JAMES E. ANDREWS Department of , University of Hawaii, Honolulu, Hawaii 96822 FRANCIS P. SHEPARD Geological Research Division, University of California, Scripps Institution of Oceanography, La Jolla, California 92037 ROBERT J. HURLEY Institute of Marine and Atmospheric Sciences, University of Miami, Miami, Florida

Great Bahama Canyon

ABSTRACT Recent surveys and sampling of the V-shaped rock, rounded cobbles, and boulders along their canyon that cuts into parts of the broad troughs axes, as well as ripple-marked sand to indicate the separating the Bahama Banks have given a greatly importance of currents moving along the canyon improved picture of this gigantic valley and the floor. Further evidence that erosion has at least processes operating to shape it. The canyon has kept the valleys open as the Bahama Banks grew two major branches, one following Northwest comes from the winding courses and the numerous Providence Channel and the other the Tongue of tributaries that descend the walls from the shallow the , which join 15 mi north of New Provi- Banks, particularly on the south side of Northwest dence Island, and continue seaward as a submarine Branch. The possibility that solution has canyon with walls almost 3 mi high. These, so lar been important comes from the finding of more as we know, are the world's highest canyon walls depressions along Northwest Branch than in other (either submarine or subaenal), and the canyon submarine canyons of the world, and the discovery length, including the branch in Northwest Provi- ol caverns along the walls by observers during deep dence Channel, is at least 150 mi, exceeded only by dives into Tongue Branch in the Alvin and two submarine canyons in the Bering Sea. Aluminaut. It seems to us highly probable that the Bottom photographs from the outer portions of modern canyons are due primarily to submarine Northwest Branch and Tongue Branch show wall erosion, partly re-excavating old filled troughs.

INTRODUCTION is joined by another true canyon, which extends in a general southeasterly direction along include 40,000 sq mi of shallow- Northwest Providence Channel. The latter we water carbonate banks, built on a submerging are calling Northwest Branch. Beyond the continental borderland east of the Florida juncture, the combined canyon continues Peninsula. The Banks are partly separated by northeastward between and Great broad-floored troughs similar to the troughs , where it has the highest canyon that lie between the Bahamas and both walls of any in the world. The total length of and Florida (Fig. 1). While these troughs are the canyon in Northwest Providence Channel, distinctly different from the winding V-shapcd added to the seaward continuation of the two submarine canyons that cut most continental branches to the point where the canyon be- slopes, two troughs, Tongue of the Ocean and comes a fan-valley, is 150 mi, which is exceeded Northwest Providence Channel, have features only by two canyons in the Bering Sea2 incised into them that have true canyon char- (Shepard and Dill, 1966, Appendix). acteristics.1 In the northern portion of Tongue Many authors have discussed the origin of of the Ocean, Athearn (1962) has shown that the troughs in the Bahama Banks, but little the broad trough changes to a V-shaped valley attempt has been made to distinguish between that deepens to the north and develops canyon the troughs and the canyon-like valleys that proportions as it passes Island are cut into parts of them. This failure has been (Fig. 2). This is called Tongue Branch. Beyond due in part to the absence of adequate contour the island, the Branch has a large entering charts to distinguish the two types. Hess (1933), tributary, and winds to the northeast where it using a 1932 survey by the U.S. Navy, was the 1 Using definitions of Shepard 2 This excludes several fan-valleys that extend (1965). hundreds of miles. Geological Society of America Bulletin, v. 81, p. 1061-1078, 10 figs., April 1970 1061

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first to call attention to the trellis pattern of and thus forming the banks. He suggested also the troughs, the continuous slope toward the that turbidity currents spread out sediments deep ocean, and to the V-shaped profile of some into the troughs and carried much of the sed- of the valleys. Based on this information, he iment from the troughs out into the deep ocean. suggested that the troughs were the result of Newell (1955) called attention to the steep submergence of river-eroded valleys cut in upper slopes of the troughs, which he explained folded rocks, accounting for the trellis pattern. as the result of upgrowth of coral, rather than Later, Hess (1960) defended the original hy- erosion of the trough. Talwani and others (1960) pothesis but brought it up to date in view of have attributed the troughs mainly to down- the discovery in the Andros Island's 14,000-ft- faulting, basing their contention on the large well of what appears to be horizontal shallow- negative gravity anomalies over the troughs, water deposition. He still explained the trellis which they believe indicate downdropping of pattern by stream erosion in an old series of rock the light upper formations, bringing them in underlying the 14,000 ft of bank sediments. He contact with heavier, deeper sections on the thought that the old drainage pattern had been unfaulted banks. Hess (1960) noted that fault- drowned by rapid submergence, and that later ing does not appear to be consistent with the slow submergence allowed the upgrowth of the trellis pattern, nor does it agree with the lack reefs on the valley walls, which preserved the of seismicity in the bank area. Many other broad features of the drainage pattern by con- reports have discussed sediments obtained in fining deposition to the area behind the reefs the Bahama troughs. These indicate that there 80

Figure 1. Index map showing the location of the two branches of Great Bahama Canyon and their relations to the Bahama Islands and Banks. The canyon axes are given as dashed lines, and the Bank margins by dotted lines.

