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Morphology of San Antonio submarine canyon on the central Chile forearc
ELSEVIER
Marine Geology 129 (1996) 197-205
Letter Section
Morphology
of San Antonio submarine canyon on the central Chile forearc
Rick A. Hagen a, Her&m Vergara b, David F. Naar ‘J a Alfred Wegener Institute for Polar and Marine Research, Postfach 120161,27515,
Bremerhaven, Germany b Instituto de Oceanologia, Universidad de Valparaiso, Casilla 13-D. Viiia de1 Mar, Chile ’ Department of Marine Science, University of South Florida, 140 Seventh Ave. South, St. Petersburg, FL 33701, USA
Received 20 March 1995; revision accepted 24 August 1995
Abstract
A multibeam survey was conducted over San Antonio submarine canyon, near Valparaiso, Chile, in April and May 1993 using the SeaBeam 2000 system on the R/V Melville. The bathymetric data from this survey reveal a canyon with an overall sinuosity of 1.25, a broad, roughly U-shaped cross-section along most of its length, and an almost constant channel slope above the forearc structural high. The course of the canyon is deflected to the north by a prominent structural high opposite the town of San Antonio. SeaBeam 2000 side-scan sonar data reveal high backscatter material on the floor of the canyon with a longitudinal fabric, reminiscent of stream braiding, and point bars formed on the inside of channel bends. We interpret this high backscatter material to be coarse sediment, transported down the canyon as turbidity currents. The source of these turbidity currents is probably the Rio Maipo, which enters the ocean near the head of the canyon.
1. Introduction Submarine canyons along the coast of Chile were discovered by early cable-laying ships, and have been surveyed by several marine research expeditions (Ziegler et al., 1957; Shepard and Dill, 1966; Fisher and Raitt, 1968; Schweller et al., 1981; Thornburg, 1985; Hagen et al., 1994). Although submarine canyons are known to be common features on active margins (Barnard, and 1978; Moore et al., 1982; Gnibidenko 1 Corresponding author 0025-3227/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDZ 0025-3227(95)00116-6
Svarichevskaya, 1984; Klaus and Taylor, 1991; Thornburg et al., 1990; Soh et al., 1990), to this point only a few have been mapped and studied in detail. In this paper we describe the morphology of San Antonio submarine canyon on the Chile forearc near the city of Valparaiso. This canyon, along with portions of the Chile Trench and forearc, was surveyed with the SeaBeam 2000 multibeam bathymetry system on the R/V Melville in April and May 1993 (Gloria Expedition legs 6 and 7). This paper is also one of the first to present the acoustic intensity data (side-scan) recorded by the SeaBeam 2000 system.
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2. Onshore and offshore geomorphology James (1971) postulates that the Farallon plate began to underthrust the North and South American plates no later than the Late Triassic or Early Jurassic. Break-up of the Farallon plate into the Gorda, Cocos, and Nazca plates occurred approximately 26 Ma (Handschumacher, 1976). The Peru-Chile trench is formed by the subduction of the Nazca plate beneath the South American plate. The dip of the Benioff zone segments the Peru-Chile margin along strike into areas with shallow (< 10’) inclination and areas where the zone dips at an angle of approximately 30” (Barazangi and Isacks, 1976). Areas above flat, relatively shallow subducted slabs tend to lack recent volcanism and tend to have mostly compressional neotectonics. Segments above steeply dipping Benioff zones correspond to areas exhibiting well-developed Pliocene and Quaternary volcanism and dominantly extensional recent tectonics (Jordan et al., 1983). The central Chile coast consists of steep headlands composed of igneous and metamorphic rocks. In the Valparaiso region these rocks are part of a coastal batholith whose main components are granodiorite, tonalite and adamelite (Corvalan and Munizaga, 1972). Remnants of marine terraces are present along the coast at 100 to 140 m above the present sea level (Paskoff, 1967). Recent sediment is deposited in small bays and near the mouths of rivers. The Chilean forearc slope has gradients that vary between 3” and 6” (Fisher and Raitt, 1962). In the San Antonio canyon area the forearc slope varies between 3.5 and 4.5”. At present there are few subsurface data in this area. Scholl et al. (1970) presented two seismic reflection profiles in the vicinity of the canyon at 31” and 33”S, and described depositional sequences along the coast. These profiles revealed two layers of sediment overlying crystalline basement. The lower layer of sediment is of variable thickness and is moderately deformed. The upper layer is 1 to 10 m thick and generally undeformed. Small basins on the upper forearc are totally filled by sediment deposits up to 800 m thick. At present, the only samples from the San Antonio Canyon area are two piston cores
from the trench axis (Thornburg, 1985), which were characterized as fan levee deposits.
