Sediment waves (giant ripples) transverse to the west coast of Mexico

Marine Geo!ogy, 20 (1976) 1--6 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

SEDIMENT WAVES ( G I A N T RIPPLES) T R A N S V E R S E T O T H E WEST COAST OF MEXICO

F. P. SHEPARD, N. F. MARSHALL, P. A. McLOUGHLIN and R. L. FISHER Scripps Institution of Oceanography, University of California, La Jolla, Calif. (U.S.A.) (Received July 28, 1975)

ABSTRACT

Shepard, F. P., Marshall, N. F., McLoughlin, P. A. and Fisher, R. L., 1976. Sediment waves (giant ripples) transverse to the west coast of Mexico. Mar. Geol., 20: 1--6. A 45-kin belt of large symmetrical sediment waves extends along the west coast of Mexico west of Manzanillo between depths of 320 and 770 m. They are thought to be the result of a strong subsurface current that changes seasonally from southeast to northwest.

INTRODUCTION Recent sounding profiles taken on the " R i o Balsas" Expedition along the upper continental slope between Cabo Corrientes and Manzanillo, Mexico, have revealed a 45-km-long zone of sea-floor features t hat record as overlapping hyperbolae. T h e y have an average height of a b o u t 20 m and an average width of 400 m (Figs.l, 2). One continuous line and a series of right-angle traverses were made to establish the trend of these features (Fig. 2A). Earlier profiles in this region have been reviewed. Fisher, on the " C h u b a s c o " Expedition, 1954, also crossed this zone (Fig.3). A n o t h e r crossing was made on the " F . D r a k e " Expedition, 1975. These additional lines have provided f u rth er i n f o r m a t i o n on the width, breadth, and depths of these hyperbolaes (Fig.4). F r o m the fathograms, we have concluded t h a t these features are giant sediment waves or dunes. T he virtual disappearance of the h y p e r b o l a e in the 030 ° course lines indicates t h a t t h e y t r end more or less normal to t he coast of Mexico. The wave field occurs between depths of a p p r o x i m a t e l y 320 and 770 m. In general, the ridges have more relief near their deeper limits. The broadest h y p e r b o l a has a wave length of a b o u t 750 m, while 65 m is the largest amplitude recorded. So far as could be determined, the hyperbol ae are roughly symmetrical. S o m e w h a t comparable hype r bol ae have been recorded in various places in the Atlantic (Ballard, 1966; Fox et al., 1968; K e n y o n and Belderson, 1973; Hollister et al., 1974; Damuth, 1975). These structures are n o t limited depthwise b u t occur on the shelf, continental slope, continental rise, and

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Fig.1. P o r t i o n of a f a t h o g r a m on a s o u t h e a s t run in " R i o Balsas" E x p e d i t i o n along the Mexican coast s h o w i n g the s e d i m e n t waves. See Fig.4 for location.

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Fig.2A. Profile with right-angle course changes in the area with sand waves s h o w n in

Fig.1. Note the sand waves virtually disappear in the legs run at right angles to the coast. even in the deep ocean. T he y were also f o u n d at the base of the slopes north of the island of Kaual. In the Indian Ocean, "giant ripples" were discovered in the Madagascar Basin (Ewing et al., 1968). ORIGIN OF THE SEDIMENT WAVES Until further study has been made of the structures recorded as intersecting hyperbolae off the Mexican coast, we can only speculate concerning their origin. One can conceive that this type of fathogram could be produced by crossing a series of small submarine valleys dissecting the upper slope of the area. However, such an explanation becomes most improbable when one compares the profiles with those taken along numerous continental slopes that are incised with submarine canyons. The latter show no such regularity in width and depth. Major and minor valleys always show great contrast in

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Fig.2B. S h o w i n g t h e small size o f the sand waves near t h e u p p e r d e p t h limit and the d i s a p p e a r a n c e o f t h e waves in t h e transverse lines.

Fig.3. Profile run west f r o m Manzanillo in t h e 1954 " C h u b a s c o " E x p e d i t i o n , giving the d e p t h limits o f the s e d i m e n t waves.

