Journal of Geosciences, Osaka City UniversityVol. 34, Art. 4, p. 85-102March, 1991

Diagenetic Effects on the Density of ArgillaceousSediments During Compaction

Koichi NAKAGAWA

(With 12 Figures and 1 Table)

Abstract

Based on data from various regions, the depth dependency of the density of sedimentarylayers of the lower and higher sedimentation rates such as for deep ocean floor and oil field in large­scale sedimentary basins, respectively, was studied. A clear relationship was found to exist be­tween the density gradient and the sedimentation rate. To clarify the geological significance ofthis observed relationship, a simple model for the compaction process is proposed. The funda­

mental equation is expressed as

ej=eoj-Cci log P-C,'i log t

where ej is void ratio of a layer under effective pressure P and elapsed time of secondary consolida­tion t, respectively, and eoj is void ratio under a specified condition, Po and to' The constantsCci and C",. are the compression index and the coefficient of secondary consolidation, respectively.

The results of theoretical analyses showed good agreement with the actual density-depth re­lation over the wide range of depth. In addition, most of the observed data plot on or near thetheoretical line on the diagram showing the relationship between density gradient and sedimenta­tion rate. The data from the trench axis in the Nankai Trough, however, deviate significantlytowards the over-consolidation area from the standard line on the diagram. This supports theidea that the accretionary prism at the convergent plate margin has not only undergone gravityconsolidation but also obvious tectonic consolidation.

Erosion of the strata also influences the original nature of the density distribution with depth.The thickness of eroded strata was estimated to be 150 meters in the Akita oil field region andover 550 meters in the Kwanto sedmimentary basin, which is known as the biggest Quaternary

basin in Japan.

Key Words: Density, Void Ratio, Compaction Model, Daiagenesis, Secondary Consolidation,Aging.

1. Introduction

Density is the most fundamental quantity among vanous kinds of physical proper­

ties. Many physical and mechanical properties of sediments strongly depend on thedensity or the void ratio.

The study of porosity changes in argillaceous sediments has been made by manyauthors since Steno's grand works in the seventeenth century. RIEKE and CHILINGARIAN

(1974) and CHILINGARIAN and WORF (1975, 1976) have broadly reviewed an enormous

Dept. Geosci., Fac. Sci., Osaka City Univ., Sugimoto, Sumiyoshi-ku, Osaka 558, Japan.

86 Koichi NAKAGAWA

amount of studies concerning the compaction of various kinds of sedimentary rocks.

Since the 1960's, many measurements of bulk densities of drill-core samples havebeen made at exploration oil fields. The spatial density structure has been revealed bymuch data in the oil fields. The characteristics of density distribution have been shown

to be storngly affected by geological structures such as the unconformity or the fault(e.g., MATSUZAWA, 1961, 1962, MIYAZAKI, 1966).

Although considerable effort has been concentrated on the search for a best fittingfunction between the density and depth, the physical meaning has yet to be revealed

clearly.

2. The factors influencing the physical property ofargillaceous sediments

The cohesive geo-materials have been undergoing remarkable changes in their physi­

cal properties and structural fabrics during various kinds of geological processes. As

these changing processes closely correspond to the physical and physico-chemicalprocesses called diagenesis, the investigation of the changing processes in the physical

properties of sediments is also considered to be important to explain the mechanisms

of diagenesis.The theory of the diffuse double layer in colloid chemistry has been introduced

into the clay-water-electrolyte system and applied to soil behavior with some success.MITCHELL (1976) has broadly reviewed physical and physico-chemical processes in the soilfrom the standpoint of soil mechanics, and he has given a consistent explanation about the.surrounding topics and applications to geotechnical problems.

The density of argillaceous sediments is thought to depend on numerous factors,

Grain's

Properties

. Grain Composition

tNineral Composi tion

Humus

Allophane

. Grain Size

[Speci fic ."reaExchangeable

Capaci ty

Shape

Surface

[Charge Densi ty

Exchangeable

ea ti on

Sedimentation

Process.

