GEOLOGY AND PALEONTOLOGY OF PALOS - Pubs Warehouse

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Foraminifera, generally represented by poorly pre­
served material or molds, were observed at different
horizons in this area. The best material, assigned to
the Siphogenerina branneri zone, was collected at local­
ity 1 in Portuguese Canyon, as noted in the preceding
section.
The Portuguese tuff bed consists of light-colored
bentonitic tuff. As shown in the view on plate 6, A, it
is well exposed in the type region, where it has a thick­
ness of 55 feet. At the landward end of Inspiration
Point the tuff is abruptly upturned on the south limb of
an anticline" (see pi. 5) and is about 60 feet thick. On
the west side of Abalone Cove tuffaceous material is ex­
posed through a thickness of 30 feet, and the debris,
indicates that it continues upward through an additional
thickness of about 30 feet. The scattered outcrops of
the Portuguese tuff that were recognized are shown on
the geologic map (pi. 1). In addition to these outcrops,
tuff debris, doubtless. derived from this bed, was found
at many localities along the north margin of the exten­
sive landslide in the Portuguese Canyon district and in
stream cuts within the landslide area. Inasmuch as the
strata, including the tuff, dip toward the landslide along
NE.
relatively soft porcelaneous shale. Foraminifera from
silty shale (locality 2) are assigned to the same faunal
zone as that in the lower part of the Altamira in the
Portuguese Canyon area.
BLUFF COVE AREA
Strata exposed in the sea cliff at Bluff Cove, at the
west end of the eastward-plunging Bluff Cove anticline,
appear to represent the lower part of the Altamira shale
but are lithologically different from those already de­
scribed. The Portuguese tuff was not recognized in
this region.
A sketch of the sea cliff at Bluff Cove is shown in
figure 5. The statigraphic relations of some of the
lithologic units are uncertain, owing to extensive slides
and talus and to incomplete knowledge of the structural
features. The schist-debris breccia forming the high
greenish cliff appears to represent the oldest strata.
Bedding is not clearly discernible in the breccia, but
there is a vague suggestion of anticlinal arching toward
the top of the cliff. The breccia consists of a rude mass
of schist slabs embedded in an unsorted greenish muddy
matrix (pi. 6, B). The schist slabs have a maximum
FIGURE 5. Sea cliff at Bluff Cove, a, Pleistocene terrace deposits; 6, basalt in middle part of Altamira member of Monterey shnle; c, silty shale and cherty shale in middle
part of Altamira shale; d, sandstone and conglomerate assigned to middle part of Altamira shale; e, silty sandstone and sandy siltstone in lower part of Altamira shale;
/, schist-debris breccia assigned to lower part of Altamira shale.
its east, north, and northwest margins, this bentonitic
tuff probably acted as a lubricant for a mass of rocks
that slid down dip into a structural basin and moving
seaward overrode the lower marine terraces.
The lower part of the Altamira shale probably crops
out in other areas along anticlines on the lower part of
the south slope of the Palos Verdes Hills east and west
of the Portuguese Canyon area. The stratigraphic re­
lations in those areas are doubtful, however, as expo­
sures are generally poor and the Portuguese tuff was not
certainly, recognized. Tuff debris, possibly derived
from the Portuguese tuff, was found at several localities
near the small landslide northwest of the extensive land­
slide just described. Some of the inadequately exposed
rocks along the anticline in the deep canyon west of
this small landslide probably represent the lower part of
the Altamira.
MIRALESTE AREA
The lower part of the strata underlying the basalt
sill on the Miraleste anticline is assigned to the lower
part of the Altamira shale. A poorly exposed benton­
itic tuff on the south limb of the anticline, about 100
feet stratigraphically below the base of the sill, is 25 or
30 feet thick and may represent the Portuguese tuff.
The strata underlying the tuff, as exposed in isolated
areas, consist of silty shale, thin-bedded sandstone, and
observed length of 3 feet, but most of them are less than
half a foot long. Greenish schist is the most abundant
rock, pieces of vein quartz are numerous, and blue
schist is a minor but conspicuous constituent. Breccia
of this type was not found elsewhere in the Palos Verdes
Hills. It resembles the San Onofre breccia described
by Woodford, 36 in the San Jpaquin Hills, 30 miles south­
east of the Palos Verdes Hills, and farther south along
the coast. Like the San Onofre breccia the breccia at
Bluff Cove may represent alluvial fan material deposited
"at the foot'of a schist highland, or landslides and ava­
lanches derived from such a highland. At all events it
is composed of rocks similar to those in the Franciscan
(?) area in the Palos Verdes Hills.
Thin-bedded, generally fine-grained silty sandstone
and sandy siltstone form the lower part of the cliff
north of the breccia. These strata are presumably
younger. than the breccia and evidently are faulted
against the breccia. A coarse-grained sandstone in
these thin-bedded rocks contains much angular schist
debris. Some layers of silty sandstone contain logs of
carbonized and silicified wood as much as \% feet in
diameter. Foraminifera from sandy siltstone (locality
2a), found by Dr. Hampton Smith, are interpreted as
representing the same faunal zone as that in the lower
^ Woodford, A. O., The San Onofre breccia: California Univ.. Dept. Qeol. Sci.
Bull., vol. 15, pp. 182-205, 1925. '