paleocurrent

7/29/2019 paleocurrent

1/15

Paleocurrent analysis for the Late PleistoceneHoloceneincised-valley ll of the Yangtze delta, China by using

anisotropy of magnetic susceptibility data

Baozhu Liua,*, Yoshiki Saitoa, Toshitsugu Yamazakia, Abdelaziz Abdeldayema,b,Hirokuni Odaa, Kazuaki Horic, Quanhong Zhaod

aMRE Geological Survey of Japan, AIST, Higashi 1-1-1, Tsukuba, Ibaraki 305 8567, Japan

bDepartment of Geology, Faculty of Science, Tanta University, Tanta 31527, Egypt

cDepartment of Geography, Graduate School of Science, University of Tokyo, Hongo 7-3-1,

Bunkyou-ku, Tokyo 113 0033, JapandMarine Geology Laboratory, Tongji University, 1239 Siping Road, Shanghai 200092, People's Republic of China

Received 9 August 2000; accepted 16 March 2001

Anisotropy of magnetic susceptibility (AMS) analysis has been conducted on samples from borehole core CM-97 from the

Yangtze River (Changjiang) incised-valley ll, China, to determine the paleocurrent directions to help in reconstructing

sedimentary paleoenvironments. Borehole CM-97 consists of uvial (Unit 1), estuarine (Units 26), and deltaic (Units 79)sediments after the Last Glacial Maximum in ascending order. The AMS results show that the paleocurrent directions for the

tide-dominated estuarine and deltaic sediments were westerly or northwesterly directed (ood-tide dominated), but give no

denite trend for the uvial sediments.

Comparison between the paleocurrent directions inferred from primary sedimentary structures and in situ AMS data shows

that they are in good agreement, conrming the applicability of AMS as a good paleocurrent indicator for sediments deposited

in coastal tide-dominated environments. Considering that these sediments were strongly tide-inuenced and the tidal pattern

since 12 kyr bp has not changed signicantly, we think that the westerly or northwesterly current direction most probably

resulted from ood-tidal currents, and the sedimentary paleoenvironment was a ood-tide dominated estuary or delta.

Additionally, it has been found that downhole changes of some AMS parameters, including the mean magnetic susceptibility

(K), the corrected anisotropy degree (Pj) and the magnetic foliation (F), clearly mark the dened stratigraphic boundaries in the

borehole. This further extends the validity of AMS as a good stratigraphic marker in addition to its long known credibility as a

sensible paleocurrent recorder.q

2001 Elsevier Science B.V. All rights reserved.Keywords: Anisotropy of magnetic susceptibility (AMS); Yangtze delta; Paleocurrent; Tide-dominated; Incised-valley ll

All materials acquire magnetization in a magnetic

eld and thus have a magnetic susceptibility. This

susceptibility is not always isotropic and varies with

the orientations of the rock (Ising, 1942). This

Marine Geology 176 (2001) 175189

0025-3227/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.

PII: S0025-3227(0 1)00151-7

www.elsevier.nl/locate/margeo

* Corresponding author. Tel.: 181-298-61-3719; fax: 181-298-

61-3747.

E-mail addresses: [email protected] (B. Liu).


2/15

spatial susceptibility variation is dened as the

anisotropy of magnetic susceptibility (AMS) and

reects the preferred orientation of magnetic

minerals in the rock or unconsolidated sediments,

i.e. its magnetic fabric (Hrouda, 1982; Tarling and

Hrouda, 1993). AMS has long been demonstrated as

a useful tool for paleocurrent determination, in parti-

cular for deep-sea sediments such as contourites

(Ellwood and Ledbetter, 1977, 1979; Ellwood et

al., 1979; Ellwood, 1980; Ledbetter and Ellwood,

1980; Abdeldayem et al., 1999), turbidites (Ellwood

and Ledbetter, 1977; Ledbetter and Ellwood, 1980),

submarine canyon and fan sediments (Rees et al.,

1968), as well as Mid-Proterozoic embayment

shales (Schieber and Ellwood, 1993) and Palaeo-

zoic ysch shales (Piper et al., 1996), modernbeach sand sediments (Rees, 1965; Taira and

Lienert, 1979), and laboratory deposited sediments

(Rees, 1965; Rees and Woodall, 1975). AMS of

experimental tidal at sediments has also been

reported (Ellwood, 1984). However, to the best of

our knowledge, no similar work has been done on

natural sediments deposited in coastal tide-domi-

nated estuary or delta environments. If AMS were

also applicable, it would be of great help for

detailed reconstruction of coastal sedimentary

paleoenvironments.AMS of a rock sample corresponds to a symmetri-

cal second-rank tensor (Hrouda, 1982) which can be

described by a triaxial ellipsoid with the principal

eigenvectors K1. K2. K3 representing the maxi-

mum, intermediate and minimum susceptibility

axes, respectively. Usually, current would be parallel

to the K1 axis and in favorable conditions its absolute

direction may be inferred from the tilting direction of

K3 axis (Rees, 1965; Tarling and Hrouda, 1993;

Tarling and Shi, 1995; Piper et al., 1996; Abdeldayem

et al., 1999). However, current could be perpendicular

to the axes ofK1 if the ow is strong enough and thegrains are very ne under traction sedimentation

(Ellwood and Ledbetter, 1977, 1979; Ledbetter and

Ellwood, 1980).

This study, therefore, attempts for the rst time to

use AMS, to determine paleocurrent direction for

sediments from the Late PleistoceneHolocene

Yangtze incised-valley ll, China, which are charac-

terized by typical tide-dominated estuarine and deltaic

sediments (Hori et al., 1999, 2001a,b).