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are two principal types consisting of the deep- on the walls where some details are better water fine-grained pelagic sediments and the developed by the Athearn survey, and these coarse-grained materials that are usually give his map considerable value. explained as turbidites. Our own sounding lines used in constructing The latest developments in the trough portions of the chart were obtained during investigation have included deep dives made by cruises on the Miami Institute of Marine the Woods Hole Alvin (Gibson and Schlee, Sciences' ships Gerda and Pillsbury, and on the 1967; Busby and Menfield, 1967) and the Scripps Institution of Oceanography's ship Aluminaut (Markel, 1968). The most interest- Thomas Washington. Some of the surveys in- ing result of these dives has been the rocks volved precision navigation with a Decca Mk obtained by Gibson and Schlee from the steep XII, but most of the work in the canyon area wall of Tongue of the Ocean near New Prov- had positions based on radar, which is not idence Island. These range in age from Miocene nearly as accurate. to Recent, and all contain faunas indicative of It will be noted that our contour chart has deep-water deposition, in contrast to the con- one area where the contours are given with tinuous shallow-water formations shown in the dashed lines. Here, the positions obtained in deep Bahama borings. This discovery appears to our own surveys were not considered very prove that the Tongue of the Ocean was a accurate, and the lines were much wider deep-water trough back at least as far as the spaced than the Navy soundings in other areas. Miocene. The dives also provided evidence of One blank area in Northwest Providence Chan- sliding of material down the slopes throughout nel was not contoured because some error this period, followed by lithification of the appears to have been made in the navigation slumped material and incorporation into the control, which made the soundings very deep-water formations. questionable. The 50-fathom contour is based This report is an attempt to synthesize largely on earlier surveys by the U. S. Navy, earlier information and to introduce some new since the precisely developed positions for the evidence that \\ ill bear on the hypotheses. We 1962 and 1963 soundings did not extend land- have coordinated various surveys to produce a ward to such shoal depths. Some contours were contour chart of the entire canyon, obtained omitted between 50 and 250 fms, or even 500 new information from dredging the walls of fms, because detailed sounding lines were not the canyon, and have taken many photo- available in these intervals. graphs that reveal hitherto unknown features from the deep floor and walls. TONGUE BRANCH The deep Tongue of the Ocean extends for BASIS FOR CONTOUR CHART more than 100 mi along the east side of Andros In constructing our chart (Fig. 2) we were Island. The southern terminus, well south of fortunate in having received copies of the orig- Andros, is in a flat-floored amphitheatre 40 mi inal survey sheets of almost the entire Great across, with bordering steep slopes that rise Bahama Canyon from the U. S. Oceanographic to reefs and calcareous sand shoals. All but the Office. These were made by the U. S. S. Prevail northern part of the Tongue of the Ocean is during 1962 and 1963, with line spacing of trough-shaped with steep walls and a flat floor 0.5 mi and precision locations obtained by that slopes to the north from about 700 fms to LORAC. In some areas, we used our own sound- 900 fms (1300 to 1650 m) in the first 70 mi with ings to supplement those of the Navy. We a gradual steepening gradient. Farther north, would have made use of the survey by Athearn two minor valleys that represent the head of (1962), which covered a considerable part Tongue Branch coalesce at 950 fms (1750 m) of the Tongue of the Ocean, but his sound- (Fig. 2), and the floor begins to develop a V- ings (not available to us) are corrected to shape, so it could be called a canyon (Fig. 3, account for the variation of sound velocity with sec. 7). At 1100 fms (2000 m) the trough-shape depth, and hence would not coordinate with has disappeared, and from there north the V- the Navy's uncorrected echo soundings, nor shape becomes still more pronounced (see Fig. with our own unless completely refigured. 3, sees. 8 and 9). Also, the Navy lines were spaced much closer About 35 mi from its head, Tongue Branch than those used by Athearn, except along por- has a gradient of 6 fms/mi. The gradient in- tions of the upper slopes. As a result, our sound- creases until it reaches a maximum of 29 fms/mi ing coverage is more complete, except locally in the zone between Calabash Kay on Andros

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and Clifton Point on New Providence. Here, island, the winding axis trends east-northeast, measuring from the lips, the canyon is 20 mi parallel to New Providence Island. The gradient across and 1350 fms (2480 m) deep. North of decreases north of the island to about 16 fims/ New Providence Island, the canyon swings mi, and then to 9 until it approaches the junc- gradually to the right, and 7 mi north of the ture with Northwest Branch; here it drops 76

Figure 3. Transverse profiles of Great Bahama Canyon and Tongue of the Ocean Trough. Profile 7 shows the slight indentation in the Trough at the head of Tongue Branch. Profile 12 shows the point of maximum wall height of the combined canyon. So far as we know, these walls rise higher than those of any other canyon, either in the sea or on land. Profile 13 shows the fan at the base of the canyon on the right side. Vertical exaggeration x 14.