Grain Arrangemen t

[Flow Veloei ty

Sedimentation Rate

Pore Hater

[

Electrolyte

Composi t io.nValence

ConcentrationpH

. Bic-Effects

CompactionProcess'

Overburden Pressure

Loading Rate

Tempera tlJ re

Stress History

[Tectonic NovementSea-level Change

Pore \\'ater

[

Electrolyte

Composition

Valence

. Concentration

pH

Weathering

process

Erosion

[ UnloadingChanging

Pore-Pressure

Underground Water

[Leaching\vash-out of

Fine Grains

Tempe ra tu rerThermal Stress~ Slaking

. Climate

Fig. 1 Geological factors affecting the physical property of argillaceoLls sediments.

Diagenetic Effects on the Density of Argillaceous Sediments During Compaction 87

including lithology, grain size, sedimentation rate, wave loading, eletro-chemical pro­

perty of pore water, geologic age and erosion (Fig. 1). Quantitative study of these fac­tors, however, has not been adequately made, and a study of the processes leading to

changing density, based on microscopic structure under geological conditions, has yet

to be made.

3. The relation between density and depth

RIEKE and CHILINGARIAN (1974) and PERRIER and QUIBLIER (1974) compiled therelation between the observed porosity of mudstone and its burial depth, using mainly

the bulk densities of specimens sampled from some basinal areas such as the oil field.Fig. 2 shows, as one of the examples, the relationship between porosity and dapth for

argillaceous sediments. There is a relation between the porosity ¢ and the wet bulk

density p as follows

( 1 )

where Ps and Pw are the density of grain and water, respectively. The general trend ofthe porosity change indicates that its ratio to depth decreases with depth. Further,

-.E

00

'" 0

10

;:a..0

15

'----'--"--...J.20----."-0----""• .,...0---' 20

Porosity

Fig. 2 Examples of porosity verSLlS depth relation for argillaceoLls sediments invarious areas (RIEKE and CHILlNGARIAN, 1974) 1: PROSHLYAKHOV (1960),2: MEADE (1966), 3: ATHY (1930), 4: Hosor (1963), 5: HEDBERG (1936),6: DICKINSON (1953), 7: MAGARA (1968), 8: WELLER (1959),9: I-lAM (1966),FOSTER and WHALEN (1966).

88 Koichi NAKAGAWA

the lines indicating the porosity versus depth relation are significantly scattered as seen

in Fig. 2. These lines tend to converge in the high-pososity zon~, that is, the depthjporosity range becomes wider with decreasing porosity and narrower with increasing

porosity.To examine the time dependency of the compaction of sediments, we have collected

data of the density distributions assocaited with relaible chronological information for

the sediments. Six samples are from sedimentary basins, namely: the Osaka Basin, OD-l(Osaka City, 1964); Osaka Basin, OD-2 (Osaka City, 1965); Akita Basin (MIYAZAKI,

1966); Niigata Basin (MATSUZAWA, 1961, 1962); Kwanto Basin (Nirei at ai., 1972); and

Po Valley (SKEMPTOM, 1969; STARER, 1959; Rocco and JABOIL, 1959). Five samples

are from DSDP Sites which were drilled by Deep Sea Drilling Projeat, namely: theDSDP Site 527, L.74 (RABINOWITZ and BORELLA, 1984); DSDP Site 357, L.39 (PERCH­NIELSEN and Supco et ai., 1977); DSDP Site 594, L.67 (FAAS, 1982); DSDP Site 474,L.64 (CURRAY and MOORE et ai., 1972); DSDP Site 298, L.31 (BUMA and MOORE, 1975).

These locations are shown in Fig. 3. We did not use the data from the stratacontaining predominantly limestone beds or diatomaceous ooze because the compaction

characteristics of calcareous or diatomaceous sediments are somewhat different from

ordinary sediments (McCrossan, 1961; Hamilton, 1960, 1976). The density distributionwith depth at each site is expressed as a single line in Fig. 4 by averaging and smoothing

the original data. As shown in Fig. 4, the curves showing those variations with depthhave a similar tendency to those seen in Fig. 2 if porosity is replaced by density. This

tendency seems to represent the presumable compaction process of argillaceous sedi­ments in the common sedimentation fields.