The Yangtze River (Changjiang) of China, the

longest river in Asia, has a length of about 6300 km,

a total catchment area of about 1.8 106 km2, meanannual runoff of 893 109 m3, and mean annual sedi-

ment discharge of 481 106 t (Milliman and Meade,

1983; Huang et al., 1996; Li and Wang, 1998; Chen et

al., 2000). It originates in the Kunlun Mountains in the

southwestern part of Qinghai Province, north of the

Tibet Plateau, and ows towards the east to its mouth

into the East China Sea, about 23 km north of Shang-

hai. The present Yangtze delta has been forming since

the maximum transgression in the Holocene at about

7 kyr bp (Liu et al., 1992; Huang et al., 1996; Chen

and Chen, 1997; Li and Wang, 1998; Li et al., 2000).The Yangtze delta plain, with low relief of 35 m, is

located at its end and faces the East China Sea and the

South Yellow Sea (Chen, 1999; Li et al., 2000).

The Yangtze delta is one of the typical tide-domi-

nated deltas in the world. At present, it is in a meso-to

macrotidal environment with an average tidal range of

2.6 m, and the maximum of about 5.0 m (Li and Wang,

1998). The present tide in the Yangtze River estuary

area is irregular semidiurnal, the ood-tidal current

direction is towards the NW, while the ebb-tidal current

is obviously diverted towards the south at SSE (Chen etal., 1988; Huang et al., 1996; Chen, 1999; Chen et al.,

2000). However, tidal currents were inferred to be

much stronger during the transgression stage in the

Holocene than at present, during which the average

tidal range was greater than 4 m (Li et al., 2001).

During the Last Glacial Maximum, the shoreline was

located near the edge of the East China Sea continental

shelf (Zhu et al., 1979), and a huge incised-valley was

formed in the present Yangtze delta area (Li et al., 2000;

Fig. 1), whereas during the postglacial sea-level rise,

most of the incised-valley had been lled, and the

present Yangtze delta had downlapped onto the estuar-ine deposits (Li et al., 2000). The incised-valley

sequence after the Last Glacial Maximum consists of

a uvial unit, estuary units, and delta units in ascending

order (Li et al., 2000; Hori et al., 2001b).

A seventy-meter-long borehole, CM-97 (latitude

B. Liu et al. / Marine Geology 176 (2001) 175 189176


3/15

31837 029 00N; longitude 121823 038 00E, the elevation of

2.48 m), was taken by rotary drilling using drilling

mud in 1997 on Chongming Island (Fig. 1). The

CM-97 site is located inside the incised-valley formed

during the Last Glacial Period (Fig. 1). Core recoveryis about 90%. Detailed core description and radio-

carbon dating are reported by Hori et al. (1999,

2001a,b).

CM-97 was divided into nine stratigraphic units

from bottom to top (Hori et al., 1999; Fig. 2). Radio-

carbon ages show that CM-97 recorded the sedimen-

tary environments of the Yangtze estuary for

approximately 10 kyr, from 11.5 to 1.5 kyr bp (Hori

et al., 1999, 2001a,b; Fig. 3). In general, borehole

sediments consist of transgressive uvial sediments

(Unit 1), transgressive estuarine sediments (Units

26), and regressive deltaic sediments (Units 79).Unit 1, formed prior to about 11 kyr bp, consists of

ne to medium sand uvial sediments with clear high-

angle trough-cross bedding. Units 26, deposited

during about 116 kyr bp, are transgressive estuarine

sediments consisting of thinly alternating silt and clay

layers with a few foraminifera. In detail, Unit 2

consists of inshore subtidal deposits, Unit 3 of

muddy intertidal to subtidal deposits, Unit 4 of trans-

gressive lag deposits, Unit 5 of muddy intertidal to

subtidal at deposits, and Unit 6 of estuarine central

basin deposits (Hori et al., 1999, 2001b). Units 79

were formed during about 61.5 kyrbp, and are

regressive deltaic sediments consisting of clayey silt

to ne sand. Sedimentary environments of these units

are prodelta for Unit 7, delta front for Unit 8, subtidal

to intertidal at and surface soil for Unit 9 (Hori et al.,

1999, 2001a).

Altogether there were 39 subcores of samples from

borehole CM-97. All the subcores were split into two

halves. A total of 2543 sequentially numbered discrete

samples were taken by continuously pressing 7 cm3plastic boxes into the face of the working half. Initial

low eld magnetic susceptibility (K) and its aniso-

tropy were rst measured using a KappaBridge

KLY-3S susceptibility meter. The natural remanent

magnetization (NRM) was then measured and demag-

netized using a three-axis 2G Enterprises cryogenic

magnetometer with an in-line alternating eld (AF)

demagnetizer with a peak eld strength of 80 mT.

All odd-numbered samples were subjected to incre-

mental AF demagnetization at steps of 0, 5, 10, 15, 20,

25, 30, 35, 40, 50, 60 and 80 mT. Following statisticaland visual analysis of this detailed demagnetization

spectrum, we found that most samples exhibited RM

stability. Therefore, it was decided to treat the remain-

ing samples (the even-numbered samples) at steps of

0, 20, 30 and 40 mT. Fig. 4 shows typical demagne-

tization behavior for these examples. The majority of

samples showed a stable magnetization expressed as a

single component that heads toward the origin of ortho-

gonal plots (Zijderveld diagram, Zijderveld, 1976, Fig.

4Ba). Although they still exhibited a general steady

decay toward the origin of the plot, the remaining

samples either acquired some spurious magnetization

at high elds (Fig. 4b) or behaved in an erratic manner

because their magnetization was too weak and

they acquired spurious magnetizations at low elds

(Fig. 4c).

Combined visual (using stereographic and orthogo-

nal plots) and statistical (using the principal compo-

nent analysis of Kirschvink, 1980) inspections of

demagnetization data indicted that 20 mT AF demag-

netization was sufcient to remove most of the


Fig. 1. Location of borehole CM-97 and map of the Yangtze

(Changjiang) delta. The area between the two dashed lines is the

huge incised-valley of the Yangtze River, which was formed during

the Last Glacial Maximum (after Li et al., 2000). The shadowed

rectangle in the inset shows the location of the study area in China.