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fms in 1.5 mi (a gradient of about 50 fms/ nautical mi) to the point where it joins the Northwest Branch. Along the entire length of the Tongue of the Ocean, the walls are relatively steep, with gradients of 90 fms/mi on the lower slopes, 1 increasing to near vertical in the upper 100 fms Z (Athearn, 1962). Vertical cliffs were seen during the Alvin dives, and samples of the cliff rock were found to be deep-water con- taining displaced shallow-water Foraminifera (Gibson and Schlee, 1967). The walls are incised ill •O by numerous small ravines and gullies, which S extend from near the bank edge to the floor (Rusnak and Nesteroff, 1964). Spur and groove structures reported in the marginal reefs (Newell and Rigby, 1957) most probably con- nect to these features. The small-scale hum- mocky topography near the base of the walls, shown in Athearn's (1962) map, apparently represent piles of sediment slumped from the banks, which presumably had neither volume nor impetus to generate turbidity currents on the gentle slopes of the floor. NORTHWEST BRANCH Northwest Branch has two V-shaped heads near the divide between the Florida Straits and Northwest Providence Channel. The O bathymetric map (Fig. 2) shows the heads cut •a 150 fms (275 m) or more below the surrounding e floor. The southernmost tributary drains east- ward with a gradient of 8 fms/mi (Fig. 4) for the first 12 mi, and then, after the entry of a subtributary from the south, bends to the northeast, running in a winding course for 25 mi, where it joins the fan formed at the mouth of the northern tributary at a depth of

850 fms (1550 m). The average gradient in the "-W ** O V 25-mi segment is also 8 fms/mi, but Navy s(A «U soundings indicate that there is a series of so rather shallow basins along the axis, and that the last 8 of the 25 mi has no appreciable gradient. The Navy soundings are recorded <- be (4-o1 Oa from fathograms at 0.5 mi intervals, but they S S may miss the maximum channel depth, and the 10 side echoes might indicate anomalously shallow depths where the canyon is narrow. Therefore, there may be somewhat fewer basin depressions than the soundings indicate. On the other hand, we ran a series of precision depth recorder pro- files approximately 1 mi apart across the Northwest Branch and, using the deepest points on the profiles, we found confirmation of the basins indicated from Navy soundings 5,13 (Fig. 5). The northern tributary runs in an

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easterly direction for 20 mi with a gradient of 37 mi of the southern tributary at the head of 5 fms/mi. Some depressions exist in this por- Northwest Branch is included, the average tion, as in the southern tributary. East of 78° gradient is 13 fms/mi, very close to that of 28' W., the northern tributary appears to be Tongue Branch. As can be seen from the filled, forming a fan, as if there had been deposi- longitudinal profile, the gradient is variable. tion where it ran into the southern tributary. There is appreciable steepening in the last 5 At the juncture, another small valley comes in mi before Northwest Branch joins Tongue from the north. Branch, but not as steep a gradient as where A north-south reflection profile across the Tongue Canyon comes into the juncture. The two canyon heads, between Great Isaac Light steepening in Northwest Branch may be and Freeport on , shows that related to a general narrowing of the canyon the canyons are incised into sediments or at Abaco Knoll. sedimentary rocks (Fig. 6). The profile suggests A series of five prominent tributaries appears successive periods of excavation and filling, east of the two heads along the northern edge followed by rejuvenation. of Great Bahama Bank, and enters the canyon Beyond the juncture, the combined canyon above the juncture with Tongue Branch. Dives follows a curving course, but bends gradually in the submarine Aluminaut to depths of 420 to the southeast, joining the Tongue Branch fms in the region of Great Stirrup Cay revealed after a course of about 78 mi. The gradient in that the tributaries here have flat floors covered this 78 mi (Fig. 4) averages 16 fms/mi, some- with sand, and steep walls (35° to 45°) 50 to 80 what steeper than the 12 fms/mi average for fms (90 to 150 m) high, with narrow ridges Tongue Branch. However, if the additional separating individual valleys (Markel, 1968).

Figure 5. Four basin depressions along Northwest Branch. Based on 13 sounding lines by Hurley and Shepard.

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Numerous ravines, but no major tributaries, impressive below the juncture of the two heads. enter the Northwest Branch from the north — They are indicated by the Navy soundings and the edge of Little Bahama Bank. Grand also in our survey on the Pilhbury (Fig. 5). Ten Bahama Island and Great Abaco Island, in basin depressions are shown in the 50-fm con- combination with the continuous reef front tour interval in the distance of 40 mi. They are between them, may block the movement of also found below the juncture with Tongue sediment from the interior of Little Bahama Branch, but are missing or insignificant in Bank so that erosion of the north side of the Tongue Branch. These basins are particularly canyon is limited. The physiographic difference interesting because they have not been detected between the northern and southern sides is in the detailed surveys oi canyons in other reflected in the slopes of the canyon walls in areas, but are reported by R. F. Dill (1969, the region. Along the "protected" north wall, oral commun.) from deep dives in the canyons the slopes are relatively steep, averaging 80 oft Baja California. fms/mi; while along the south wall, the Twelve mi south of the tip of Great Abaco development of the tributary system and more Island, Abaco Knoll rises to a summit of 855 active erosion have resulted in more gentle fms from the ridge that forms the angle between slopes, averaging 50 fms/mi. At the top of both Northwest Branch and the Northeast Prov- walls, very steep slopes were found next to the idence Channel continuation of the canyon. On reefs. the north, the knoll is joined to Little Bahama In contrast to the U-shape of Tongue of the Bank by a saddle 1250 fms deep. On the other Ocean Trough, most of the transverse profiles three sides, it borders directly on the sections of the canyon are V-shaped (Fig. 3), but differ of the canyon that bend around the knoll. from those ot other submarine canyons in that the walls extend up to the bank or island The Highest Canyon Walls in the World margins virtually all along the length of the Below the juncture of Northwest and Tongue canyon. Branches, Great Bahama Canyon has a gorge The basin depressions found in the southern through Northeast Providence Channel that tributary of Northwest Branch are even more may well be the greatest erosional cut on earth

