Paper submitted to "GSA
Bulletin" Revision Date:
11/16/82
Mudflow deposition and horizonation in
Holocene soils near Point Conception, California
Glenn Borchardt, Salem Rice
California Division of Mines and Geology, Ferry Building,
San Francisco, California 94111
Jerome Treiman
California Division of Mines and Geology, 107 S. Broadway,
Los Angeles, California 90012
ABSTRACT
Trenching at the site of the proposed LNG (liquefied natural
gas) terminal near Point Conception
revealed much detail
regarding late Quaternary
continental deposition along uplifted
marine terraces. The uppermost unit at the site is a
distinctive
light gray (10YR6/1d) loam which is
up to 60 cm thick. The unit
forms an abrupt contact with an
underlying very dark grayish
brown (10YR3/2d) clay soil
horizon. The origin of the light gray
loam is a matter of some
controversy in dating the age of latest
fault movement. It is either an A2 soil horizon formed
mostly as
a result of pedogenesis during the
last 10,000 years, an aeolian
deposit accumulating since the last
high stand of the sea about
5,000 years ago, or a young mudflow
about 1,000 years old.
We support the mudflow interpretation because: 1) the unit
is absent from localities where its
deposition would have been
interrupted by an intervening drainage
way, 2) the unit contains
pebbles and cobbles, 3) its
thickness bears little relation to
the thickness of what we believe to
be an underlying vertisol
(expansive soil), and 4) the
mineralogy of the mudflow and the
vertisol indicates little pedogenic
relationship between them.
Soil development within the mudflow is about 10% of that in
the vertisol, which began
developing between 8,000 and 12,000
years ago. Thrust faults at the site do not cut the
mudflow, and
at one exposure a shallow
equivalent of the vertisol is absent on
the headwall of a fault. Thus the latest fault movement at the
site occurred prior to mudflow
deposition about 1,000 years ago.
INTRODUCTION
Investigations of the seismic safety of the proposed LNG
terminal near Point Conception,
California (Figs. 1, 2), have
developed much detailed information
concerning late Quaternary
deposition on uplifted marine
terraces associated with
neotectonism of the Transverse
Ranges (Dames & Moore, 1980; Rice
and others, 1982). The uppermost unit, dubbed the "gray
layer"
for purposes of discussion, is a
distinctive light gray
(10YR6/1d) loam, up to 60 cm thick
and forming an abrupt contact
with an underlying very dark
grayish brown (10YR3/2d) clay soil
horizon (Figs. 3, 4). The origin of the light gray loam is a
matter of some controversy in
dating the age of latest fault
movement because it is the youngest
unit not displaced by
faulting. It is either an A2 soil horizon formed primarily
through pedogenesis during the last
10,000 years, an aeolian
deposit accumulated since the last
high stand of the sea about
5,000 years ago, or a young mudflow
deposit less than 1,000 years
old. The origin of the gray layer is important, not only for
dating the most recent movements on
the bedding-plane thrust
faults at the site, but also for
developing techniques for
distinguishing between pedogenic,
aeolian, and fluvial features.
Description of the Gray Layer
Detailed descriptions of the gray layer and the underlying
soil horizons have been given for
the "Beach Fault" trench soil
excavated at the LNG site (Johnson,
1981, p. 33) and for the
Concepcion1 soil series
described at a locality on Hollister
Ranch about 4 km to the east
(Shipman, Rabey, and Mann, 1981, p.
20). Unfortunately, neither of these recognizes the two-story
nature of the alluvial deposition
and of the soil. A generalized
description, complete with the
genetic interpretations of our
study, is given in Table 1. In brief, we consider the gray layer
to be a mudflow fan which was
deposited about 1,000 years ago on
the weakly-dissected surface of a
vertisol (Borchardt, 1977) that
had been forming since the end of
the Pleistocene. The
characteristics of these two units
indicate that the gray layer
is indeed a young mudflow.
_______________
1Spelling used by Shipman, Rabey,
and Mann (1981) as well as
Johnson (1981).