4/15

viscous remanence and isolate the stable magnetic

north direction for most samples. For the remaining

samples, magnetic north had to be computed using the

principal component analysis (Kirschvink, 1980) by

tting a line through a minimum of three consecutive

steps and toward the origin of the orthogonal plot.

Thus we could obtain the magnetic north of each

sample. Then the relative magnetic north direction


Fig. 2. Column section with sedimentological features of borehole CM-97 (after Hori et al., 1999, 2001a,b).


5/15

at each sample level was calculated using the equation

determined from linear tting of obtained magnetic

north versus depth in order to correct the affection

of paleosecular variation. This linear tting was

done on each subcore as it has the same cut direction.

And this relative magnetic north of each sample was

used for reorientation of AMS directions of the rela-

tive sample to their geographic coordinates so as to

obtain the absolute paleocurrent directions (Abdel-

dayem et al., 1999).

Following the recommendation of Jelinek (1981),

Ellwood et al. (1988) and Tarling and Hrouda(1993), the following set of AMS parameters that

dene the mean magnetic susceptibility (K), the

corrected anisotropy degree (Pj), the magnetic

lineation (L), the magnetic foliation (F) and the

ellipsoid shape (q) were calculated and used to

evaluate the magnetic fabric of borehole CM-97

sediments:

K K1 1 K2 1 K3=3

(Mean magnetic susceptibility, Nagata, 1961)

Pj exp

{2n1 2 nm

2 1 n2 2 nm2 1 n3 2 nm

2}

q;

(Corrected anisotropy degree, Jelinek, 1981)

where n1 ln K1; n2 ln K2; n3 ln K3; nm

n1 1 n2 1 n3=3

L K1=K2

(Magnetic lineation, Balsley and Buddington, 1960)

F K2=K3

(Magnetic foliation, Stacey et al., 1960)

q K1 2 K2=K1 1 K2=22 K3

(Shape factor, Granar, 1958)

Magnetic fabric of laboratory deposited materials

have shown that the magnetic grains are mostly

aligned within or close to the bedding plane, with

their longer axes in the direction of ow, with some

degree of imbrication (Rees, 1965; Rees and Woodall,

1975). More specically, the magnetic fabric para-

meters fall within specic ranges, such as

0.06, q, 0.7 and the imbrication angle, the angle

between horizontal and the plane of maximum-inter-mediate susceptibility, is less than 208 (Hamilton and

Rees, 1970). These ranges have been widely adopted

as being diagnostic of primary sedimentary fabrics

when they are found in natural sediments, while

values outside these ranges can generally be attributed

to coring disturbances, bioturbation and the like or

specically secondary fabrics. On the other hand,

the depositional magnetic fabrics of most deposited

sediments are characterized by clearly oblate suscept-

ibility ellipsoids (Hrouda, 1982). In the present study,

a foliated ellipsoid with q values ,0.7 and K3 direc-

tions lying within 258 of the vertical were considered

indicative of a primary fabric that is credible in

providing information on paleocurrent direction and

depositional conditions (Hamilton and Rees, 1970;

Hrouda, 1982; Tarling and Hrouda, 1993). Fig. 5

shows the features of F versus L of all the samples

with primary AMS, from which we can see that all the

samples have oblate ellipsoids, indicating primary

depositional nature of these samples (Hrouda,

1982). Thus, only those samples that have a primary


Fig. 3. Accumulation curve of borehole CM-97 and sea-level curve

in the East China Sea since 12 kyr bp. The accumulation curve is

after Hori et al. (1999), while the sea-level curve is after Saito

(1998).


6/15

magnetic fabric were used in paleocurrent determina-

tion for borehole CM-97 samples. There were a total

of 664 samples with secondary AMS, which were

mostly distributed on the top part of each subcore.

Description of the primary sedimentary structures

was done on the basis of detailed examination of

X-ray photographs of samples. Paleocurrent direc-

tions, relative to the subcore section, were inferred

from the dipping directions and angles of the foresets

of cross lamination. Then paleocurrent direction of the

sample located at the same horizon as the cross lami-

nation was determined by using the in situ (un-reor-iented) AMS data, so that we were able to compare the

results inferred from the sedimentary structures and

the in situ AMS data. In this case, we assumed that the

cut section of each subcore was along the `eastwest'

line, and the sample box was pressed into the subcore

section towards the `north'. Thus, paleocurrent direc-

tion from both the primary sedimentary structures and

the in situ AMS data were in the same coordinate

system, and were comparable.

5.1. Comparison between the paleocurrent directions

determined from the primary sedimentary structures

and the in situ AMS data

Sedimentary structures, especially the cross lami-

nations with clear foresets that have current direction

implications (Reineck and Singh, 1980; Allen, 1984;

Reading, 1996) were observed and described based on

detailed examination of X-ray photographs. In order

to demonstrate the applicability of AMS in paleocur-rent determination of CM-97 sediments, we selected

one subcore (Subcore B30) to conduct detailed

comparison between the paleocurrent directions

from the primary sedimentary structures and the in

situ AMS data, respectively (Fig. 6). Subcore B30

was located at the depth of 36.80 37.50 m within

stratigraphic Unit 6, and consisted of thinly inter-

bedded coarse silt and silty clay.

Firstly, we reconstructed the paleocurrents from


Fig. 4. Typical examples of directional and intensity changes during AF demagnetization of samples from borehole CM-97. (A) Normalized

NRM intensity versus AF peak amplitudes. Sample number and NRM of each sample are shown in the plots. (B) Orthogonal projections

(Zijderveld diagrams) of stepwise demagnetization of the same samples. Units of each sample are shown in the plots. The NRM measurement

for each sample is marked with a larger symbol. Horizontal projections are marked with solid squares and vertical projections are marked with

open squares.


