September 1993 The Huapinghsu Channel / Canyon System off Northeastern Taiwan : Morphology , Sediment Character and Origin

A study of the Huapinghsu Channel/Canyon System was conducted with bathymetric profiles, 3.5 kHz echograms and box cores, to investigate mor­ phology and sedimentary features. This channel/canyon system consists of two distinct units: (1) a broad trough·shaped channel which cuts into the shelf, and (2) a narrow V-shaped canyon with steep walls on the upper slope, which merges shoreward with the first channel. It extends from the shelf to the slope and has a total length of about 120 Km. Core samples indicate that coarse-grained sediments are the dominant surface sediments in the channel and adjacent shelves but muddy sediments occur in front of the canyon mouth and nearby slopes. Erosional processes of lateral widening predominated in the channel course on the shelf and intense downcutting was prevalent in the canyon on the upper slope. This channel/canyon system probably was initiated by sediment failure at the paleo-shelf edge. The canyon head then began to extend shoreward and resulted in a broad channel on the shelf. After later submergence the channel/canyon system has been preserved and enlarged by marine processes and mass wasting. The Huapinghsu Channel/Canyon System on the shelf and slope was a part of seaward sediment-transport systems during the late Pleistocene low­ sea-level stand. The subsequent transgression, some 5,000 to 7,000 years ago, inhibited the landward erosion of the channel. Because of cut-off from its main sediment source during the transgression, this channel/canyon system can no longer act as a modern conduit transporting sediments to the sea. The inadequate sediment supply from the China mainland and Taiwan has also prevented modern sediments from filling the submerged Huapinghsu Channel/Canyon System. 1 Marine Geology and Geophysics Division, Institute of Oceanography, National Taiwan University, Taipei, Taiwan, R.O.C. "2 Department of Oceanography, National Taiwan Ocean University, Keelung, Taiwan, R.O.C.


INTRODUCTION 1.1 Canyon Setting
The continental margin off northeastern Tai wan consists mainly of a broad East China Sea continental shelf, a narrow East China Sea continental slope and the deep southern Okinawa Trough (Figure 1).Submarine canyons and canyon-like incisions (narrow linear depressions) are recognized as the most prominent physiographic features on the shelf-slope region off northeastern Taiwan.Bathymetric contours indicate the presence of three major submarine canyons which indent the shelf, cross the slope and extend south-eastward into the southern Okinawa Trough (Figure 1).1\vo of these canyons were named the Chilung Canyon and the Huapinghsu Canyon by Yu (1992), after a nearby port city and an island near the head of the canyon on the shelf, respectively (Figure 1).The third canyon remained unnamed because of inadequate bathymetric mapping (Yu, 1992). 121.9 121.8 122.9 122.B 129.9 Fig. 1 .

Purpose
Location map showing the major physiographic units and the principal canyons of the northeast coast off Taiwan.Small dots are volcanic is lands.P=Pengchiahsu, M=Menhuahsu, H=Huapinghsu, CH=Chilung Tao and K=Kueishan Tao.(After Yu and Hong, 1992, Song, 1992).
These submarine canyons have been known for twenty years (Wang and Hilde, 1973, Boggs et al., 1979, Kimura, 1983, Marssett et al., 1987, Song, 1992) but the morphology, sediment dispersal, and origin of these canyons have not been fully investigated.Up until 1992, not a single paper specifically describing these canyons has been published.This has resulted in limited understanding.
The purpose of this paper is to determine the area.describe the morphology.and discuss the possible origin of the Huapinghsu Channel/Canyon System .This study primarily presents a plane view and a cross-sectional examination along the course of the channel/canyon system.Selected 3.5kHz echograms and sediment samples are also integrated into the interpretation of its origin.This paper intends to result in a better understanding of the Huapinghsu Channel/Canyon System, and lead to an investigation of other aspects of the chann el/canyon systems.

Data
The bathymetric data, high frequency (3.5 kHz) subbottom profi les and five box cores in areas off northeastern Taiwan were acquired during three cruises aboard RIV Ocean Re searcher I during 1991-1992 (Figure 2).Bathymetric data were recorded by Simrad EK 500 Sonar.High frequency 3.5 kHz profiles were collected by ORE echo sounders.The survey line totalled about 2,000 km ,of which eleven transects across the Huapingshu Chan nel/Canyon System and surr ounding areas were used in this study.

