A Preliminary Study of the Echo Characters and Sedimentary Processes along the Continental Margin , Northeast of Taiwan

The regional mapping of the 3.SkHz echo characters in the areas off northeastern Taiwan provided valuable information about sedimentary pro­ cesses on the sea floor. Seven distinctive echo types were recognized. These echo types were grouped into four major classes: indistinct, distinct, hy· perbolic and irregular echoes. Indistinct echoes had three sub-classes: pro­ longed, prolonged with irregular saw-toothed surface and prolonged or semi· prolonged with inclined sub-bottom reflectors cropping out laterally. Besides, the hyperbolic class included the large irregular hyperbolae and the broad gently rolling hyperbolae. The irregular echoes were similar to the distinct type but with a jumbled morphology. The echo character map showed that each of the physiographic provinces was characterized by a distinctive echo type. The very prolonged echo occur­ ring on the narrow llan continental shelf indicated that strong current activ· ities prevailed in this region. The upper part of the continental slope, char­ acterized by large irregular hyperbola, was dominanted by slumping and/or sliding processes. Meanwhile, the turbidity currents and mass-wasting pro­ cesses had apparently been the primary agents shaping the lower part of the slope as indicated by the distinct echoes.


INTRODUCTION
Off the northeast of Taiwan, the Kuroshio current crosses the Ilan Ridge and then turns northeastwards along the East China Sea continental slope.The crossing over and the turning have caused the Kuroshio to change tremendously in its flow regime.Therefore, the northeast offshore region of Taiwan (Figure 1) has long been the focus of studies of many oceanographic disciplines.However, only a few studies deal with the geological aspect (Niino and Emery, 1961;Emery et al., 1969;Wangeman et al., 1970;Wang and Hilde, 1973;Chen, 1979;Lee et al., 1980;Chern, 1983;Lin and Chen, 1983;Shyu and Chiao, 1983).
The purpose of this study is to interpret the sedimentation processes in the northeastern offshore region of Ta iwan.Since previously studies (Damuth, 1975(Damuth, , 1978(Damuth, , 1980;;Klaus and Ledbetter, 1988 ;McClennen, 1989 ;Pratson and Laine, 1989) can potentially reveal the bedforms responsible for specific echo characters and the near bottom processes which create these bedforms, certain preliminary conclusions could be drawn from the echo character map of the study area.

CLASSIFICATION OF 3.SkHz ECHOES
Seven distinctive echo types were recognized in the study area.The echo types were grouped into four major classes: indistinct, distinct, hyperbolic and irregular echos.Type Ia.Very prolonged bottom echoes with no sub-bottom reflectors (Figure 3a).These echoes occur at most parts of the Ilan continental shelf.
Type lb.Prolonged to semi-prolonged echoes with a irregular saw-toothed surface (Figure 3b).Amplitudes generally range from 5 to 30 meters and wavelengths are less than 15 meters.These echoes occur only in the areas off northeastern Taiwan around 200 meters depth contour.
Type le.Semi-prolonged echoes with inclined sub-bottom reflectors cropping out later ally (Figure 4a).These echoes are returned exclusively from the rugged terrain of Ilan Ridge, submarine troughs and channels.

Distinct echoes
Distinct echoes are characterized by a sharp, continuous, smooth bottom echo.Type II.Distinct echoes with parallel, continuous sub-bottom reflectors (Figure 4b).The main region returning this type of echoes is from the lower part of the continental slope.

Hyperbolic echoes
Two different types of hyperbolic echoes are gathered in this category.Type Illa.Large, irregular hyperbolic echoes with widely varying vertex elevations (Figure 5a) characterize the upper part of the continental slope.Hong , Yu and Chen Type IIIb.This type is a kind of isolated large, broad, gently rolling hyperbolae with conformable parallel sub-bottom reflectors (Figure Sb).These echoes are observed from the lowermost part of the continental slope adjacent to the western tip of the Okinawa Trough.

Irregular echoes
Type N. The echo character is similar to Type II having continuous, sharp, bottom echoes with continuous, sharp, parallel sub-bottom reflectors.However, it has a jumbled morphology (Figure 6).This type is confined in a narrow zone, extending roughly along the 1400 meters depth contour , at the middle part of the East China Sea continental slope .

