Leg 159 Logging Summary
Shipboard Scientific Party
Leg 159 addressed the evolution of the Cote d'Ivoire/Ghana transform margin, one of the best-known examples of a former transform boundary between continental and oceanic crust. Transform faults are not as thoroughly understood as divergent and convergent margins; transform continental margins such as those in the Eastern Equatorial Atlantic have not previously been investigated by drilling. The specific objectives of this project were to investigate the deformation and sedimentation associated with the Cote d'Ivoire/Ghana transform margin and its development, to quantify its nature, structure and deformational history, and to place constraints on the opening of the oceanic gateway between the central and south Atlantic during the Cretaceous era.
Bridging and poor hole conditions prevented logging in the upper segment of Hole 959D but downhole measurements were successfully obtained over the lower part of the hole (395-1077 mbsf). The Schlumberger Quad, Formation MicroScanner (FMS), and Geochemical (GLT) toolstrings were run and the logs were of generally excellent quality apart from limited deterioration due to borehole wash-out near the top (405-424 mbsf) and bottom (1025-1045 mbsf) of the logged interval. The sonic and density logs were used to create a synthetic seismogram, which allowed increased precision of time/depth conversions of the seismic data. The boundaries separating the lithological units defined from core measurements are confirmed by changes in the logs at depths coinciding with these boundaries. An increase in micritization and a decrease in porcellanite, coinciding with a short term drop in clay content, is well-defined by a sharp rise in velocity and resistivity and an accompanying drop in porosity at approximately 750 mbsf (within Subunit IIC). The transition from the black claystone of Unit III to the sandstones at the top of Unit IV is also well delineated by most of the logs. Preliminary interpretation of the FMS data shows bedding planes dipping consistently towards NNW with dips increasing downhole from 5 to 14 degrees. Postcruise processing should improve the data and provide core orientation information which will allow the structural geologists and paleomagnetists to orient their measurements. The geochemical log data obtained at this site show good characterization of variable clay and carbonate compositional changes.
Neither the FMS nor GLT toolstrings were run at Hole 960A. At Hole 960C, a reduced Quad string measuring only natural gamma ray, resistivity, and sonic came within 5 m of total depth (logging 121.2-92.4 mbsf and 374.6-159.7 mbsf). Unfortunately, the borehole again was shown by the caliper curve to be washed out for well over half of the logged interval, resulting in poor quality sonic data. The gamma ray anomaly seen in Hole 960A was again well-defined in this hole at approximately 205 mbsf. Because of the importance of collecting structural data from the hole, the FMS was run in Hole 960C over the interval 354.5-173.7 mbsf despite the poor borehole conditions. Preliminary results indicate that the bedding is dipping NE with no evidence for increasing dip with depth. Although data quality was understandably poor, it may improve when processed onshore.
Lee Ewert and Carlos Goncalves
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