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