Abyssal erosion and scarp retreat: Deep Tow observations of the Blake Escarpment and Blake Spur

Deep Tow surveys conducted at two sites at the base of the Blake Spur and B lake Escarpment serve to delineate the contact between Mesozoic platform ca rbonates of the Blake Plateau and adjacent abyssal strata. The continental margin in the surveyed areas consists of steep limestone cliffs and headlan ds with over 2 km of relief. This extreme topography distorts the hydrograp hic structure of the water column and intensifies abyssal current flow in t he surveyed areas. Significant lateral scarp retreat is indicated by the pr esence of broad, nearly horizontal benches of Mesozoic limestone presently exposed at the base of the Blake Spur and Escarpment in 4.5-5 km water dept h; and by angular, apparently truncated limestone ledges that crop out on t he cliff face. At the Blake Spur, the contact between Mesozoic platformal l imestones and basinal Tertiary strata does not occur directly at the foot o f the cliff, but is exposed on the abyssal plain within a 50 m deep erosion al moat. Here, strong (> 2 knots) south-flowing bottom currents occur withi n 100 In of the sea floor. The coincidence of this vigorous current flowing within the moat near the base of the escarpment and the eroded character o f the sea floor suggests that currents are an important mechanism in sculpt ing this abyssal headland and contributing to lateral scarp retreat. Simila r erosional processes are indicated along the Blake Escarpment near 29 degr ees 03'N, where the relatively straight cliff, face is interrupted by occas ional steep-walled box canyons. On the headland between the canyons, the Me sozoic outcrop projects into the Blake-Bahama Basin for several kilometers as a broad, arcuate:, gently-sloping bench. These outcropping benches of Me sozoic strata at the base of the escarpment are analogous to larger-scale b uried benches observed in the subsurface that were formed by pre-Miocene er osion. Neogene and Quaternary scarp retreat is suggested by the location of the contact between Mesozoic rocks and younger strata seaward of the steep er cliff face, and is probably the result of current-induced erosion. Focus ed erosion may be facilitated by the presence of a thin (similar to 50 m) f ilament of corrosive Antarctic Bottom Water (AABW) that flows vigorously al ong the base of the cliff at both sites. The recessed position of the canyo ns shelters them from the main flow of this abyssal current, but the steepn ess and morphology of the canyon walls suggest that additional unidentified erosional processes are focused at the canyon heads. (C) 1999 Elsevier Sci ence B.V. All rights reserved.