3. San Antonio Canyon San Antonio Canyon is located just south of the port of Valparaiso, near the small fishing port of San Antonio. The canyon apparently begins in San Antonio harbor itself (based on unpublished charts from the Chilean Hydrographic Service), and extends about 170 km to the northwest, where it enters the trench axis near 33”05’S, 72”45’W (Fig. 1). The canyon has an overall sinuosity of approximately 1.25 (channel length/valley length), making several sharp bends along its route to the trench axis. The canyon exhibits a broad, shallow, U-shaped cross-section along most of its length (Fig. 2). The width of the approximately flat canyon floor (in a cross-sectional sense) varies from 1 km (profile C) in about 1100 m water depth, to about 4 km (profile W) at about 3400 m depth. Below profile W, a cross-section view of the canyon displays a fairly flat and smooth inner valley floor, but the walls of the canyon open out into a broader V-shape on the lower forearc slope versus the roughly U-shape cross-section observed further upslope. Between profiles Q and W there is a fairly long and straight stretch (about 28 km) where the canyon trends at 352”, roughly parallel to the N-S trench axis. The forearc west of the canyon along this stretch is several hundred meters higher than the equivalent area of the forearc north of the canyon. The canyon was apparently deflected north by this barrier before finding a low area and making a 115” counter-clockwise turn to the southwest (245”). This sharp change in the canyon’s trend occurs at a major break in the forearc slope at a depth near 3500 m. It also corresponds to a significant change in the slope of the canyon floor (Fig. 3) from an almost constant slope of 35 m/km (2”), to a downslope gradient of 110 m/km (6.3”) where it then gradually decreases to about 2” at the trench axis. At this sharp canyon bend and increase in slope near -3500 m, a bifurcation of the canyon can be seen in the SeaBeam 2000 bathymetric data
RA. Hagen et al./Marine 73
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SAN ANTONIO SUBMARINE CANYON SEABEAM 2000 BATHYMETRY 33” 30’ s
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Fig. 1. SeaBeam 2000 multibeam bathymetry of San Antonio Canyon.
(Fig. 4). An obstruction about 200 m in height (and 2 x 5 km in area) occurs in the center of the canyon. The bathymetric data show that the channel splits and passes to both the north and south sides of the obstruction (Profile AA, Fig. 2). The southern channel is deeper and extends further up the western edge of the canyon axis by curving towards the right (south), thereby creating a shallow step of about 100 m on the floor of the west side of the canyon (profile X, Fig. 2). In addition to multibeam bathymetry, the SeaBeam 2000 system provides acoustic intensity imagery similar to towed side-scan sonar. The
SeaBeam 2000 imagery data have 12 bit resolution stored as 16 bit words, but presently the side-scan data provided to us and displayed are downgraded to 4 bit resolution. Software is being developed by SeaBeam Instruments Inc. to process and display the 12 bit resolution data (D. Caress, personal comm., 1995). However, the SeaBeam 2000 side-scan sonar data (Chit resolution) we have processed and displayed have sufficient resolution to show that the floor of the canyon has a relatively high degree of strong acoustic backscatter with respect to the surrounding seafloor (Fig. 5b). It is important to
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I San Antonio Canyon Profiles
Vertical Exaggeration= 3.6 t
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0 (6)
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Horizontal Distance (km)
Fig. 2. (a) Synthetic profiles across the canyon generated from the SeaBeam 2000 bathymetry. Profiles were created by sampling the SeaBeam data every 2 km along the canyon. The profiles are oriented perpendicular to the canyon axis and consist of about 60 to 120 narrow-beam depth along h 120” swath (depending on depth). (b) Map showing profile locations.
note that the geometry of the canyon and seafloor will influence the acoustic backscatter, but because the system transmits and collects the data at the sea surface and the backscatter in Fig. 5b do not correlate to the morphologic structure (bathyme-
try) of the canyon, it is safe to interpret that the backscatter pattern is indicative of bottom roughness related to sediment type and is not a representation of the surface geometry. The boundary of this high backscatter area is sharp and varies in
R A. Hagen et al./Marine Geology 129 (1996) 197-205
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72" 3O'W
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JJ
EE
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Distance (km) Fig. 3. Axial profile along San Antonio Canyon showing changes in channel slope with the location of where the profiles in Fig. 2 cross the axial profile.