4. Theoretical model of density variation with depth

Figs. 2 and 4 indicate that the density versus depth curve varies at each site andthat their curves tend to converge at near the ground surface.

In order to discuss density changes under the geological conditions, it is necessary to

introduce a simple consolidation model. As a stratum generally consists of a set of thesingle-layers, we can assume that the void ratio of i-th layer ei, is expressed by a functionof the effective overburden pressure P at the normal consolidation stage, namely:

( 2 )

where eoi is the void ratio at effective overburden pressure Po, and Cc is a constant namedas compression index which depends on such factors as lithology, grain size and soilconsistency. Let Vsi be an equivalent thickness of the solid part of the i-th layer.Then the bulk thickness of i-th layer under effective overburden pressure P, is

Vi = Vsi(l+e j ) ( 3 )

where ei=eoi-Cci log (PIPo). Consequently, the thickness of whole strata is

Fig. 3 Locality map of the deep drilling sites from which data relating to density variation with depth is citedin this study. A: Akita Basin, B: Lake Biwa, K: Kwanto Basin, iigata Basin, 0: Osaka Basin, P: PoValley, T: ankai Trough ( ite 1 0 298), umber is the ite o. of DSDP.

60'

O'

•;} 57

I30·

IO·

90' 120'I

150'

140'

150·

14S'E

120·

120·

90'

90·

60·

60·

90 Koichi NAKAGAWA

1.6

ME()

"'-~

1.8:=VIc:III

0-"::J 2.0

to~

III

~

2.2

0 1.0 2.0

Dept h (km)

Fig.4 Density variation with depth at various localities. 1) DSDP Site 527, L.74(RABINOWITZ and BORELLA, 1980). 2) DSDP Site 357, L.39 (PERCH-NIELSENand Supco et at., 1974). 3) DSDP Site 'f94, L.67 (FAAS, 1984). 4) DSDPSite 474, L.64 (CURRAY and MOORE et at., 1978). 5) DSDP Site 298, L.31(BUMA and MOORE, 1973). 6) Osaka Basin, OD-1 (Osaka City, 1964). 7)Osaka Basin, OD-2 (Osaka City, 1965). 8) Akita Basin (MIYAZAKI, 1966).9) Niigata Basin (MATSUZAWA, 1961). 10) Kwanto Basin (NIREI et at., 1972).11) Po Valley (SKEMPTOM, 1969; STARER, Rocco and JABOLl, 1959).

( 4 )

The void ratio of the whole strata can be expressed as

( 5 )

If a stratum exists at sufficiently great depth so that the effective pressure can be re­

garded as constant in a stratum, we have

e = k Vsi eodkVsi-(k VsCcdk Vsi ) log (PIPo)

= eo-Cc log (PIPo) ( 6 )

where eo is a bulk void ratio of the strata under effective pressure Po and IS equal to

(k V si eodk V s;). Furthermore,

(7)

Therefore, the index Cc is found to be the weighted average for a set of layers as

proportional to the thickness of solid part in the each layer.

The above equation (6), however, should be valid in a special case, namely, at some

Diagenetic Effects on the Density of Argillaceous Sediments During Compaction 91

constant time after the termination of the primary consolidation. Hence, the variationof void ratio with time under a constant effective pressure can be expressed as

( 8 )

where C"i is a constant called the coefficient of secondary consolidation, which exhibits

the degree of elapsed time-effect on the consolidation.

Consequently, the void ratio at elapsed time t under a condition of constant pressure

p in the case of a single layer may be given by combining the equations (6) and (8), as

follows,

( 9 )

When applying the above equation to the whole strata, the subscript i in the equation

should be removed wherever it occurs. Namely,

(10)

where eo is an initial void ratio under the consolidation condition at which the effective

pressure is Po and the elapsed time of the secondary consolidation is to' and C" is the

coefficient of secondary consolidation for the whole strata as follows,

(11)

The coefficient C", is a weighted average, similar to the compression index Cel for each

layer. Therefore, the void ratio of a stratum which consists of many layers can also be

analysed similarly to the case of a single layer.Fig. 5 shows e-log p-log t diagram representing some paths of the variation of

e

log P

'"~i::>,

Fig. 5 Some consolidation paths on the void ratio-mean stress-time diagram.