7/15

primary sedimentary structures. We examined the X-

ray photos of Subcore B30 and found that there were

three horizons with clear cross lamination at depths of

(A) 36.86036.864 m, (B) 37.06837.071 m and (C)

37.07337.080 m, respectively (Fig. 6). LaminationA has a thickness of 4 mm, foreset laminae dipping

to the right with an apparent dip angle of 158, indicat-

ing a paleocurrent directed to the right; Lamination B

has a thickness of 3 mm, foreset laminae dipping to

the right with an apparent dip angle of 108, also indi-

cating a paleocurrent owing to the right; Lamination

C has a thickness of 7 mm, foreset laminae dipping to

the left with an apparent dip angle of about 308, indi-

cating a paleocurrent towards the left (Fig. 6). These

data clearly show that there were bi-directional ows

and these sediments were deposited under bi-direc-

tional currents (Fig. 6).

Secondly, we reconstructed the downhole paleocur-

rents of Subcore B30 based on the in situ AMS data,i.e. axes of the magnetic ellipsoid were in the subcore

coordinate system, to compare the paleocurrent direc-

tions between sedimentary structures and the in situ

AMS data. These downhole paleocurrent directions

were determined from the declination ofK1 (Fig. 6).

Results show that the paleocurrent of the sample at

Horizon A was to the right, and that of combined

sample B and C was to the left (Fig. 6), indicating

that paleocurrent directions from sedimentary structures


Fig. 5. Flinn-type (Fversus L) plot of the samples with primary AMS. It is very clear that all the samples have oblate ellipsoids, indicating their

primary depositional nature.


8/15

and AMS were in good accordance. Laminations B

and C were combined within one single AMS sample

(the thickness of one AMS sample is 20 mm), so only

one average paleocurrent direction could be inferred

from the AMS data. B was much thinner than C, thus

the paleocurrent direction from AMS showed the

predominated C direction.

Furthermore, there were a total of 41 horizons

where primary sedimentary cross laminations were

observed, among which 27 (about 65.9%) had similarpaleocurrent directions to those determined from the

in situ AMS data (Table 1). Among the 41 cross lami-

nations, 16 has a thickness of equal to or greater than

10 mm, of which 15 (about 93.8%) has similar paleo-

current directions to those determined from the in situ

AMS data (Table 1). Even 56% of those cross lamina-

tions with a thickness of less than 10 mm shows

similar paleocurrent directions to those determined

from the in situ AMS data (Table 1). These results


Fig. 6. Detailed comparison between the paleocurrent directions from primary sedimentary structures and the in situ AMS data for Subcore B30

samples from CM-97. Photos of the primary sedimentary structures were enlarged so as to make them clearer. Paleocurrent directions relative

to the core section from these sedimentary structures are shown with arrows: A, right; B, right; C, left. The downhole paleocurrent directions are

inferred from in situ AMS data (K1). A and C are in good agreement with the AMS results. In situ declinations and inclinations at 20 mT AF

eld demagnetization of the corresponding samples are also shown in the right part of the plot.

Table 1Statistics of primary cross laminations that have similar paleocur-

rent directions to those determined from the in situ AMS data

Cross

lamination

numbers

Numbers

with similar

paleocurrent

directions

Percentage

Totally 41 27 65.9

Thickness$ 10 mm 16 15 93.8

Thickness, 10 m 25 14 56.0


9/15

indicate that the absolute paleocurrent directions

inferred from the reoriented AMS data represent the

true current directions.

5.2. Paleocurrent reconstruction for each

stratigraphic unit

The paleocurrent directions for the nine strati-

graphic units were established from the reoriented

AMS data, based on paleomagnetically oriented K1and K3 for samples that satised the afore-mentioned

criteria for a primary fabric. K1 axes display a moder-

ate to complete girdle on the lower hemisphere equal-

area projections ofK1 and K3 (Fig. 7). Absolute paleo-

current direction was estimated from the imbrication

of K3 in the plot of lower hemisphere equal-areaprojections of reoriented K1 and K3 (open arrows in

Fig. 7). Rose diagram ofK1 was also done to show the

azimuthal distribution of K1 axes (Fig. 7). Preferred

imbrication that enables absolute current estimation

could also be observed for quite a few units (Fig. 7).

Note that there is a slight disagreement between the

absolute paleocurrent directions (open arrows in

Fig. 7) and the azimuthal K1 axes rose diagrams

(Fig. 7). This is because the absolute directions take

into account imbrications and are more realistic. The

following is a summary of AMS patterns and paleo-current evaluation for each unit in borehole CM-97

(Fig. 7).

Unit 1: K1 axes display a girdle. K1 axes are mostly

along the NS in the rose diagram, which also shows

a subordinate EW trend. The K3 axes, on the other

hand, are very steep to vertical with no clear imbrica-

tion. Such a pattern reects a predominant NS

running paleocurrent that seems to have oscillated

back and forth for short intervals, yet no absolute

current direction could be inferred from the AMS

data (Fig. 7).

Unit 2: K1 axes display a moderate girdle, while its

azimuthal distribution in the rose diagram shows a

good grouping along the SEENWW. K3 axes slightly

tilted toward the NW away from vertical, which

shows a weak imbrication indicating a probable abso-

lute NW paleocurrent direction (Fig. 7).

Unit 3: K1 axes are grouped along the NEE while K3axes are clearly tilted toward NWW indicating a

predominant NWW paleocurrent direction (Fig. 7).

Unit 4: K1 axes display a moderate girdle. K1 axes

are mostly to the east with a clear imbrication of K3axes that indicates a strong paleocurrent trending

generally westward. Rose diagram also shows a

subordinate NWWSEE running paleocurrent that

might have occurred for short periods (Fig. 7).

Unit 5: K1 axes display a moderate girdle. Although

extending over a wide range of azimuths, the K1 axes

show a higher concentration along the EW trend.

The K3 axes, on the other hand, show a slight tilt

toward the west marking a predominant paleocurrent

in this direction (Fig. 7).

Unit 6: Similar to Unit 5 but with a better

constrained ow.

