Summary for Field Activity Number 1971-005-FA

Alternate Leg ID: UGEO Leg 5; CORPUS 20

Area of Operations: Liberian continental margin, Liberia, Africa, North Atlantic

Dates: October 29, 1971 to November 20, 1971

Objectives: The objectives of the geophysical and geological studies were to investigate the structural-stratigraphic framework of the continental margin adjacent to Liberia; to determine the nature of the transition from the pre-Cambrian shield area on land to the ocean floor; and to determine how deep sea fracture zones of the Atlantic Ocean tie to the continental shelf and to the continental geology of Africa. The cruise was divided into two main parts, geophysical profiling along the Liberian continental margin the first 17 days and dredging plus profiling the remaining 5 days of the cruise.

Type of Activity: Seismic profiling
Sampling

Information to be derived: Morphology

Summary: With only shipboard examination of data, our results are preliminary and sketchy. 1. Pronounced magnetic anomalies on inner shelf are probably the seaward continuation of diabase intrusives on land, detected in aeromagnetic survey published by Behrendt and Wotorson. 2. A progradation of the shelf and slope along the central third of the Liberian continental margin. 3. Presence of a submarine canyon oblique to the coast off Cape Palmas. The same area is marked by steepened eroded slope, crumpling and faulting of strata, and marked gravity and magnetic anomalies. 4. A thick section of sedimentary rocks beneath much of the outer continental shelf, capped by gently dipping Pleistocene (?) terrace deposits at the shelf break. Sparker and 3.5 kHz profiles: 5400 km (37 tracklines); Sonobuoy refraction profiles: 5; Dredge stations: 7 (chain broke on last station, sample lost, dredge recovered); Magnetics; Gravity.

USGS Project Number: IDOE

Contact: Jim Robb (jrobb@usgs.gov)