92 Koichi NAKAGAWA

void ratio. The path (1) on the figure exhibits the relation as an e-log p one obtainedfrom the step-loading type consolidation test. On the other hand, the path (2) exhibits

the relation as an e-log t one after primary consolidation at a stage for the commonconsolidation test. This path corresponds to a void ratio variation with time in the

process of secondary consolidation under a constant effective stress.Furthermore, the path (3) exhibits the relation for the layer at which applied effec­

tive stress is increasing proportionally with time, such as the case of underlying sediment

in a stationary sedimentation basin. As the actual process seems to be as in the case ofpath (3) or nearly this, it is considered to be significant to examine the change of void

ratio in the stationary sedimentation field by means of this simple relation.If the sedimentation rate is constant, the effective overburden pressure on a layer

which was formed before a time t, is expressed as

(12)

where g, Pw and Ps are the acceleration of gravity, the density of water and the densityof the solid part, respectively, and g(pw/Ps) is buoyancy applied to unit mass of thesolid part. The parameter m is a sedimentation rate referred to as the accumulationflux which is defined as the amount of the accumulating solid mass per unit area and

per unit time. This parameter is also called the mass accumulation rate, MAR.It is presumed that the period required for primary consolidation is significantly

short in the scale of geologic time. Accordingly, the equation (10) seems to be valid inthe actual strata and may be reformed to

e(t) = eoi-Cci log (m'tg/Po)-C"'i log (t/to)

= {eOi+Cci log (Po/g)+C,.; log to}-Cci log m'(Cci+C",;) log t (13)

where m' is an apparent mass measured in water, that is, m'=m(1-pw/ps)' Theefffective overburden pressure is P=m'~I? As the first term of the right-hand memberof the latter equation (13) does not depend on the time, the equation (13) becomes

The thickness of the layer consequently decreases with the elapsed time, namely,

o(t) = os{1+e(t)}

(14)

(15)

where o(t) is the thickness variation of a layer formed in a unit time, and Os is the

equivalent thickness of a solid part in the layer, that is, 0s=m/ps'In these sedimentation fields, the underlying layers decrease their thickness with

time through the effects of the increasing overburden pressure and secondary consolida­tion. Integrating the equation (15), we can obtain the thickness variation of the strataduring elapsed time, thus,

Diagenetic Effects on the Density of A,'gilLaceous Sediments Du.ring Compaction 93

D = )~ 0s{l+e(t-r)}dr

= Do+(mtjp s)[l+eo' -OA34{Cc In 111.' +(Cc+C",)(ln (t-1)} ] (16)

where Do is the initial value of the thikcness of the strata; but in this case we can denote

Do=O when t=O, From above equation, the relation between void ratio and depth canbe obtained by means of substituting an adequate value for the sedimentation rate m

and changing the variable t.

5. Application to the compiled data

Fig. 6 shows the actual relationship between age and depth in places where sedi­mentation seems to have continued at a constant rate up to Recent times. The ages of

data plots indicated by open circles or solid lines on the figure have been determinedprecisely by means of microfossil analysis, fission-track technique, identification of paleo­magnetic polarity and other methods. The values of parameter m attached to the dottedline on the figure exhibit the sedimentation rate, kgj(m2 X 103 yrs), and correspond toeach relationship between the age and depth dederived from equation (16).

Some assumptions in this computation are as follows: the sedimentation rate is con­

stant, and the values of each constant are eo'=O.4, Cc=0.05, and Ps=2.65 gjcm3• These

values are nearly equal to those of the silty soils; they seem also to be a mean value for aseries of strata accumulated in submerged area, such as a sedimentary basin.

