GEOLOGY AND PALEONTOLOGY OF PALOS - Pubs Warehouse

24
Miraleste tuff. On the preliminary geologic map 42
part of the diatomaceous rocks overlying the Miraleste
tuff was assigned to the Valmonte diatomite. That
member is now thought to be overlapped by the Plei­
stocene San Pedro sand and Lomita marl.
MIBALESTE-SAN PEDRO HHI AREA
In the deep canyons on the south slope of San Pedro
Hill and in Averill Canyon beds of sandstone, some of
which contain lenses of conglomerate made up chiefly
of schist pebbles, are conspicuous constituents of the
middle part of the Altamira. Altered diatoms occur
in a thin zone included in moderately soft laminated
shale exposed on Palos Verdes Drive East, about 40
feet stratigraphically above the thick basalt sill on the
south limb of the Miraleste anticline. Foraminifera
from silty- shale at locality 4, on Crest Koad, are
assigned doubtfully to the Siphogenerina reedi zone.
A Collection from buff siltstone at locality 11, south­
east of San Pedro Hill, is assigned to the Siphogenerina
nuciformis zone.
East of the Cabrillo fault the Miraleste tuff is exposed
on the road leading from Ninth Street to Miraleste
and at two nearby localities in a tributary of San
Pedro Canyon, where there are many minor folds.
Diatomaceous silt at locality.5 contains Foraminifera
of the Siphogenerina reedi zone. In San Pedro Canyon
and its tributaries isolated outcrops show cherty shale
and limestone dipping in various directions. The
structure and stratigraphy are obscure, but presumably
these strata underlie the Miraleste tuff.
The rocks penetrated in the Whites Point tunnel in
the Miraleste area north of the Cabrillo fault, consist
of shale containing many thin beds of greenish sand
and siltstone containing scattered phosphatic nodules.
Two thin sills of altered basic igneous rock, one of
which is too thin to be shown on the structure section
(section E E', pi. 1),, were encountered. South of the
Cabrillo fault, beds of greenish sandstone and conglom­
erate several feet thick, containing schist pebbles as
much as 6 inches long, are interbedded with silt-
stone. Higher in the section phosphatic siltstone, thin
beds of sandstone, cherty shale, and limestone were
encountered.
SAN PEDRO AREA
The Miraleste tuff was .recognized in a ravine ad­
joining the west edge of Peck Park in the northwestern
part of San Pedro and also in the next canyon to the
north. At both localities there appear to be two beds
of tuff. Foraminifera of the Siphogenerina reedi zone
were found in steeply dipping calcareous sandstone at
locality 7, on the north side of the main ravine in Peck
Park, opposite a great mass of chert on - the south side.
The steeply dipping strata are probably faulted against
flat-lying diatomaceous shale cropping out upstream.
Downstream diatomaceous shale and limestone are
presumably in the same part of the section as the
diatomaceous shale upstream.
Along San Pedro Canyon and its main tributary,
just north of Seventh Street, the complexly folded
cherty shale and limestone mentioned under the head­
ing "Miraleste-San Pedro Hill area" are overlain by
diatomaceous silt and limestone, with which some
cherty shale is interbedded. The lower part of the
diatomaceous silt contains pebbles and slabs of schist.
The largest pebble observed has a length of 10 niches,
«Idem. «
but an occasional slab of schist is as much as 2 feet
long. Even as far east as a locality in San Pedro
Canyon 300 feet upstream from the projection of
Leland Avenue diatomaceous silt contains pieces of
schist a foot long. Foraminifera from the lower part
of the diatomaceous silt at locality 6 represent a fauna
of small, possibly immature forms assigned doubtfully
to the Siphogenerina reedi zone. The diatomaceous
silt at locality 6 is estimated to be at about the horizon
of the Miraleste tuff, which, however, was not found
in this area nor farther south.
Diatomaceous silt, limestone, and thin beds of blue-
schist sandstone exposed along Averill Canyon and
nearby in western San Pedro represent presumably the
same part of the section as that just described.
The relative abundance in the San Pedro. area of
diatomaceous silt in strata now assigned to the middle
part of the Altamira led to the assignment of part of.
this section to the Valmonte diatomite.member on the
preliminary geologic map. 43
POINT FERMIN AREA
In the Point Fermin area the upper part of the
Altamira shale includes a coarse-grained detrital facies,
less phosphatic shale, and more cherty shale than
elsewhere. Consequently lithologic differentiation
between the middle and upper parts of the member
is indefinite. One lithologic type, however, may serve
in distmg-ukhirig the two parts of the Altamira.
Wherever!massive siltstone was found it is referable
to the middle part on both stratigraphic and faunal
grounds. In the sea-cliff section on the east side of
Point Fermin the base of the upper part of the Altamira
is drawn, arbitrarily at the base of the lower of the
two thick units of blue-schist sandstone.
The strata along the Point Fermin anticline inland
from the coast consist of thin-bedded blue-schist
sandstone, silty shale, siltstone, cherty shale, and thin
beds of phosphatic shale. The sandstone is generally
medium-grained. Locally, however, it is coarse-grained
and conglomeratic, as at Thirty-fifth Street and Patton
Avenue, where a 1-foot bed contains pebbles and
angular pieces of schist half an inch long. Foraminifera
assigned to the Siphogenerina collomi zone were
collected from massive buff siltstone at locality 12,
on Alma Street.
Porcelaneous shale, silty shale, calcareous phosphatic
shale, and minor beds of blue-schist sandstone form the
100-foot cliff along the crest of the Point Fermin anti­
cline where it emerges on the coast. A thin layer of
silty shale about 4 feet above the foot of the cliff
(locality 8) yielded a collection of small, possibly
immature, Foraminifera assigned doubtfully to the
Siphogenerina reedi zone. The stratigraphic relations
of these strata to those in western San Pedro north of
the Cabrillo fault are doubtful, but they are presum­
ably the equivalent of part of the section in western
San Pedro that includes much diatomaceous silt. A
local discontinuity or a bedding-plane fault was visible
formerly at the foot of the cliff, about 100 feet east of
the abandoned oil well (pi. 7, B).
The Cabrillo fault is well exposed in the sea cliff
(pi. 6, .D). South of the fault are massive buff silt-
stone and lenticular limestone in the middle part of the
Altamira; to the north are porcelaneous shale, phos-
« Woodring, W. P., Bramlette, M. N., and Kleinpell, B. M., op. cit. (Am. Assoc.
Petroleum Geologists Bull., vol. 20), pp. 128-129), fig. 1.