10th Asian Regional Conference of IAEG (2015)

Sediment supply and transport pass inferred from magnetic susceptibility of submarine
fan deposits of the Kazusa Group
Naoya SUGIYAMA(1) and Yuichiro MIYATA(2)(1) J Power Design, Japan
E-mail:sugiyama@jpde.co.jp(2) Yamaguchi University, Japan
Abstract
Field study using magnetic susceptibility instrument enables to detect deposit
properties of the lower Pleistocene Otadai Formations of the Kazusa Group in Boso
Peninsula, central Japan, that is, time series variation and regional variation of submarine
fan deposits in relation to sea-level change. Oxygen isotope stages of the Otadai Formationof submarine fan have been determined in detail by Tsuji et al. (2005).
In upper submarine fan, maximum magnetic susceptibility of sandstones is almost
ten times higher in lowstand interval than highstand intervals. Maximum magnetic
susceptibility of turbidite sands is poorly controlled by grain size. In addition, turbidutesands are thick and more frequent during lowstand stages. Such a trend is also observed in
the JNOC TR-3 drill site at 5km east. There is little sandstone layer in western slope facies.
It is, therefore, thought that the sediment properties are different between lowstand andhighstand stages. Magnetic susceptibility of mudstones is higher in the lowstand intervals
than in highstand intervals. Although there is no distinct difference in grain size (sandcontent) between them, magnetic susceptibility of mudstones of submarine fan is higher
than that of shelf and slope in the lowstand stages. This evidence suggests magnetite–richsource material for submarine fan during lowstand stages.
At that time of 1 Ma, submarine fan reflected change in sediments source andtransport path from land areas corresponding to a minor change in sea-level at most 90m,
and showed a remarkable variation in magnetic susceptibility of sediments. Such eustaticchange thus can be detected by magnetic susceptibility of submarine fan deposits other thanshallow marine or marine biogenic evidence.
Keywords: magnetic susceptibility, turbidite, sea-level change
1.

Introduction

In the present study, we derive correlativity
between maximum magnetic susceptibility ofsandstones and glacio-eustatic sea-level change. Wethen deduce sediment supply and transport pass ofsubmarine fan deposits of the Kazusa Group.
Examples discussed in this paper are the Lower
Pleistocene Otadai Formation in the Boso Peninsula,
Japan (Fig. 1)

Modeling distributed of turbidite sands out of
which a reservoir of petroleum and a methane hydrate
is formed occupies the important location inresource-searching. Submarine fan has been studied
by seismic prospecting, chemical analysis, and isotopic
measurement with a main method. But it took for time
and the expenses before these methods got a result, andthe individual variation was also big in a judgement of aresult.

2.

This study used magnetic susceptibility for thepurpose of efficiency, quantification and labor-saving.
The susceptibility of the submarine fan deposits
shows the content of the magnetite and shows theaspect of the mineral composition. Magneticsusceptibility meter enables to measure directly at thesite.

Geologic setting

The Otadai Formations constitute the lower partof the Kazusa Group (Fig1). The group is up to3000-m thick and represents the infill of thePliocenePleistocene Kazusa forearc basin.
The Otadai Formation is up to 540-m thick andincludes a great number of sandstone beds andhemipelagic siltstone beds. The Otadai Formation1

10th Asian Regional Conference of IAEG (2015)

consists of interbedded some volcanic ash beds (suchas O26 , O18 and O7 ) which are found in large
region (Ito and Katsura, 1992).
Oxygen isotope stages of the Otadai Formation ofsubmarine fan have been determined in detail byTsuji et al. (2005).

total sum of siltstone bed thickness.(m)Yomaximum magnetic
susceptibility

TR-3Fig. 1 Geological sketch map of the central part ofthe Boso Peninsula.
Modified from Ito (1998). Yo-Section shows study
area. TR-3 shows bowling site (Tsuji et al., 2005)

Fig. 2 Maximum magnetic susceptibility andsea-level change.
Oxygen isotope curve of Tsuji et al. (2005).
Stratigraphic positions of some volcanic ash beds
are indicated on the right side of the oxygen isotope
curve.

3.

MethodMagnetic susceptibility was measured at theoutdoors using magnetic susceptibility meter (WSL-B
made by Tanaka geological consultant). Since putting
it in the magnetic measurement, we formed anoutcrop face and made the magnetic susceptibility
meter body stick to an outcrop perpendicularly. We
measured the magnetic susceptibility of most beds inthe study area. Highest magnetic susceptibility invertical direction in sandstone layer is called
maximum magnetic susceptibility in this paper.
4.

4.2 Maximum magnetic susceptibility is poorly
controlled by grain size
A relation between magnetic susceptibility
variation in vertical direction in sandstone layer andsedimentary structure can be classified into 3 types.

Resulta) Graded bedding
Magnetic susceptibility is most high at thebottom mostly, and the upper part is low.
b) Massive
Whether magnetic susceptibility is almost
fixed, the upper part is low.
c) Thickened very fine sand in layer
Magnetic susceptibility is highest at thethickened very fine sand.

The following 4 points became clear as a result ofthe study.
4.1 The susceptibility (the content of the magneticmineral) becomes higher in lowstand interval thanhighstand intervals
Fig.2 draws up a chronology of maximum
magnetic susceptibility of sandstones. Maximum
magnetic susceptibility of sandstones shows themaximum of the vertical direction in sandstone bed.
Axis of ordinate shows total sum of siltstone bed
thickness in study area. The study area is comparable
to marine isotope stage 29 to 33 (Tsuji et al., 2005).
The susceptibility becomes higher in lowstandinterval than highstand intervals. Maximum magnetic
susceptibility of sandstones is almost ten times higher
in lowstand interval than highstand intervals.