South North

0 i Naut. Miles Figure 6. Seismic reflection profile across the two tributaries at the head of Northwest Branch. The subsurface layers, indicative of deposition in the trough, are apparently truncated by the southern tributary, and very likely also by the northern. Scale marks represent 0.1 sec of reflection time. Vertical exaggeration about x7.

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(Fig. 3, sees. 11, 12). Between the shoal border- north side of Abaco Knoll, and is bordered on ing northwest Eleuthera and Abaco Knoll, the the south by a steep escarpment, suggesting floor attains 2276 fms (4162 m) and rises faulting. essentially to on the east and to 835 fms (1527 m) on the west, so that the east wall Bottom Photographs Along the Canyons is 13,646 ft (4162 m) high. Ten mi farther During several of the University of Miami downcanyon, another 20-mi-wide section shows expeditions, bottom photographs were taken a bottom depth of 2377 fms (4347 m), and along or near the axis of Great Bahama Canyon. the walls rise nearly to sea level, attaining The two most successful stations, 6 and 9, were heights up to 14,262 ft (4347 m), making them taken, respectively, in Tongue Branch at about almost 3 mi high. This compares with the Grand 1900 fms (3475 m) and in Northwest Branch Canyon, which is 14 mi wide and has wall at about 1830 fms (3347 m). Station 9 was heights of, respectively, 5000 and 6000 ft (1524 notable in that the echo soundings indicated and 1829 m). The Bahama Canyon walls do not that while the photographs were being taken have a steep average slope, but the same is true we were drifting almost directly downaxis of the Grand Canyon. A few miles outside this for the 2-hr duration of the station.3 deep cut, the wall heights decrease rapidly, The pictures suggest that the floors of both and only a shallow fan-valley forms the con- canyons are covered with a veneer of rounded tinuation of the canyon (Andrews, 1970). cobbles and boulders (Fig. 7). It is difficult to However, a series of true submarine canyons determine whether some of the larger rocks are are found along the north slope of Eleuthera displaced boulders fallen from above or out- (see Shepard and Dill, 1966, Fig. 95). These crops. Some rounded rocks are largely buried also terminate in the deep fan at the mouth of in the finer sediment, and others seem to be Great Bahama Canyon. resting on the surface as if they had recently Along the outer section of the canyon, the 3 This is a phenomenon that has been observed by channel is winding and two tributaries enter, Shepard on a number of occasions while operating over one on each side. These tributaries are some- submarine canyons, and appears to be related to the what trough-shaped with amphitheater heads. general dynamics of the currents in the underlying deep The tributary on the west extends up along the valleys.

Figure 7. Rounded cobbles and coarse sediment in the axis of Great Bahama Canyon at 2000 fm depth. Note that these cobbles are lying on a sand surface. Scale is provided by the 35 cm vane attached to compass.