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Fig. 4. SeaBeam 2000 bathymetry of San Antonio canyon at the point where it crosses the forearc slope break. Contour interval is 100 m.
width, becoming narrow at the apex of the three bends and wider in the four straight reaches of the canyon shown in Fig. 5. This high backscatter material is not homogeneous, and it shows a fabric of longitudinal “streaks” and occasional blockiness. We believe that this high backscatter material is a sedimentary deposit on the floor of the canyon, and that the observed fabric is caused by current or sediment flow. Erosion or collapse of the canyon walls is suspected in at least three places along the surveyed portion of the canyon (e.g., profiles E, H, and M), and slumped material may be responsible for the blocky side-scan fabric occasionally observed on the canyon floor as a dark block, elongated parallel to the canyon axis surrounded by low (lighter shade) backscatter imagery (i.e., Fig. 5b north of 33”2O’S). Near 33”23’S, 72”12’W, an area of low backscatter and low bathymetric relief on the canyon floor is interpreted to be a point bar existing on the inside of the sharp canyon bend (Fig. 5). This bar is about 80 m high, 600 m wide, and extends about 5000 m in a curved manner from the sharp bend, suggesting it is part
of the canyon dynamics and not a terrace. Other point bars may exist at other bends but they are less apparent in our data set.
4. Discussion San Antonio Canyon is distinctive, among other active margin canyons that have been wellsurveyed, in having a broad (up to 5 km wide) roughly U-shaped cross-section versus the more commonly observed narrow V-shaped crosssection. However it is similar to the lower portions of the Monterey Canyon, which has a similar flatfloored valley cross-section up to 3 km wide (Greene et al., 1989). The broad roughly U-shaped floor of San Antonio Canyon may be caused by turbidite and debris deposits covering the canyon floor, as proposed by Greene et al. (1989) for the lower Monterey Canyon. This is consistent with the side-scan data showing relative high backscatter signal along the floor of San Antonio Canyon, as well as with the evidence (discussed below) of a frequent supply of sediment to the head of the
R A. Hagen et aLlMarine Geology 129 (1996) 197-20s -72” 10
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Fig. 5. (a) SeaBeam 2000 bathymetry of the central portion of San Antonio Canyon. Contour interval is 100 m. (b) SeaBeam 2000 side-scan sonar data for the same area (Chit resolution). Dark shades represent low-backscatter areas; light shades represent high-backscatter areas. The canyon floor displays relatively high-backscatter and longitudinal flow fabric. Lowerbackscatter areas along the canyon floor appear to be point bars, with one clear example in an inside bend of the canyon near the center of the figure at 33”23’S, 72”12’W. Note some angular sharp contrast features near sharp canyon turns (which also correspond to ship turns) are data artifacts, e.g. near 33”28’S, 72”08’W.
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canyon. The interpretation of point bars and a longitudinal fabric in the side-scan data reminiscent of channel braiding (Belderson et al., 1984) lead us to suggest that the material displaying the relatively strong backscatter pattern on the canyon floor is fairly coarse sediment. Although the canyon may be partially fed by intermittent esteros (seasonally active drainage gullies) that extend into San Antonio harbor, most of the sediment flowing down the canyon to the trench axis is apparently derived from the Rio Maipo, about 5 km south of San Antonio. In their paper, de1 Canto and Paskoff (1983) present an aerial photograph (Fotografia 8) which shows a large cloud of sediment-laden water being discharged from the mouth of the Rio Maipo and swept along the coast to the north by currents. They also observe that dark volcanic sediments are washed up on Cartagena beach about 15 km north of the Rio Maipo after such an event. The Rio Maipo experiences two periods of peak discharge every year. The maximum average peak discharge is in August at the peak of the winter rainy season. A second, smaller, peak discharge occurs in January during the summer thaw in the Andes (Empresa National de Electricidad, 1968). Bornhold et al. (1994) found a high correlation between periods of peak river discharge and turbidity current events in a study in British Columbia, Canada. They favor a model in which high river discharge events erode sediments that were deposited in the river channel and delta during low discharge periods, sweeping the sediment out onto the delta front and initiating turbidity currents. We believe that a similar (but less direct) mechanism may cause turbidity currents in the San Antonio canyon, such that sediment flushed into the ocean by high discharge events from the Rio Maipo encounters strong longshore currents that sweep the fine grained sediment northward along the coast to the head of the canyon, as observed by de1 Canto and Paskoff (1983). Presumably, the coarser grain sediment along with the settling finer sediments would be trapped at the head of the canyon and serve as the source for episodic turbidity currents to flow down the San Antonio Canyon. The mechanism may be similar to the Bomhold et al. (1994) driving mechanism, although other
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R A. Hagen et al. IMarine Geology I29 (1996) 197-205
factors may be involved such as the intermediate to large earthquakes that occur near this area associated with the active Chile trench. The remarkably large width of the canyon with respect to its cross-sectional depth may be caused by the apparent resistance to downcutting erosion at the forearc slope break. As Fig. 3 shows, the canyon above the slope break has a very constant slope of about 2”, whereas the canyon below the slope break has slopes of up to 6.3”. It is likely that the downcutting of the upper canyon has been arrested for quite some time. This may have allowed the width of the canyon to increase through mass-wasting and bank undercutting, producing the observed broad, U-shaped canyon profile. If this interpretation is correct, it implies a potentially large pulse of (probably coarse) sediment could be deposited rapidly downslope, if the structural obstruction suddenly becomes fully breached by a major earthquake, for example. The shape of the canyon, with its sharp, nearly right-angle bends, might suggest that its path is controlled by faulting. Certainly, as discussed earlier, the long N-S stretch of the canyon is caused by a confining seaward structural high. Whether or not this high is a fault block we are unable to say. There is some evidence in the SeaBeam bathymetry (visible in color shaded-relief views) for linear features trending at 310” at the point where the canyon crosses the slope break. These structures may be faults, but if so, we are uncertain if they played any role in canyon formation.