INTERPRETATION OF THE ECHO-CHARACTER MAP
3.5kHz echoes were first identified, classified and then mapped.During mapping, the regional bathymetry was used to constrain echo-type distribution.Constiu:ction of the echo-character map was mainly based on guidelines of Damuth (1980).The interpretation of depositional processes operating in the study areas is inferred mainly from the echo characters and their aerial distribution.
• It appears that different physiographic provinces exhibit their own distinct echo types (Figure 7).Accordingly the associated sedimentary processes will be discussed physiograph ically province by province.

Ilan continental shelf
The shelf is mainly characterized by very prolonged echoes with no sub-bottom re flectors (Type Ia).Recent studies utilizing near-bottom sound sources suggest .that the pro longed nature is the result of reflections from small erosional/depositional bedforms which have wavelengths that are too small to be resolved into discrete hyperbolae (Ewing et al., 1973;Damuth, 1978).Sub-bottom reflections are not recorded because there are high con centrations of bedded silt/sand in the upper meters of the sea floor (Damuth, 1980;Addy et al., 1982).
It had been reported that the Han continental shelf is covered by fine-to medium-grained sandy sediments (Boggs et al., 1979).Above these, the velocities of tidal curr ent and the Kuroshio current are measured up to 100 cm/sec (Chen and Lin 1990).The region is also often disturbed by seasonal storm activities.These observations are consistent with the interpretation of the prolonged echo type in previous studies (Ewing et al., 1973;Damuth and Hayes, El77;Damuth, 1980;Addy et aL, 1982).

East China Sea continental shelf
Only the southern end of the East China Sea continental shelf was covered in this study.This area, around 200 meters in depth, is characterized mainly by prolonged to semi prolonged echoes with a irregular saw-toothed surface (Figure 3b � Type lb).Based on mutually crossed seismic profiles these saw-toothed features seem to be dune-like bedforms Their amplitude is ranged from 5 to 30 meters with wavelength generally less than 15 meters.
The sediments of the saw-toothed bed forms were mainly composed of very coarse shell debris and a few quartzose sands and pebbles as recovered by three box cores.There were two possibilities for developing such kinds of bed forms.First, the current velocities should be significantly larger than those in the Ilan continental shelf.Alternatively, these features were probably the relict bed-forms sculptured by sedimentary and/or erosional processes during low-stand of eustatic sea-level in the Pleistocene (Boggs et aL, 1974(Boggs et aL, , 1979;;Niino and Emery, 1961).

Continental slope
This region involves the southeastern part of the East China Sea continental slope and the Ilan continental slope provinces.The gradient is greater in the upper part of the slopes (1 to 6 degrees) than that of the lower part of the slopes(0.5 to 4 degrees).

(a) Upper continental slope
These slopes are characterized by large, irregular hyperbolic echoes with widely varying vertex elevations (Figure Sa; Ty pe Illa).These type of echoes have been reported from very rugged terrains such as the steep sides of seamounts and plateaus, continental slopes, canyon walls, as well as rugged mid-ocean ridges (Damuth andHayes, 1977� Damuth, 1980;Pratson and Laine, 1989;Satterfield and Behrens, 1990).The other studies (Jacobi, 1976;Klaus and Ledbetter, 1988) also indicate that the echo character could also be return from regions with slide and slump blocks.
As mapped by Song (1992), the upper continental slope in part of the study area was characterized by steep and rugged topography.This is compatible with the irregular hyperbolic nature of the observed 3.5kHz echoes.