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fallen from above. Interestingly, the rounded ary fades—mud, pellet mud, grapestone, oolite, boulders appear to be narrowly confined to the and coralgal (Purdy and Imbrie, 1964). The axis of the canyon; a short distance up the side shelf lagoon is the area of muds, pellet muds, slopes, the photographs show only angular and grapestone facies. The oolite and coralgal debris. facies both appear on the barrier rim and outer Outcropping of rock is indicated in many of platforms, while the coralgal facies extend along the photographs by the angularity of the blocks the marginal escarpment. with parallel orientation (Fig. 8, A and B). In Fine sediments of aragonite needles ac- some places, the rock wall appears to be either cumulate in limited areas on the banks, mostly vertical or overhanging (Fig. 8, C and D). in the shelf lagoon area west of Andros, and in There can be little doubt that these precipitous the protected areas west of Great Abaco. slopes show the importance of relatively recent During normal periods, the fines produced on erosion along the canyon floor. other parts of the banks are swiftly removed Currents are revealed on the floor of the by wave and current action. During periods canyons by ripple marks, streaks, banking ol of storm-wave activity, they are also lost from sediment against boulders, and excavation of the more protected areas (Cloud, 1961). This depressions next to boulders (Fig. 9). The material forms most of the fine fractions of the direction of currents is remarkably consistent sediments found in the troughs and canyon, in a downcanyon direction. Although the reaching the floor either slowly through the direction varies from point to point, the varia- , or entrained in sand or mud tion is only what would be expected from a flows down the walls. winding canyon floor. In both stations, the The marginal facies are of primary interest average direction is toward the east, being to canyon studies since these are supplying most somewhat more southeast in Northwest Branch, of the material to the canyon. The coralgal and more directly east in Tongue Branch. At facies is the more prominent of the two in this camera station 4, in the upper portion of North- area, covering about 90 percent of the margins. west Branch, a few pictures again suggest It is characterized by extensive areas of exposed downcanyon current direction. The predom- rock surfaces on which grow sediment-formers, inant movements of currents downcanyon has such as coralline algaes, Millepora, alcyonarians, also been observed in other canyons during and corals (Purdy, 1963b). Oolites are present deep dives in submersibles. The current records around the amphitheater at the head of the now being obtained by Shepard and collabora- Tongue of the Ocean, around the Joulters Cay tors in the canyon heads near La Jolla, Califor- area (north of Andros Island), and on Little nia, also show most but not all of the stronger Bahama Bank (near Mores Island). currents are downcanyon (Shepard and Mar- In the Tongue of the Ocean, the sediment shall, 1969). distribution has been described by Athearn (1962), Rusnak and Nesteroff (1962), Pilkey CANYON SEDIMENTS and Rucker (1966) and by Gibson and Schlee The sediments of the Bahama Banks have (1967). On the flat floor to the south, according been examined by many scientists. The to Rusnak and Nesterofl, 70 percent to 90 principal writings include: Newell and others percent of the sediment was deposited by (1951); Illing (1954); Newell (1955): Newell turbidity currents and slumps, which occur at and Imbne (1955); Lowenstam and Epstein the rate of one every 500 to 10,000 yrs, depend- (1957); Newell and Rigby (1957); Cloud ing on the location in the canyon. Athearn (1962; Imbrie and Purdy (1962); Purdy (1963a, reports that the more rapidly accumulating 1963b); and Purdy and Imbrie (1964). The deposits near the base of the slope result in a studies were made in various sections of the hummocky topography, and the mounds have Great Bahama Bank, but with the similarity coarse-grained reel debris that grade laterally of conditions it is possible to use them as guides into the basin. In Tongue Branch, the transport to the general sedimentary environments of of fines away from the initial slump mounds the banks. occurs along the two tributary channels (shown Topographically, the banks have been better in the 1962 Athearn map), which run divided into four regions: the shelf lagoon, the near the base of the walls. barrier rim, the terraced outer platform, and The coarse deposits are explained by Rusnak the marginal escarpment (Newell, 1955). and Nesteroff (1964) as material coming more Superimposed on these areas are five sediment- or less constantly over the bank edge and ac-

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/4/1061/3442868/i0016-7606-81-4-1061.pdf by guest on 26 September 2021 Figure 9. Current ripples on floor of Great Bahama Canyon near 1900 fm depth. Current moving down- canyon from northwest to southeast shown by compass. Photo taken in axis southwest of Abaco Knoll.

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cumulating at some midpoint on the wall, until came directly downslope from the banks, but they are triggered into a slide when sufficient still traveled a distance of at least 12 mi to sediment has accumulated. They estimate the reach the canyon axis. Beds-of oolite up to 18 rate of accumulation of pelagic sediments in cm thick were sampled at 1000 fms (1829 m) the troughs at 1 to 3 cm/1000 yrs, and suggest along the northern extension of Tongue Branch that much of this material is reworked into (T. Washington box core No. 1), and farther passing turbidity currents, since the thickness east at the junction of the Tongue and North- of pelagic material between dated turbidite west Branches at about 2000 fms (3658 m) layers in the Tongue of the Ocean is less than (G-6514-1 and 2). The nearest known oolite should be found at the relatively high rates of source is 30 mi away along the axis of Tongue deposition. This erosion and reworking should Branch at joulters Cay, north of Andros Island, also encompass turbidite deposits in the deeper but the cored layers are well-sorted oolite parts of the canyon, so that while the interval sands, with very little fine pelagic material or of 500 yrs near the walls may be a measure of reef debris. Other turbidite layers contained incidence of local slumping, the interval of abundant bank fines, along with bryozoans, 10,000 yrs in the steep sections of the canyon coral, shells, Hahmeda, benthic Foraminifera has been lengthened beyond the actual rate by (notably Homotrema), and reworked pelagic the removal and reworking of one or more of Foraminifera and pteropods. The cleanness of the intervening deposits. these layers may be due to the action of normal Fine-grained sediments in the southern bottom currents in the canyon, indicated by section of Tongue Branch contrast with the ripple marks (Fig. 9). Another box core (T. coarser material to the north, where the Washington No. 3, at 2013 fms [3682 m]) taken gradient of the floor increases, and rapid southeast of Abaco Knoll in the Northeast deposition is replaced by some degree of win- continuation, contains a 1- to 2-cm-thick layer nowing (Athearn, 1962). Oolites are found in of nearly pure pteropod tests on top of a rippled the Tongue ol the Ocean only in the cul-de-sac surface. Apparently, rapid burial has pre- of the head where they have a source in the served the ripples, otherwise the normal cur- marginal oolite shoals. Examination of the fine rents might have destroyed them. fractions of sediments by Pilkey and Rucker Fine-grained turbidities are generally found (1966) has shown that they are principally at the surface in the fan-valley continuation bank-derived in both the pelagic and the of the canyon. Carbon-14 dates from these turbidite sections, the having layers indicate a flow activity through the fan been originally deposited on the banks. channel of approximately one flow per 5000 We have not obtained many samples from yrs (Andrews, 1970). This is somewhat in line the canyon, but some free-fall cores and a few with the suggested interval between flows of box cores with undisturbed layers have added about 10,000 yrs reported in the northern sec- inlormation concerning the nature of the floor tion of Tongue Branch (Rusnak and Nesteroff, sediments. These cores show that the canyon 1962). sediment consists primarily of two principal Total sediment accumulation in the southern components: pelagic material forming a gray flat-floored Tongue of the Ocean, according to ooze and containing pteropods, coccoliths, and a seismic profile, is in excess of 1.5 sec of reflec- Foraminifera (mainly Globigerinoides ruber and tion time (approximately 5000 ft [1.5 km]). Orbulma universa]; and coarse material general- No basement reflections were observed on the ly referred to as "turbidites," which has graded record. A single seismic reflection profile shows bedding and consists primarily of material several discontinuities in horizontal reflectors reworked from bank-derived sediments or from suggestive of rotational slump faults. Where sediments of the canyon walls. the Tongue Branch becomes V-shaped, the Gravel-sized fragments of coral and shells sediment thins, and dredging shows that rocks have been sampled in axial cores at two loca- crop out on the lower walls. tions: (1) from the top of the core taken north of the Berry Islands in Northwest Branch (core PHYSIOGRAPHY AND DREDGING ON G-6514-9 at 1600 fms [2926 m]); and (2) ABACO KNOLL beneath a 6-cm layer of ooze m the outermost Bottom photographs taken on the Abaco canyon (core P-6601-2 at 2430 fms [4326 m]). Knoll show steep cliffs with thin horizontal Material in both of these samples probably beds and laminations that are grooved by ero-

