BULLETIN OF MARINE SCIENCE OF THE GULF AND

V01.UME 12 1962 NUMBER 3

BATHYJVIETR Y OF THE STRAITS OF AND THE BAHAMA ISLANDS.

PART 1. NORTHERN STRAITS OF FLORIDA)

ROBERT J. HURLEY, VIOLET B. SIEGLER, AND L. KENNETH FINK, JR. Institute of Marine Science, University of Miami

ABSTRACT A new chart showing the results of recent surveys of bottom topography of the northern portion of the Straits of Florida from 27° 30' N to 24° 30' N is presented. A smoothly graded axial valley which deepens towards the south and west has its head near the northern end of the Straits. The slope of the valley is interrupted by a 60 mile long plain at 462 fathoms. The Florida or western margin is more gently sloping, has a prominent ridge at 200 fathoms and terraces not found on the Bahama side which is everywhere steep and concave.

PREFACE This is the first of a series of papers intended to describe the sub- merged topography of the Florida Straits and the deeper basins and troughs of . The purpose of these presentations is to provide students of this region with a better conception of that most elementary observation for the land geologist, the topography. As the only oceanographic institution in the area, the Institute of Marine Science of the University of Miami has been conducting various in- vestigations in this region for nearly ten years. The charts accompanying the papers in this series are not intended as navigational charts. Rather, it is hoped that they will present, through the critical and free use of unpublished data from profes- sional cartographic agencies as well as of those assembled on a more limited scale by this Institute, a somewhat less pre,cise but morpholo- gically more informative picture of the area. lContribution No. 414 from The Marine Laboratory, University of Miami. 314 Bulletin of Marine Science of the Gulf and Caribbean [12(3)

INTRODUCTION The Straits of Florida is the longest of the several channels or valleys of the Bahamas region. It is of interest for several reasons among which, in particular, is the fact that it is occupied by the . This current, consisting of much of the water of the , sw.eeps this trough with velocities of as high as 4 knots and occasionally even higher, contributing greatly to the difficulties of all oceanographic work here. The geologic history and the significance of this current to the morphology of Florida Straits are as yet unknown. Very little echo sounding of the Florida Straits north of had been done until surveys by the University of Miami were initiated in 1958 (Fig. 1). A small amount of work had been done in this region by the U. S. Coast and Geodetic Survey between 1869 and 1919 by Platt, Sigsbee, Pillsbury and Yates (Anon., 1962), but more modern surveys have been in progress since 1951. Most of the completed surveys are south and west of the charted area. Siegler (1959) made a reconnaissance report on the bathymetry of the area that included a small scale chart and a number of profiles. During the last year a series of cruises of the R/V GERDA of the Institute of Marine Science of the University of Miami have been made in this ar.ea. These cruises have been directed toward check- ing older lines, filling in gaps in coverage and investigating certain features of interest. Navigational errors have often been large and in at least one instance a previously undete.cted echo sounder failure resulted in soundings about 10 percent too deep. With the greater number of sounding lines now available it is probable that the large features ar.e correctly located (PI. 1) within a mile or two and that smaller features may be located with an error of perhaps as much as four miles. Depths shown in the chart are calculated assuming a sound velocity of 4800 feet per second, except northwest of Cay Sal Bank where a large number of corrected depths (averaging about 2 percent deeper) obtained from the U. S. Coast and Geodetic Survey were used. No slope corrections have been applied. All of the soundinQ;s by the Institute of Marine Science were made using the Edo Corp. AN IUQN Ie and the Times Facsimile Corp. Precision Depth Recorder. 1962] Hurley, et al.: Bathymetry 315

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FIGURE 1. Index chart of University of Miami sounding lines. 316 Bulletin of Marine Science of the Gulf and Caribbean [12(3)

