Page 14 - wintererdurham1962

282 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

greatly thinned section that contains several uncon grained and range in composition from granite to
formities. quartz diorite.
Within the province between the San Gabriel and An important unit of the sequence of pre-Cretaceous
Santa Susana faults, the formations of late Tertiary crystalline rocks, present on the northeast side of the
age thin markedly and become coarser grained toward San Gabriel fault but not present in the mapped area,
the east, where successively younger formations rest is a large body of anorthosite and norite containing
directly on the lower Tertiary and pre-Cretaceous rocks irregular-shaped magnetite-ilmenite bodies. This oc
of the San Gabriel Mountains. The Modelo and the currence of the anorthosite-norite suite is unique in
Towsley formations and the lower part of the Pico California, with the exception of a possible occurrence
formation were deposited in a marine environment, in a small area in or near the San Andreas fault zone
probably at depths greater than 600 feet. During more than 100 miles southeast, on the north side of
deposition of the Pico the water became gradually Coachella Valley. The anorthosite-norite suite is easy
shallower, and the overlying Saugus formation repre to recognize as clasts in conglomerates, and its presence
sents chiefly nonmarine conditions. The contact be in a conglomerate is presumptive evidence of a San
tween the Pico and the Saugus, that is, the marine Gabriel Mountains provenance, even though by a
nonmarine interface, rises stratigraphically toward the circuitous and interrupted route.
West.
TERTIARY SYSTEM
South of the Santa Susana fault system the strata
EOCENE SERIES
of late Cenozoic age are thin or even absent in many
places, although a very thick succession of Pliocene Eocene rocks exposed in a small area in Elsmere
and Pleistocene strata is present in a narrow belt along Canyon are the oldest sedimentary rocks that crop
the southern margin of the San Gabriel Mountains. out within the area shown on the geologic map. The
possible Eocene age of these rocks was first realized by
PRE-CRETACEOUS ROCKS
Homer Hamlin, who directed W. L. Watts to the out
crops. Watts (1901, p. 56–57) noted the resemblance
The San Gabriel Mountains consist of a complex
of these rocks to sandstones of the Sespe district now
assemblage of igneous and metamorphic rocks that
assigned to the Domengine stage, but he apparently
constitute the oldest rocks in the mapped area. No
found no fossils.
attempt was made during fieldwork to distinguish
The rocks consist chiefly of light- to medium-gray
units within these rocks for this report, and statements
well-indurated fine- to medium-grained sandstone that
concerning them result from incidental observations
weathers grayish orange, interbedded with medium- to
made along their contacts with the younger sedimen
dark-gray siltstone that weathers light to moderate
tary rocks, from brief reconnaissance trips into the San
brown, and grayish-orange conglomeratic sandstone
Gabriel Mountains, and from a perusal of published
that weathers light gray. The coarser beds are gen
and unpublished maps and reports on the area.
erally very thick bedded. Graded bedding is a prev
The oldest exposed rocks in the mapped area are
alent feature in the sandstone layers.
assigned to the Placerita formation of Miller (1934)
In Elsmere Canyon the Eocene rocks are in fault
and consist of schist, gneiss, quartzite, and marble.
contact with the pre-Cretaceous crystalline rocks and
This formation of metamorphosed sedimentary rocks
has been intruded by the Rubio diorite of Miller (1934) are overlain unconformably by rocks of early Pliocene
age. Neither the base nor the top of the Eocene
which consists chiefly of hornblende and biotite dio
sequence is exposed. Many wells near Newhall have
rite gneiss. In many places the diorite and metamor
penetrated Eocene rocks. One well, Continental Oil
phic rocks are so intimately associated that separation
Phillips 1, was drilled about 6,500 feet through Eocene
into mappable units is impossible. Miller (1934) in
rocks (pl. 45) before reaching crystalline rocks. The
cluded such mixed rocks in his San Gabriel formation
dip of the beds in this well is mainly between 20° and
(1931). These older rocks are intricately crumpled
60°, but the possibility that reverse faults repeat parts
and fractured, but some of the marble units can be
of the succession makes it difficult to estimate the
traced continuously for many hundreds of feet. At
thickness of the sequence.
some places, particularly in the Grapevine Canyon Some of the more friable sandstone beds in Elsmere
area, dark mylonite is common.
Canyon are saturated with tar, and heavy oil oozes
Plutonic igneous rocks that are probably much young from some fractures. Shows of oil in the Eocene rocks
er than the Placerita formation of Miller (1934) and have been reported from many wells, and a few wells
the Rubio diorite of Miller (1934) intrude the older in Whitney Canyon have produced high-gravity light
formations. These younger rocks are medium to coarse green oil, apparently from Eocene rocks.