Near-Bottom Sedimentation Near-Bottom Sedimentation Offshore Southwestern Taiwan Offshore Southwestern Taiwan

from Echo Character Studyfrom Echo Character Study

Jui-kun ChiuJui-kun Chiu, Cher-Shine Liu, Cher-Shine Liu

Institute of OceanographyInstitute of Oceanography

National Taiwan UniversityNational Taiwan University

2009/03/252009/03/25

AcknowledgementAcknowledgement

• The crew of the R/V Ocean Research I

• Central Geological Survey, MOEA

Offshore SW Taiwan

Liu et al., 1998

TAIWANTransition from Subduction toCollisionAccretionary Wedge: folds and faultsPassive margin: normal faultsDeformation frontRapid sediment depositionSlumps on continental slope

Liu et al., 1997

Deform

ation Front

AccretionaryWedge

Passive

Continen

tal

Margin

PurposePurpose• To find the possible mechanisms of the

different sedimentary processes and their geological implications offshore area Southwestern Taiwan

http://www.marum.de/

How to conduct researches How to conduct researches in this big area?in this big area?

Yellow Sea

South China Sea

(Chough et al., 2002)(Damuth, 1980)

Echo Character

1B

2A

2B

Yellow Sea

(Chough et al., 2002)

Echo Character

Methods and DataMethods and Data• Bathymetry data• Chirp sonar profiles(Bathy-2000P on OR1)

Taiwan

Ship Track Map

> 7 cruises>10,000 km

Unconformity

Chirp Sonar ProfileChirp Sonar Profile

2 km

100mSW NE

We identified and classified discrete echo types on the basis of reflection characters (e.g. clarity, continuity, amplitude and geometry of bottom and sub-bottom echoes).

Four categories of echo types are recognized in this study:

(1) Distinct echoes (Type I-1 and I-2)

(2) Indistinct echoes (Type II-1 and II-2)

(3) Hyperbolic echoes (Type III)

(4) Irregular echoes (Type IV-1, IV-2 and IV-3)

Echo Characters (1)Echo Characters (1)

Echo Characters (2)Echo Characters (2)

Type Line drawing Description Occurrence Interpretation

I-1

Sharp surfloor reflector without or with few subbottom echoes

Passive and active continental shelf

Coarse-grained sediments (Chen, 1983; Lee et al., 2004)

I-2

Distinct bottom echo with continuous, parallel internal reflectors; the penetration depth ranges from 40 to 100 m; seafloor is generally flat

Intra-slope basin of the Kaoping Slope and the lower part of the passive continental slope

Pelagic deposition (Lee et al., 1999); submarine fan

II-1Indistinct bottom echo and bushy, prolonged sub-bottom echoes

Upper China continental slope Creep sediments

II-2Wavy, prolonged sub-bottom echoes; the penetration depth varies from 10 to 50 meters

Troughs or depressions of the Kaoping Slope

Turbidites

III

Single or irregular overlapping hyperbolae with widely varied vertex elevations and no sub-bottom reflectors

Summits and outcrops of the Kaoping Slope and small hills or rugged area of the upper China continental slope

Basement high or outcrops (Damuth, 1980) , Escarpments

IV-1

Slope failure planes within a short distance; down-slope slumps by the gravity sliding

Steep slope and gullies of the SCS continental margin and canyon systems of the Kaoping Slope

Slope failure (Lee et al., 2002; Lee et al., 2005)

IV-2Acoustic blanking zone between continuous reflectors; the width of the blanking signals varied from 200 meter to several kilometers

Mud dirpir zone of the upper Kaoping Slope

Gassy sediments (Chiu et al., 2006)

IV-3Uppermost transparent blanketing layer below the sea floor

lower SCS continental slope and around the lower section of the Penghu Canyon

Debris flow deposits (Lee et al., 1999)

Echo Character MapEcho Character Map

What we observedWhat we observed1. In the submarine Taiwan orogenic wedge offshore SW Taiwan,

and fill-and-spill processes have smoothed the seafloor topography of the upper Kaoping slope there. In the lower Kaoping slope, steep slopes of the ridge flanks and submarine canyon walls generate frequent slope failures.

2. In the passive SCS continental margin, the steep slope of the upper continental slope forms an erosional environment while the mass wasting materials are deposited in the lower continental slope.

Accretionary Wedge

Kaoping River

NE

Kaoping Submarine Canyon

Penghu Submarine Canyon

100 m

2 km

Flood Layer thinner

(OR1-647)

Kaoping Submarine Canyon

Taiwan

Type 1-2

（ Huh ， 2008 ）

Kaoping River

OR1-716-4

Kaoping Submarine Canyon

Penghu Submarine Canyon

Gullies

Penghu Submarine Canyon Kaoping Submarine Canyon

Passive margin

• Differences in the occurrence of slope failures and sea-floor gradients from NE (near Taiwan) to SW (away from Taiwan) along the passive SCS continental margin.

Line ANW SE

Continental Shelf

Clift

Mass movement

2 km

100m

SE NW

• This reflects the influence of the loading of Taiwan orogenic wedge on the passive SCS continental margin.

(Lin & Watts, 2002)

• Continuous, parallel, general conformable sub-bottom reflections are observed on the lower SCS continental slope

Continuous Reflectors

Mass movement

2 km

100m

SE NW

Summary(1)Summary(1)• There is a major difference between the sedimentary

environments of the passive SCS continental margin and the submarine orogenic wedge offshore southwestern Taiwan.

• The echo distribution map clearly indicates that in the passive SCS continental margin province, the sedimentary environment in the upper to mid-slope is erosional, whereas the lower continental slope of the SCS continental margin is depositional.

Summary(2)Summary(2)

• On the other hand, in the active submarine Taiwan orogenic wedge, the upper slope domain appears to be a depositional environment, whereas the lower slope domain is erosional in nature.

• The main reason for this big contrast is the supply of terrigenous sediment.

Thanks for your attention!Thanks for your attention!