MORPHOLOGY AND EROSIONAL DEVELOPMENT OF THE BLAKE ESCARPMENT, A MASSIVE SUBMARINE CARBONATE CLIFF

The Blake Escarpment is an exceedingly steep cliff eroded in horizontal limestone strata of a drowned carbonate platform by deep-sea processes. It lies about 400 km east of Florida, extends north-south for more than 600 km in water depths of 1200-5000 m and probably exposes the entire Lower Cretaceous section. Long-range GLORIA sidescan sonar images disclose the morphology of the escarpment, multichannel seismic reflection profiles indicate its structural and stratigraphic framework, and a series of ten ALVIN submersible dives afford direct observations, photography, and rock samples that provide ground truth. The face of the original bank has been eroded back by as much as 15 km and scour has been concentrated at the foot of the cliff, where moat-like depressions in the deep sea floor are commonly present. However, the balance of mechanical, chemical and biological effects in creating erosion is still poorly defined. The escarpment displays four morphological types. 1. Smooth, steep faces occur at the Blake Spur, a salient at the north end of the escarpment where erosional effects are greatest. 2. Regions of shallow valleys with tributary gullies coincide with areas of minimal erosion, one area just south of the Blake Spur, and another south of Abaco Canyon extending into Northeast Providence Channel. 3. Terraced sections are present where varying lithologies are being differentially eroded by current scour along the main, 300 km-long, straight section of the Blake Escarpment. 4. A box canyon area with collapsed blocks may reflect fracturing caused by continued compaction at an old fracture zone in the region of Abaco Canyon, just north of the Bahamas. These escarpment types correspond to similar types observed in the Gulf of Mexico on the West Florida and Campeche Escarpments and reflect the stratigraphic, structural and morphological influences that can control the development of steep submarine escarpments in carbonate rocks.