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sion (Fig. 10). The less coherent laminated solution cavities and karst topography is dis- beds have been more strongly eroded, under- cussed by Malloy and Hurley (in prep.). Such cutting more resistant beds. Small gullies or a process may well explain the occurrence of chutes run down the slope, crossing both this calcite. resistant and softer beds. The scarps are so Although coarse sediments are seen in photo- straight and steep as to suggest faulting, but graphs taken of ledges around the knoll, the there is little other indication of this origin, cores collected contained only pelagic ooze. and they might have been eroded as ravines or X-ray diffraction analysis of this material and tidal channels during a period of temporary of fines from the canyon floor showed (qualita- emergence. Submarine and erosion tively) a considerably higher content of bank are also possibilities, but the knoll would seem fines (aragonite needles used for index) in the to lack sufficient area to provide the sediment turbidites from the axis than in the pelagic required for such results, and it is far removed material from the knoll. It would appear that from the zone of active sediment production most of the material formed on the banks near sea level. The rocks dredged from the passes seaward along the canyon, rather than knoll at depths at or above 1600 fins (2926 m) being drifted off from the banks to deep water consist of chalks of varying degrees of indura- as fine sediment in suspension. tion and widely differing ages, all coated by manganese oxide up to 1 mm thick. A soft ORIGIN OF GREAT BAHAMA CANYON white porous chalk contains coccohths of The deep troughs with their locally incised Pleistocene age, while a sample of an indurated canyons, which separate the various banks of yellowish chalk contains coccoliths and Fora- the Bahamas, have been explained by two minifera of age (M. N. Bramlette, hypotheses: (1) that they represent very 1966, written commun., L. Lidz, 1967, oral ancient drowned river valleys, which developed commun.). a trellis pattern because they originally were cut One fragment of the Cretaceous chalk was into an old series of folded and eroded forma- cut by a vein of clean, coarsely crystalline, tions, and that this pattern has been preserved low-magnesium calcite. Oxygen isotope analy- by turbidity currents as the banks grew up on sis of samples from the vein gave an average either side (Hess, 1933, 1960; Newell, 1955), deviation of-4.66 per mil 018/016from SMOW or (2) that they are essentially fault grabens (Standard Mean Ocean Water). This composi- between horsts that form the banks (Talwani tion indicates either formation of the crystals and others, 1960). These explanations have in equilibrium with rain water (that is, sub- been made without trying to differentiate aerial formation), or in equilibrium with between the broad-floored troughs and the V-- volcanic waters (post-Cretaceous volcanic shaped canyons that appear to have been incised activity in the region) (Epstein, 1959). Since into them. The explanations are not necessarily there is no evidence of volcanism either in the the same for the two features. Our investiga- geomagnetic data or in the sedimentary record tion has been concerned with the canyons, but in Andros or in Florida drillings, the volcanic we cannot overlook the significance of the hypothesis seems quite unlikely. Four cycles troughs into which the canyons are cut. In of emergence and submergence of the banks fact, our discussion of Tongue of the Ocean in post-Cretaceous time have been postulated shows that the trough grades northward into a on the basis of dolomitization in the Andros canyon. The abundant new information from Deep Test Well (Goodell and Carman, 1969). the intensive bathymetric surveys, seismic This would have provided ample opportunity reflection profiles, bottom sampling, and deep- for lormation of solution cavities and precipita- diving vehicle descents into the branches of the tion of the calcite vein. Another possibility is canyons allows us to make some evaluation of solution and precipitation of the calcite by these hypotheses. The recent discoveries that fresh ground water. At present, in Florida the bear especially on the canyon origin include: Floridian Aquifer apparently outcrops at (1) bottom photographs and box cores show depths of 100 to 170 fms (183 to 311 m) near that currents strong enough to produce ripple Miami (compare with Parker and others, 1955). marks and transport sand-sized material are Kohout (1967) has suggested that the salinity moving down the canyons; (2) the bottom of water in this aquifer is recycled sea salt, due photographs show the presence of abundant to mixing with sea water intruding at deeper rounded cobbles along the canyon floors and depths. The possible submarine production of steep rock walls, indicating active erosion or

