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Summary for Field Activity Number 1971-004-FA

Alternate Leg ID: UGEO Leg 4; CORPUS 19

Area of Operations: Venezuela continental borderlands, Netherlands Antilles, Aruba, Curacao, Bonaire, Caribbean Sea, North America, North Atlantic

Dates: August 18, 1971 to October 01, 1971

Objectives: Scientific objectives of this cruise leg were: (a) to determine the structure, crustal thickness, and composition of the continental margin; (b) to tie this structure to that of the Venezuelan mainland, the Leeward Antilles and the central Caribbean area, where structural information has been provided by Deep Sea Drilling; (c) to determine the nature and history of the boundary between the Caribbean and South American plates; and (d) to outline areas of potential mineral resources on the continental margin.

Type of Activity: Magnetometry

Information to be derived:

Summary: Approximately 8,000 km of survey lines (44 lines) were run during the geophysical cruise, and in addition the United States and Dutch scientists conducted a 10-day geological reconnaissance of the islands of Aruba, Curacao and Bonaire. Observations of pillow basalts were extended from the well-known exposures on Curacao to previously unknown occurrences on Aruba and Bonaire. The abundance of pillow basalt, diabase, chert and siliceous sedimentary rocks on the islands and the presence of positive gravity anomalies of as much as 100 mgals indicate that this island area is a raised segment of oceanic rocks. The Curacao Ridge is a broad submarine upwarp that is highly deformed only on the north side west of Las Aves, but to the east of the island the entire ridge is deformed. The deformation is in sharp contrast to the eastward-thickening ponded turbidites north of the continental slope. All profiles between Aruba and Las Aves show layer "B", the basaltic or diabasic layer under the sediments of the Venezuelan Basement. This layer, which was cut by drilling in DSDP site 146 (Edgar and others, 1971), passes smoothly beneath the deformed deposits of the continental slope for several kilometers southward. East of Las Aves islands the sediment thickness beyond the continental slope approaches 4 km, and layer "B" could not be followed under the slope. In profiles from this region, however, and in other profiles to the west, the outermost deformed sediment of the Curacao Ridge is seen to be continuous with the undeformed layered strata to the north in the Venezuela Basin. In this basin, the thickness of the pelagic section is fairly uniform, but the section is reduced in thickness where it underlies horizontally bedded reflectors of possible turbidity-current origin in the trough north of the slope. The great increase in sediment thickness in the Venezuelan Basin north of La Orchila suggests that the pathway of sediment transport was through Los Roques Canyon. A wide basin of deposition, the Bonaire Trough, lies between the island chain and the Venezuelan coast. A zone of deformation is seen on its south flank. A continuous reflector passes beneath the deformed zone in one profile.

USGS Project Number: IDOE

Contact: Eli Silver ()

Map for Area of Operations :

Map of field activity boundary.

Platform Info:
photo of UNITED GEO I
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