scan sonar data suggest the existence of at least one point bar on the inside of a channel a bend. The course of the canyon is deflected to the north by a prominent forearc structural high. Where the canyon breaches the structural high, the basement material is resistant to erosion, resulting in an almost constant canyon slope of 2” above the structural high. This resistance to down-cutting erosion may have led to increased lateral erosion, resulting in the unusual broad U-shaped (versus the more common V-shaped) canyon crosssectional profile.
Acknowledgements We wish to thank Captain Eric Buck and the crew of the R/V Melville for their assistance in collecting these data during expeditions GLOR06MV and GLOR07MV. Their superior ship handling skills amidst hordes of small fishing boats made this study possible. We appreciate the logistical support from Bob Knox and Rose Dufour, and data processing efforts by Uta Albright and Stu Smith, all at Scripps Institution of Oceanography. Most of the figures were generated using the GMT software (Wessel and Smith, 199 1). We thank Zhengrong J. Liu and Yoav Rappaport for assistance at sea and at USF. We also thank the government of Chile for permission to collect these data. This work was partially funded through NSF grants OCE9116012 and OCE9302802, and a USF Research Council Grant. This is a contribution of the Alfred Wegener Institute for Polar and Marine Research.
5. Conclusions San Antonio canyon is over 170 km long, extending from the coast, near the fishing village of San Antonio, to the Chile trench axis. The canyon has a sinuosity of 1.25, and has a broad, roughly U-shaped cross-section along much of its length. The canyon is apparently fed by sediment carried into the ocean by the Rio Maipo during periods of high river discharge. The high backscatter deposit on the floor of the canyon with a longitudinal fabric reminiscent of stream braiding is interpreted to be coarse sediments. The side-
References Barazangi, M. and Isacks, B.L., 1976. Spatial distribution of earthquakes and subduction of the Nazca plate beneath South America. Geology, 4: 686-692. Barnard, W.D., 1978. The Washington continental slope: Quatemary tectonics and sedimentation. Mar. Geol., 27: 799114. Belderson, R.H., Kenyon, N.H., Stride, A.H., Pelton, C.D., 1984. A “braided” distributary system on the Orinico deepsea fan. Mar. Geol., 56: 195-206. Bomhold, B.D., Ren, P. and Prior, D.B., 1994. High-frequency
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turbidity currents in British Columbia fjords. Geo-Mar. Lett., 14: 238-243. Corvalan, J. and Munizaga, S., 1972. Edades radiomtrices de rotas intrusivas y metamorhcas de la Hoja Valparaiso-San Antonio, Inst. Invest. Geol. Bol., 28, 4Opp. Del Canto, S. and Paskoff, R., 1983. Caracteristicas y evolution geomorfologica actual de algunas playas de Chile central entre Valparaiso y San Antonio, V Region. Rev. Geogr. Notre Grande, 10: 3145. Empresa National de Electricidad S.A., 1968. Anuario Hidrologico de Chile, Division de Hidrologia de ENDESA (Empresa National de Electricidad S.A.). Santiago de Chile, 3 17 pp. Fisher, R.L. and Raitt, R.W., 1962. Topography and structure of the Peru-Chile Trench. Deep-Sea Res., 9: 423-443. Gnibidenko, H.S. and Svarichevskaya, L.V., 1984. The submarine canyons of Kamchatka. Mar. Geol., 54: 277-307. Greene, H.G., Stubblefield, W.L. and Theberge, A.H., Jr., 1989. Geology of the Monterey Submarine Canyon system and adjacent areas, offshore central California. U.S. Geol. Surv., Open-File Rep., 89-221, 30 pp. Hagen, R.A., Bergersen, D.D., Moberly, R. and Coulboum, W.T., 1994. Morphology of a large meandering submarine canyon system on the Pert-Chile forearc. Mar. Geol., 119: 7-38. Handschumacher, D.W., 1976. Post Eocene plate tectonics of the eastern Pacific. In: The Geophysics of the Pacific Ocean Basin and its Margin. Am. Geophys. Union Monogr., 19: 1177202. James, D.E., 1971. Plate tectonic model for the evolution of the central Andes. Bull. Geol. Sot. Am., 82: 3325-3346. Jordan, T.E., Hacks, B.L., Alhnendinger, R.W., Brewer, J.A., Ramos, V.A. and Ando, C.J., 1983. Andean tectonics related
