The Miami Terrace is the primary seafloor feature of the southeastern margin of the Florida Peninsula north of the Florida Keys. Its geology has been investigated in substantial detail via high-resolution seismic reflection profiling, rock dredging, and submersible and ROV observations. In 1959, Siegler first reported the Terrace as "an old coral reef" following a University of Miami echo-sounding survey in 1958. Subsequent soundings and seismic reflection profiles revealed a drowned Late Paleogene to Early Neogene, narrow carbonate platform that interrupts the smooth profile of the Florida-Hatteras slope for about 120 km between Fort Lauderdale and northern Key Largo. The Terrace covers ~740 km2; reaches its widest extent, 22.2 km, off Miami, and tapers to the north and south where it disappears under prograding sediments.
A distinct upper terrace, which extends from ~200 to 375 m, is characterized by irregular, mostly sediment-free, karst-like topography that was most likely produced by subaerial exposure during the middle to late Miocene. Almost all exposed surfaces are dense, dark gray to black, conglomeratic phosphorites and phosphatic limestones. The shallower western portion in <290 m is chiefly pavements, low-relief outcrops and gravel and rubble fields overlain by extensive patches of sediment in some areas. It is often unclear whether what appear to be loose rocks or rubble are actually free or cemented to the underlying hard substrate. More extensive and often continuous exposed surfaces appear in >290 and include the same hard substrates as well as moderate to high-relief slabs, outcrops, boulders, ledges, steep slopes and escarpments. A narrower, discontinuous lower terrace in ~600-700 m apparently formed as a result of submarine erosion in the middle Miocene. In 1979, Mullins & Neumann suggested that intensification of the Gulf Stream/Florida Current system associated with closure of the Isthmus of Panamá provided the erosional mechanism. A discontinuous ridge, perhaps a drowned bank margin complex, may exceed 80 m in vertical relief and separates the upper and lower terraces.
The most abundant organisms on hardgrounds are demosponges (e.g., Phakellia sp., Pachastrellidae, Desmacellidae, Spongosoritidae, Geodiidae, Raspailliidae, Lithistida), hexactinellid glass sponges (Aphrocallistes beatrix, Vazella sp.), sea anemones (Actinauge sp., Actinoscyphia sp., Liponema sp., Sagartiidae, Corallimorphus sp.) soft corals (Pseudodrifa nigra), colonial anemones (Zoanthidea) and lace corals (Stylasteridae), with fewer octocorals (primnoids Callogorgia americana and Plumarella pourtalesi, and isidid bamboo corals Isidella sp. and, on deeper slopes, Keratoisis flexibilis). Some areas are relatively barren with little more than scattered Liponema sp. anemones and zoanthids, while projecting ledges and boulders of the escarpment may support dense assemblages of large stylasterid fans, a variety of sponges, P. pourtalesi , crinoids, snakestars and basketstars. A sea pen (~0.5 m tall), either Pennatula sp. or Ptilosarcus sp., occurs chiefly in 280-305 m, both on sediments, in gravel/rubble fields and cracks in pavements. An echiuran spoonworm, ?Ochetostoma sp., is common on low-relief substrates. Isolated colonies of the branching coral Lophelia pertusa generally appear in >275 m. Although this species does not build the massive mounds found further north, it may produce extensive talus deposits on outer Terrace slopes. In some places, the edges of projecting Terrace ridges may bear extensive thick ramparts of L. pertusa accompanied by another stony branching coral, Madrepora oculata, and stylasterids, while the flat ridge tops support meadows of Phakellia fan sponges, and P. pourtalesi. Although the valley between the two Terrace ridges is relative barren, it also has scattered large black coral trees, Leiopathes sp. Mobile invertebrates include a variety of sea stars (Tremaster mirabilis, Tosia parva) and crustaceans (Eumunida picta).The most common fish is a codling (Moridae), most likely Laemonema barbatulatum. Others include blackbelly rosefish (Helicolenus dactylopterus), Scorpaenidae, wreckfish (Polyprion americanum), red dory (Cyttopsis rosea), and alfonsino or red bream (Beryx decadactylus).