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FIGURE2. Longitudinal Profile of the northern Straits of Florida. 1962] Hurley, et at.: Bathymetry 317 MAJOR FEATURES The most striking discovery is the existence of a smoothly graded valley on the floor of the Straits. This valley has a gentle gradient (Fig. 2) of about 0.6 fms/nautical mile (also approximately parts per thousand or m/km) as far south as about 25° 30' N where the valley empties onto an "abyssal" plain at the not quite abyssal depth of 462 fms. For about 60 miles this plain has almost no slope until, near Cay Sal Bank, the valley narrows, steepens to about 4.0 fms/n. mile and becomes more irregularly shaped. Preliminary data suggest that this slope continues uninterrupted to the basin of the . The most unsatisfactorily defined part of this valley is its shallow- er northern part. Here it is a few miles wide, irregularly floored, and at a depth very clos.e to 400 fathoms. The significance of this last fact is that the PDR pings every second, or every 400 fathoms, so that an echo from this particular depth is often lost in the following ping. Further difficulty is caused by some small, unsystematic dis- crepancies in maximum depth observed on crossings at the same latitude that are suspected to have been caused by temporal changes in the vertical sounding velocity. In summary, all evidence suggests a continuous grade south from near 27° N, but basin depressions of a few fathoms are possible in the northern part of the channel. It is also possible that there is a divide in the area, and a very gentle grade toward the north occurs in the northern part of the chart area. Further studies will seek to clarify this question with the use of im- proved equipment. Santaren Channel, between Cay Sal Bank and Great Bahama Bank, has a smooth profile. It has steep concave slopes on the sides and a low, gentle rise in the center. The valleys on either side of this rise both slope gently to the north and merge with the plain at grade. The southern part of this channel is not explored. Northwest Providence Channel has an axial channel that slopes to the south and east, the head of which appears near 26° 45' N in the Florida Straits. It has several small tributary gullies cutting into the broad smooth nose north of Great Bahama Bank. The slopes along the side of the valley of the Florida Straits vary considerably in character. In general, the slope on the Florida side of the Straits is more gentle, terraced, and differs more in appearance from place to place. With the exeption of the broad nose extending northeast of the Great Bahama Bank, all the slopes of the Bahamas 318 Bulletin of Marine Science of the Gulf and Caribbean [12(3) including Cay Sal Bank are steep, concave and typically have rough surfaces near the base. As shown in PI. 1, these rough areas, how- ever, are absent in several places. The Florida side of the Straits north of about 26° 25' N is a smooth, straight, ,even slope. Here are located three of the few small gullies found in this entire area. South of 26° 15/ N the upper slope be- comes convex and steeper and a terrace becomes evident. This terrace at depths of 120 to about 200 fathoms is flatter, deeper and rougher near the north, and smoother and more sloping toward the southern and inner part. Off Miami the smooth inner and rough outer parts are separated by a V-shaped channel at the boundary. This channel tr,ends somewhat away from the shore until it reaches a very irregular ridge parallel to the shore, which then becomes its eastern bank. This ridge, or zone of ridges, forms a nearly straight outer boundary of the terrace southward from about 26° 25' N to 250 20' N where the ridge and terrace disappear. This ridge crests at 180 to 200 fathoms and typically appears in profile as one or several nearly rectangular blocks, the tops of which tilt gently sea- ward. The east side of this ridge slopes rapidly downward to a parallel, narrow, V-shaped basin about 20 fathoms deeper than the axial valley described earlier and separated from it by a smooth, asym- metrical rise 50 fathoms high. Southeast of the slope resembles that in the northern end of the Straits except for a smooth but steeper section from about 200 to 300 fathoms. Toward the southwestern part of the charted area, this slope be- comes steeper again, higher and a depression reappears at the base of the slope. Here, however, this depression is drained to the south- west. A small patch of irregular bottom here appears to be analo- gous to the ridge farther north (PI. 1). A shallower terrace is also evident here between 100 and 150 fathoms. A band of very irregular topography with relief of near 100 fathoms crosses the terrace near 80° 30' W. DISCUSSION The large number of echograms in the northern Florida Straits (Fig. 1) permit mapping the distribution of small scale irregularities on the sea floor. The individual "bumps" are too small to be consider- ed separately but areas of their occurence can be mapped. Typically they have relief of less than 10 fathoms and are often steep enough to produce hyperbolic side-echos on echograms (Hoffman, 1957). 1962] Hurley, et al.: Bathymetry 319 On the shelf off Miami, on the terrace off Key Largo, and on the steep upper slopes of the Banks, these irregularities most probably indicate outcrop of rock. Sloping areas of rough topography at the base of steep slopes may indicate talus-like accumulations. The significanc.e of the large area of irregular topography in the northern part of the Florida Straits channel is quite uncertain but it may in- dicate outcrop of bedrock. The majority of the charted area has a very smooth rounded ap- pearance in profile that most probably indicates a cover of sediments overlying bedrock. The processes of transport and accumulation whereby much of this sediment was deposited must, at least in several cases, have been under circumstances not prevalent today since several apparently thick accumulations of sediment are topographi- cally isolated from shallow source areas. Notable examples are the swell north of Great Bahama Bank and the low rise that encloses the depressions at the base of the slope off Miami. The channels and passages of the Bahamas have been attributed to a variety of processes. L. Agassiz (1883, p. 75) and A. Agassiz (1888, p. 75) suggest submarine erosion of the Florida Straits and Blake Plateau by the Gulf Stream. Hess (1933) suggested subaerial erosion of folded sedimentary rocks as the origin of the basins of the Bahamas. Field (1931) and Siegler (1959) have believed that faulting was of great significance in the origin of these troughs. Jn a recent summary of geophysical data, Talwani (1960) concluded that faulting was a probable factor but not unequivocally required by the data. While Pressler (1947, p. 1858) concluded that, "from the configuration of submerged areas . . . . Florida Peninsula is bounded on the east and south by major fault zones", clear evidence of such a fault zone is lacking. The only suggestion of such a fault may be the depression at the base of the slope off Miami and this feature is discontinuous. Since the late Mesozoic and Tertiary sec- tions of shallow water carbonate rocks are similar in depth and thick- ness in south Florida (d. Applin, 1959) and in the one deep well in the Bahamas (Spencer 1949, reported in Eardley, 1951, p. 573), more recent large vertical displacements, except by symmetrical dip slip faulting, are unlikely. There is no surviving evidence of a fault trace on the Bahamas side of Florida Straits. Newell (1955) suggested that the Bahama Banks have been built up by sedimentary accretion on a sinking basement. The valleys 320 Bulletin of Marine Science of the Gulf and Caribbean [12(3) thus are regions of less active deposition which may result from submarine transport of sediment along the axial valleys. If, then, the channels have persisted for a long time, the time correlatives of the largely neritic rocks of Florida and the Bahama Banks will be found in and beneath the Florida Straits and Bahama Channels con- taining evidence of deeper water and more abundent pelagic organ- isms. The difference in appearance of the slopes of the Florida side of the Straits may reflect the longshore contribution of sediment from the north (Vaughan, 1910, p. 161), not available in the Bahama Banks region.