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to geometry of subducted Nazca plate. Geol. Sot. Am. Bull., 94: 341-361. Klaus, A. and Taylor, B., 1991. Submarine canyon development in the Izt-Bonin forearc: A SeaMARC II and seismic survey of Aoga Shima Canyon. Mar. Geophys. Res., 13: 131-152. Moore, G.F., Curray, J.R. and Emmel, F.J., 1982. Sedimentation in the Sunda Trench and forearc region. In: J.K. Leggett (Editor), Trench-forearc Geology. Blackwell, Oxford, pp. 245-258. Paskoff, R., 1967. Recent state of investigations of Quatemary sea level along the Chilean coast between lat. 30”~33”S. J. Geosci., Osaka City Univ., 10: 107-113. Scholl, D.W., Christensen, M.N., Von Huene, R. and Marlow, M.S., 1970. Pert-Chile Trench sediments and sea-floor spreading, Bull. Geol. Sot. Am., 81: 1339-1360. Schweller, W.J., Kulm, L.D. and Prince, R.A., 1981. Tectonics, structure, and sedimentary framework of the Peru-Chile trench. Mem. Geol. Sot. Am., 154: 323-349. Shepard, F.P. and Dill, R.F., 1966. Submarine canyons and other sea-valleys. Rand McNally, Chicago, 381 pp. Soh, W., Tokuyama, H., Fujioka, K., Kato, S. and Taira, A., 1990. Morphology and development of a deep-sea meandering canyon (Boso Canyon) on an active plate margin, Sagami Trough, Japan. Mar. Geol., 91: 227-241. Thomburg, T.M., 1984. Sedimentation in the Chile Trench. Ph.D. Dissert., Oregon State Univ., 182 pp. Thomburg, T.M., Kulm, L.D. and Hussong, D.M., 1990. Submarine fan development in the southern Chile Trench: A dynamic interplay of tectonics and sedimentation. Bull. Geol. Sot. Am., 102: 165881680. Wessel, P. and Smith, W.H.F., 1991. Free software helps map and display data. Eos, Trans. Am. Geophys. Union. 72: 44l4.46. Ziegler, J.M., Atheam, W.D. and Small, H., 1957. Profiles across the Pert-Chile Trench. Deep-Sea Res., 4: 238-249.
Marine Geology 129 (1996) 197-205
Letter Section
Morphology
of San Antonio submarine canyon on the central Chile forearc
Rick A. Hagen a, Her&m Vergara b, David F. Naar ‘J a Alfred Wegener Institute for Polar and Marine Research, Postfach 120161,27515,
Bremerhaven, Germany b Instituto de Oceanologia, Universidad de Valparaiso, Casilla 13-D. Viiia de1 Mar, Chile ’ Department of Marine Science, University of South Florida, 140 Seventh Ave. South, St. Petersburg, FL 33701, USA
Received 20 March 1995; revision accepted 24 August 1995
Abstract
A multibeam survey was conducted over San Antonio submarine canyon, near Valparaiso, Chile, in April and May 1993 using the SeaBeam 2000 system on the R/V Melville. The bathymetric data from this survey reveal a canyon with an overall sinuosity of 1.25, a broad, roughly U-shaped cross-section along most of its length, and an almost constant channel slope above the forearc structural high. The course of the canyon is deflected to the north by a prominent structural high opposite the town of San Antonio. SeaBeam 2000 side-scan sonar data reveal high backscatter material on the floor of the canyon with a longitudinal fabric, reminiscent of stream braiding, and point bars formed on the inside of channel bends. We interpret this high backscatter material to be coarse sediment, transported down the canyon as turbidity currents. The source of these turbidity currents is probably the Rio Maipo, which enters the ocean near the head of the canyon.
1. Introduction Submarine canyons along the coast of Chile were discovered by early cable-laying ships, and have been surveyed by several marine research expeditions (Ziegler et al., 1957; Shepard and Dill, 1966; Fisher and Raitt, 1968; Schweller et al., 1981; Thornburg, 1985; Hagen et al., 1994). Although submarine canyons are known to be common features on active margins (Barnard, and 1978; Moore et al., 1982; Gnibidenko 1 Corresponding author 0025-3227/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDZ 0025-3227(95)00116-6
Svarichevskaya, 1984; Klaus and Taylor, 1991; Thornburg et al., 1990; Soh et al., 1990), to this point only a few have been mapped and studied in detail. In this paper we describe the morphology of San Antonio submarine canyon on the Chile forearc near the city of Valparaiso. This canyon, along with portions of the Chile Trench and forearc, was surveyed with the SeaBeam 2000 multibeam bathymetry system on the R/V Melville in April and May 1993 (Gloria Expedition legs 6 and 7). This paper is also one of the first to present the acoustic intensity data (side-scan) recorded by the SeaBeam 2000 system.