Hypothesis No. 1: PEDOGENESIS
In the forests of northern climes, particularly in
Wisconsin, Minnesota, and France,
there arises a soil horizon,
light gray in color, eluviated of
its inherited clays, and
leached of free iron oxides (Soil
Survey Staff, 1975, p. 506,
Plates 4C and 6D). In the ideal case, this horizon, properly
designated an A2 or albic horizon,
forms when organic chelates
reduce the iron in the layer and
move together with the iron to
reprecipitate as a Bhir horizon at
some lower depth. By this
process--one that is entirely
pedogenic—the original colors of
the sand and silt grains are
recovered from the parent material
and the horizon appears gray or
white. According to Soil
Taxonomy (Soil Survey Staff, 1975,
p. 44), intervenor in these
matters: "The albic horizon is
one from which clay and free iron
oxides have been
removed."
They go on to caution, however, that "Deep deposits of
pure
white sand can be formed by wind or
wave action. Although these
deposits have the apparent
morphology of an albic horizon, they
are in fact parent material. The white sand in such a deposit
does not overlie a B horizon or any
other soil horizon except, in
some places, a buried soil"
(Soil Survey Staff, 1975, p. 44). We
contend that the gray layer that
forms the upper portion of the
Concepcion soil is an analogous
parent material, in contradiction
of the interpretations of Shipman,
Rabey, and Mann (1981, p. 20)
and Johnson (1981). We agree with Shlemon (1980) that the gray
layer is a young mudflow, but do
not agree that it should be
considered an albic horizon.
Physical and Chemical Analysis
The excellent detailed work of Johnson (1981) affords us an
opportunity to review some of the
laboratory data which show the
two-story nature of the Concepcion
soil. Particle size analyses
provide an excellent means for
detecting soil horizons derived
from two or more parent materials (Borchardt,
Hole, and Jackson,
1968; Borchardt and Hill,
1982).
In the "Beach Fault" trench (Fig. 3), depth
functions for
sand, silt, and clay show little
change within the gray layer,
while clay contents decrease with
depth in the vertisol below it
(Fig. 5). Nearly vertical depth functions are indicative of
minimal exposure to soil
development, and in this case, the
rather uniform content of 20% clay
in the gray layer also
contradicts the claim that its
lower portion is an eluvial
horizon--one from which clay has
been removed through
pedogenesis. Thin sections prepared from the lower
portion of
the gray layer show "abundant,
very small iron concretions and
films (ferrans) plus a few
argillans [clay films]" (Johnson,
1981, p. 9). The concretions are a clue to why the
mudflow
remains gray: it is subject to poor
drainage produced by the
clayey vertisol upon which it was
deposited. The argillans
indicate that this young deposit is
currently undergoing
incipient illuviation rather than
longstanding eluviation. The
so-called albic horizon in the
Concepcion soil is not a zone of
significant iron removal, but one
of periodic iron solubilization
and reprecipitation occurring more
or less in situ.
Johnson (1981, p. 15) found that over 15% of the cation
exchange capacity (CEC) within the
buried vertisol was occupied
by sodium (Fig. 6). Soluble salts were absent in the gray layer,
but increased dramatically with
depth within the vertisol. Total
sodium was about 150 ppm in the
gray layer, whereas it was about
1500 ppm in the vertisol. We believe that, because most of this
sodium is probably oceanic, the
amount of sodium present is a
fair indicator of the duration of aerial
exposure to sea spray
and other forms of salt-laden
precipitation. In young soils near
the coast, sodium thus can serve as
a chronometer. According to
this “sodium chronometer”, the
vertisol is at least ten times older
than the gray layer above it.
The x-ray diffraction patterns of the clay fraction also
demonstrate that the Concepcion
soil is a two-story soil (Fig.
7). Being derived from mudflows and alluvial fan materials from
the same source in the Santa Ynez
Mountains, clays from both the
gray layer and the vertisol contain
quartz, kaolinite, and mica.
The kaolinite and mica is probably
inherited from the well-
drained soils of the uplands, while
the poorly crystalline
smectite predominant in the
vertisol may have formed in situ.
Kaolinite and mica increase with
depth in the gray layer and in
the vertisol (Fig. 7), behaving as
if each had been weathered or
diluted by other minerals during
two different periods of aerial
exposure. Of all the horizons examined, the greatest contrast in
mineralogy exists between the
bottom of the gray layer and the
top of the vertisol. This is a likely result of geologic
deposition rather than of pedogenic
translocation.
