A. CONRAD NEUMANN School of Marine and Atmospheric Sciences, University of Miami, Miami, MAHLON M. BALL Florida 33149 Submersible Observations in the Straits of Florida: Geology and Bottom Currents ABSTRACT An interesting finding of the dives in the Straits of Florida is the observation, based on The submarine slopes chat border the Straits both current measurements and sedimentary of Florida off Miami and Bimini were traversed structures, that the bottom current on the by the submersible Aluminaut, in August and western side of the Straits flows southward at September of 1967. The Bimini escarpment is observed velocities of 2 to 50 cm/sec. This characterized by a three-part zonation con- southerly bottom flow is opposite to either the sisting of relatively strong northerly bottom northerly Florida Current above or to the currents in both deep and shallow zones, and an bottom current on the Bahama side of the intermediate zone of low northerly bottom Straits. The nature and orientation of the sedi- current velocities. From 538 m (the bottom of mentary structures, plus the combined observa- the traverse) to 222 m, there is a sloping, tions of several Aluminaut dives in the same smooth, rock surface veneered with sand, rip- area, indicate that the southward bottom ple-marked by northward bottom currents of counterflow is persistent and not a temporary 50 cm/sec or more. The middle, low velocity tidal reversal. An extensive sedimentary anti- zone, from 222 m to 76 m, exhibits a muddy cline in the west-central sector of the Straits slope of largely bank-derived material. The may have been built by this bottom counter sediment surface exhibits tracks, burrows and current bringing material from the north. mounds, which indicates that currents here are never as strong as in the rippled zones above INTRODUCTION and below. Observed current velocities in the In August and September of 1967, three middle zone were only 5 to 10 cm/sec. Above dives by the DSRV Aluminaut were made in 76 m, a steep, vertical to overhanging cliff with the Straits of Florida off Gun Cay, Bimini, and large talus blocks at its base rises to a crest at Miami (Fig. 1). A later dive was made off 30 m. Currents in the upper zone are north- Miami to 290 m in May, 1969. The 1967 ward at 50 to 150 cm/sec. The inverted situa- Bimini and Miami dives lasted about 10 hrs tion of higher energy bottom current condi- each and traversed the escarpments at the tions and associated sedimentary features and eastern and western margins of the Straits textures existing in the same area, but at a (Figs. 2 and 3). Observations and collections greater depth than low energy surface features were made of rock and sediments, and biologi- and fine sediments, is of significance to the cal and sedimentary processes were noted. Sedi- stratigraphic interpretation of ancient rocks. ment samples were taken with small flap- On the western side of the Straits, at the base fronted scoops. Bottom currents were measured of the Miami Terrace, is an elongate trough by timing the passage of dye or free-floating 825 m deep. The bottom here is characterized detritus past aim bar placed on the bottom by ridges and mounds of muddy sand capped by and oriented parallel with the current. Four to thickets of living deep-water branching coral. eight current measurements were made over a The eastward-facing escarpment of the Miami 10 to 20 minute interval at each locality. Terrace exhibits ledge-like outcroppings of Gravity measurements were made with two dark phosphatic limestone from depths of 719 Worden gravimeters; these data and their m to the crest at 457 m where the traverse geophysical significance have been reported ended. elsewhere (Ball and others, 1968). Geological Society of America Bulletin, v. 81, p. 2861-2874, 9 figs., October 1970 2861 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/10/2861/3432638/i0016-7606-81-10-2861.pdf by guest on 29 September 2021 2862 NEUMANN AND BALL—SUBMERSIBLE OBSERVATIONS IN FLORIDA STRAITS Figure 1. Map of Straits of Florida showing location of 1967 Aluminum dives. The May 1969 dive was at 25°38.6' N./80°01.7' W., or near the "V" in "DIVE" on the left side of the Figure. "D" and "U" refer to the down and up locations, respectively. Among the observations of greatest oceano- Previously published observations of sub- graphic and geologic interest are the following: mersible dives in or near the Straits of Florida In the western Straits off Miami, there exists include a general narrative of an Aluminaut beneath the northward Florida Current a deep dive off Miami in November, 1965 (Neumann counter-flow to the south. On the Bahama side and Hull, 1966), and another in which the value off