Sediment slides and turbidity currents on the ...

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Sediment slides and turbidity currents on the Laurentian Fan: Sidescan sonar investigations near the epicenter of tlhe 1929 Grand Banks earthquake David J. W. Piper Atlantic Geoscience Centre, Geological Survey of Canada, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth Nova Scotia B2Y 4A2 Canada

Alexander N. Shor Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964

John A. Farre,* Suzanne O'Connell Lamont-Doherty Geological Observatory of Columbia University, Palisades, New York 10964 and Department of Geological Sciences, Columbia University

Robert Jacobi Department of Geological Sciences, State University of New York at Buffalo, Amherst, New York 14226 and Lamont-Doherty Geological Observatory of Columbia University ABSTRACT The epicenter area of the 1929 Grand Banks earthquake on the continental slope south of Newfoundland has been investigated using Sea MARC I, a deeply towed, midrange sidescan sonar with a 4.5-kHz subbottom profiler. Shallow slides pass downslope into debris flows on the muddy continental slope east of the epicenter. At the head of the Eastern Valley of the Laurentian Fan, west of the epicenter, arcuate slide scars cut undisturbed upper-slope sediment and lead downslope to a lineated erosional seabed. At a water depth of about 1600 m, this erosional seabed passes into extensive fields of 100-m-wavelength gravel waves situated on the broad, irregular valley floor. The gravel waves become better developed downslope and extend at least to water depths of 3000 m. All these morphological features appear fresh on the sidescan sonograms, suggesting that they date from the 1929 earthquake event, and the distribution of slides corresponds to the area of instantaneous cable breaks in 1929. The upper limit of erosion on valley walls suggests that the 1929 turbidity current was less than 300 m thick. Timing of cable breaks downfan suggests that flow velocities were sufficient to rework gravel deposits into large bedforms during waning flow stages over elevated areas of the valley floor. Similar cross-bedded coarse sands and gravels are common in ancient channel deposits. 58'

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53" INTRODUCTION We have made a Sea MARC I sidescan sonar and high-resolution subbottom survey in water depths of 500-3000 m around the epicenter of the 1929 Grand Banks earthquake (Doxsee, 1948; Stewart, 1979), on the eastern Canadian continental slope above the Laurentian Fan (Fig. 1). The major physiographic regions studied are, from west to east, the Intervalley Divide between the Western and Eastern fan valleys of the Laurentian Fan (Piper et al., 1984); the Eastern Valley of the Laurentian Fan; and the St. Pierre Slope, which is traversed by a tributary valley termed the St. Pierre Valley (Fig. 2). The Eastern Valley has a central elevated region bounded by a thalweg channel adjacent to each valley wall.

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Figure 1. Location map showing area of survey (box; see Fig. 2), approximate epicenter of 1929 earthquake, location of cable breaks, and core sites containing gravel. 538

In the 1929 earthquake, submarine cables were broken instantaneously within 100 km of the epicenter, presumably because of sediment failure (Heezen and Ewing, 1952; Heezen et al., 1954). Downslope on the Laurentian Fan and northern Sohm Abyssal Plain, cables broke sequentially from 1 to 13 h after the earthquake. These cable breaks are attributed to a turbidity

current that reached a velocity of at least 18 m / s on the upper fan. We describe here surface-sediment instability features in the area around the 1929 earthquake epicenter and interpret the sedimentary processes responsible for them. SIDESCAN SONAR INTERPRETATION Sea MARC I is a deeply towed instrument package containing a 4.5-kHz subbottom profiler and 27- and 30-kHz sidescan sonar transducers producing records with slant-range corrections and swath width of up to 5 km (Kosalos and Chayes, 1983). Navigation was by LORAN C, and Sea MARC I towbody positions were estimated from acoustic ranging with the ship. Sidescan and subbottom profiler records have been used to distinguish nine morphogenetic types of seabed, the distribution of which is described below and illustrated in Figure 2. Intervalley Divide The Intervalley Divide comprises gullied ridges and s purs dissected by a network of sinuous valleys. Areas of flat, undisturbed seabed are rare. Submarine headward erosion through a variety of mass-wasting processes is the mechanism most widely cited for the development of this type of morphology (e.g., Twichell and Robert!;, 1982; Farre et al., 1983). It is presumed that these processes act over considerable periods of time, so that gullied ridge and spur morphology is one that is relatively mature. Eastern Valley The uppermost slope above the Eastern Valley in water depths less than 800 m is covered by undisturbed sediments that are cut by arcu*Present address: Exxon Production Research Co., Houston, Texas 77252. GEOLOGY, v. 13, p. 538-541, August 1985


Figure 2. Surface morphology of study area around epicenter of 1929 earthquake, based on interpretation of Sea MARC I sidescan sonograms and subbottom profiles. Inset shows three main physiographic regions, Sea MARC I tow-vehicle tracks, and location of areas shown in Figures 3, 4, 5, and 6.