Lastly, if the vertisol was a product of clay eluviation
from the gray layer, then wherever
the gray layer is especially
thick we would expect to find an
especially thick vertisol
beneath it. There are numerous exposures near Point
Conception
where precisely the opposite is the
case (Fig. 8). Regardless of
the nomenclature used to describe
it, the primary characteristics
of the gray layer did not originate
through significant
pedogenesis.
Hypothesis No. 2: AEOLIAN DEPOSITION
Yerkes, Greene, Tinsley, and Lajoie (1981, p. 5)
hypothesized an aeolian origin for
the gray layer. They assigned
it a maximum age of 5,000 B.P.
based upon Recent sea level
changes and guessed that it was
still accumulating today. They
correctly recognized that the unit
had very little soil
development and that it was the
only one not displaced by bedding
plane faults at the site.
The case against the aeolian hypothesis was made earlier by
Shlemon (1978, p. A-7), who pointed
out that the gray layer "does
not pervasively blanket the modern
surface, but rather occupies
topographic lows," contains
lag gravels, and has no easily
recognizable aeolian source. Indeed, subsequent excavations for
an LNG tank site uncovered a local,
but laterally extensive layer
of angular cobbles near the base of
the gray layer (Fig. 9).
These were at first thought to be
of archeological significance,
but no artifacts were found in
direct association with the
cobbles (Ancient Enterprises, Inc.,
1981). To those upholding
either the pedogenesis or the
aeolian hypothesis, this cobble
layer was disturbingly anomalous,
even though Johnson and
Rockwell (1982) did propose a novel
combination of anthropogenic
and biogenic forces to explain its
presence. Although faunal
turbation is common in soils (Wood
and Johnson, 1978), a more
likely explanation is that the
cobble layer is an instance of the
sporadic occurrence of lag gravel
and angular cobble in what are
otherwise fine-grained mudflows.
Hypothesis No. 3: MUDFLOW DEPOSITION
Marine terrace deposits near Point Conception accumulate
primarily by alluvial fan
deposition upon uplifted wave-cut
platforms (Dames & Moore, 1980;
Rice and others, 1982). Although
much of this material consists of
coarse gravels and sands
deposited in the high-energy
environment of the late Pleistocene,
the fine-grained mudflows produced
during the Holocene followed
similar paths from the Santa Ynez
Mountains to the north.
Indeed, the recent soil survey map
of the area nicely delineates
the Concepcion series (Cg),
characterized by the gray layer, in
the form of lobate fans headed in
the intermediate drainages
(Fig. 10).
The gray layer is absent from some of the interfluves where
Diablo clay (Da) is mapped (Figs.
10, 11). In these areas the
vertisol remains unburied by the
young mudflow and continues to
shrink and swell with the annual
changes in moisture. The young
mudflows, having been intercepted by
headward-eroding arroyos
(Fig. 12) and lacking sufficient
capacity to fill them, failed to
invade these terrace surfaces.
It is clear, then, that hypotheses 1 (pedogenesis) and 2
(aeolian deposition) should be
rejected, and that hypothesis 3,
mudflow deposition, best explains
the characteristics of the
Concepcion soil and its
distribution. The implications are that
the gray layer is very young and
does not contain an albic
horizon or A2 horizon formed
through pedogenesis. Instead, the
light gray horizon that occurs at
the base of the thickest
mudflow deposits is just what the
good book (Soil Survey Staff,
1975, p. 44) says it is:
"parent material," that is, a C horizon.
The Concepcion series is an excellent
example of the initial
development of a bisequum: a soil
profile containing two
superimposed sets of genetically
related soil horizons.
RECENCY OF FAULTING AT POINT CONCEPTION
The young mudflow, alias the "gray layer," is
crucial for
dating the most recent fault
movement at the proposed site for an
LNG terminal (Fig. 1). North-south compression in this part of
the Transverse Ranges has produced
a parallel series of bedding
plane faults within the south limb
of an anticline in the
actively folding Sisquoc Formation
(Dames & Moore, 1980; Rice and
others, 1982). At Arroyo Central an 80,000 year-old wavecut
platform and its associated marine
sand deposits have been offset
almost a meter along a structure now
known as the Arroyo fault.
