Stratigraphic Evolution of Blake Outer Ridge

Abstract

Multichannel seismic data from a reconnaissance survey of the Blake Outer Ridge reveal the seismic stratigraphy down to oceanic basement, as well as intracrustal and mantle reflections. The depositional history of the outer ridge can now be subdivided into four principal phases, based on seismic stratigraphic style: (1) Jurassic-Early Cretaceous sedimentation which filled in basement irregularities and leveled the sea floor by horizon s (Barremian) time, (2) Early Cretaceous-Late Cretaceous deposition of a seaward-thinning wedge typical of Atlantic-type margins; its deeply eroded surface is probably equivalent to the Late Cretaceous-Miocene hiatus reflected by horizon Au, (3) earliest Miocene current-controlled deposits preferentially deposited along the xis of the incipient Blake Outer Ridge; these prograding strata, which thin and dip downridge and laterally away from the ridge axis, are attributed to the interaction of the Florida Current and Western Boundary Undercurrent, and (4) continuing early Miocene preferential deposition on the ridge axis and sculpting by the Western Boundary Undercurrent; this phase is characterized by strata thinning and dipping upridge and toward the ridge axis. The reversal of dip is explained to a first approximation by a steady-state contour-current model in which the current position is progressively shifted by the deposition. Five angular unconformities and associated bathymetric terraces west of the ridge crest are mapped using all Lamont-Doherty seismic data and interpreted as having formed peneconte poraneously during phase four. The areal extent of the gas hydrate (clathrate) horizon is also delineated; it is continuous across the northern Blake Outer Ridge, and the multichannel seismic data presented here show it to extend onto the shallow Blake Plateau as well.