GEOLOGY AND PALEONTOLOGY OF PALOS - Pubs Warehouse

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GEOLOGY AND PALEONTOLOGY OF PALOS VERDES HILLS, CALIFORNIA
Owing to the change from cherty shale and phosphatic
shale to diatomaceous rocks, the Altamira and Valmonte
members are well defined in the western part of the
hills. On the northeast and east slopes differentiation
of the two members is difficult, as the middle and
upper parts of the Altamira include diatomaceous
rocks. In those areas only relatively pure, diatoma­
ceous. strata overlying diatomaceous rocks interbedded
with cherty shale, blue-schist sandstone, and phosphatic
shale are now assigned to the Valmonte.
Diatomite occurs in the Valmonte as; layers or
laminae of varying thickness. In some areas thinly
laminated diatomite forms units several feet thick
alternating with massive diatomaceous mudstone. In
other ,areas the entire exposed thickness of the Valmonte
consists of laminated diatomite and diatomaceous
shale containing varying proportions of diatoms. Some
of the thick units and also some of the thin laminae
in areas where thick units are absent consist of a
feltlike mass of the pelagic hairlike diatom Thalassio-
thrix. In most of the layers and laminae, however,
relatively large discoidal diatoms are the most abundant
constituents noted in field examination.
N.
, STRATIGRAPHY AND LITHOLOGY
MALAGA COVE AREA
The strongly deformed formations in the sea cliff at
Malaga Cove include the Valmonte diatomite (pi. 10,
A; fig. 7). The strata at that locality assigned to the
Valmonte consist of laminated diatomite and diato­
maceous shale containing exceptionally abundant
layers, stringers, and elongate nodules of soft phos­
phatic material. A thickness of about 100 feet is. ex­
posed on the north limb of the syncline containing the
overlying Malaga mudstone member and apparently
about 200 feet on the south limb. In the faulted areas
beyond the limb of the syncline stratigraphic relations
are uncertain.
The dry diatomite and diatomaceous shale, including
the phosphatic layers, are white (pi. 11, B}. At the
base of the cliff, where the rocks are kept wet by waves
and spray, the diatomite and diatomaceous shale are
almost black and the phosphatic layers are light gray,
the two types-of rock being strongly contrasted.. As
shown on plate 10, C, many phosphatic layers pinch and
swell, and others are broken into minute displaced
FIGURE 7. Sea cliff at Malaga Cove, a, Nonmarine terrace cover and dune sand (Pleistocene to Recent); 6, San Pedro (?) sand (Pleistocene); c, Repetto siltstone
(lower Pliocene); d, Malaga mudstone member of Monterey shale (upper Miocene); e, Valmonte diatomite member, of Monterey shale (upper Miocene).
At least locally, thin layers of phosphatic material
are interbedded with laminated diatomite and diatoma­
ceous shale. Perhaps phosphatic material is more
prevalent than is apparent, for the. thin layers may
be difficult to recognize in weathered outcrops. Lime­
stone is less abundant than in the Altamira shale. In
the lower part of the Valmonte of most areas limestone
occurs in lenticular beds, as in the Altamira. In the
upper part and in some places throughout the member
it occurs-as concretions. In laminated rock laminae
pass generally without interruption into the concre-
. tions. Locally zones of cherty shale are included in
the Valmonte, but they are rare. Black chert in the
form of discontinuous thin layers and stringers parallel
to the bedding and as veins cutting across the bedding
is found as a" minor constituent in the Valmonte.
Vitric volcanic ash in layers a few inches thick, appro­
priately called silver sand by the diatomite quarrymen,
is another minor constituent.
The soft rocks of the Valmonte member are exposed
for the most part only in cliffs and artificial excavations.
No exposed section of the entire member was found, and
the thickness is difficult to estimate. The estimate of
750 feet for the north slope of the hills cited in the pre­
liminary paper 70 may be excessive, owing to inclusion
of soft shale in the upper part of the Altamira; 500 feet
is perhaps a better estimate. In San Pedro the
thickness is only about 250 to 300 feet according to the
classification now adopted.
Foraminifera of the Bolivina hughesi zone were found
in the Valmonte at localities in the San Pedro area.
70 Woodriug, W. P., Bramlette, M. N., and Kleinpell, R. M., op. cit.
blocks. The generally consistent relation of offsets
suggests that the blocks are the result of shearing pro­
duced by differential movement during deformation
rather than the result of penecontemporaneous breccia-
tion. Limestone occurs in the form of concretions,
25 Feet
NE.
FIGURE 8. Valmonte diatomite and Altamira shale members of Monterey shale in
sea cliff near mouth of Malaga Canyon, a, Pleistocene terrace deposits; b, Val­
monte diatomite member of Monterey shale (laminated diatomaceous and .phos­
phatic shale); c, Altamira shale member of Mouterey shale (phosphatic shale,
limestone, and cherty shale).
generally laminated, of varying dimensions. The only
hard siliceous rock observed consists of thin lenticular
layers, stringers, and veins of black chert. On the
south limb of the syncline a 4-inch layer of vitric vol­
canic ash is interbedded with diatomacepns shale.
The base of the Valmonte is exposed in the sea cliff
200 feet southwest of the mouth of Malaga Canyon.
The strata are abruptly upturned at that locality, but