Unit 7: K1 axes display a moderate girdle. K1 axes

extend over a wide range with a pronounced SWNE

concentration, accentuated by the rose diagram. Aslight imbrication that reects a NW owing paleo-

current can also be observed for the K3 axes (Fig. 7).

Unit 8: K1 axes are broadly grouped around an east-

erly direction with a general tendency for the K3 axes

to slightly imbricate toward the NWW, marking a

relatively strong paleocurrent in this direction (Fig. 7).

Unit 9: K1 axes are widely distributed around an

easterly trend. K3 axes display a pronounced imbrica-

tion that marks a strong paleocurrent owing toward

the west (Fig. 7).

In summary, the predominant paleocurrent direc-tions for the tide-dominated estuarine and deltaic sedi-

ments in the borehole were westerly or northwesterly,

and generally parallel or oblique to the azimuth ofK1axes shown in the rose diagrams. However, no abso-

lute paleocurrent direction could be obtained for the

uvial sediments from CM-97.

5.3. The mean magnetic susceptibility (MS), the

anisotropy degree, the magnetic lineation, the

magnetic foliation, the ellipsoid shape parameter and

their sedimentological implications

The mean magnetic susceptibility (K, or MS)

changes positively with grain size (Fig. 8). For the

whole borehole section, the maximum MS value is

3139 1026 SI, minimum is 228 1026 SI, and the

average is about 560 1026 SI. The lowest values of

MS occur in stratigraphic Unit 7 (Fig. 8), which is the

nest in whole borehole section (Fig. 2), with an aver-

age value of about 346 1026 SI. The highest values of

MS occur in stratigraphic Unit 1 (Fig. 8), which is the



10/15


Fig. 7. Paleocurrent directions from the reoriented AMS data for the nine stratigraphic units in borehole CM-97. Upper: Lower hemisphere

equal-area stereographic projections of K1 (solid square) and K3 (solid circle), showing absolute paleocurrent directions with open arrows.

Lower: Rose diagrams showing the azimuthal distribution of K1 axes.


11/15


Fig. 8. Downhole changes of the mean magnetic susceptibility (K), the corrected anisotropy degree (Pj), the magnetic lineation (L), the

magnetic foliation (F) and the magnetic ellipsoid shape parameter (q). These changes clearly mark the dened stratigraphic boundaries (after

Hori et al., 1999, 2001a,b), which are shown with dashed lines. The stratigraphic units are shown on the right hand side with numbers.


12/15

coarsest in the borehole (Fig. 2), with an average value

of about 955 1026 SI. In general, the mean MS exhi-

bits clear changes at horizon where lithology changes.

Both the corrected anisotropy degree (Pj) and the

magnetic foliation (F) also change with grain size, i.e.

the general trend increases with a decrease in grain

size (Fig. 8). The lowest, highest and average values

are 1.015, 1.208 and 1.089 for Pj, and 1.009, 1.173 and

1.067 for F, respectively.

Unlike the mean MS, Pj and F, there seem to be no

change in the magnetic lineation (L) and the shape

parameter (q) with grain-size, although there are

large downhole changes in both L and q (Fig. 8).

The lowest, highest and average values are 1.000,

1.067 and 1.015 for L, and 0.005, 0.700 and 0.226

for q, respectively.The eight stratigraphic boundaries from strati-

graphic Units 19 can be recognized clearly from

the characteristic downhole changes in K, Pj, L, F

and q, but particularly in K, Pj and F (Fig. 8). The K

value increases at the horizon where grain size

increases for each stratigraphic unit, even at the

erosion surface within Unit 4 (Figs. 2 and 8). Sharp

boundaries in K are identical to changes in lithology.

For example, the boundary between stratigraphic

Units 1 and 2, and the boundary between the Units 7

and 8 (Fig. 8) show marked changes in K. The chan-ging patterns of Pj and F also correspond to changes

between stratigraphic units although both parameters

have changes within individual units, e.g. within Unit

7 (Fig. 8). Although the downhole changes ofLand q

are not so clearly related to lithology, some bound-

aries can also be recognized from their curves, such

as, the boundaries between Units 1 and 2, and between

Units 8 and 9 (Fig. 8). All of these changes made it

possible for us to easily recognize the stratigraphic

boundaries throughout the CM-97 borehole (Figs. 2

and 8).

6.1. Paleocurrents and estimated sedimentary

paleoenvironments

It has been demonstrated that core samples could be

reoriented to their geographical coordinates by using

remanent magnetization (Hailwood and Ding, 1995;

Rolph et al., 1995). In this study, paleocurrent direc-

tions were determined from the reoriented AMS data

by using the relative magnetic north instead of the in

situ magnetic north. Relative magnetic north of each

sample was calculated from the equation determined

from the linear tting of obtained magnetic north

versus depth on each subcore. We used the relative

magnetic north in order to correct the affection of

paleosecular variation.

The AMS data indicates that most of the Yangtze

estuarine and deltaic sediments were deposited under

a relatively strong current that mostly owed toward

the west to northwest direction. Sedimentary facies

showed most of the CM-97 sediments were inuenced

by tidal uxes (Hori et al., 1999; Hori et al., 2001a,b).

Moreover, other borehole data from the Yangtze deltaarea also showed similar effects on sediment facies (Li

et al., 2000; Hori et al., 2001a,b). Present sedimentary

environments in the Yangtze estuary indicate that

strong tidal currents are dominant and therefore fora-

minifera living in coastal seas of the East China Sea

are transported into the estuary by ooding tidal

currents (Li and Wang, 1998). Moreover, paleo-tidal

patterns at 6 and 10 kyr bp for the Yangtze estuary

estimated through numerical simulation, also showed

tidal domination and similar pattern, but with a differ-

ent magnitude (Uehara et al., 2000). Thus it can beconcluded that the estimated west to northwest paleo-

current directions were induced from the ood-tidal

currents throughout the last 12 ka.

Based on the above-mentioned conclusion, the

sedimentary paleo-environments and their paleocur-

rent directions have been estimated for each of the

nine stratigraphic units, and are as follows.

