Grain fabric of experimental gravity f low deposits

Tetsuya Sakai a,*, Miwa Yokokawa b,1, Yu’suke Kubo c, Noritaka Endo b, Fujio Masuda a

aDepartment of Geology and Mineralogy, Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University,

Kyoto 606-8502, JapanbDepartment of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan

cDeep-Sea Research Department, Japan Marine Science and Technology Center, Yokosuka 237-0061, Japan

Accepted 24 January 2002

Abstract

Grain fabric of deposits accumulated from a high-density surge-type gravity (turbidity) current in an experimental flume was

measured. Vertical sequential change (0.2-mm interval) in imbrication shows that a bed can be divided into lower, middle, upper

and uppermost parts. The lower part is characterized by both up-current and down-current imbrication with a wide range of

angles. Dominant up-current imbrication and rare down-current imbrication characterize the middle part. The imbrication angle

of this part tends to be smaller than in the lower part. The upper part is represented by intervals with up-current imbrication.

Nearly flat imbrication is dominant in the uppermost intervals. Statistically significant preferred orientation was observed from

the lower, middle and upper parts, and it deviates up to 13j in both clockwise and anticlockwise directions from the current

direction. The lower and middle parts of the bed may correspond to the Bouma A-division judging from the wider range of

imbrication angles and the presence of down-current imbrication which have been reported from natural turbidite beds. The

upper part, which is characterized by up-current imbrication, is interpreted to be the Bouma B-division. The uppermost interval

may coincide with the D-division. The episodically appearing down-current imbrication in the lower and middle parts can

probably be attributed to oscillation of an interface between a denser basal layer and superjacent low-density layer of the

turbidity current. Measurement of grain orientations in this and previous studies implies that at least 30j of deviation from the

flow axis should be considered for paleoflow analyses based on grain fabrics.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Gravity flow; Flume; Grain fabric; Imbrication; Orientation; Turbidite

1. Introduction

The purposes of this study are to describe vertical

changes in grain orientation and imbrication within a

turbidite bed accumulated from a single, unsteady,

high-density gravity (turbidity) current in an exper-

imental flume, where complicated topographic control

is almost negligible, and to discuss their origin.

Analysis of grain fabric has been applied to esti-

mation of hydraulic regime of various types of flows,

as well as for determination of the paleoflow direction

(e.g. Pettijohn, 1962; Sestini and Pranzini, 1965;

Parkash and Middleton, 1970; Hiscott and Middleton,

1980; Pickering and Hiscott, 1985; Taira and Niit-

suma, 1986; Cheel, 1991; Yokokawa and Masuda,

0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0037 -0738 (02 )00106 -9

* Corresponding author. Tel.: +81-75-753-4158; fax: +81-75-

753-4189.

E-mail address: sake@kueps.kyoto-u.ac.jp (T. Sakai).1 Present address: Lab. of Geoenvironments, Faculty of

Information Science and Technology, Osaka Institute of Technol-

ogy, Osaka 573-0196, Japan.

www.elsevier.com/locate/sedgeo

Sedimentary Geology 154 (2002) 1–10

1991; Yagishita et al., 1992; Jo et al., 1997). Grain

fabric of turbidite beds, especially the massive or

graded division (Bouma A-division), has been studied

in detail since the 1950s (e.g. Crowell, 1955; Bouma,

1962; Taira and Scholle, 1979; Hiscott and Middleton,

1980). Early works focused on the determination of

paleoflow and on testing the validity of grain orienta-

tion within turbidite beds as an indicator of paleoflow.

Studies since the 1960s mainly discussed depositional

mechanisms (e.g. Parkash and Middleton, 1970; Taira

and Scholle, 1979; Hiscott and Middleton, 1980).

Grains settled from suspension fallout and from

highly concentrated sediment dispersions such as

grain flows tend to align their long axis parallel to

the fluid motion, as shown by both theoretical analysis

and experimental results (e.g. Rusnak, 1957; Rees,

1968; Hamilton et al., 1968; Rees and Woodall, 1975;

Arnott and Hand, 1989). Actual measurements of the

orientation of sand grains or contained plant frag-

ments show rough correspondence with paleoflows

shown by sole marks (e.g. McBride, 1962; Sestini and

Pranzini, 1965). However, there are several reports of

grain orientation oblique or perpendicular to sole

mark lineations (e.g. Bouma, 1962; Spotts, 1964;

Taira and Scholle, 1979). These have been explained

by change in flow direction (Spotts, 1964), rapid

deposition from high-density current or debris flow

(Hiscott and Middleton, 1980) and traction occurring

before deposition of the next layer (Ballance, 1964).