(b) Lower continental slope
The distinct echoes with parallel, continuous subbottom reflectors (Figure 4b ; Type II) characterized the lower slope areas.Studies by previous investigators indicated that the areas returned by this echo type contained mainly fine sediments with or without bedded sand or silt ( Damuth, 1975( Damuth, , 1980;;Mayer, 1980;Addy et al.;Pratson and Laine, 1989).
So far, our knowledge is still not sufficiently advanced to permit positive correlations between specific echo character and specifi c sediment property.However, regions charac terized by the above echoes were often found to be dominated by weak waning turbidity currents and/or by pelagic sedimentation processes (Damuth, 1980).Consequently, the lower continental slopes in these areas were most likely shaped by turbidity currents and pelagic depositions.
A same type of reflector but with jumbled morphology (Figure ;Type IV) was observed from the upper part of the lower East China Sea continental slope.A few seismic reflection profiles were taken parallel and perpendicular to the continental margin.From these seis mograms, the region looked like to be irregular surfaces of slide or slump blocks cutting by small valleys.This interpretation was encouraged• for the same type of surface morphology and acoustic reflection had been well documented from many other parts of the world ocean (Jacobi, 1976;Embly and Jacobi, 1977;Mcgregor, 1977;Pratson and Laine, 1989).

Okinawa Trough
Tue seismic lines only slightly cover the western tip of the basin floor of the Okinawa Trough.A small portion of the lower most continental slope and this part of the basin floor ( Figure 5) return a kind of isolated large, broad, gently rolling hyperbolae (Type IIIb).Rolling hyperbolae looks like sediment waves reported in many parts of the oceans (Jacobi et al., 1975;Embley and Langseth, 1977;Damuth, 1979;Normark et al., 1980;Flood and Shor, 1988;Klaus and Ledbetter, 1988;Carter et al., 1990).However, it needs further study to verify what the echoes really are.
Finally, the prolonged or semi-prolonged echoes with inclined sub-bottom reflectors cropping out laterally (Type le) are returned exclusively from the rugged terrain of llan Ridge or areas adjacent to the submarine trough or channel.The lateral crop-out sub-bottom reflectors seem to be formed by the erosion and truncation of the sediment gravity flows during low stand of eustatic sea level in Pleistocene time.Further sedimentary and paleontologic data are required in the future studies.

CONCLUSIONS
3.SkHz echograms have been applied successfully to study the near-bottom sedimen tation processes world-wide for more than 20 years.The same type of echograms recorded from the northeast off Taiwan were classified and mapped in an attempt to study the regional sedimentation.Preliminary conclusions have been drawn as follows: 1. Seven echo types were recognized on 3.SkHz profiles from the areas off northeastern Taiwan.Each of the physiographic provinces was characterized by its own distinct echo types indicating different sedimentary processes prevailed.2. Prolonged bottom echoes were recorded from the Ilan continental shelf.Strong tidal and storm currents prevailed as evidenced by medium to coarse sandy sediments and current data.The correlation between sediments and echo type is consistent with other studies.3. Very coarse shell debris, along with a few rounded quartzose sands and pebbles, were recovered from the southern end of the East China Sea continental shelf.Distinct saw toothed reflectors had been found here.The causes for this echo type need further study.4. The upper parts of the continental slopes were characterized by very steep and rugged topography which returned large irregular hyperbolic echoes.5. Distinct sharp echoes with parallel sub-bottom reflectors and those with jumbled mor phology distributed over the lower part of continental slopes.Tue gentle rolling hyper bolae were only recorded from the western tip of the Okinawa Trough.Tue appearance of these different types of echoes indicated that the gravity-induced down slope pro cesses were probably the main agents shaping the lower slopes and the western tip of the Okinawa Trough.

Fig. 1 .
Fig. 1.Bathymetric map of the northeastern offshore of Ta iwan.The contour interval is 200 meters.The box shows study area.Solid lines indicate locations of echogram profiles.

2. 1
Indistinct echoes 437 Indistinct echos have a prolonged or sometimes semiprolonged bottom echo, and occur in areas of flat and locally rugged sea-floor.There are three subclasses that have been distinguished as follows:

Fig. 3 .
Fig. 3. Examples of echo types: (a) Ia.Very prolonged bottom echoes with no sub-bottom reflectors.(b) lb.Prolonged to semi-prolonged echoes with a irregular saw-toothed surface.

Fig. 6 .
Fig. 6.Echo type IV. is similar to type II but with a jumbled morphology.

Fig. 7 .
Fig. 7. Echo character map of the study area.