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2. Onshore and offshore geomorphology James (1971) postulates that the Farallon plate began to underthrust the North and South American plates no later than the Late Triassic or Early Jurassic. Break-up of the Farallon plate into the Gorda, Cocos, and Nazca plates occurred approximately 26 Ma (Handschumacher, 1976). The Peru-Chile trench is formed by the subduction of the Nazca plate beneath the South American plate. The dip of the Benioff zone segments the Peru-Chile margin along strike into areas with shallow (< 10’) inclination and areas where the zone dips at an angle of approximately 30” (Barazangi and Isacks, 1976). Areas above flat, relatively shallow subducted slabs tend to lack recent volcanism and tend to have mostly compressional neotectonics. Segments above steeply dipping Benioff zones correspond to areas exhibiting well-developed Pliocene and Quaternary volcanism and dominantly extensional recent tectonics (Jordan et al., 1983). The central Chile coast consists of steep headlands composed of igneous and metamorphic rocks. In the Valparaiso region these rocks are part of a coastal batholith whose main components are granodiorite, tonalite and adamelite (Corvalan and Munizaga, 1972). Remnants of marine terraces are present along the coast at 100 to 140 m above the present sea level (Paskoff, 1967). Recent sediment is deposited in small bays and near the mouths of rivers. The Chilean forearc slope has gradients that vary between 3” and 6” (Fisher and Raitt, 1962). In the San Antonio canyon area the forearc slope varies between 3.5 and 4.5”. At present there are few subsurface data in this area. Scholl et al. (1970) presented two seismic reflection profiles in the vicinity of the canyon at 31” and 33”S, and described depositional sequences along the coast. These profiles revealed two layers of sediment overlying crystalline basement. The lower layer of sediment is of variable thickness and is moderately deformed. The upper layer is 1 to 10 m thick and generally undeformed. Small basins on the upper forearc are totally filled by sediment deposits up to 800 m thick. At present, the only samples from the San Antonio Canyon area are two piston cores
from the trench axis (Thornburg, 1985), which were characterized as fan levee deposits.
3. San Antonio Canyon San Antonio Canyon is located just south of the port of Valparaiso, near the small fishing port of San Antonio. The canyon apparently begins in San Antonio harbor itself (based on unpublished charts from the Chilean Hydrographic Service), and extends about 170 km to the northwest, where it enters the trench axis near 33”05’S, 72”45’W (Fig. 1). The canyon has an overall sinuosity of approximately 1.25 (channel length/valley length), making several sharp bends along its route to the trench axis. The canyon exhibits a broad, shallow, U-shaped cross-section along most of its length (Fig. 2). The width of the approximately flat canyon floor (in a cross-sectional sense) varies from 1 km (profile C) in about 1100 m water depth, to about 4 km (profile W) at about 3400 m depth. Below profile W, a cross-section view of the canyon displays a fairly flat and smooth inner valley floor, but the walls of the canyon open out into a broader V-shape on the lower forearc slope versus the roughly U-shape cross-section observed further upslope. Between profiles Q and W there is a fairly long and straight stretch (about 28 km) where the canyon trends at 352”, roughly parallel to the N-S trench axis. The forearc west of the canyon along this stretch is several hundred meters higher than the equivalent area of the forearc north of the canyon. The canyon was apparently deflected north by this barrier before finding a low area and making a 115” counter-clockwise turn to the southwest (245”). This sharp change in the canyon’s trend occurs at a major break in the forearc slope at a depth near 3500 m. It also corresponds to a significant change in the slope of the canyon floor (Fig. 3) from an almost constant slope of 35 m/km (2”), to a downslope gradient of 110 m/km (6.3”) where it then gradually decreases to about 2” at the trench axis. At this sharp canyon bend and increase in slope near -3500 m, a bifurcation of the canyon can be seen in the SeaBeam 2000 bathymetric data
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Fig. 1. SeaBeam 2000 multibeam bathymetry of San Antonio Canyon.