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ate slide scars, typically about 1 km across. The seabed immediately below the scars has irregular relief; scattered isolated sidescan targets are interpreted as large blocks. From 1000- to 1600-m depth, the seabed is characterized by downslope-trending lineations (Fig. 3) that have relief of less than 30 m. This upper-slope region is also traversed by several shallow channels (Figs. 2, 3). Below about 1600-m depth, the Eastern Valley floor becomes relatively flat, although there is local irregular relief of up to 50 m. Fields of large bedforms, interpreted as gravel waves, GEOLOGY, August 1985

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first occur near the 1500-m isobath at the ends of the shallow channels crossing the upper slope, and they become more widespread farther downvalley (Fig. 2). These bedforms are discussed in detail below. Sediment waves are not common in the valley margin thalweg channels, where the sediment is less reflective, because there probably is a mud veneer over sand. St. Pierre Slope Much of St. Pierre Slope below 500-m water depth shows surficial sediment failures. Dis-

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sected stratified sediments have bedding-plane slide scarps, regressive rotational slumps (Fig. 4), and thin acoustically transparent debris flows (Fig. 5). St. Pierre Valley shows erosional features similar to those in Eastern Valley and some gravel waves in its distal part. Gravel Waves The sediment waves in the Eastern Valley are highly asymmetrical, typically 2 - 5 m high, 50-100 m in wavelength, the slip slope facing downvalley. Although a few waves are observed in water depths less than 1500 m, the 539


main field (illustrated in Fig. 6) occupies a V-shaped area, which widens from an apex at 2000-m depth to more than 10 km across on our deepest line at 3000 m, more than 50 km farther downvalley. In this main field, waves are best developed on the elevated plateau in the center of the valley (Fig. 2), which rises

about 50 m above the thalwegs near the valley margins. Three cores from the area of sediment waves (Fig. 1) recovered gravel and coarse sand at the surface. The very high acoustic reflectivity also suggests that the bedforms are composed of gravel. Additional new data show that the gravel waves extend farther downvalley

Figure 3. A 5-km-swath sidescan sonogram of head of Eastern Valley. In northeast, undisturbed seabed on upper slope is cut by shallow slide detachment scarps, partly filled by irregular reflective debris. In central area, seabed appears eroded and has downslope lineations and widespread irregular blocks. In southwest, there are gullied ridges and spurs and a flat channel floor in extreme southwest.

to about 4500-m depth where piston cores contain graded gravel and sand up to 6.5 m thick (Stow, 1981; see Fig. 1). The surface of the gravel-wave field is crossed by several long streaks subparallel to the valley trend and with lower reflectivity; cores show that they are composed of sand (Fig. 6). Streaks are subparallel to the valley trend and are typically 100-500 m across and up to 25 km or more in length. The streaks emanate from small channels within the upper part of Eastern Valley and from the St. Pierre Valley. They do not exhibit significant relief nor subbottom reflectors. The streaks are similar in appearance and size to sand ribbons on gravel substrate, such as those illustrated from the English Channel by Belderson et al. (1972). DISCUSSION A N D CONCLUSIONS Age of the S ea-Bed Features Unequivocal evidence as to the age of the sediment slumps and gravel waves is lacking. The slump features appear sharp in sidescan images, the gravel waves have very high reflectivity, and except for the local debris flow shown in Figure 5, no surficial sediment drape is visible either in subbottom profiles or in cores. In contrast, sidescan images from similar slumps on the Scotian Slope (Piper et al., 1985) are muted, subbottom profiles show a 1-2-mthick drape of mud over slump features, and dating of cores shows that these slumps are about 10000 yr old. High-resolution sparker and airgun profiles from the St. Pierre Slope (Piper and Normark, 1982; and unpub. data) show that buried subbottom slumps (in the upper 50-100 m of sediment) are very rare, in contrast to the widespread surface slumping. These data suggest that the widespread slumping resulted from an unusual event in the past few hundred years. Cable breaks indicate that the 1929 earthquake caused sediment failure within 100 km of the epicenter, and the resulting turbidity current achieved high velocities in the Eastern Valley. Thus, the 1929 earthquake could be responsible for all the sediment slumps and gra vel waves seen in the study area.

