The main goal for Site 29, that of obtaining a complete biostratigraphic sequence of pelagic foraminifera and calcareous nannofossils in the upper Carib beds (sediments above Horizon A"), was not realized due to the paucity of carbonate in the post-Eocene sediments in the deeper parts of the Venezuelan Basin. For this reason a nearby site at a higher level, which would certainly have remained above the carbonate compensation depth since the Cretaceous, was required to fulfill the biostratigraphic goals. This site lies on the Aves Ridge in an area which should correlate with the well-known stratigraphy of the nearby Barbados, Trinidad and Margarita sections. The Aves Ridge or Swell, with crestal depths about 500 fathoms, parallels the Lesser Antilles island arc and separates the Grenada Basin on the east from the Venezuelan Basin on the west. Reflection profiler records crossing the Ridge show the eastern and western flanks draped with sediments which are apparently continuous with those in the basins on both sides. On the Grenada Basin side, the sediments descending from the Ridge appear to be buried by turbidite sediments in some places. The sediments on the Ridge flanks resemble the Carib beds of the Venezuelan Basin, both in character and thickness. From geophysical measurements, the Aves llidge is believed to represent a deformation of the crust by warping under stress of sea-floor spreading, accompanied by intrusive or near-surface volcanic activity suggested by magnetics studies of the area (Edgar, 1968). Geophysical studies suggest that the Aves Ridge appears to show a common positive gravity axis with the Dutch West Indies and may have been elevated in early Tertiary time (Ewing, Talwani and Ewing, 1965). Dredging results from depths of 1000 to 2000 meters from the western slopes of the Aves Ridge (Fox et ah, 1970)

The main goal for Site 29, that of obtaining a complete biostratigraphic sequence of pelagic foraminifera and calcareous nannofossils in the upper Carib beds (sediments above Horizon A"), was not realized due to the paucity of carbonate in the post-Eocene sediments in the deeper parts of the Venezuelan Basin. For this reason a nearby site at a higher level, which would certainly have remained above the carbonate compensation depth since the Cretaceous, was required to fulfill the biostratigraphic goals. This site lies on the Aves Ridge in an area which should correlate with the well-known stratigraphy of the nearby Barbados, Trinidad and Margarita sections. The Aves Ridge or Swell, with crestal depths about 500 fathoms, parallels the Lesser Antilles island arc and separates the Grenada Basin on the east from the Venezuelan Basin on the west. Reflection profiler records crossing the Ridge show the eastern and western flanks draped with sediments which are apparently continuous with those in the basins on both sides. On the Grenada Basin side, the sediments descending from the Ridge appear to be buried by turbidite sediments in some places. The sediments on the Ridge flanks resemble the Carib beds of the Venezuelan Basin, both in character and thickness. From geophysical measurements, the Aves llidge is believed to represent a deformation of the crust by warping under stress of sea-floor spreading, accompanied by intrusive or near-surface volcanic activity suggested by magnetics studies of the area (Edgar, 1968). Geophysical studies suggest that the Aves Ridge appears to show a common positive gravity axis with the Dutch West Indies and may have been elevated in early Tertiary time (Ewing, Talwani and Ewing, 1965). Dredging results from depths of 1000 to 2000 meters from the western slopes of the Aves Ridge (Fox et ah, 1970)

The northern margin of the Demerara Abyssal Plain is bordered by a low ESE-WNW trending rise known as the Barracuda Ridge. North of South America, the eastern margin of the Demerara Abyssal Plain is formed by the Barbados Ridge. An extensive sedimentary section of the Barbados Ridge is exposed on the island of Barbados and consists of a series of radiolarian to planktonic foraminiferal and calcareous nannofossil biogenic sediments interlayered with ash beds. This series, known as the Oceanic Formation, has been described in detail by Jukes, Brown and Harrison (1892), Beckmann (1953) and Saunders and Cordey (1968). The pelagic sediments of the Oceanic Formation range in age from Lower or Middle Eocene through Miocene, and rest with angular and erosional unconformity on complex contorted flysch-like sedimentary strata, almost devoid of fossils, referred to as the Scotland Group. The nature of the sediments and basement in the northern part of the Demerara Abyssal Plain to the east of the Barbados Ridge has been thought to differ markedly from those to the west and south. Profiler records along a traverse at about 16°N latitude, made by H.M.S. Vidal in 1965 (Collette et al, 1969), show the sediments to the east, in the northern part of the Demerara Abyssal Plain, to have a different appearance from those of the Barbados Ridge; the underlying "basement" is not as smooth as is typical for regions adjacent to the continental margin off eastern North and South America (Ewing et al., 1966). To determine the nature of the sediments and "basement," Site 27 was selected on the northern margin of the Demerara Abyssal Plain, south of the Barracuda Ridge. At the site, a rather rough "basement" is overlain by 0.3 to 0.6 second of acoustically transparent sediment. The site also lies in the region traversed by the

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The Shipboard Scientific Party 1 SETTING AND PURPOSEThe Outer Ridge, which lies between the north wall of the Puerto Rican Trench and the Nares Basin to the north, is covered for the most part with thick, acoustically "transparent" sediments, diminishing in thickness northward where they dip beneath turbidites of the Nares Abyssal Plain (Ewing and Ewing, 1962; Ewing etal, 1965).Along the southern portion of the Outer Ridge, reflection profiles reveal that the basement layer reaches a maximum elevation, while the sediment cover diminishes to less than 0.2 second acoustical thickness near the north wall of the Puerto Rican Trench. Seismic refraction measurements show that the Outer Ridge crustal layers are unusually thin and that the 8.2 km/sec mantle reaches within less than 5 kilometers of the ocean bottom in this area. This thinning of the crust has been cited by Worzel (1965) as evidence of extensional forces on the seaward side of island arc trenches. The same effect is shown by Ludwig et al. (1966) for the Japan Trench. Isaacs et al. (1968) show that extensional stresses are predicted on the convex side of the bend of the downmoving lithosphere beneath island arcs, even though the principal stress, deeper in the lithosphere, may be compressional. Hersey (1966) described the typical Outer Ridge sedimentary section (from seismic refraction and reflection measurements) as having an upper zone of unconsolidated, acoustically "transparent" sediments separated from basement rock by a semi-transparent layer having a considerably higher compressional wave velocity (4.2 km/sec), assumed to be layered rock or sediment. Figure 1 shows the topography of the Outer Ridge, the position of Site 28, and the location of a reflection profiler transect which is presented in Figure 2. Figure 3, from Ewing and Ewing (1962), shows a section drawn