MORPHOLOGY
The bathymetric chart (Figure 3) and cross-sectional morphology (Figure 4) indicate that this linear depression consists of two distinct units: (1) a broad trough-shaped channel which cuts into the shelf, and (2) a narrow V-shaped canyon with steep walls on the upper slope which merges shoreward into the channel.This submarine depression is herein named Huapinghsu Channel/Canyon System.The head of the Huapinghsu ChannneVCanyon System is located close to the Huap inghsu island where the water depth is approximately 120 m (Figure 3).This channeVcanyon system follows a relatively straight course extending across the shelf and the upper slope in a nearly E-W direction.It then changes its course at around 25°371N and 122°30'E, and continues its path southeastward and finally ends approximately at the isobath of 900 m on the continental slope.The Huapinghsu ChanneVCanyon System, with its length of about 120 km, has a relief exceeding 580 m.The width of the channeVcanyon system is approximately 5 km close to the head, and increases to about 26 km in the middle and then narr ows to about 16 km around the canyon mouth.
Morphological characteristics, depth, width, relief and gradient of this chann eVcanyon system, are summ arized in Table 1.There are considerable variations in elevation of the channeVcanyon rims and the floors.The depth of the floor ranges from 120 to 825 m, and the difference in depth between the rim and floor of the chann el/canyon ranges from 110 to 580 m.The average slope of both flanks varies from 0.36 to 4.67 degrees.Clearly, the canyon segment has steep walls.The gradients of the channeVcanyon floors ranges from 0.04 to 2.28 degrees and the floor of the canyon is steeper than that of the the channel.Thus the basic channel/canyon shape is that of a broad trough-shaped channel cutting into the broad continental shelf and a narrow well-defined• V-shaped thalweg of "the canyon downcuts the upper slope region.
The cross-sections of the Huapinghsu ChanneVCanyon System are presented in Figure 4. Profile A, running approximately in a NE-SW direction, shows that the head of the channel  and a relief exceeding 580 m, as represented by Profiles J and K.

Longitudinal Profile and Channel/Canyon Relief
The axial slope of the channel changes gradually and maintains around 0.16 degree (Figure 5).A sharp increase of the slope up to 2.28 degrees occurs at the canyon segment and thereafter there is a decrease to 0.75 degree or less along the canyon course on the upper

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slope.The variations of axial slope along the channel are generally in acc ordance with the surrounding shelf floor, which has an average gradient of about 0.07 degree (Yu and Being, 1992).The greater gradients of the canyon segment also reflect the steepness of the East China Sea slope off northeastern Taiwan which has gradients ranging from 1/50 to 1/100 (Wang and Hilde, 1973), in places exceeding 10 degrees (Yu and Hong, 1992).
It is suggested that a boundary can be placed •between profiles H and I to separate the chann el from the canyon.The slope between profiles A and H has gradients ranging from 0.04 to 0.54 degree and is represented by a broad shallow depression while the slope be tw een profiles H and K has greater gradients (0.6 to 2.3 degrees) and is characterized by a sharp V-shaped trough.Apparently, lateral erosion of the channel course on the shelf and downcutting into the upper slope during its development have shaped and modified the linear trough into its present form.
A plot of the incision depth of the channel/canyon system against its course indicates that the incision depth changes very little on the shelf and greater incision depth talces place at the lower reaches of the channet and the cany on segment (Figure 6).Variations of the incision depth, together with the longitudinal profile, also suggest that the erosional processes of lateral widening predominated in the channel but the downcutting was prevalent at the canyon on the upper slope.

SEDIMENT PROPERTIES AND DISPERSAL
Five box-cored samples in and around the channel/canyon, supplemented by the bottom sediment facies maps (Boggs et aL, 1979) and the echo character map (Hong et aL, 1992), were used to delineate the sediment distribution and dispersal around the channel/canyon region.Core samples (Figure 7) indicate that coarse-grained sediments are dominant at the channel surface and on the adjacent shelf, whereas muddy deposits occur around the canyon mouth and nearby slopes.Sediments deposited on the shelf immediately off northeastern Taiwan are characterized by coarse-grained gravel and pebbles, usually larger than 2 cm in diameter (Figures 7A & 7B).Further northeast, the shelf sediments (Figure 7C) are primarily composed of coarse-grained shell fragments.These biogenic sediments consist principally of mollusk shells and fragments and foraminifera tests.The sediments in the Huapinghsu ChanneVCanyon are characterized by coarse-grained rock fragments and mud clasts (Figure 70).These coarse-grained shelf sediments have been interpreted a8 relict sediments ( Boggs et al., 1979).However, sediments at the canyon mouth are mainly silts and clays (Figure 7E).

