Amazon Cone Research Articles

Gravity and Magnetic Investigations in the Guiana Basin, Western Equatorial Atlantic

A free-air gravity map of the Guiana Basin between 15° N. and 6° S. in the western equatorial Atlantic, using all available shipboard and pendulum data, is presented. The gravity field is interpreted in terms of short wave-length components directly related to topographic features and a long wave-length regional field which is independent of surface or basement relief. The regional field is negative throughout the survey area, varying from −15 to −40 mgal. The magnetic anomalies over the large equatorial fracture zones indicate that the fracture zone trough is an area of zero or greatly reduced magnetization within a zone in which the magnetization is induced rather than remanent. Only about half of the gravity anomaly over the fracture zone can be assigned to topographic relief implying the presence of excess mass under the fracture zones. The gravity and magnetic evidence together suggest that large fracture zones serve as the site of intrusion of ultrabasic rocks from depth. The deformation of the lithosphere due to the sediment load of the Amazon cone and the resulting gravity anomalies were computed for various flexural rigidities, using two-dimensional elastic beam theory. The value giving the best fit to the observed gravity anomalies in both wave length and amplitude is 2 × 1023 Newton meters (nt m) (2 × 1030 dyne cm). This implies an effective lithospheric thickness of 30 km. It is suggested that the lithosphere behaves somewhat as a Kelvin (viscoelastic solid) material in its response to imposed long-term loads, approaching a minimum apparent flexural rigidity of 2 × 1030 dyne cm asymptotically in a period of a few million years.

The Vidal deep-sea Channel and its relationship to the Demerara and Barracuda Abyssal plains

The topography and sediments of the northern Guiana Basin, including the northern section of the Demerara Abyssal Plain and the Barracuda Fault are discussed. A seismic profile from the Amazon Cone to the eastern end of the sediment-filled Barracuda Fault trough (the Barracuda Abyssal Plain) reveals a series of three ponded turbidite levels, the deepest being the Barracuda Abyssal Plain. The main source of the turbidites appears to be the Amazon region. A JOIDES hole in the turbidites of the Vema Fracture Zone has revealed very high sedimentation rates but as yet it is unclear what their relationship is to the Demerara Abyssal Plain sediments. A major deep-sea channel (the Vidal Channel) runs across the two levels of the northern Demerara Abyssal Plain and ends in the eastern Barracuda Abyssal Plain. It is the principle access for turbidites to the latter. The main channel has a length of at least 800 km and an average gradient of 1:1100. Several tributaries are also present. In its northern end there is an abrupt gradient change, and reflection records indicate this to be due to differential erosion. Backcutting has apparently been a major factor in the formation of the channel.

North Brazilian Ridge and Adjacent Continental Margin

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. 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. 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.