Figure 4. A 5-km-swath sidescan sonogram of a cross section of St. Pierre Valley. In extreme northeast, seabed appears only slightly disturbed. To west, large area of "wrinkled" or "thumbprint" seabed is interpreted as a regressive slump, similar to those described on land from St. Lawrence Lowlands of Canada by Larochelle et al. (1970) and Mollard (1975, Figs. 11.6,11.7). Floor of St. Pierre Valley is lineated in east and smooth in west. Steep walls of St. Pierre Valley are dissected by gullies. 540

Significance of Gravel Waves Little is known about the flow conditions necessary to form the bedforms observed in the Eastern Valley of the Laurentian Fan. No similar features have been reported in this type of modern setting. "Gravel dunes" of comparable texture and dimension were reported by Winn and Dott (1977) from channel deposits associated with flysch in the Upper Cretaceous Lago Sofia conglomerate of southern Chile. Winn and Dott estimated an overlying flow velocity of 7 - 8 m/s to move the largest clasts of 20-30-cm diameter (after Walker, 1975, using the formulation that mean flow velocity is equal to 15 times friction velocity). GEOLOGY, August 1985 539


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Figure 5. A 1-km-swath sidescan sonogram from 2500-m water depth at eastern margin of Eastern Valley. Gravel waves in west are in sharp juxtaposition with smooth, low-reflectivity seabed in northeast. Subbottom profile (4.5-kHz) shows that smooth seabed is a 10-m-thick debris flow that overlies gravel waves. (Thick line above subbottom profile is artifact produced by outgoing acoustic signal.) T h e f l o w velocity in excess of 18 m / s ( 6 5 k m / h ) e s t i m a t e d for t h e initial p h a s e of t h e turbidity current that w a s generated by the G r a n d Banks earthquake (Heezen and Ewing, 1 9 5 2 ) c o u l d h a v e t r a n s p o r t e d gravel in s u s p e n sion. A s velocity d e c r e a s e d , b e d - l o a d t r a n s p o r t c o u l d h a v e built t h e gravel i n t o w a v e s . A l t h o u g h large-scale b e d f o r m s a r e generally a b sent in t u r b i d i t e s ( A l l e n , 1 9 7 0 ) , single sets a t t h e t o p of m a s s i v e c o a r s e c h a n n e l d e p o s i t s a r e relatively c o m m o n ( P i p e r , 1970; Hiscott a n d M i d d l e t o n , 1 9 7 9 ) a n d a p p e a r t o r e p r e s e n t tract i o n a l r e w o r k i n g a f t e r t h e m a i n p h a s e of sediment deposition. T h e s m a l l a r e a of gullied ridge a n d s p u r m o r p h o l o g y a t t h e crest of t h e high a r e a in t h e s o u t h w e s t p a r t of E a s t e r n Valley (Fig. 2 ) s h o w s n o e v i d e n c e of h a v i n g b e e n e r o d e d b y t h e f l o w , i m p l y i n g a m a x i m u m f l o w thickness of 3 0 0 m in this a r e a . F u r t h e r e v i d e n c e f o r a relatively Figure 6. A 5-km-swath-width sonogram of gravel waves'at 3000-m water depth in Eastern Valley of Laurentian Fan.

thin flow is f o u n d in St. Pierre Valley, w h e r e t h e c h a n g e f r o m lineated valley floor t o gullied valley w a l l o c c u r s a b o u t 150 m a b o v e t h e thalweg depth.

U s i n g a similar a p p r o a c h , w e c a n m a k e velocity e s t i m a t e s f r o m grain-size d i s t r i b u t i o n in gravels r e c o v e r e d in c o r e s f r o m t h e L a u r e n t i a n F a n . T h e m o d a l size of gravel w i t h i n t h e survey a r e a is a b o u t 2 c m , i m p l y i n g a friction velocity of 14 c m / s , c o r r e s p o n d i n g t o a m e a n c u r r e n t velocity of s o m e 4 m / s ( 1 4 k m / h ) f o r b e d - l o a d t r a n s p o r t ( D y e r , 1972; W a l k e r , 1975). T h i s is consistent with simultaneous suspended-load t r a n s p o r t of c o a r s e s a n d . H o w e v e r , t h e thick, s o r t e d a n d g r a d e d b e d s in c o r e s in 4 5 0 0 - m d e p t h ( S t o w , 1 9 8 1 ) in such a relatively distal setting i m p l y high t r a n s p o r t rates resulting f r o m initial t r a n s p o r t in s u s p e n s i o n t h a t w o u l d req u i r e a velocity of a t least 10 m / s ( 3 6 k m / h ) .