DISTANCE (KM)
Our findings of the sediment distribution are compatible with previous studies in this region (Boggs et al, 1979, Chen et al., 1992, Hong et al., 1992).For instance, 3.5 kHz echograms covering the channeVcanyon system and adjacent shelves were dominated by prolonged echoes (Figure 8).Numerous studies (e.g.Damuth, 1980, McClennen, 1989) have shown that prolonged echoes usually indicate coarse-grained sediments.
Eustatic change of sea level was the main factor controlling the dispersal and •nattJre of the sandy sediments on the shelf around Taiwan (Niino andEmery, 1961, Boggs et al., 1979).During the late Pleistocene (15,000 years B.P.), sea level was probably about 140 m below present level, exposing most of the Taiwan continental shelf.Rivers on the broad shelf deposited sediments of gravels, sands and muds.The sea level rose later and reached its present level some 5,000 to 7,000 years ago (Boggs et aL, 1979).Niino and Emery (1961) found widespread sandy and gravelly sediments on the broad shelf west and north of Taiwan and interpreted them as relict sediments which have not been covered by younger sediments since the late Pleistocene.In contrast, the bottom sediments in the inshore area within about 40 km of Taiwan consist mainly of fine-grained Holocene to modem sediments (Boggs et aL, 1979).
We interpreted the coarse-grained sediments in the ch.annel to be reworked relict sed iments.Current velocities along the channel course are not available at present.However, near-bottom currents around the •channel area generally flow from shelf edge toward shelf with a velocity averaging around 20 emfs and the near-surface currents, in turn, flow seaward  (Chem, personal communication).The currents pattern suggests that the flow seems not to be capable of transporting coarse-grained sediments in the channel seaward.We suggest that the coarse-grained sediments, either relict or eroded from the channel edges, were reworked and confined in the channel, but the clays were removed and laid down in the canyon segment.This suggests that the channel is no longer a conduit for transporting coarse-grained materials seaward. •

ORIGIN
The submarine canyons in the shelf-slope region off northeastern Taiwan have been interpreted to be glacially eroded, submerged valleys developed in the late Pleistocene (Boggs et al., 1979).However, Shepard (1981) cautioned that the origin of submarine canyons is of multiple causes and their development history may be long.Based on bathymetry, morphology, sediment characteristics and regional geology, the origin of the Huapinghsu Chann eVCanyon System may be inferred.
Since the Huapinghsu Channel/Canyon System is related neither to the present river drainages of Taiwan nor to those of the China mainland directly, it does not belong to the river-extension type (Shepard, 1981).However, morphological characteristics suggest that the erosional process of lateral widening dominated in forming the channel (profiles A to H) on the shelf, as evidenced by the wide and shallow troughs and low gradients of the chann el floor and lack of steep walls.Apparently, the intensity and duration of downcutting of headward erosion in the shelf areas were not strong and long enough to cut the shelf strata into a canyon fonn.It is apparent that steep canyon walls begin to appear at profile I, which marks a turning point in the channel course near the transition from submarine channel to b'ue submarine canyon.The canyon segment (profiles I to K) is characterized by higher gradients (0.57 to 2.28 degrees) than those of the channel (0.04 to 0.5 degree).The trough on the upper slope is a relatively narr ow, linear, deep depression with steep sides which fit Bouma's 1990 definition of the submarine canyon.
During the late Pleistocene (15,000 years B.P.) the broad shelf north of Taiwan was exposed subaerially and the Pleistocene shoreline was probably located along Longitude 120 E (Boggs et al., 1979).Following the models of submarine canyon evolution (Shepard, 1981, Farre et al., 1983), we speculate that the infant Huapinghsu canyon was initiated at the paleo-shelf edge where significant sediment failure and mass-wasting occurr ed.Once initiated, the canyon head acted as a sediment conduit, where erosion took place and indented the shelf landward.The canyon head erosion did not penetrate deeply into the shelf.Instead, it formed a broad -channel probably due to the low gradient of the shelf.On the o.ther hand, substantial submarine mass-wasting proce�ses cut the slope into steep walls and deepened and widened the canyon course downslope.The subsequent rising of sea level stopped the landward erosion of the chann el.The fine clays in the channel were removed and coarse grained sands and gravels remained.Based on the finding that Holocene and modem sands from nearby Tawian drainages were restricted to the nearshore areas (Boggs et al., 1979), we suggest that very few sediments were deposited in the Huapinghsu ChanneVCanyon System and those found there are relicts.Inadequate sediment supply prevented the infilling of the submerged troughs of the Huapinghsu channeVcanyon system.

