The Equatorial Mid-Ocean Canyon: A relict deep-sea channel on the Brazilian continental margin

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The Equatorial Mid-Ocean Canyon is an erosional-depositional deep-sea channel which was formed by turbidity-current activity, was active during the Miocene, but is now a relict feature. The canyon parallels the Brazilian continental rise between 3°–5°S and 33°15′ to 28°W and descends eastward for at least 1,200 km with a gradient of 1:1,000. The canyon is 5 to 8 km wide and as much as 200 m deep. Although the bathymetric expression of the canyon on the sea floor begins at about 33°15′W, the canyon has been traced westward (up-slope) from this longitude in subsurface for at least 75 to 150 km. The trend of this buried portion of the canyon indicates that originally the head of the canyon was on the upper continental rise. The stratigraphic relationship between the canyon and a lower Miocene acoustic reflector of widespread regional extent suggests that the canyon formed during the late early Miocene period. Piston cores from the canyon floor, walls, and levees consist largely of pelagic foraminiferal marls and brown clays which indicate that the canyon has been inactive for at least the past million years. The exact time when the canyon became inactive is still uncertain, but it was probably middle to late Miocene. When active, the canyon channeled terrigenous sediments southeastward along the trend of the Fernando de Noronha Basin. The canyon apparently was abandoned and subsequently buried when the predominant direction of sediment dispersal shifted from southeastward along the Fernando de Noronha Basin to northeastward into the adjacent Guiana Basin, thus making the canyon path obsolete.