Flow-normal grain orientation formed in viscous flow

without grain interaction was demonstrated by both

theoretical and experimental studies (Jeffrey, 1922;

Taylor, 1923). Rees (1983) also found that flow-

normal grain orientation was formed by intermediate

density flow in an experimental flume.

In flow parallel vertical section, up-current-dipping

imbrication of sand grains is predominant in the A-

division of many turbidite beds (e.g. Spotts, 1964;

McBride, 1962; Sestini and Pranzini, 1965; Taira and

Scholle, 1979); however, down-current imbrication is

also detected from some turbidite beds (Sestini and

Pranzini, 1965; Hiscott and Middleton, 1980). Hiscott

and Middleton (1980) found that such down-current

imbrication appears in intervals with bimodal orienta-

tion in horizontal plane, and explained its formation

by rapid sedimentation from a high-density current.

Bouma (1962) described a bed which starts with up-

current imbrication and grades upward into down-

current imbrication; he attributed the formation of the

upward imbrication change to flow reversal of the

turbidity current.

As cited above, grain fabric of turbidites, especially

in the graded or massive division, apparently varies

bed by bed. This is probably because (i) there have

been no studies which measured continuous fabric

change of a whole bed and (ii) the complexities of

natural turbidity currents, such as topographic control

affecting the flow path and characteristics of the

turbidity current itself, cause wide variety of grain

fabric. In previous studies, several depositional

hypotheses for the A-division have been proposed

such as deposition from a high-density current, debris

flow and so on (e.g. Stauffer, 1967; Middleton, 1967;

Walker, 1967; Lowe, 1982; Arnott and Hand, 1989;

Allen, 1991; Kneller and Branney, 1995; Shanmugam,

1996). Sediment transport and depositional process in

turbidity currents differ with the grain size of the

material contained in currents and with the flow

character itself (e.g. Lowe, 1982; Kneller and Bran-

ney, 1995; Sohn, 1997). We believe that there are

several mechanisms that may result in deposition of

the A-division, as pointed out by Sohn (1997), but

there has been no discrimination on the basis of the

fabric analysis. Neither has there been any completely

satisfactory model explaining the formation of the

fabric of turbidite beds. Therefore, an analysis of the

simplest case is needed and its results, we believe, will

contribute to the understanding of fabric formed under

more complicated conditions.

2. Methods

2.1. Flume experiment

Turbidity currents were generated in an experimen-

tal flume of Osaka University which is 10 m long, 0.2

m wide and 0.5 m deep (Fig. 1). A clear small tank

with a lock gate was set at the upstream end of the

flume. Tap water was mixed with about 10 vol.%

sediments in the closed tank. Sediment used was

artificial sand consisting mainly of quartz particles

(almost 100%). The sediment has mean grain size of

2.93/, r/ of 0.62 (moderately well sorted) and con-

tains about 4.5% of muddy material (Fig. 2). The

sediment and water mixture was well dispersed by

T. Sakai et al. / Sedimentary Geology 154 (2002) 1–102

hand until the sediment was completely suspended.

Then, the gate was unlocked and the turbidity current

was released from the tank (Fig. 1).

We carried out two series of experiments (Series 1

and 2). Three currents were generated in each series;

the first flow was for measuring change in flow

velocity using the Laser Doppler Anemometry (for

detailed measurement methodology, see Kneller et al.,

1997); the second was to accumulate a bed for fabric

analysis; and the third was for accumulation of a bed

to protect the top of the second bed from the destruc-

tion of its fabric by bed fluidization while water was

drained from the flume. The time interval between

each flow was about 5 h, which was believed to be

sufficient for settling of fine suspended particles.

After three beds were accumulated, water was

carefully drained from the flume. The bed surface

was dried until it became moderately moist and

suitable for sampling (approximately 7 days). The

samples were collected at 250 cm (Series 1) and

200 cm downstream points (Series 2) from the gate.