Most of the magnetic susceptibility variation invertical direction in sandstone layer is highest at thebottom or the thickened very fine sand. But,maximum magnetic susceptibility is unrestricted insedimentary structure. The maximum magnetic
susceptibility also becomes different respectively at
the same type of sandstone bed. The maximum
susceptibility becomes high in lowstand interval at
every type of sandstone bed.

210th Asian Regional Conference of IAEG (2015)5.
4.3 Periodic sand rich alternate layer appears
only in submarine fan deposit
Submarine fan facies are compared with shelf andslope. Stratigraphic positions of some volcanic ash
beds are indicated on the oxygen isotope curve.
Shelf facies are affected at bioturbation.
Sandstone bed accumulate only a little in slope facies.
Sand rich alternate layers and silt rich alternate
layers are repeated in submarine fan. Sand richalternate layers grow in lowstand stages. Silt richalternate layers grow in highstand stages. Submarine
fan facies show a variation which corresponds tosea-level change.

In upper submarine fan, maximum magnetic
susceptibility of sandstones is higher in lowstandinterval than highstand intervals. Maximum magnetic
susceptibility of turbidite sands is poorly controlled
by grain size. The maximum susceptibility becomeshigh in lowstand interval. Sandstone bed accumulate
only a little in slope. It is suggested that there was asubmarine valley. Sediment supply was directlytransported by way of submarine valley to submarine
fan in lowstand stages. In addition, turbidute sands
are thick and more frequent during lowstand stages.
On the other hand, depositional facies in shelf andslope do not show a variation which corresponds tosea-level change. Turbidute sands accumulate only alittle. Sediment supply of submarine fan deposits isdifferent from sediment supply of shelf and slope. It
is suggested that sediment supply of submarine fan
deposits is transported to submarine fan through asubmarine valley every time.
There are generally few heavy minerals including
magnetite in river sand at the offshore more than aforeshore much. Therefore while a little sand ofmagnetite of shallow sea was supplied in highstandstages, a possibility that sand rich in magnetite in land
was supplied is considered in lowstand stages. Shelfand slope Deposits were supplied by a current and astorm to a submarine valley in highstand stages. In
lowstand stages, river sand has been supplied to asubmarine valley directly for approach (ordirectly-connected) with an estuary and a submarine
valley by emergent of river plain and delta or bed
slope steeply-inclined.
At that time of 1 Ma, submarine fan reflectedchange in sediments source and transport path fromland areas corresponding to a minor change insea-level at most 90m, and showed a remarkablevariation in magnetic susceptibility of sediments.
Such eustatic change thus can be detected bymagnetic susceptibility of submarine fan deposits
other than shallow marine or marine biogenic
evidence.

4.4 Turbidute is frequently transported tosubmarine fan in lowstand stages
Thickness of sandstone bed and number ofsandstone bed per 1 meter of siltstone are compared(Fig.3). Turbidute sands are more than 3 times thicker
in lowstand interval than highstand intervals. Numberof turbidute sand bed is almost 2.5 times in lowstandinterval than highstand intervals. When deposition
rate of siltstone was fixed, amount of turbidute sandis increase and grow in frequency. Turbidute sand isfrequently transported to submarine fan in lowstandstages.
(b)thickness of sandstone bed

number of sandstone bed

per 1 meter of siltstone (m)per 1 meter of siltstone

total sum of siltstone bed thickness.(m)(a)

Consideration

6.

Conclusions

This study is discussed sediment supply andtransport pass inferred from magnetic susceptibility.
Magnetic susceptibility is easy at the outdoors
using magnetic susceptibility meter and, can measurequickly, and is available as a numerical data.
Magnetic susceptibility becomes high in lowstandinterval. Maximum magnetic susceptibility is poorly
controlled by grain size.
Magnetic susceptibility shows a possibility that it
can fit as an index of sea-level change and a climatechange.

Fig. 3 Amount of sandstone.
Fig. 3(a) shows thickness of sandstone bed per 1
meter of siltstone. Fig. 3(b) shows number ofsandstone bed per 1 meter of siltstone.

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10th Asian Regional Conference of IAEG (2015)

Acknowledgements
We thank Technology Research Center of Japan
Oil, Gas and Metals National Corporation(JOGMEC) for allowing us to the TR-3core
observance.
References
Ito, M., Katsura, Y.( 1992): Inferred glacio-eustatic
control for highfrequency depositional sequences ofthe Plio-Pleistocene Kazusa Group, a forearc basin
fill in Boso Peninsula, Japan., Sediment. Geol. 80,
pp. 67-75.
Ito, M., (1998): Submarine fan sequences of thelower Kazusa Group, a Plio-Pleistocene forearc
basin fill in the Boso Peninsula, Japan., Sediment.
Geol. 122, pp. 69-93.
Tsuji, T., Miyata, Y., Okada, M., Mita, I., Nakagawa,H., Sato, Y. and Nakamizu,M. (2005) :High-resolution chronology of the lower Pleistocene
Otadai and Umegase Formations of the Kazusa
Groupe, Boso Peninsula, central Japan –Chronostratigraphy of the JNOC TR-3 cores basedon oxygen isotope, magnetostratigraphy andcalcareous nannofossil -., Jour. Geol. Soc. Japan,
Vol. 111, No.1, pp. 1-20.

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