North Brazilian Ridge and Adjacent Continental Margin

A geologic and geophysical reconnaissance of the continental margin of the north coast of Brazil led to the discovery of a narrow basement ridge, herein called the North Brazilian Ridge. The ridge closely parallels the coast of Brazil between the Amazon cone and the easternmost tip of Brazil and is essentially a continuous topographic and/or structural feature across a distance of 1,300 km. The ridge lies 150-200 km seaward from the base of the continental slope. Its topographic relief ranges from about 300 m to about 4 km. Although a conspicuous sediment ponding is observed on the continental side of the ridge, there is no marked contrast in type for the upper few meters of sediment in piston cores from opposite sides of the ridge. The primary process of sediment transport and deposition thus appears to be acting parallel with the ridge rather than downslope. A prominent acoustic reflector, which may be synchronous with Horizon "A" in the North Atlantic, abuts the north flank of the ridge. This observation, together with ages extrapolated from sedimentation rates determined on the ridge at JOIDES drill sites 25-25A, suggests that the ridge is relatively old, of the order of 100 m.y. The ridge is gravimetrically compensated, and gravity values are roughly the same in the basins on opposite sides of the ridge. Magnetic anomalies over the ridge are typically of low amplitude and have an apparently random character. Rock outcrops were photographed along the steep flanks of the ridge and are very similar to outcrops of mafic igneous rocks on the flanks of the Mid-Atlantic Ridge. The North Brazilian Ridge is not continuous with the major fracture zones of the equatorial Atlantic. A collective examination of all data leads us to the conclusion that the ridge probably formed by oceanic volcanism shortly after the separation of Africa and South America. Its location and extent may be the direct consequence of the manner and geometry of the opening of the equatorial South tlantic Ocean.

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