Shears extend through the overlying
continental deposits and into
the base of of the Concepcion loam
described above (Figs. 13,
14). The footwall below one of these shears contains a
moderately well-developed B horizon;
the headwall contains a
slightly weathered C horizon. The offset of the B horizon is
roughly 35 cm, and, as recognized
previously by Yerkes and others
(1981, p. 5), the overlying mudflow
is "the only geologic or
pedologic unit apparently not displaced
over the Arroyo fault."
Dating the Mudflow
No direct date is available for the mudflow overlying the
vertisol in the Concepcion soil,
but it is unlikely that the soil
carbon within it is more than 500
years old. Charcoal from the
basal C horizon of the Concepcion
soil has been dated at 11,745
B.P. (Geochron Lab. No. GX-5569,
Shlemon, 1978, p. A-9). We
obtained a soil carbon date of
4,330 +_190 B.P. (Isotopes Lab. No.
I-11,121) for the top 10 cm of the
vertisol buried by a 60 cm-
thick mudflow in the northeast
corner of trench S-B excavated by
Dames & Moore (1980; Fig. 1 and
near the site of Fig. 4). An
assessment of the soil carbon date
is complicated because it
represents a mean residence time
(MRT) (Yaalon, 1971) plus a
burial time. That is, it consists of carbon that was
accumulating throughout the entire
period that the vertisol
remained unburied by the
mudflow. Soil carbon sampled at greater
depths in the vertisol undoubtedly
would give even older dates
(Yaalon, 1971, p. 83). This is because, even in vertisols, the
most recently deposited plant
remains are unlikely to be
throughly represented at
depth. Thus the 4,330 B.P. date
provides a minimum for mean residence
time and burial time for
soil carbon in the vertisol.
Based upon relative soil development and the aforementioned
propensity of the Concepcion soil
to act as a crude "sodium
chronometer," we estimate the
vertisol portion of the two-story
soil to have undergone at least ten
times as much soil
development as the mudflow
portion. Assuming a 10:1 development
ratio, the charcoal date yields a
maximum age of 1,068 B.P.
(11,745 B.P./11) and the soil
carbon date yields a minimum age of
722 B.P. ([(2 X 4,330 B.P.) - 722
B.P.]/11) for the mudflow.
Because the mudflow is nowhere
offset at the proposed site for
the Point Conception LNG terminal,
the latest fault movement
there occurred about 1,000 years
ago.
ACKNOWLEDGMENTS
We thank the California Public Utilities Commission (CPUC)
for financial support. Western LNG Associates, Holister and
Bixby Ranches, and Dames &
Moore kindly provided access to trench
excavations and arroyo
exposures. The staff and consultants to
Dames & Moore, Envicom, CPUC,
and the staff of the U.S Geological
Survey provided us with the
invigorating intellectual climate
without which this report would not
have been written.
REFERENCES CITED
Ancient Enterprises, Inc., 1981,
Archaeological test excavations
and some lithic analysis of
materials from TC and TW localities,
unpublished report prepared for
Western LNG Terminal Associates,
Suite 3300, 700 S. Flower St., Los
Angeles, CA, 90017.
Borchardt, G. A., 1977, Montmorillonite
and other smectite
minerals, in J. B. Dixon and S. B. Weed, editors, Minerals in
soil environments: Soil Science
Society of America, Madison, WI,
p. 293-330.
Borchardt, Glenn, and Hill, R. L.,
1982, Soil development under
poor drainage associated with the
Raymond fault, San Marino,
California: Geological Society of
America Abstracts with Programs
(Cordilleran Section), v. 14, no.
4, p. 151.
Borchardt, G. A., Hole, F. D., and
Jackson, M. L., 1968, Genesis
of layer silicates in
representative soils in a glacial landscape
of southeastern Wisconsin: Soil
Science Society of America
Proceedings, v. 32, p. 399-403.
Dames & Moore, 1980, Final
geoseismic investigation, proposed LNG
terminal, Little Cojo Bay, California:
Summary and conclusions.