A clay or a shale, however, may have more important meaning than other kinds of

geo-materials. Hence the density for the more plastic material is a better indicator of

the maximum pressure applied in the past. Therefore, we use other parameters, Cc '

and C",', instead of Cc and C"', in the equation (14). Namely, the void ratio of clay or

shale is expressed by similar equation as (14)

e(t) = eo"-Cc' log 112' -(Cc'+C",') log t

If the sediments are saturated by water, their density is

(17)

(18)

where Ps and p.. are the density of solid and wattr, respectively. Then we can easilyobtain the density from the void ratio based on a general assumption that p,=2.65 gjcm3

and p ..= 1.0 gjcm3•

As a necessary preliminary to the analysis of density distribution, let us comparethe results of observation and computation for density versus depth relation. Fig. 7shows the density versus depth relations of a clayey sediment computed from equations(16) and (17). It is clear from the figure that the density versus depth relation is strongly

influenced by parameter 111, and that the increasing of m leads to the decreasing of thedensity gradient. In contrast to this, it may also be possible to estimate the sedimenta­tion rate from the density gradient referring to this figure.

94 Koichi NAKAGAWA

m=4000

2

1.5

,......E

-=-1

i;!!i<:<:c:;:-'-.... <

6

S

Q..0

om=2000

111 =1100

2 3

(Mal

..c:......Cl.Cl.l

o

.5

aa

Ag e

....

S.474

.. 10=400

S.494

.S:}?7 .. m =100S.527,m= 10

Fig. 6 The relation between the age of strata and its depth (unit of sedimentationrate m is kg/(m2 X 103 yrs)). Niigata Basin: Boundary between the Nishi­yama Formation and the Kaizume F., 0.85 Ma (TSUCHT, 1979); Po valley:Boundary between the Calabrian and Upper Pliocene strata, 18.2 Ma (SELLI,1967); Akita Basin: Boundary between the Tentokuji F. and Sasaoka F.,0.90 Ma (TSUCHT, 1979); Osaka Basin: Azuki tuff, 0.87 Ma (NTSHIMURAand SASAJIMA, 1970); Lake of Biwa: Compiling of fission track age (TAI­SH[, 1986); DSDP S. 474 (Leg 64): (CURRAY and MOORE et ai., 1978);DSDP S. 494 (Leg 67): (AUBOUTN and von HUENE et ai., 1980); DSDPS. 357 (Leg 39): (PELCH-NIELSEN and SUPKO, 1974); DSDP S. 527 (Leg74 : (MOORE and RABINOWITZ, 1980).

Fig. 8 shows the comparison between theoretically computed and observed relationsof density variation with depth. Those two results are in good agreement with each other

over wide range, as seen in the figure, with the exception of two curves, no. 8 and no. 10.

If the curves no. 8 and no. 10 are shifted by about 150 meter and 550 meter, re­

spectively, in the direction of deeper depth, that is, on the right side in Fig. 8, the theo­

retical curves are almost identical with the observed ones.

The above theoretical curves are obtained by substituting adequate values for the

constants of the equations (16) and (17). A set of optimum values for the parameters

Diagenetic Effects on the Density of Argillaceous Sediments During Compaction

..-..E 1.6

~0>

>-+-" 1.8OIlc:QJ

0

-"":::J

2.0m

2.2

95

1

Depth (km)2

Fig. 7 Theoretical computation of the effect of sedimentation rate, 111 {kg/(m2 X 103

yrs)}, on the density variation with depth .

..-....,1.6E

~0>

>-1.8

OIlc:QJ

0

-"":::J

2.0m

QJ

::::2.2

0 1

Dept h (km)2

Fig. 8 Comparison of theoretical computation results and observed relation for thedensity variation with depth. Heavy solid line: observed relation, Lightsolid line: theoretical relation. Broken line: observed relation at the localitywhere the region seems to have been eroded significantly (refer to text).

96 Koichi NAKAGAWA

can be obtained by comparing the results of theoretical calculation and the actual densitydistribution shown in Fig. 8. This is shown in Table 1. Each value of the parameteradopted shows that it is a common value in general kinds of sediments.