Bimini, the muddy talus sediments from of the submersible as a scientific instrument is the banktop appear to be restricted to a narrow discussed (Neumann, 1968). Emery and others zone of low current velocity at intermediate (1970) have described the observations of a 4.5 depths and do not accumulate at the base of the km traverse in depths of 32 to 165 m off West slope as the topography suggests. The base of Palm Beach made aboard the submersible Ben the slope exhibits, instead, a smooth, hard, bed- Franklin. A subsequent dive on the Miami rock surface, over which a veneer of rippled Terrace off Miami has been described by sand is moving northward under the influence Ballard and others (in prep.). of relatively strong bottom currents. A variety The physiography of the cliffs bordering the of sedimentary structures usually associated Tongue of the Ocean off Nassau and the nature with shallow water characterizes this thin, deep, of the sediments and rocks exposed there have sand blanket between 215 and 540m (Neumann been described by Gibson and Schlee (1967) and Ball, 1968). Thus, in making stratigraphic from the DSRV Alvin. Submarine lithification paleoenvironmental interpretation of the an- of slope sediments and subsequent fracture and cient counterpart of this sediment distribution, downfaulting were recognized as the processes it would be hazardous to assume that deeper responsible for the fact that the rocks that environments of deposition are usually charac- veneer the slope are younger than those at terized by relatively lower energy conditions. similar depths from the platform interior. The Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/10/2861/3432638/i0016-7606-81-10-2861.pdf by guest on 29 September 2021 OBSERVATIONS 2863 (50-150 cm/sec) 7%'9'" WEST CURRENT /TNNORTH EAST U7 ^, ^ ^ BIMINI SHORT TERRACES ^ ~j Ft. LARGE ROCKS FEW STREAKS NO RIPPLES BURROWS AND MOUNDS TRACKS AND TRAILS I- 1200 RIPPLES STARVED RIPPLES CURRENT CRESCENTS "STREAKS" SAND RIDGES 3-5' HIGH NAUTICAL MILE 25'46'N 79'23'W Figure 2. Schematic profile of submersible dive traverse off Bimini, Bahamas, indicating major observations. Bottom current direction is north. steep to overhanging, cave-studded cliffs of the shows a bimodal distribution of particle sizes upper 200 m off the east coast of Andros Island with peaks in the 250 to 62 micron and 8 to 4 have been described by Busby and others micron ranges. The coarse and medium sand (1966) from the DSRV Perry Cubmarine PC3- fractions are composed largely of planktonic B. pteropod and Foraminifera remains. The presence of calcareous algae, shallow benthonic OBSERVATIONS Foraminifera, and pelecypod fragments indi- cates some contribution from shallow bank- Gun Cay Dive top environments. Sedimentary structures During the dive off Gun Cay, Bahamas (Fig. called "current crescents" (Potter and Petti- 1), the density of suspended organic matter ap- john, 1963) were developed at the bases of peared to be high and increased with depth. obstacles, such as attached sponges and heavy Diving through these suspended aggregates fragments of deep water corals. Fragments of gave the impression of gliding in a snowstorm. Thalassia, a marine grass, were common and Visibility was so reduced that the bottom at drifted over the bottom in the current. The 800 m was not apparent until it was 6.1 m loose sand of the streaks was not moving at the (20 ft) away from the lower viewing port. The time of observation, despite a northward cur- bottom appeared flat, and exhibited a striped rent measured at 20 cm/sec (0.4 kts). appearance because of the north-south streak- ing out of a thin cover of mobile, dark colored Bimini Dive material over a firmer, finer, lighter colored The major observations of the Bimini dive substrate. Size analysis of a scooped sample traverse are summarized in Figure 2. The over- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/81/10/2861/3432638/i0016-7606-81-10-2861.pdf by guest on 29 September 2021 2864 NEUMANN AND BALL—SUBMERSIBLE OBSERVATIONS IN FLORIDA STRAITS 25°36'N 25°36'N 25°37'N WEST 79°54'W 75°52.5'W 79°50'W EAST MIAMI 7 SEPT 67 Ft. M TERRACE 1000- "ALUMINAUT" DIVE -300 WESTERN STRAITS OF FLA. 1200 -400 1400- VERTICAL ROCK LEDGES 1600- WALL BROKEN SLABS ON TOP STEEP 8 -500 ROCKY 525m. TRENCH - LIKE 1800 SILTY SED. AT \O BURROWS STEEP SLOPE CURRENT CRESCENTS -600 2000 RIPPLES COMMON 'AND STREAKS" ALL SOUTH SMALL DARK |DETRITUS MOVING SOUTH NODULES 2200- FIRST RIPPLES ALL FACE SOUTH -700 2400- ROCK BEGINS LARGE SLABS AND LEDGES 2600- SERIES OF MOUNDS -8OO CORAL THICKET ON CRESTS 2800- NAUTICAL MILES Figure 3. Schematic profile of submersible dive traverse off Miami, Florida, indicating major observations.