Near-Bottom Sedimentation Offshore Southwestern Taiwan from Echo Character Study Jui-kun Chiu, Cher-Shine Liu Institute of Oceanography National Taiwan University 2009/03/25

Acknowledgement The crew of the R/V Ocean Research I Central Geological Survey, MOEA

Offshore SW Taiwan Liu et al., 1998 TAIWAN Transition from Subduction to Collision Accretionary Wedge: folds and faults Passive margin: normal faults Deformation front Rapid sediment deposition Slumps on continental slope Liu et al., 1997 Deformation Front Accretionary Wedge Passive Continental Margin

Purpose To find the possible mechanisms of the different sedimentary processes and their geological implications offshore area Southwestern Taiwan

How to conduct researches 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 Data Bathymetry data Chirp sonar profiles (Bathy-2000P on OR1) Taiwan Ship Track Map > 7 cruises >10,000 km

Unconformity Mud Volcano Flow disturb Fault Chirp Sonar Profile 2 km 100m SW 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 (2)

Type Line drawing DescriptionOccurrenceInterpretation 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-1 Indistinct bottom echo and bushy, prolonged sub-bottom echoes Upper China continental slopeCreep sediments II-2 Wavy, 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-2 Acoustic 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-3 Uppermost 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 Map

What we observed 1.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 OR Kaoping Submarine Canyon Penghu Submarine Canyon

Gullies Penghu Submarine CanyonKaoping 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 A NW 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) 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) 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.

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