We now examine the density versus depth relation from the another point of view.Fig. 9 shows the relation between depth of the layer aged 1 Ma and depth of the layer

having density of 2.0 g/cm3• This relation approximately correspond~ to the relation

between a sedimentation rate and a density gradient of subsurface strata.The observed data plot on or near the solid line derived from the theoretical com­

putation, except the data from Site 298 of DSDP.The above solid line is obtained by theoretical calculation and by using the same

values of the parameters as those shown in Table 1. In Fig. 7, we can draw a straightline indicating the density of 2.0 g/cm3 parallel to the depth axis. This line gives thedepth of a layer having density of 2.0 g/cm3 corresponding to each value of sedimenta­

tion rate m. Applying this relation to the age versus depth relation (equation (16) or

2.0

Nankai Troughs. 2 98 •

E~

1.5

11 1.0

'"'"

.<:a. 0.5

'"o

0.5

o

km1.0 1.5

Depth of layer having density 2.0 g/cm 3

Fig. 9 Relation between depth of the layer aged 1 Ma and depth of the layer havingdensity of 2.0 gjcm3• Open circle: observed data from land area. Solidcircle: observed data from wells of DSDP. umber is Site 1 o. Solid line:theoretical relation using optimum value of C,.'jCc' =0.6. Broken line: the­oretical relation using empirical common value of Co/ICc' =0.05.

Table 1. The values of parameters used by computation

0.4 0.1

C<»

0.005 4.7

Cc'

1.0 0.6

Diagenetic Effects on the Density of Argillaceous Sediments During Compaction 97

Fig. 6), we can obtain the depth of layer aged 1 Ma corresponding to each value of m,eventually obtaining the theoretical curve shown in Fig. 9.

In this case, the ratio of secondary consolidation coefficient to compression index

C"" ICc' is equal to 0.6. This value is a order of magnitude larger than the commonvalue of 0.05 reported from the general consolidation test in the laboratory. If the value

of 0.05 is adopted for the analysis, the theoretical curve is represented as a broken line

as shown in Fig. 9. As the time effect obtained from the laboratory is too small, we

cannot explain the consolidation process found in actual strata. This discrepancy should

be attributed to the difference of conditions on the consolidation process between the

conventional testing method in laboratory and the in situ conditions.

Concerning the datum at Site 298 of DSDP, on the Nankai Trough, it is clear that

the point is plotted in an extreme over-consolidated region at a good distance from the

standard line in Fig. 9. The sedimentation at the site is very rapid, and the thickness of

sediment has been estimated to be over 900 meters during 1 Ma to present (INGLE et al.]975).

As pointed out by MOORE and KARIG (1976) and BRAY and KARIG (1985), the fact

is that as the drilling point is located exactly on the trench axis, the accretionary prism

existing at the convergent plate margin has been strongly deformed, with desiccation not

only by the gravimetric compaction but also by the obvious tectonic compaction.

6. Influence of erosion

Though a relative uplift associated with tectonic movement or change of sea level

leads to erosion of the strata, an underlying stratum has the density corresponding to

applied maximum overburden pressure, and an alteration of the relationship between

density and depth takes place.The density variation with depth at two typical regions in the Osaka Basin is shown

in Fig. 10. Closed and open circles represent the data from drilling OD-], which is

situated in the depression area (Minato district, Osaka Pref.), and from drilling OD-2,

which is situated in the uplift area (Miyakoijma district, Osaka Pref.), respectively. These

two areas are regarded as being of similar geological constitution, from the observation

of their drill cores. The upland region including OD-2, however, has been considered

to be significantly eroded by uplift during the Late Pleistocene (Hl!ZITA, 1983), and the

displacement is estimated to be about 370 meter from the offset of the intercalated key

bed called the Azuki Tuff. If the density distribution of OD-2 is shifted to deeper depth

by 370 meters in Fig. 10, we can see a good agreement between the two density distribu­

tions with depth.