Report submitted to the Public
Utilities Commission of California
on behalf of Western LNG Terminal
Associates (Los Angeles), Dames
and Moore Job No. 00011-168-02,
Vol. 1.
Johnson, D. L., 1981, Report and
analysis on the Beach Fault
trench soil, LNG site, Point
Conception, California, in Analysis
of data: Marine terrace studies and
age dating, Final geoseismic
investigation, proposed LNG
terminal, Little Cojo Bay,
California: Unpublished report for
Western LNG Terminal
Associates by Dames and Moore, Job
No. 00011-168-02, March 9,
1981, Appendix A.4, 38 p.
Johnson, D. L., and Rockwell, T.
K., 1982, Soil Geomorphology:
Theory, concepts and principles
with examples and applications on
alluvial and marine terraces in
coastal California. Geological
Society of America, Abstracts with
Programs (Cordilleran Sec.),
v. 14, no. 4, p. 176.
Rice, S. J., Treiman, J. A.,
Borchardt, Glenn, Jones, A. L.,
Mualchin, Lalliana, Chapman, R. H.,
and Sherburne, R. W., 1982,
Geologic and seismic hazards
evaluation of the proposed Little
Cojo Bay LNG terminal site, Point
Conception, California:
California Division of Mines and
Geology Open-file Report 82-00
SF, 68 p.
Shipman, G. E., Rabey, D. F., and
Mann, L. D., 1981, Soil survey
of Santa Barbara County,
California, South Coastal Part, U. S.
Department of Agriculture Soil
Conservation Service and Forest
Service in cooperation with the
University of California
Agricultural Experiment Station,
144 p.
Shlemon, R. J., 1978,
Soil-stratigraphy of late Pleistocene-
Holocene deposits, proposed LNG
site, Point Conception,
California. Appendix A in
Dames & Moore, Addendum report,
geological investigations, proposed
LNG terminal, Point
Conception, California. Report submitted to Dames & Moore (Los
Angeles) on behalf of Western LNG
Terminal Associates (Los
Angeles), Dames and Moore Job No.
00011-168-02, 17 p.
Shlemon, R. J., 1980,
Soil-stratigraphy of the proposed LNG site,
Little Cojo Bay, Santa Barbara
County, California. Report
submitted to Dames & Moore (Los
Angeles) on behalf of Western LNG
Terminal Associates (Los Angeles),
Dames and Moore Job No. 00011-
168-02, 26 p.
Soil Survey Staff, 1975, Soil
taxonomy: A basic system of soil
classification for making and
interpreting soil surveys: USDA
Handbook 436, U.S. Government
Printing Office, Washington, D.C.,
754 p.
Wood, W. R., and Johnson, D. L.,
1978, A survey of disturbance
processes in archaeological site
formation. Advances in
Archaeological Method and Theory,
v. 1, p. 315-381.
Yaalon, D. H., 1971, Paleopedology:
Origin, nature and dating of
paleosols: International Society of
Soil Science and Israel
University Press, Jerusalem, 350 p.
Yerkes, R. F., Greene, H. G.,
Tinsley, J. C., and Lajoie, K. R.,
1981, Seismotectonic setting of the
Santa Barbara Channel area,
southern California. U.S.
Geological Survey Miscellaneous Field
Studies Map MF-1169, 25 p.
LIST OF TABLES AND FIGURES
Table 1. Generalized soil description for the mudflow/vertisol
sequence (Concepcion loam) near
Point Conception.
Figure 1. Subparallel bedding plane faults at the proposed LNG
terminal site south of Point
Conception.
Figure 2. Oblique aerial view of the geological exploration
trenches in the marine terraces at
the proposed LNG site. The
Santa Ynez Mountains of the
Transverse Ranges appear in the
background (Photo by Paul L.
McClay).
Figure 3. The "Beach Fault" trench studied in detail by Johnson
(1981) and Shlemon (1980), showing
the local uniformity of
mudflow depth and vertisol
development on the terrace surface.
Figure 4. Close up of the Concepcion soil showing a 40-cm thick
mudflow overlying the
vertisol.