Unit 1: The reason why no absolute ow direction

could be inferred from AMS for the uvial sediments

of this unit may be due to the complicated riverine

processes characterized with a strong transverse circu-

lation (Huang et al., 1996) or coarser grain size withhigh-angle trough cross bedding.

Units 2 6: The westerly or northwesterly paleocur-

rent directions inferred from the AMS data may have

resulted from a ood tidal current-induced fabric

structure. This is evident from the sedimentological

and paleontological characteristics of these units that

indicate a strong tidal inuence during the last trans-

gression at about 116 kyr bp (Hori et al., 2001b;

Figs. 2 and 3). Furthermore, tidal currents during the



13/15

Holocene transgression stage were inferred to be much

stronger than those at present, with an average tidal

range of more than 4 m at that time (Li et al., 2001).

Units 7 9: These units constitute a regressive

deltaic succession. An estimated west to northwest

paleocurrent reects an onshore ow direction,

consistent with ood-tidal current. Because the

modern environment in the Yangtze estuary and

delta is tide-dominated, ood tidal currents transport

marine particles like foraminifera into the estuary for

a distance of up to 200 km (Li et al., 1983). Flood-

tidal currents during spring tide were measured

230 km upstream from the Yangtze River mouth in

the March of 1982 (the dry season), while 200 km

upstream in the August of 1983 (the ood season)

(Li and Wang, 1998). As shown by sedimentary struc-tures of wave ripples in Unit 9 (Hori et al., 2001a),

other processes such as waves action and river

currents may have also inuenced these sediments.

6.2. MS and AMS parameters

In this study, we considered that the primary AMS

represented the primary deposition nature of sedi-

ments, thus could supply us with reliable paleocurrent

directions. The above-mentioned criteria for a

primary magnetic fabric have been widely acceptedas indicator of depositional nature of sediments

(Hamilton and Rees, 1970; Hrouda, 1982; Tarling

and Hrouda, 1993). CM-97 Samples with secondary

AMS were mostly distributed on the top part of each

subcore, which may most probably resulted from

coring disturbance, similar to deep sea sediments

reported elsewhere (Abdeldayem et al., 1999). Inter-

estingly, it has been found that there were better

`primary' AMS features with the development of

bioturbation in experimental tidal at sediments,

which was induced by the re-alignment of magnetic

minerals under the action of bioturbation and pore

water (Ellwood, 1984). However, there is not so

much bioturbation in the natural CM-97 sediments

(Hori et al., 2001a,b). Therefore, we thought that the

above-mentioned criteria for primary AMS were still

valid for this study, and the primary AMS results are

also valid and reliable.

Magnetic susceptibility has been successfully used

in stratigraphic division and correlation in the Chinese

and American loesspaleosol sequences (Heller and

Liu, 1982; Heller et al., 1991; Bloemendal et al.,

1995; Grimley et al., 1998), continental shelf sedi-

ments (Arai et al., 1997), deep-sea sediments (Barthes

et al., 1999), and in Paleozoic marine sequences

(Crick et al., 1997; Ellwood et al., 1999). In this

study, we found that not only magnetic susceptibility,

but also other AMS parameters could be used in

stratigraphic division. K and other AMS parameters

reect the grain size and perhaps mineral assemblage

as well. This was inferred from the fact that samples

where demagnetization behavior showed very low

intensities (Fig. 4c) mainly distributed in the strati-

graphic Unit 1 (Fig. 2), but quite few such samples

occurred in other stratigraphic units. This may extend

the use of AMS in stratigraphic division and maybe

also in stratigraphic correlation in addition to paleo-current determination.

Remarkable similarity in downhole changes of Pjand Findicates that the AMS of the sediments studied

here was induced by magnetic foliation that probably

formed due to compaction processes, a mechanism

that has been recognized in deep-sea sediments

(Ellwood, 1979; Abdeldayem et al., 1999).

The AMS analysis was applied to tidal-dominated

coastal sediments of late PleistoceneHolocene

incised-valley ll from the Yangtze delta, China

(borehole CM-97). Based on the comparison between

the paleocurrent directions from the primary sedimen-

tary structures and the in situ AMS data, we have

shown that AMS analysis is applicable in determining

paleocurrent directions for these sediments.

Throughout borehole CM-97 sediments, the domi-

nated paleocurrent directions were westerly to north-

westerly for transgressive estuarine and regressive

deltaic sediments. This is estimated to be mainly

due to ood tidal currents in the paleo- and modern

Yangtze (Changjiang) estuary. No absolute paleocur-

rent direction could be inferred from AMS for the

uvial coarse sediments. These results indicate that

the sedimentary paleoenvironment since about

11 kyr bp was a ood-tide dominated estuary or delta.

In addition, we have demonstrated that AMS can be

useful in stratigraphic division and may also be in

stratigraphic correlation. Our work indicates that in



14/15

addition to its long known credibility as a sensible

paleocurrent indicator, AMS may also be useful as a

stratigraphic marker.

The authors are grateful to staff of Marine Geology

Laboratory of Tongji University, Shanghai, China for

their great help during the coring and subsampling.

The authors are also grateful to Dr Brooks B. Ellwood

and an anonymous reviewer whose critical review has

greatly improved the manuscript. This research is

funded by the Global Environment Research Fund

of the Environment Agency of Japan. B. Liu would

like to express his special thanks to STA/JST/JISTECof Japan that made it possible for him to conduct this

study under the STA Fellowship. Demagnetization

data analysis was carried out by using Dr R. Enkin 0s

PC program (http://www.pgc.emr.ca/tectonic/

enkin.htm). Fig. 1 was created by using GMT

(http://www.soest.hawaii.edu/gmt/).

Abdeldayem, A.L., Yamazaki, T., Ikehara, K., 1999. Magnetic

susceptibility anisotropy and remanence of some deep-seasediments of the Tokai Basin. In: Yuasa, M. (Ed.), Marine

Geological Investigations of the Tokai Offshore Area.

