Current-controlled sedimentation in the Equatorial Atlantic: examples from the southern margin of the Guinea Plateau and the Romanche Fracture Zone

Abstract

3.5 kHz and seismic reflection data were used in a study area of bottom-current control of sedimentation in two areas of the Equatorial Atlantic: a 100 km long segment of the Romanche Fracture Zone (RFZ) and a 30 km wide sector of the southern margin of the Guinea Plateau. The RFZ is the most important conduit for Antarctic Bottom Water (AABW) into the eastern Atlantic. At mid depths (ca. 1500–4500 m), North Atlantic Deep Water (NADW) spreads southwards and eastwards, whereas at the surface, directly overlying the RFZ, is the Equatorial Divergence, a zone of high biological production. On the Guinea margin, a mixture of AABW/NADW moving north washes the foot of the continental rise below 4250 m and the overlying NADW hugs the slope contours. Another divergence zone lies just to the north of this area. On the southern margin of the Guinea Plateau, sediment accumulation is controlled by contour currents within the NADW. Deposits are concentrated into a series of slope-parallel sediment drift, the surfaces of which are moulded into standing sediment waves in the slack region between areas of eroded or condensed sequences marking the loci of higher velocity current cores. The outer edge and upper slope of the Plateau, under the influence of high velocities in the Equatorial Surface Water, are swept bare of unconsolidated sediment. The lower-rise sediments, washed by the weak, mixed AABW/NADW water mass are generally flat-surfaced. In the RFZ, restricted sediment deposition and accumulation, as well as the reworking of sediment surfaces, document the influence of bottom currents. Flat-surfaced sediments deposited on the floor of the axial valley (below the ca. 4500 m CCD), are ponded into basins at the foot of the valley walls by a weak AABW. On the northern flank of the North Ridge (Pillsbury Seamount) and on the South Ridge, the overlying NADW has moulded the thin sediment cover into sediment waves. A thicker accumulation of generally flat-surfaced, ponded sediments in the suspended valley were apparently deposited under a weaker current regime in the lee of the Pillsbury Seamount. Stronger bottom currents, as well as steepness of slope, appear to prevent sediment deposition on both the upper slope and crest of the North Ridge.