The sampling procedure was as follows: accumulated

sediment was excavated except for a 5-cm-wide strip

along the flume centerline. The sample surface was

cemented by using rapidly solidifying low-viscosity

glue. The rapidly cemented samples were carefully

taken from the flume. They were then completely

dried and were impregnated slowly and carefully with

epoxy-type resin. Completely cemented samples were

split into specimens for imbrication measurement

(vertical flow parallel section) and orientation meas-

urement (horizontal section).

2.2. Fabric measurement

The imbrication and orientation were automatically

measured by using software (NIH Image) released by

NIH, USA. The procedure for imbrication and ori-

entation angle measurement was as follows. Image

data were recorded as 256 gray-scale digital data (Fig.

3) using a CCD camera. Image data from the whole

bed were divided into 0.2-mm intervals for imbrica-

tion analysis, as this interval is approximately equiv-

alent to the diameter of two grains. Only grains of

elliptical shape, with apparent a/b axial ratios greater

than 3:2, were measured. Grains that occupied more

than one interval were incorporated into the interval in

which the largest area of the grain was observed.

Directions of grain elongation were drawn manually

as black line on the data image (Fig. 4B). The thresh-

old value of the image data was then increased to the

Fig. 1. Experimental flume of Osaka University.

Fig. 2. Grain-size distribution of sediment used in the experiments.

T. Sakai et al. / Sedimentary Geology 154 (2002) 1–10 3

highest level so that only the drawn lines remained

(Fig. 4C and D). The angles between the lines and a

reference horizon were automatically measured using

the software and resultant data were summarized into

rose diagrams (Fig. 4E) and tested statistically using

the Turkey chi-square test (Middleton, 1965).

Fig. 4. Method of grain fabric measurement. See text for details.

Fig. 3. Photomicrograph of an experimental gravity flow deposit.

White arrows represent down-current imbrication. Black spots re-

present parts where grains were removed during sample polishing.

T. Sakai et al. / Sedimentary Geology 154 (2002) 1–104

3. Results

Average density of the released flows was less than

10% because large volume of the sediment accumu-

lated inside the tank. Accumulation of the sediments

inside the tank suggests, however, that higher density

of sediment and water mixture might be created in the

basal part of the tank because of sediment settling

during a short-time lag between stopping of the

mixing and unlocking the gate; density of the head

was probably higher than 10% and might attain

that of high-density flows as reported by Middleton

(1967).

Flow character around the sampling points (200–

250 cm downstream points from the gate) was as

follows: the flow reached near the sampling points

about 5 s after the gate was unlocked and head

velocity around the site was about 30 cm/s. The flow

had a denser layer appeared at about 20 cm behind the

head in the basal part of the flow. The thickness of the

basal density layer was about 5–10 mm (Fig. 5A) and

the interface between density layer and superjacent

Fig. 5. Photograph of experimental gravity flow (A) and its deposit (B).

T. Sakai et al. / Sedimentary Geology 154 (2002) 1–10 5

flow formed a wave train. The wavelength of each

wave was about 20 cm. The wave trains moved in the

down-current direction and were sustained in about 2

s around the sampling points and then sediment bed

was formed.

Accumulated sand beds tend to thicken toward the

ca. 60-cm downstream point from the gate, and then

thin in the down-current direction. Each sand bed has

a distinct graded massive division (Fig. 5B), followed,

in a few beds, by faint parallel lamination near the top

of the bed. A < 1-mm-thick inversely graded unit was

recognized at the bed base (Fig. 3). The Series 2 bed is

slightly thicker than that of Series 1 probably due to

slight delay of opening the gate.

Fig. 6. Upward change in imbrication and orientation of Series 1. Statistically significant unimodal patterns were shown by using the Turkey

chi-square test at a= 0.01 level (Middleton, 1965). V.M. = vector mean, s0 = circular variance (see Davis, 1986).

T. Sakai et al. / Sedimentary Geology 154 (2002) 1–106

The resultant fabric of both series is broadly

similar; here, we show rose diagrams for the entire

vertical profile from Series 1 (Fig. 6). Upward

changes in the vector means and modes of imbrication

angles of both series are shown in Fig. 7. Sampled

sand bed thickness is 5.4 mm in Series 1, which is

divided into 27 intervals (P1–P27) in the vertical

section for imbrication measurement, and 8.2 mm in

Series 2, which is divided into 41 intervals.