CONCLUSION
The postulation of submerged valleys for the origin of the canyons off northeastern Tawian (Boggs et al, 1979) was partly incorrect due to insufficient bathymetric data.We present an alternative model for the development of the submerged linear depressions.Our results suggest that the narrow linear, trough on the shelf-slope region comprises a chan neVcanyon system.Morphologically.the canyon head portion on the shelf shows typical channel features which have resulted genetically from the headward erosion of the canyon.
The axial gradient of the channel generally is 0.15 degree or less.This channel generally has an irregular floor with a maximum width of about 26 km.The canyon segment on the upper slope has an axial gradient about 0.8 degree higher than that of the channel.The canyon walls have a steep slope of about 4.5 degrees.The relief of the canyon ranges from 160 to 580 m.The width of the canyon averages about 15 km.
The Huapinghsu Chann eVCanyon System apparently originated on the East China Sea shelf-slope region as a part of a seaward sediment -transport system during the late Pleistocene low-sea-level stand.The subsequent transgressive sea drowned the chann eVcanyon system and cut off its sediment source.This chann eVcanyon system can no longer act as a modem sediment conduit to the sea.The limited sediment supply from the China mainland and Taiwan prevents the filling of the chann eVcanyon system.

Fig. 2 .
Fig. 2. Location of echo-sounding tracks, 3.5 kHz subbottom profiles and five box-cored samples.The surveys covered most of the areal extent of the Huapinghsu Channel/Canyon System.

Fig. 3 .
Fig. 3.The bathymetric chart shows a linear depression .extending on the shelf slope region in an east-west direction and ending approximately at the isobath of 900 m on the slope.The solid triangle indicates the box-cored samples.

*
Rell£ is the depth difference between the chann el/canyon rim and the chann el/canyon floor is a relatively small Y-shaped trough.The width of the channel head is around 5 km and the depth of the channel floor is about 220 m.As shown in Profiles B, C, D, and E in a downchannel direction, the channel increases its width to about 22 km, but the channel floor maintains its depth of about 250 m.The cross-section of the channel changes.from a single Y-shaped trough to a shallow and relatively wide depression with irregular floor surfaces.Further downchannel, it then bifurcates into two depressions with irregular floor surfaces and reaches the maximum cross-sectional area.The channel increases its depth slightly over 300 m as shown in Profiles F and G.These two depressions then merge into a Y-shaped trough and continue along its course eastward as shown in Profiles H and I.The segment between Profiles I and J is the transitional zone ,where the channel-dominated system evolved into a narrow (15 km) and deep (600 m) canyon with decreasing cross-sectional area.The canyon then changes its course at Profile J from a E-W direction to a southeast direction..This SE-NW segment of the canyon is characteristically a sharp Y-shaped trough with steep walls , .t , , ! , , 1 1 , 1 , , r , , t I I I � 1 � I I ' i I I I ' I I ' I I I I I I I I I I l I I I I I l

Fig. 5 .
Fig. 4. The cross-sectional morphology of the Huapinghsu Channel/Canyon System comprises a broad trough shaped channel on the shelf and a narrow V-shaped canyon with steep walls on the upper slope.

Fig. 6 .
Fig. 6.Relief plotted against distance from the channel head area to the canyon mouth.Note the Huapinghsu Channer maintains a uniform incision of 100-200 m, increasing rapidly to over 600 m at the canyon mouth.

Fig. 7 .
Fig. 7. Five box-cored samples indicate that coarse-grained sediments are the dominant surface sediments in the channel and adjacent shelves and muddy' deposits occur around the canyon mouth and nearby slopes.(HC-A) and (HC-B) Pebbly sands and gravels, (HC-C) Coarse-grained shell fragments, (HC-D) Gravelly mudclasts and rock fragments and (HC-E) Fine-grained silts and clays.Locations of the cores are shown in Fig. 3.

Fig. 8 .
Fig. 8.The 3.5 kHz record of profi le B shows that the channel areas are dom inated by prolonged echoes which indicate coarse-grained (from silt to gravel) sediments.The inserted echo character map at the right shows that the areas surr ounding the Huapinghsu Channel/Canyon System are characterized by prolonged to semi-prolonged echoes.(After Hong et al .•1992).

Table 1 .
Morphological variables of the Huapinghsu ChanneVCanyon System.Water Depth Relief* Width Wall Slope Axis Slope Line Oiannel/c.anyon