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Fig.4. Map of locations of the lines used to show the sediment waves off the Mexican coast. depth. Furthermore, the profiles that were obtained on the same expedition, both northwest and southeast of the 45 km of hyperbolae, show an almost continuous series of obvious canyons. Examples are included in Fig.5 coming from either side of the area with the hyperbolae. Therefore, we are inclined to believe that the absence of canyons along this 45-km stretch indicates that th ey have been filled with recent sediments, i.e., the smoothness of the b o t t o m is constructional in origin. The alternate hypothesis is that the features are the result of deposition from currents, like the c o n t o u r currents first described by Fox et al. (1968) that sweep along the continental rise of f the East Coast. Most other examples of hyperbolae have been explained as due to currents. The regularity in dimensions and the virtual absence of fill or flat floors in the depressions argues for a dynamic balance of an ongoing process maintaining the "wave train". Such an explanation seems particularly applicable to the area south of Cabo Corrientes {meaning currents) because surface currents have considerable effect on navigation along this coast. The area is also notable for its upwelling, which in turn is f o u n d in areas of the Northern Hemisphere, where the coastal currents are in the proper direction, as off California nort h of Point Conception. A not he r example of sediment waves transverse to the coast is observable f r om the air along the west coast of Florida south of Tampa Bay (Shepard, 1952). Two features of the giant ripples are puzzling. One is the fact that t h e y appear to be confined to a depth range between 320 and 770 m. Perhaps

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the fiat outer shelf has been scoured clean of coarse sediment while the currents are ineffective at the greater depth. The other enigma is that most of the waves are apparently symmetrical. Sand waves due to currents, as for example in bays or over shallow banks (Jordan, 1962}, have decidedly asymmetrical profiles, with the steep side always downcurrent like the lee slope of dunes. Further studies are needed to explain these characteristics. It might be suggested that the symmet~T is the result of deep currents flowing alternately northwest and then southeast along this slope. In fact, studies of the area do show that the surface currents have seasonal changes of this sort (Wyrtki, 1965). It is difficult to explain why these dunelike structures are not found in shoaler water, despite surface currents that flow along the coast. Perhaps there is a stronger current, like the Cromwell Current, that flows only below the surface.

NEEDED INVESTIGATIONS The area is certainly one requiring m o r e investigation. Because it is inside the Mexican 12-mile limit, where investigations require r a t h e r difficult and lengthy n e g o t i a t i o n s for nationals o f o t h e r c o u n t r i e s to o b t a i n permission to operate, it c o u l d best be studied t h r o u g h some c o o p e r a t i v e w o r k with Mexican scientists. D e e p - t o w i n s t r u m e n t a t i o n (Spiess and Mudie, 1 9 7 0 ) w o u l d be helpful in d e t e r m i n i n g if these features are actually s e d i m e n t waves by s h o w i n g the internal s t r u c t u r e o f the deposit. The m e t h o d m a y also establish if s e d i m e n t in this area has filled old s u b m a r i n e c a n y o n s . T h e n a t u r e of the s e d i m e n t c o u l d be d e t e r m i n e d best b y the use o f b o x corers o r p r e f e r a b l y by vibration c o r i n g ( K u d i n o v , 1957). ACKNOWLEDGEMENTS The " R i o Balsas" E x p e d i t i o n was s u p p o r t e d b y National Science F o u n d a t i o n grant D E S 7 4 - 2 2 0 8 9 a n d the Office of Naval R e s e a r c h c o n t r a c t N 0 0 0 14-69A-0200-6049.

REFERENCES Ballard, J. A., 1966. Structure of the lower continental rise hills of the western North Atlantic. Geophysics, 31 (3): 506--523. Damutb, J. E., 1975. Echo character of the western equatorial Atlantic floor and its relationship to the dispersal and distribution of terrigenous sediments. Mar. Geol., 18" 17--45. Ewing, M., Aitken, T. and Eittreim, S., 1968. Giant ripples in the Madagascar Basin. Trans. Am. Geophys. Union, 49 (1). Fox, P. J., Heezen, B. C. and Harian, A. M., 1968. Abyssal anti-dunes. Nature, 220 (5166): 470--472. Hollister, C. D., Flood, R. D., Johnson, D. A., Lonsdale, P. and Southard, J. B., 1974. Abyssal furrows and hyperbolic echo traces on the Bahama Outer Ridge. Geology, 2 (8): 395--400. Jordan, G. F., 1962. Large submarine sand waves. Science, 136 (3519): 839--848. Kenyon, N. H. and Belderson, R. H., 1973. Bed forms of the Mediterranean undercurrent observed with a side-scan sonar. Sediment. Geol., 9: 77--99. Kudinov, E. I., 1957. Vibro-plston core sampler. Tr. Inst. Okeanol. Akad. Nauk U.S.S.R., 25:143--152 (in Russian). Shepard, F. P., 1952. Revised nomenclature for depositional coastal features. Am. Assoc. Pet. Geol. Bull., 36 (10): 1902--1912. Spiess, F. N. and Mudie, J.D., 1970. Small-scale topographic and magnetic features. In: A. E. Maxwell (Editor), The Sea. Wiley-Interscience, New York, N.Y., 4 (part 1): 205--250. Wyrtki, K., 1965. Surface currents of the eastern tropical Pacific Ocean. Inter-Am. Trop. Tuna Comm. Bull., 9 (5): 271--294.