Lithologic Interpretation from Geophysical Logs in Holes 737B, 738C, 739C, and 742A

Abstract

During Leg 119 a full suite of geophysical logs was recorded in four holes drilled in the southern Indian Ocean. Holes 737B and 738C are located on the northern and southern Kerguelen Plateau, respectively, whereas Holes 739C and 742A are located on the Antarctic Shelf in the Prydz Bay area. The sediments encountered on the Kerguelen Plateau consist mainly of fine siliceous or calcareous biogenic deposits accumulated during the past 100 m.y. On the Antarctic Shelf, the formations encountered are of glacial origin and consist principally of diamictites. The geophysical logs were correlated with the core descriptions in order to supplement the lithologic column in the four holes. The in-situ measured parameters used for lithologic interpretation in Holes 737B, 738C, and 742A are the natural radioactivity, the neutron porosity, the bulk density, the photoelectric effect, the P-wave sonic transit time, and the thorium, uranium and potassium content. Only a limited number of geophysical logs (natural radioactivity, />-wave sonic transit time, and resistivity) were recorded in Hole 739C. A lithologic interpretation is, however, proposed for this hole on the basis of the existing logs. The lithology inferred from the logs is determined by comparing the position in standard diagrams of plotted parameters, either untreated or calculated, with the position of mineralogic and lithologic poles (calcite, quartz, etc.). Also, lithologic information from cores was extrapolated in the non-recovered intervals, according to the log changes observed between recovered and non-recovered sections. In the Kerguelen Plateau holes the radioactivity and uranium, thorium, and potassium logs were used to determine the amount and the nature of clays. Six logging units are recognized in Hole 737B in the depth interval 296.5-701.0 meters below seafloor (mbsf) and three in Hole 738C in the depth interval 113.0-455.0 mbsf on the basis of the shaliness and the porosity of the calcareous formations. Almost no clay was found from the logs in Hole 738C, whereas a significant amount of illite and montmorillonite, up to 38.5% of the dry bulk, was found in Hole 737B. Unlike the biogenic sediments of the Kerguelen Plateau, the deposits encountered in the holes of the Antarctic Shelf show a very high radioactivity rate (up to 120 GAPI in Hole 742A) due to the presence of granitic eroded basement rocks. Two kinds of glacial sediments were identified from the log records in Hole 742A and 739C. The first kind corresponds to compacted massive diamictite units deposited beneath grounded ice; the second kind corresponds to less compacted, stratified diamictite units deposited seaward of the grounding line. The basal glacier deposits are compacted because the weight of the ice has acted upon the sediments since the time of their deposit whereas distal diamictites are less affected by ice loading. In both Holes 739C and 742A, the alternation of basal diamictites and distal diamictites suggests several cycles of ice advance and recession. At least two cycles seem...

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