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JAMES E. ANDREWS Department of Oceanography, University of Hawaii, Honolulu, Hawaii 96822 FRANCIS P

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JAMES E. ANDREWS Department of Oceanography, University of Hawaii, Honolulu, Hawaii 96822 FRANCIS P JAMES E. ANDREWS Department of Oceanography, 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 Ocean, 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 limestone 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 The Bahamas 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 Eleuthera and Great broad-floored troughs similar to the troughs Abaco Islands, where it has the highest canyon that lie between the Bahamas and both Cuba 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 New Providence 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 submarine canyon 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 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 1062 ANDREWS AND OTHERS—GREAT BAHAMA CANYON 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. 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 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 TONGUE BRANCH 1063 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.
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