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/4/1061/3442868/i0016-7606-81-4-1061.pdf by guest on 26 September 2021 Figure 10. (A) Well-bedded limestones with narrow laminated zones on Abaco Knoll, showing erosional grooves running downslope and undercutting of resistant beds. Depth 1500 fms. (B) Steep slopes and slump phenomena on the north side of Abaco Knoll at about 1400 fathoms. Small pocket to right of compass suggests that block has slumped out from the slope.

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mass wastage and slumping, as do the rock Alvin and the Aluminaut. During the erosion cliffs seen by Gibson and Schlee (1967) in the of the submarine canyons in this limestone, it Alvin dives; (3) the seismic profile across the would be possible to encounter some of these heads of Northwest Branch shows that these caves producing sink-hole types of depressions valleys were cut through older formations, along the axis, which would eventually fill with which are slightly down-bowed as if deposited debris carried by marine currents. Alternative- in a trough; (4) the grooves in the sides, also ly, it may be possible that some of the materials seen in the Alvin dives as well as in our bottom are soluble in sea water. It is evident that we photographs on the side of Abaco Knoll, sug- need much more information concerning these gest that sediment is moving actively down the apparent basins before they can be clearly walls; (5) Mio-Pliocene deep-water formations interpreted. A deep-diving vehicle, such as the in the wall rock of Tongue Branch, collected Alvin, might be able to follow much of the during Schlee's Alvin dives, have shown that canyon axis to look for further evidence. the Tongue of the Ocean is an old valley that The hypothesis of graben origin of the has been partly filled and re-excavated; (6) the troughs is somewhat supported by the gravity precipitous canyons that cut the steep slope anomalies that indicate the presence of low- along the north side of Eleuthera (Shepard density material under the troughs, such as and Dill, 1966, Fig. 95) are further evidence would result from down-dropping of light of the importance of submarine erosion in cut- material (Talwani and others, I960). The bowl- ting deep into the limestone, and they could shaped troughs with straight walls at the head not be related to the Bahama troughs; and (7) of Tongue of the Ocean and in Exuma Sound the finding of basin depressions along the axis also somewhat resemble grabens, and the of Northwest Branch and in the outer com- straight scarp on the north side of Abaco Knoll bined canyon, in contrast to the usual continu- could be a fault scarp. Periodic emergence ous deepening of submarine canyons. related to Cuban orogenesis, as suggested by The basin depressions that are so pronounced Goodell and Garman (1969), provides a along the axis of Northwest Branch and just mechanism for block faulting. On the other below the juncture of the two branches, appear hand, as Hess (1960) has indicated, the trellis to indicate that Great Bahama Canyon has pattern of drainage is not characteristic of fault undergone a different history from other well- valleys. Furthermore, the winding course of sounded submarine canyons where the basins the canyons (Fig. 2) is certainly indicative that are not shown in the surveys. Basins can be these are the result of erosion, as is the trunca- formed by slumping of wall sediment or rock tion of layers shown in the seismic reflection falls that build barriers along the axis; however, profile (Fig. 6). The various bottom photo- other well-known canyons have had the same graphs and observations from the deep dives in influences. It is perhaps hazardous to venture Alvin and Aluminaut support this contention. the suggestion that the difference may be due In addition, there are no records of recent in some way to the presence of soluble limestone seismicity to suggest that faulting along the in the Great Bahama Canyon. So far as we walls is continuing, if it ever occurred. know, no other well-surveyed submarine can- yon has been cut in equally soluble rock. This CONCLUSIONS raises the question of how ground water could Much more work is necessary before we can produce solution cavities that would lead to the make clear interpretations of the origin of the development of these basins. For active ground- gigantic cleft that cuts through the Bahama water circulation, there should be a consider- ridge between Great Abaco and Eleuthera. able hydraulic head, but the low Bahama According to information now available, it Islands could scarcely develop anything of this seems highly probable that this is primarily an sort unless perhaps some time in the past there erosion valley and that the erosion is still con- were episodes of temporary uplift. In any case, tinuing. The idea, long held by Harry Hess, the evidence from the well logs (Goodell and that streams cutting through an old eroded Garman, 1969) indicates that unconformities series of folded rocks account for the trellis underlain by weathered rocks exist in the pattern of the Bahama troughs, still seems stratigraphic column. Also, the drilling logs more reasonable than that the troughs are the suggest that cavities were found at various result of faulting. Following the latest ideas depths (Spencer, 1967), Caverns were seen of Hess, the pattern could have persisted during along the walls during the dives of both the the deposition on the banks of the more than