GEOLOGY, August 1985

REFERENCES CITED Allen, J.R.L., 1970, The sequence of sedimentary structures in turbidites, with special reference to dunes: Scottish Journal of Geology, v. 6, p. 146-161. Belderson, R.H., Kenyon, N.H., Stride, A.H., and Stubbs, A.R., 1972, Sonographs of the sea floor: New York, Elsevier, 185 p. Doxsee, W.W., 1948, The Grand Banks earthquake of November 18, 1929: Publications of the Dominion Observatory, Canada, v. 7, p. 323-336. Dyer, K.R., 1972, Bed shear stresses and the sedimentation of sandy gravels: Marine Geology, v. 13, p. M 3 1 - M 3 6 . Farre, J.A., McGregor, B.A., Ryan, W.B.F., and Robb, J.M., 1983, Breaching the shelf break: Passage from youthful to mature phase in sub-

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marine canyon evolution: Society of Economic Paleontologists and Mineralogists Special Publication 33, p. 25-39. Heezen, B.C., and Ewing, M., 1952, Turbidity currents and submarine slumps, and the 1929 Grand Banks earthquake: American Journal of Science, v. 250, p. 849-873. Heezen, B.C., Ericson, D.B., and Ewing, M., 1954, Further evidence for a turbidity current following the 1929 Grand Banks earthquake: DeepSea Research, v. 1, p. 193-202. Hiscott, R.N., and Middleton, G.V., 1979, Depositional mechanics of thick-bedded sandstones at the base of a submarine slope, Tourelle Formation (Lower Ordovician), Quebec, Canada: Society of Economic Paleontologists and Mineralogists Special Publication 27, p. 307-326. Kosalos, J.G., and Chayes, D.N., 1983, A portable system for ocean bottom imaging and charting, in O C E A N S 83: Oceanographic Data Systems Symposium, 3rd, Proceedings, p. 1-8. Larochelle, P., Changnon, J.Y., and Lefebvre, G., 1970, Regional geology and landslides in marine clay deposits of eastern Canada: Canadian Geotechnical Journal, v. 7, p. 145. Mollard, J.D., 1975, Landforms and surface materials of Canada: Regina, Saskatchewan, 421 p. Piper, D.J.W., 1970, A Silurian deep sea fan deposit in western Ireland and its bearing on the nature of turbidity currents: Journal of Geology, v. 78, p. 509-522. Piper, D.J.W., and Normark, W.R., 1982, Effects of the 1929 Grand Banks earthquake on the continental slope off Eastern Canada: Geological Survey of Canada Paper 82-IB, p. 147-151. Piper, D.J.W., Stow, D.A.V., and Normark, W.R., 1984, The Laurentian Fan—Sohm Abyssal Plain: Geomarine Letters, v. 3, p. 141-146. Piper, D.J.W., Farre, J.A., and Shor, A.N., 1985. Late Quaternary slumps and debris flows on the Scotian Slope: Geological Society of America Bulletin (in press). Stewart, G.S., 1979, The Grand Banks earthquake of November 18, 1929 and the Bermuda earthquake of March 24, 1978. A comparative study in relation to their intraplate location: E O S (American Geophysical Union Transactions), v. 60, p. 312. Stow, D.A.V., 1981, Laurentian Fan: Morphology, sediments, processes, and growth pattern: American Association of Petroleum Geologists Bulletin, v. 65, p. 375-393. Twichell, D.C., and Roberts, D.G., 1982, Morphology, distribution and development of submarine canyons on the United States Atlantic continental slope between Hudson and Baltimore Canyons: Geology, v. 10, p. 4 0 8 - 4 1 2 . Walker, R.G., 1975, Generalized facies models for resedimented conglomerates of turbidite association: Geological Society of America Bulletin, v. 86, p. 737-748. Winn, R.D., Jr., and Dott, R.H., Jr., 1977, Largescale traction-produced structures in deep-water fan-channel conglomerates in southern Chile: Geology, v. 5, p. 4 1 - 4 4 .

ACKNOWLEDGMENTS Supported by Canada Program of Energy Research and Development. W e thank Keith Manchester, Bill Ryan, Dale Chayes, Roy Sparkes, and the officers and crew of CSS Hudson. Lamont-Doherty Geological Observatory Contribution No. 3832. Manuscript received December 13, 1984 Revised manuscript received May 13, 1985 Manuscript accepted May 23, 1985

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Geology Sediment slides and turbidity currents on the Laurentian Fan: Sidescan sonar investigations near the epicenter of the 1929 Grand Banks earthquake David J. W. Piper, Alexander N. Shor, John A. Farre, Suzanne O'Connell and Robert Jacobi Geology 1985;13;538-541 doi: 10.1130/0091-7613(1985)132.0.CO;2

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