(Fig. 4). An obstruction about 200 m in height (and 2 x 5 km in area) occurs in the center of the canyon. The bathymetric data show that the channel splits and passes to both the north and south sides of the obstruction (Profile AA, Fig. 2). The southern channel is deeper and extends further up the western edge of the canyon axis by curving towards the right (south), thereby creating a shallow step of about 100 m on the floor of the west side of the canyon (profile X, Fig. 2). In addition to multibeam bathymetry, the SeaBeam 2000 system provides acoustic intensity imagery similar to towed side-scan sonar. The
SeaBeam 2000 imagery data have 12 bit resolution stored as 16 bit words, but presently the side-scan data provided to us and displayed are downgraded to 4 bit resolution. Software is being developed by SeaBeam Instruments Inc. to process and display the 12 bit resolution data (D. Caress, personal comm., 1995). However, the SeaBeam 2000 side-scan sonar data (Chit resolution) we have processed and displayed have sufficient resolution to show that the floor of the canyon has a relatively high degree of strong acoustic backscatter with respect to the surrounding seafloor (Fig. 5b). It is important to
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R A. Hagen et al. Jkfarine Geology 129 (1996) 197-205
I San Antonio Canyon Profiles
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Horizontal Distance (km)
Fig. 2. (a) Synthetic profiles across the canyon generated from the SeaBeam 2000 bathymetry. Profiles were created by sampling the SeaBeam data every 2 km along the canyon. The profiles are oriented perpendicular to the canyon axis and consist of about 60 to 120 narrow-beam depth along h 120” swath (depending on depth). (b) Map showing profile locations.
note that the geometry of the canyon and seafloor will influence the acoustic backscatter, but because the system transmits and collects the data at the sea surface and the backscatter in Fig. 5b do not correlate to the morphologic structure (bathyme-
try) of the canyon, it is safe to interpret that the backscatter pattern is indicative of bottom roughness related to sediment type and is not a representation of the surface geometry. The boundary of this high backscatter area is sharp and varies in
R A. Hagen et al./Marine Geology 129 (1996) 197-205
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Distance (km) Fig. 3. Axial profile along San Antonio Canyon showing changes in channel slope with the location of where the profiles in Fig. 2 cross the axial profile.
R. A. Hagen et al./Marine Geology 129 (I 996) 197-205
202 -72"40
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Fig. 4. SeaBeam 2000 bathymetry of San Antonio canyon at the point where it crosses the forearc slope break. Contour interval is 100 m.
width, becoming narrow at the apex of the three bends and wider in the four straight reaches of the canyon shown in Fig. 5. This high backscatter material is not homogeneous, and it shows a fabric of longitudinal “streaks” and occasional blockiness. We believe that this high backscatter material is a sedimentary deposit on the floor of the canyon, and that the observed fabric is caused by current or sediment flow. Erosion or collapse of the canyon walls is suspected in at least three places along the surveyed portion of the canyon (e.g., profiles E, H, and M), and slumped material may be responsible for the blocky side-scan fabric occasionally observed on the canyon floor as a dark block, elongated parallel to the canyon axis surrounded by low (lighter shade) backscatter imagery (i.e., Fig. 5b north of 33”2O’S). Near 33”23’S, 72”12’W, an area of low backscatter and low bathymetric relief on the canyon floor is interpreted to be a point bar existing on the inside of the sharp canyon bend (Fig. 5). This bar is about 80 m high, 600 m wide, and extends about 5000 m in a curved manner from the sharp bend, suggesting it is part
of the canyon dynamics and not a terrace. Other point bars may exist at other bends but they are less apparent in our data set.
4. Discussion San Antonio Canyon is distinctive, among other active margin canyons that have been wellsurveyed, in having a broad (up to 5 km wide) roughly U-shaped cross-section versus the more commonly observed narrow V-shaped crosssection. However it is similar to the lower portions of the Monterey Canyon, which has a similar flatfloored valley cross-section up to 3 km wide (Greene et al., 1989). The broad roughly U-shaped floor of San Antonio Canyon may be caused by turbidite and debris deposits covering the canyon floor, as proposed by Greene et al. (1989) for the lower Monterey Canyon. This is consistent with the side-scan data showing relative high backscatter signal along the floor of San Antonio Canyon, as well as with the evidence (discussed below) of a frequent supply of sediment to the head of the
R A. Hagen et aLlMarine Geology 129 (1996) 197-20s -72” 10
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Fig. 5. (a) SeaBeam 2000 bathymetry of the central portion of San Antonio Canyon. Contour interval is 100 m. (b) SeaBeam 2000 side-scan sonar data for the same area (Chit resolution). Dark shades represent low-backscatter areas; light shades represent high-backscatter areas. The canyon floor displays relatively high-backscatter and longitudinal flow fabric. Lowerbackscatter areas along the canyon floor appear to be point bars, with one clear example in an inside bend of the canyon near the center of the figure at 33”23’S, 72”12’W. Note some angular sharp contrast features near sharp canyon turns (which also correspond to ship turns) are data artifacts, e.g. near 33”28’S, 72”08’W.