Geological Survey of Japan, Japan, pp. 127146.

Allen, J.R.L., 1984. Sedimentary Structures Their Character and

Physical Basis. Elsevier, Amsterdam.

Arai, K., Sakai, H., Konishi, K., 1997. High-resolution rock-

magnetic variability in shallow marine sediment: a sensitive

paleoclimatic metronome. Sedim. Geol. 110, 723.

Balsley, J.R., Buddington, A.F., 1960. Magnetic susceptibility

anisotropy and fabric of some Adirondack granites and

orthogneisses. Am. J. Sci. 258, 6 20.

Barthes, V., Pozzi, J.P., Vibert-Charbonnel, P., Thibal, J., Melieres,

M.A., 1999. High-resolution chronostratigraphy from downhole

susceptibility logging tuned by palaeoclimatic orbital frequen-

cies. Earth Planet. Sci. Lett. 165, 97 116.

Bloemendal, J., Liu, X.M., Rolph, T.C., 1995. Correlation of the

magnetic susceptibility stratigraphy of Chinese loess and the

marine oxygen isotope record: chronological and palaeoclimatic

implications. Earth Planet. Sci. Lett. 131, 371380.

Chen, Z., 1999. Geomorphology and coastline change of the Lower

Yangtze delta plain, China. In: Miller, A.J., Gupta, A. (Eds.),

Varieties of Fluvial Form. Wiley, New York, pp. 427443.

Chen, Z., Chen, Z., 1997. Quaternary stratigraphy and trace-element

indices of the Yangtze delta, Eastern China, with special refer-

ence to marine transgressions. Quat. Res. 47, 181191.

Chen, J., Shen, H., Yu, C., 1988. Process of Dynamics and Geomor-

phology of the Changjiang Estuary. Shanghai Scientic and

Techical Publishers, Shanghai.

Chen, Z., Song, B., Wang, Z., Cai, Y., 2000. Late Quaternary evolu-

tion of the sub-aqueous Yangtze delta, China: sedimentation,stratigraphy, palynology, and deformation. Mar. Geol. 162,

423441.

Crick, R.E., Ellwood, B.B., El Hassani, A., Feist, R., Hladil, J.,

1997. Magnetosusceptibility event and cyclostratigraphy

(MSEC) of the EifelianGivetian GSSP and associated

boundary sequences in north Africa and Europe. Episodes 20,

167175.

Ellwood, B.B., 1979. Particle occulation: one possible control on

the magnetization of deep-sea sediments. Geophys. Res. Lett. 6,

237240.

Ellwood, B.B., 1980. Application of the anisotropy of magnetic

susceptibility method as a indicator of bottom-water ow

direction. Mar. Geol. 34, M83M90.

Ellwood, B.B., 1984. Bioturbation: minimal effects on the magneticfabric of some natural and experimental sediments. Earth

Planet. Sci. Lett. 67, 367376.

Ellwood, B.B., Ledbetter, M.T., 1977. Antarctic bottom water

uctuations in the Vema Channel: effects of velocity changes

on particle alignment and size. Earth Planet. Sci. Lett. 35,

189198.

Ellwood, B.B., Ledbetter, M.T., 1979. Paleocurrent indicators in

deep-sea sediment. Science 203, 13351337.

Ellwood, B.B., Ledbetter, M.T., Johnson, D.A., 1979. Sedimentary

fabric: a tool to delineate a high-velocity zone within a deep

Western Indian Ocean Bottom Current. Mar. Geol. 33,

M51M55.

Ellwood, B.B., Hrouda, F., Wagner, J., 1988. Symposia on magneticfabrics: introductory comments. Phys. Earth Planet. Inter. 51,

249252.

Ellwood, B.B., Crick, R.E., El Hassani, A., 1999. The magneto-

susceptibility event and cyclostratigraphy (MSEC) method

used in geological correlation of Devonian rocks from Anti-

Atlas Morocco. AAPG Bull. 83, 1119 1134.

Granar, L., 1958. Magnetic measurements on Swedish varved

sediments. Arkiv. f. Geofysik 3, 1 40.

Grimley, D.A., Follmer, L.R., Donald McKay, E., 1998. Magnetic

susceptibility and mineral zonations controlled by provenance

in loess along the Illinois and Central Mississippi River valleys.

Quat. Res. 49, 2436.

Hailwood, E.A., Ding, F., 1995. Palaeomagnetic reorientation of

cores and the magnetic fabric of hydrocarbon reservoir sands.In: Turner, P., Turner, A. (Eds.), Palaeomagnetic Applications

in Hydrocarbon Exploration and Production. Geological Society

Special Publication No. 98, pp. 245258.

Hamilton, N., Rees, A.I., 1970. The use of magnetic fabric in

palaeocurrent estimation. In: Runcorn, S.K. (Ed.), Palaeo-

geophysics. Academic, London, pp. 445464.

Heller, F., Liu, T.S., 1982. Magnetostratigraphical dating of loess

deposits in China. Nature 300, 431 433.

Heller, F., Liu, X.M., Liu, T.S., Xu, T.C., 1991. Magnetic suscept-

ibility of loess in China. Earth Planet. Sci. Lett. 103, 301310.

Hori, K., Saito, Y., Zhao, Q., Cheng, X., Wang, P., Li, C., 1999.



15/15

Sedimentary characteristics of post-glacial deposits beneath the

Changjiang River delta. In: Saito, Y., Ikehara, K., Katayama, H.,

(Eds.), Proceedings of an International Workshop on Sediment

Transport and Storage in Coastal Sea-ocean System, Tsukuba,

Japan, March 1519, 1999, pp. 3943.Hori, K., Saito, Y., Zhao, Q., Cheng, X., Wang, P., Sato, Y., Li, C.,

2001a. Sedimentary facies and Holocene progradation rates of

the Changjiang (Yangtze) delta, China. Geomorphology (in

press).