The derived rose diagram patterns (Fig. 6) suggest

that a bed can be divided into four segments: (1)

lower, (2) middle (3) upper and (4) uppermost parts.

(1) In the lower part (P1–P5 intervals of Fig. 6), most

of the resultant rose diagrams have modes showing

both up-current and down-current imbrication with

larger circular variance (typical in the P5 interval of

Fig. 6), even though they have vector means showing

up-current imbrication. High-angle imbrication is fre-

quently observed in this part (e.g. P4 interval). (2) In

the middle part (P6–P21 intervals of Fig. 6), intervals

are also dominated by up-current imbrication, but

intervals in which down-current imbrication predom-

inates also occur (e.g. P15 interval). The boundary

between lower and middle parts is placed at the first

interval represented by low-angle imbrication and

smaller circular variance (i.e. P6 interval). Imbrication

angles < 40j are dominant in this part; however, high-

angle imbrication also occurs in a few intervals (e.g.

P14 of Fig. 6). (3) The upper part (P22–P25) is

represented by rose diagrams showing up-current

imbrication. The middle and upper part boundary is

also not sharply defined. High-angle imbrication and

down-current vector means cannot be recognized

above the P21 interval, which may be of the upper

limit of the middle part. Bimodal imbrication like P25

interval is also observed in this part. However, modes

suggesting down-current imbrication are smaller than

15j, which should be recognized as statistically insig-

nificant (Hiscott and Middleton, 1980). (4) Nearly

flat, very low-angle imbrication was observed in the

uppermost part (intervals P26 and P27) (Fig. 6). Plots

of mean imbrication angles and modes throughout

each interval from both series (Fig. 7) also exhibit the

upward changes in imbrication pattern described

above and show the similarity in patterns between

the series.

Orientation was measured at 1, 4 and 5 mm from

the base of the second bed of Series 1. The height of

measured points corresponds to the lower, middle and

upper parts of the bed. Statistically preferred unimodal

orientation was derived at these three points (Fig. 6).

Preferred orientation deviates up to 13j in both clock-

wise and anticlockwise directions from the flume axis

(Fig. 6).

4. Discussion

Here, we discuss the following topics: (1) the

corresponding divisions in natural turbidite beds, (2)

Fig. 7. Fluctuation of vector mean and mode of imbrication angle.

Modes are represented by intermediate value of the classes. (A)

Series 1; (B) Series 2.

T. Sakai et al. / Sedimentary Geology 154 (2002) 1–10 7

the genesis of changes in imbrication angle and (3)

deviations in preferred orientation.

(1) A single bed accumulated from an experimental

turbidity current is interpreted to correspond to the A-,

B- and D-divisions of the Bouma sequence. The lower

and middle parts are probably equivalent to the

Bouma A-division because the wider variations in

imbrication angle and down-current imbrication

observed in these parts have previously been de-

scribed from the A-division of natural turbidite beds

(e.g. Bouma, 1962; Colburn, 1968; Onions and Mid-

dleton, 1968; Taira and Scholle, 1979; Hiscott and

Middleton, 1980). Upward decrease of imbrication

angle in the lower and middle parts may reflect

changes in the rate of deposition as demonstrated by

Arnott and Hand’s (1989) experiment, which detected

higher angle imbrication in beds accumulated under

higher rates of sediment rain-out.

The upper part may correspond to the Bouma B-

division because the fabric of this part is similar to

that of parallel stratification (e.g. Taira and Scholle,

1979). The uppermost intervals with lower angle

imbrication than underlying intervals are interpreted

to be of the Bouma D-division. Taira and Scholle

(1979) described the imbrication pattern of the D-

division which has low-angle imbrication and current

parallel orientation. This type of fabric is interpreted

to be a result of suspension fallout under weak

influence of current shear stress (Taira and Scholle,

1979).

(2) Our detailed imbrication measurements

detected down-current imbrication from a bed accu-

mulated in the experimental flume. The origin of the

down-current imbrication has been rarely discussed in

previous studies. Hiscott and Middleton (1980)

explained formation of bimodal or isotropic orienta-

tion and variable imbrication by rapid deposition from

highly concentrated flow. However, such variable

orientation was not observed in this case.