ACKNOWLEDGEMENTS The assistance and guidance of Prof. F. F. Koczy is gratefully acknowledged. This work was supported by the Office of Naval Re- search under contract Nom 840 (01).

REFERENCES

AGASSIZ, A. 1888. Three cruises of the United States Coast and Geodetic Survey steamer "Blake", vol. I. Bull. Mus. compo Zool. Harv., 14: xxii + 314 pp., 194 figs.

AGASSIZ, L. 1869. Report upon deep-sea dredgings in the Gulf Stream, during the third cruise of the U. S. Steamer Bibb, addressed to Professor Benjamin Pierce, SuperiJ;tendent U. S. Coast Survey. Bull. Mus. compo Zool. Harv., 1 (13): 363-386.

ANONYMOUS 1962. U. S. Coast & Geodetic Survey index to Hydrographic Surveys Sheets 1002 & 1007.

ApPLIN, P. L. 1951. Preliminary report on buried Pre-Mesozoic rocks in Florida and adjacent states. Circ. U. S. geo\. Surv., No. 91: 1-28.

EARDLEY, A. 1. 1951. Structural Geology of North America. Harper & Bros., New York, 624 pp.

FIELD, R. M. 1931. Geology of the Bahamas. Bull. geo\. Soc. Amer., 42: 759-784. HESS, H. H 1933. Interpretation of geological and geophysical observations, in Navy Princeton Gravity Expedition to the West Indies in 1932. U. S. Navy Hydro. Office, pp. 27-53. 1962] Hurley, et al.: Bathymetry 321

HOFFMAN, J. 1957. Hyperbolic curves applied to echo sounding. International hydro Rev., November, J957, 13 pp.

NEWELL, N. D. 1955. Bahamian Platforms. Spec. Pap. geo!. Soc. Amer., 62: 303-316.

PRESSLER, E. D. 1947. Geology and occurrence of oil in Florida. Bull. Amer. Ass. Petrol. Geo!.,31 (1O):1851-J862.

SIEGLER, V. B. 1959. Reconnaissance survey of the bathymetry of the Straits of Florida. Marine Lab., Univ. Miami, Technical Report 59-3: 9 pp. (mimeo- graphed) .

TALWANI, M. K. 1960. Gravity anomalies in the Bahamas and their interpretation. Unpub- lished Ph.D. Thesis, Geology, 1960, Columbia University, 89 pp.

VAUGHAN, T. W. 1910. A Contribution to the geologic history of the Floridian Plateau. Pap. Tortugas Lab., 4: 99-185, 15 pis.