Lithoherms on the Florida-Hatteras slope

Sections of the Florida-Hatteras Slope were investigated during dives of th e Johnson Sea-Link I submersible and US Navy's submarine NR-1, Numerous cal careous coral-capped lithoherms, often exceeding 40 m in elevation, were en countered in the subphotic depths below 440 m. One huge lithoherm complex w as explored that forms a long, slope-parallel ridge perched on the seaward edge of mid-slope terrace. This lithoherm belt is similar to 400 m wide, mo re than 4.4 km long and similar to 150 m high on its down slope flank. Dire ct observations and side-scan imaging indicate that the long axes of indivi dual ridge crest mounds trend perpendicular to the regional current flow. A way from the ridge individual lithoherms trend more parallel with regional current flow and occur without obvious bedrock control. All of these structures have very steeply sloping flanks (frequently 30-60 degrees) where cemented crusts are exposed by erosion. Thickets of living a hermatypic coral coat the crest and up-stream flanks of these topographic f eatures. Living deep-water coral have modem C-14 ages (700 +/- 80 year sp) and trap older (3250 +/- 100 year sp) sediment rich in fine-grained aragoni te. Elsewhere, older deep-water coral rubble (20, 230 +/- 230 year sp) trap s younger sediment (13, 760 +/- 140 year sp). The trapped sediments contain >50% aragonite fines (<62.5 mu m) The friable crusts exposed on these flan ks are generally similar in composition to the sediments that accumulate on the crests of these structures, they are older (14C ages of 17, 770 +/- 33 0 and 32, 710 +/- 570 year sp) and contain less aragonite (similar to 20%) and more high-magnesium calcite. Apparently, lithoherms grow upwards by tra pping fine sediments within ahermatypic coral thickets on their crests and upstream flanks. Cemented crusts may be a consequence of earlier paleoceano graphic conditions or a result of gradual and ongoing cementation and diage nesis. The large size, platform edge position, and linear trend of this dee p-water lithoherm complex are strikingly similar to a shallow marginal reef system. (C) 2000 Elsevier Science B.V. All rights reserved.