Figure 5. Depth functions for particle size analyses of
Concepcion loam at the "Beach
Fault" trench (from Johnson, 1981).
Note the uniform clay content
within the "gray layer" (upper 60
Figure 6. Soluble salts, adsorbed sodium (Na+), and total sodium
in Concepcion loam at the
"Beach Fault" trench (modified from
Johnson, 1981).
Figure 7. X-ray diffraction patterns from Johnson (1981)
illustrating the youthfulness of
the gray layer in comparison to
the vertisol beneath it. Kaolinite and mica had nearly
disappeared from the upper portion
of the vertisol prior to
deposition of the gray layer.
Figure 8. Trench exposure showing the occasional inverse
relationship between mudflow and
vertisol thickness.
Figure 9. Archeological excavation uncovering angular cobble
anomalous to the pedogenic and
aeolian hypotheses for the origin
of the "gray layer."
Figure 10. Soil survey map showing the lobate fans
produced by
young mudflows delineated by the
Concepcion series (Cg) at the
LNG site (modified from Shipman,
Rabey, and Mann, 1981). Note
the interfluvial bodies of Diablo
clay (Da) (a vertisol)
untouched by the mudflows.
Figure 11. Trench S-D showing the unburied vertisol on
one of
the interfluves.
Figure 12. Arroyo Oeste capped by a young mudflow and
actively
eroding headward from the Pacific
Ocean across the coastal
terrace.
Figure 13. Shears extending from the Arroyo fault
through the
alluvial section and into the base
of the modern soil, a shallow
equivalent of the Concepcion
loam. Erosion and channel
development on the headwall allowed
an extra-thick mudflow
accumulation there.
Figure 14. Log of the soil profile in the previous
figure
showing a shallow equivalent of the
young mudflow as the only
unit not offset by faulting at
Point Conception.
TABLE 1. GENERALIZED SOIL DESCRIPTION FOR THE MUDFLOW/VERTISOL
SEQUENCE
(CONCEPCION LOAM) NEAR POINT CONCEPTION
Composite of descriptions from the "Beach Fault" trench
(Johnson, 1981) and the northeast
corner of trench S-B at the proposed LNG terminal site, Point Conception. Abruptic Natriaquoll (Soil Survey
Staff, 1975) at 34o 27.28'
latitude, 120o 24.62' longitude and 25 m elevation. Mediterranean climate. This two-story soil consists of an upper
unit developed in a very young mudflow and a lower unit developed primarily as
a vertisol in generally fine-grained
Holocene alluvium derived from the Santa Ynez
mountains. Slope 2%; aspect
south. Moderately well to poorly drained. The pH is slightly acid
to
strongly acid.
The 2ABb horizon contains soluble salts and
15-23% sodium on the exchange complex
(Johnson, 1981).
________________________________________________________________________________
Horizon Depth, cm Description
________________________________________________________________________________
A1 0-30 Very dark
grayish brown (10YR3/2m; 10YR6/2d)
loam; massive; hard; clear wavy boundary
C1 30-40 Very dark
gray (10YR3/1m; 10YR6/1d) loam; massive; many fine vesicular pores; gradual
wavy boundary; few clasts to 4 cm
C2 40-60 Very dark
gray (10YR3/1m; 10YR6/1d) loam; numerous yellowish brown(10YR4/6m; 10YR5/6d)
mottles and fine iron
concretions; massive; many fine
vesicular pores; very abrupt
boundary; rare angular cobbles to 20 cm
at heads of fans
2ABb 60-130 Very dark
grayish brown (10YR3/2m;
10YR4/1d) to dark
brown(10YR3/3m; 10YR5/3d) clay,
strong coarse prismatic; extremely hard; common slickensides and pressure faces;
few to common thin to moderately thick clay films on peds; clear smooth
boundary
2BAb 130-160 Dark brown
(10YR4/3m; 10YR5/3d) sandy clay loam;
moderate coarse prismatic;
common moderately thick clay films on peds
2C 160+
Yellowish brown (10YR5/4m) to
brown (10YR4/3m) loam to loamy coarse sand grading laterally to sandy cobble;
massive
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[Download Figure Set B (4.7 mb)]
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