In the case of Akita Basin, if the line in Fig. 8 represented by linked small circles

is shifted downward by 150 meters from the dotted line (no. 8) in the figure, it almost

coincides with the theoretical line corresponding to m=2,000 (kg/(m2 .103 yrs» and the

amount of the erosion is estimated to be 150 meters. The estimated value is concordant

98 Koichi NAKAGAWA

00·1o 00·2

1.6

Eu.,Ol

2.0

~VIc:

"<:>

oo

Depth (m)

Fig. 10 Comparison of difIerent relations between density and depth at two local­ities in the Osaka Plain. OD-l: well at Minato-Ku, near shore line of theOsaka bay (region of tectonic depression). OD-2: well at Uemachi Uphill(center of tectonic upheaval zone).

1.6......."'E

'ZOl'--" .'..~1.8U)

c:.,o

2,0

o

'. : ....,

.' .: .550 m __

2

Depth (km)

Fig. 11 Estimation of the thickness of the eroded strata in Kwanto Basin deducedfrom the density versus depth relation together with the present model.

with the evidence of at least the two erosional periods during the Late Pleistocene, as

seen from the stratigraphic examinations (IKEBE and IWAsA, 1963).

The comparison of density distributions as between observation and computation In

Diagenetic Effects on the Density of Argillaceous Sediments During Compaction 99

Kwanto Basin, which is greatest scale of the Quaternary basins in Japan, is shown inFig.l1. The observed data are from Funahashi City in Chiba Pref. (NIREI et al., 1972).Some data from Tokyo tend also to be similar to those (ENDO, 1979). The amounts oferosion and the sedimentation rate m are estimated to be over 550 meters and 8,000 (kgj

(m2 .103 yrs», respectively. The data on geologic age of the subsurface strata related tothe above results have not yet been obtained.

7. Influence of environment

As mentioned above, physico-chemical factors play an important role in the clay­water electrolyte system. Factors such as the specific area of clay, surface characteristicsof grain and chemical characteristics of pore water affect the physical properties, especial­

ly the elastic moduli of argillaceous sediment (NAKAGAWA et al., 1977, NAKAGAWA and

OKUDA, 1978). The void ratio of clay consisting of montmorillonite is thought to be

affected by the concentration of electrolyte in pore water. Accordingly, it is expectedthat the density or the density distribution with depth of lacustrine clayey sediments dif­fers from that of marine clayey sediments.

Fig. 12 shows the comparison between the bulk density of the clayey sediments

under Lake Biwa and under Osaka Bay, with exception of sediemnts whose liquid limitis smaller than 70%. The void ratio of marine sediments tend to decrease more than

those of lacustrine sediments, and it is difficult to know whether this difference is sig­

nificant or not.

Wl > 70"o Lake Biw3

• Osaka Bay (No. 57-30) Site B

• Osaka Bay (No. 56-9 j Site A

M

E 1.2u

"-on 1."

~~00 •

• ca ~ 0 CP 0

::: 1.6 ~ • '!:PCb o't~ 0"0 ~cP=b0 .- 0.\ ~ ~.r..~ ~ ." . . ..... .:,.: :. .;. .. 1.8

0 .. . .:: 2.0 0 0,<Il

2.2

oo

100 200 300 "00

Depth(m)

500 600 700 800

Fig. 12 Comparison of density variations with depth for marine sediments andlacustrine sediments. The plots are data from core samples of whichliquid limit is larger than 70%. The original data are obtained by Akaiet al. (1986).

100 Koichi NAKAGAWA

Acknowledgements

The author acknowledges the helpful advice and continuing encouragement of Prof.K. WADATSUMI, and wishes to express his gratitude to Prof. M. HIRANO and Dr. K.

SHIONO for frequent stimulating and helpful discussion.

References

AKAI, K., KAMON, K., and IDA, S., 1986:, Geotechnical properties of deep lake sediments fromLake Biwa (in Japanese with English abstract)., Annual Disas. Prevo Inst., Kyoto Univ., No.29B-2, 1-11.

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Received Dec. 3, 1990Accepted Dec. 22, 1990