As also described in Middleton’s (1967) experi-

ments about turbidity current, there was a relatively

distinct surface between the near-bottom dense layer

(‘‘quick bed’’ sensu Middleton, 1967) and the over-

lying low-density layer behind the head of the high-

density turbidity current, which was formed before

sand grains in the current were completely deposited.

This interface had a wave train that was attributed to

the strong return flow in the superjacent low-density

part near the upper surface of the high-density part

(Middleton, 1967; Postma et al., 1988). Flow velocity

measurements, inferred to have been taken just above

the near bottom high-density layer, detected distinct

flow oscillations in the flow parallel direction (Fig. 8)

which caused the interface wave trains. According to

Middleton (1967), such wave trains, ‘‘Helmholtz

waves’’, produce cyclical fluctuations in shear stress

which extend down into the deeper part of the dense

layer.

We believe that the shear oscillation applied by the

wave train remobilized grains to form down-current

imbrication. Intense oscillation which causes stronger

shear oscillation has been observed not just behind the

head, but at 20–150 cm behind the advancing head

and the wave moved in the down-current direction.

During the early phase of the high-density layer

formation, before the interface becomes clear, grains

are probably inclined in the up-current direction with

flow parallel orientation (cf. Rees, 1968) due to the

stronger down-current-directed shear stress within the

flow head based on flow patterns observed in experi-

ments (Middleton, 1966). As soon as an interface

wave train was formed, short periods of up-current-

directed shear stress due to oscillation affect part of

the high-density layer, during which local down-

current imbrication might be formed. The A-division

of natural turbidite beds, deposited from a ‘‘quick

bed’’ as observed in the flume, may have distinct

down-current imbrication in several horizons of the

Fig. 8. Velocity fluctuations in flow parallel and normal components

in the horizontal plain of a turbidity current ca. 1 cm above the bed

at ca. 1.5 m downstream point from the gate. x= Flow parallel

component, y= flow normal component.

T. Sakai et al. / Sedimentary Geology 154 (2002) 1–108

bed, which can be visualized by detailed analysis such

as the method applied in this study. These may arise

because shear oscillation of a natural current induced

by the return flow of the superjacent low-density part

can be much stronger than that in the experimental

flume.

(3) Our results detected nearly flow parallel ori-

entation and this is consistent with other studies which

measured orientation of experimental density current

deposits (Middleton, 1967; Rees and Woodall, 1975;

Arnott and Hand, 1989). Measured modes of orienta-

tion deviated up to 13j from the flume axis. This

deviation was probably induced by local flow diver-

gence, which is recorded in the velocity profile (Fig.

8). According to the results of this study and previous

analyses that detected grain orientation deviations

from the flume axis (e.g. Arnott and Hand, 1989;

Hughes et al., 1995), at least 30j of deviation should

be taken into account when discussing paleoflow

directions based on grain orientation analysis.

5. Conclusion

Grain fabric of experimental gravity flow deposits

was analyzed in detail.

(1) Based on variations of imbrication angle and

appearance of down-current imbrication, we divided a

bed into four parts, which consist of the lower part

with high-angle imbrication and both up- and down-

current imbrication intervals, the middle part with

lower-angle imbrication and few down-current imbri-

cation intervals, the upper part with up-current imbri-

cation intervals and the uppermost part in which

nearly flat imbrication was measured. Accumulated

experimental turbidite beds correspond to A-, B- and

D-divisions of natural turbidite beds.

(2) Down-current imbricated intervals, reported

from previous studies, were also found in the lower

and middle parts. Their formation may be attributed to

the wave trains which are observed at the interface

between the dense near-bottom layer and the over-

lying low-density layer of the current. The interface

waves may remobilize grains in the dense bottom

layer of the current, resulting in partial down-current

imbrication in the lower and middle parts of the bed.

(3) Deviation of the preferred grain orientation

from the flow direction was up to 13j. Paleoflow

analysis based on fabric measurement should consider

deviations of at least 30j from true flow direction.

Acknowledgements

We would like to thank journal reviewers, Dr. B.C.

Kneller and Dr. Y.K. Sohn, and journal editor, Dr.

K.A.W. Crook, who gave us constructive comments

for the paper and corrected grammatical mistakes in

the early version of the manuscript.

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