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canyon. The interpretation of point bars and a longitudinal fabric in the side-scan data reminiscent of channel braiding (Belderson et al., 1984) lead us to suggest that the material displaying the relatively strong backscatter pattern on the canyon floor is fairly coarse sediment. Although the canyon may be partially fed by intermittent esteros (seasonally active drainage gullies) that extend into San Antonio harbor, most of the sediment flowing down the canyon to the trench axis is apparently derived from the Rio Maipo, about 5 km south of San Antonio. In their paper, de1 Canto and Paskoff (1983) present an aerial photograph (Fotografia 8) which shows a large cloud of sediment-laden water being discharged from the mouth of the Rio Maipo and swept along the coast to the north by currents. They also observe that dark volcanic sediments are washed up on Cartagena beach about 15 km north of the Rio Maipo after such an event. The Rio Maipo experiences two periods of peak discharge every year. The maximum average peak discharge is in August at the peak of the winter rainy season. A second, smaller, peak discharge occurs in January during the summer thaw in the Andes (Empresa National de Electricidad, 1968). Bornhold et al. (1994) found a high correlation between periods of peak river discharge and turbidity current events in a study in British Columbia, Canada. They favor a model in which high river discharge events erode sediments that were deposited in the river channel and delta during low discharge periods, sweeping the sediment out onto the delta front and initiating turbidity currents. We believe that a similar (but less direct) mechanism may cause turbidity currents in the San Antonio canyon, such that sediment flushed into the ocean by high discharge events from the Rio Maipo encounters strong longshore currents that sweep the fine grained sediment northward along the coast to the head of the canyon, as observed by de1 Canto and Paskoff (1983). Presumably, the coarser grain sediment along with the settling finer sediments would be trapped at the head of the canyon and serve as the source for episodic turbidity currents to flow down the San Antonio Canyon. The mechanism may be similar to the Bomhold et al. (1994) driving mechanism, although other
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factors may be involved such as the intermediate to large earthquakes that occur near this area associated with the active Chile trench. The remarkably large width of the canyon with respect to its cross-sectional depth may be caused by the apparent resistance to downcutting erosion at the forearc slope break. As Fig. 3 shows, the canyon above the slope break has a very constant slope of about 2”, whereas the canyon below the slope break has slopes of up to 6.3”. It is likely that the downcutting of the upper canyon has been arrested for quite some time. This may have allowed the width of the canyon to increase through mass-wasting and bank undercutting, producing the observed broad, U-shaped canyon profile. If this interpretation is correct, it implies a potentially large pulse of (probably coarse) sediment could be deposited rapidly downslope, if the structural obstruction suddenly becomes fully breached by a major earthquake, for example. The shape of the canyon, with its sharp, nearly right-angle bends, might suggest that its path is controlled by faulting. Certainly, as discussed earlier, the long N-S stretch of the canyon is caused by a confining seaward structural high. Whether or not this high is a fault block we are unable to say. There is some evidence in the SeaBeam bathymetry (visible in color shaded-relief views) for linear features trending at 310” at the point where the canyon crosses the slope break. These structures may be faults, but if so, we are uncertain if they played any role in canyon formation.
scan sonar data suggest the existence of at least one point bar on the inside of a channel a bend. The course of the canyon is deflected to the north by a prominent forearc structural high. Where the canyon breaches the structural high, the basement material is resistant to erosion, resulting in an almost constant canyon slope of 2” above the structural high. This resistance to down-cutting erosion may have led to increased lateral erosion, resulting in the unusual broad U-shaped (versus the more common V-shaped) canyon crosssectional profile.
Acknowledgements We wish to thank Captain Eric Buck and the crew of the R/V Melville for their assistance in collecting these data during expeditions GLOR06MV and GLOR07MV. Their superior ship handling skills amidst hordes of small fishing boats made this study possible. We appreciate the logistical support from Bob Knox and Rose Dufour, and data processing efforts by Uta Albright and Stu Smith, all at Scripps Institution of Oceanography. Most of the figures were generated using the GMT software (Wessel and Smith, 199 1). We thank Zhengrong J. Liu and Yoav Rappaport for assistance at sea and at USF. We also thank the government of Chile for permission to collect these data. This work was partially funded through NSF grants OCE9116012 and OCE9302802, and a USF Research Council Grant. This is a contribution of the Alfred Wegener Institute for Polar and Marine Research.
5. Conclusions San Antonio canyon is over 170 km long, extending from the coast, near the fishing village of San Antonio, to the Chile trench axis. The canyon has a sinuosity of 1.25, and has a broad, roughly U-shaped cross-section along much of its length. The canyon is apparently fed by sediment carried into the ocean by the Rio Maipo during periods of high river discharge. The high backscatter deposit on the floor of the canyon with a longitudinal fabric reminiscent of stream braiding is interpreted to be coarse sediments. The side-
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