terraces are about 5 feet above the present
In La Placerita Canyon, near U.S. Highway 6, the river bed. This terrace level merges into the alluvium
three lowest terrace levels are about 50, 125, and 175
of the present river bottom both upstream and down
feet above the adjacent canyon bottom. Along the
stream from the area of its distinct development.
Santa Clara River several somewhat similar terrace
At the mouths of numerous small valleys and gullies,
levels can be distinguished below extensive higher fans with comparatively steep slopes are forming on
terraces.
the surface of the older alluvium that veneers the floors
Just east of Saugus are remnants of two small
of the Santa Clara River valley and its major tribu
Valleys formed during an earlier cycle of erosion. taries. º
The lower parts of these valleys are being destroyed LANDSLIDES
by streams of the present erosion cycle, but their
On the flanks of the Santa Susana Mountains and in
upper parts are still preserved.
the hills in the northwestern part of the area, the shale
A nearly undissected erosion surface, cut on the
and siltstone of the Modelo and Pico formations are
Crystalline rocks of the western flank of the San Gabriel
exposed in steep slopes. Soil creep, slumping, and
Mountains in the vicinity of Elsmere Canyon, is a
landsliding are prevalent in these areas. Many of
resurrected pre-Pliocene surface exposed by the recent
the hills near the Del Valle and Ramona oil fields have
removal of the sedimentary rock cover.
dip slopes that are especially susceptible to movements
- The Santa Susana Mountains are made up of a con
of the superficial rock and soil cover. This has been
tinuous northwestward-trending ridge with steep,
the source of much trouble in the maintenance of oil
deeply dissected flanks. The relatively narrow summit
well drill sites and roads.
region has rounded knolls and gentle slopes; streams
there have gentle gradients but steepen markedly STRATIGRAPHY
where they begin to descend the mountain flank. At
one time these streams must have had gentle gradients The area described in this report includes parts of
throughout their length comparable to those now found three depositional provinces; each province has a dif
"long the uppermost parts of their courses. The crest ferent geologic history, but the three are now crowded
ºf the Santa Susana Mountains is a remnant of the old close together along faults of large displacement.
Northeast of the San Gabriel fault a thick succession
surface across which they flowed.
At the head of Rice Canyon, near the eastern end of of pre-late Miocene nonmarine strata comprising the
the mountains at an altitude of 2,750 feet, boulders of Vasquez and Mint Canyon formations overlies pre
granitic rocks rest on the Modelo formation. These Cretaceous igneous and metamorphic rocks. South
"oulders are the remains of a river-terrace deposit— west of the fault, strata of roughly equivalent age are
the oldest such deposit recognized in the area. At chiefly marine. The thick conformable upper Miocene,
ºther places on the flanks of the mountains, gently Pliocene, and lower Pleistocene section southwest of
"oping bench areas and accordant ridge levels give the fault is represented northeast of the fault by a