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14,575 ft (4442 m) of calcareous sediment, if canyon. Perhaps not, but there can be little marine erosion continued to offset deposition doubt that it would be given such a name if it along the old valley system. The large supply were exposed above sea level. Perhaps other of sediment falling from the precipitous margins submarine canyons owe their large vertical of the growing banks should provide turbidity dimensions to such a combined action of build- and other types of bottom currents with the ing on the walls while the valley was preserved cutting tools that account for the canyon by marine processes. This was first suggested excavation and preservation. However, the by Shepard (1934) for the New England valleys appear to have had episodes of partial canyons. filling, accounting for the flat-floored troughs and for the deep-water formations on the walls ACKNOWLEDGMENTS of the Tongue Branch, discovered by Gibson This research was supported by ONR Grant and Schlee. These filling episodes were inter- 4008(02) and Nonr 2216(23), and by NSF spersed by cutting that re-excavated the valleys Grants GB 2462 and GP 3815. The authors in the final episode, but left much of the troughs wish to thank the officers and crews of the with deep fills, as shown by the seismic profiles. research vessels Thomas Washington, John If the preceding interpretations are correct, Elliot Pillsbury, and Gerda for their cooperation the 3-mi-high walls of the lower portion of and assistance. Radiocarbon dates were kindly Great Bahama Canyon do not represent a provided by Gote Ostlund (University of single excavation, such as is usually considered Miami). We are indebted to Edmund Fisher to have been the case for land canyons, but are for his efforts in obtaining bottom photographs. a combination of wall growth with alternating We also appreciate the help in field and labor- fill and excavation. Judging from the 14,575 ft atory work done by Neil F. Marshall (Scripps (4442 m) of limestone deposition that apparent- Institution of Oceanography). We are also ly has occurred since the original trellis devel- grateful for the careful reading and constructive oped, the original valley floor should be found criticism of the earlier versions of this article below even the deepest part of the canyon. This made by D. G. Moore, R. F. Dill, John Schlee, raises the question of whether we should refer to and George Keller. the great notch, with its 3-mi-high walls, as a REFERENCES CITED Andrews, James E., 1970, Structure and sedi- 14th Ann. Mtg., p. 234-235. mentary development of the outer channel of Gibson, T. G., and Schlee, J., 1967, Sediments and the Great Bahama Canyon: Geol. Soc. fossiliferous rocks from the eastern side of the America Bull., v. 81, no. 1, p. 217-226. Tongue of the Ocean, Bahamas: Deep-Sea Athearn, W. D., 1962, Bathymetric and sediment Research, v. 14, p. 691-702. survey of TOTO, Pt. 1-Bathymetry and Goodell, H. G., and Carman, R. K., 1969, Car- sediments: unpub. rept., WHOI, 17 p. bonate geochemistry of Superior Deep Test 1963, Bathymetry of the Straits of Florida Well, Andros Island, Bahamas: Am. Assoc. and the Bahama Islands, Pt. II-Bathymetry Petroleum Geologists Bull., v. 53, no. 3, p. of the Tongue of the Ocean, Bahamas: Gulf 513-536. and Caribbean Marine Sci. Bull., v. 13, no. 3, Hess, H. H., 1933, Interpretation of geological p. 365-377. and geophysical observations in Navy-Prince- Busb?, R. F., and Merrifield, R., 1967, Undersea ton Gravity Expedition to the West Indies in studies with the D. S. R. Alvin, Tongue of the 1932: U. S.' Hydrographic Office, p. 26-38. Ocean, Bahamas: U. S. Navy Oceanographic 1960, The origin of the Tongue of the Ocean Office, I. R. no. 67-51, 54 p. and other great valleys of the Bahama Banks: Cloud, P. E., 1961, Environment of calcium car- 2d Caribbean Geol. Conf. Trans., Puerto Rico bonate deposition west of Andros Island, Univ., p. 160-161. Bahamas: U. S. Geol. Survey Prof. Paper 350, Hurley, R. J., and Shepard, F. P., 1964, Submarine 138 p. canyons in the Bahamas (abs.): Geol. Soc. Epstein, S., 1959, The variations of the 018/016 America 77th Ann. Mtg. Prog., p. 99. ratio in nature and some geologic implications, Illing, L. V., 1954, Bahamian calcareous sands: in Abelson, Ph. H., Editor, Researches in Am. Assoc. Petroleum Geologists Bull., v. 38, Geochemistry: p. 217-240, John Wiley & Sons, no. l,p. 1-95. Inc., New York, 511 p. Imbrie, J., and Purdy, E. G., 1962, Classification of Field, R. M., and Hess, H. H., 1933, A bore hole modern Bahamian carbonate sediments, in in the Bahamas: Am. Geophys. Union Trans., Ham, W. E., Editor, Classification of Car-

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MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 19, 1969 REVISED MANUSCRIPT RECEIVED NOVEMBER 6, 1969 CONTRIBUTION No. 300 FROM THE HAWAII IN- STITUTE OF GEOPHYSICS CONTRIBUTION No. 1133 FROM THE INSTITUTE OF MARINE AND ATMOSPHERIC SCIENCES, UNI- VERSITY OF MIAMI

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