Hori, K., Saito, Y., Zhao, Q., Cheng, X., Wang, P., Sato, Y., Li, C.,

2001b. Sedimentary facies of the tide-dominated paleo-

Changjiang (Yangtze) estuary during the last transgression.

Mar. Geol. 177, in press.

Hrouda, F., 1982. Magnetic anisotropy of rocks and its application

in geology and geophysics. Geophys. Surv. 5, 37 82.

Huang, H., Tang, B., Yang, W., Yuan, Y., Zhou, C., Zan, Y., 1996.

Sedimentary Geology of the Yangtze Delta. China Geological

Publishing House, Beijing (in Chinese).

Ising, G., 1942. On the magnetic properties of varved clay. Ark.Mat. Astr. Phys. 29, 137.

Jelinek, V., 1981. Characterization of the magnetic fabric of rocks.

Tectonophysics 79, T63T67.

Kirschvink, J.L., 1980. The least-squares line and plane and the

analysis of palaeomagnetic data. Geophys. J. R. Astron. Soc.

62, 699718.

Ledbetter, M.T., Ellwood, B.B., 1980. Spatial and temporal changes

in bottom-water velocity and direction from analysis of particle

size and alignment in deep-sea sediment. Mar. Geol. 38,

245261.

Li, C., Wang, P., 1998. Late Quaternary Stratigraphy of the

Changjiang Delta. China Science Press, Beijing (in Chinese).

Li, C., Li, P., Cheng, X., 1983. The inuence of marine factors on

sedimentary characteristics of Yangtze River channel below

Zhenjiang. Acta Geographica Sinica 38, 128140 (in Chinese,

with English abstract).

Li, C., Chen, Q., Zhang, J., Yang, S., Fan, D., 2000. Stratigraphy

and paleoenvironmental changes in the Yangtze Delta during

the Late Quaternary. J. Asian Earth Sci. 18, 453 469.

Li, C., Fan, D., Hori, K., Zhao, Q., Saito, Y., Cheng, X., 2001. Some

problems on postglacial transgressionregression and environ-

mental change in the Changjiang delta. J. Nanjing Univ. (in

Chinese, with English abstract) (in press).

Liu, K.-B., Sun, S., Jiang, X., 1992. Environmental change in the

Yangtze River delta since 12,000 years bp. Quat. Res. 38,

3245.

Milliman, J.D., Meade, R.H., 1983. World-wide delivery of riversediments to the ocean. J. Geol. 91, 121.

Nagata, T., 1961. Rock Magnetism, 2nd ed. Maruzen, Tokyo.

Piper, J.D.A., Elliot, M.T., Kneller, B.C., 1996. Anisotropy of

magnetic susceptibility in a Paleozoic ysch basin: the

Windermere Supergroup, northern England. Sedim. Geol. 106,

235258.

Reading, H.G., 1996. Sedimentary Environments: Processes, Facies

and Stratigraphy, 3rd ed. Blackwell, Osney Mead, Oxford.

Rees, A.I., 1965. The use of anisotropy of magnetic susceptibility in

the estimation of sedimentary fabric. Sedimentology 4,

257271.Rees, A.I., Woodall, W.A., 1975. The magnetic fabric of some

laboratory-deposited sediments. Earth Planet. Sci. Lett. 25,

121130.

Rees, A.I., von Rad, U., Shepard, F.P., 1968. Magnetic fabric of

sediments from the La Jolla submarine canyon and fan,

California. Mar. Geol. 6, 145178.

Reineck, H.-E., Singh, I.B., 1980. Depositional Sedimentary

Environments (Second, revised and updated edition). Springer,

Berlin.

Rolph, T.C., Shaw, J., Harper, T.R., Hagan, J.T., 1995. Viscous

remanent magnetization: a tool for orientation of drill cores.

In: Turner, P., Turner, A. (Eds.), Palaeomagnetic Applications

in Hydrocarbon Exploration and Production. Geological Society

London Special Publ. 98, 239243.

Saito, Y., 1998. Sea levels of the Last Glacial in the East China Sea

continental shelf. Quat. Res. (Jpn) 37, 235242 (in Japanese,

with English abstract).

Schieber, J., Ellwood, B.B., 1993. Determination of basinwide

paleocurrent patterns in a shale succession from anisotropy of

magnetic susceptibility (AMS): a case study of the mid-

Proterozoic Newland formation, Montana. J. Sedim. Petrol.

63, 874880.

Stacey, F.D., Joplin, G., Lindsay, J., 1960. Magnetic anisotropy and

fabric of some foliated rocks from SE Australia. Geophys. Pura.

Appl. 47, 3040.

Taira, A., Lienert, B.R., 1979. The comparative reliability of

magnetic, photometric and microscopic methods of determiningthe orientations of sedimentary grains. J. Sedim. Petrol. 49,

759772.

Tarling, D.H., Hrouda, F., 1993. The Magnetic Anisotropy of

Rocks. Chapman & Hall, London.

Tarling, D.H., Shi, H., 1995. Magnetic anisotropy of borehole core

samples. In: Turner, P., Turner, A. (Eds.), Palaeomagnetic

Applications in Hydrocarbon Exploration and Production.

Geological Society London Special Publ. 98, 273280.

Uehara, K., Saito, Y., Hori, K., 2000. Numerical simulation of

paleotides around the Changjiang estuary at 10 and 6 kyr bp.

Abstracts of the International Workshop on Deltas: Their

Dynamics, Facies and Sequences, 1617 March, 2000,

Tsukuba, Japan, p. 12

Zhu, Y., Li, C., Zeng, C., 1979. On Late Pleistocene low stand of sea

level in East China Sea. Chin. Sci. Bull. 24, 317320 (in

Chinese).

Zijderveld, J., 1976. AC Demagnetization of rocks: analysis of

results. In: Collinson, D. (Ed.), Methods in Paleomagnetism.

Elsevier, New York, pp. 254286.


paleocurrent

Documents

Transcript of paleocurrent