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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: [email protected] (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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