Abstract

The submarine slopes that border the Straits of Florida off Miami and Bimini were traversed by the submersible Aluminaut, in August and September of 1967. The Bimini escarpment is characterized by a three-part zonation consisting of relatively strong northerly bottom currents in both deep and shallow zones, and an intermediate zone of low northerly bottom current velocities. From 538 m (the bottom of the traverse) to 222 m, there is a sloping, smooth, rock surface veneered with sand, ripple-marked by northward bottom currents of 50 cm/sec or more. The middle, low velocity zone, from 222 m to 76 m, exhibits a muddy slope of largely bank-derived material. The sediment surface exhibits tracks, burrows and mounds, which indicates that currents here are never as strong as in the rippled zones above and below. Observed current velocities in the middle zone were only 5 to 10 cm/sec. Above 76 m, a steep, vertical to overhanging cliff with large talus blocks at its base rises to a crest at 30 m. Currents in the upper zone are northward at 50 to 150 cm/sec. The inverted situation of higher energy bottom current conditions and associated sedimentary features and textures existing in the same area, but at a greater depth than low energy surface features and fine sediments, is of significance to the stratigraphic interpretation of ancient rocks.

On the western side of the Straits, at the base of the Miami Terrace, is an elongate trough 825 m deep. The bottom here is characterized by ridges and mounds of muddy sand capped by thickets of living deep-water branching coral. The eastward-facing escarpment of the Miami Terrace exhibits ledge-like outcroppings of dark phosphatic limestone from depths of 719 m to the crest at 457 m where the traverse ended.

An interesting finding of the dives in the Straits of Florida is the observation, based on both current measurements and sedimentary structures, that the bottom current on the western side of the Straits flows southward at observed velocities of 2 to 50 cm/sec. This southerly bottom flow is opposite to either the northerly Florida Current above or to the bottom current on the Bahama side of the Straits. The nature and orientation of the sedimentary structures, plus the combined observations of several Aluminaut dives in the same area, indicate that the southward bottom counterflow is persistent and not a temporary tidal reversal. An extensive sedimentary anticline in the west-central sector of the Straits may have been built by this bottom counter current bringing material from the north.

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