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On DSDP Legs 59 and 60, sites were drilled in the South Philippine Sea along an east-west transect at 17°N lat. Clay minerals from Cenozoic samples at DSDP sites 447A, 449, 450, 452, 453, and 454A were analyzed by X-ray diffraction methods. A semiquantitative determination of the percentages of clay minerals was made for each sample. In addition, illite crystallinities and relative iron contents in illites and chlorites were analyzed using X-ray diffraction data.
At site 447A, on the western flank of the Palau-Kyushu Ridge, the drill penetrated an undated surficial clay zone into underlying early Miocene pelagic clay. The clay is smectite-rich (averaging 95% smectite). The remaining percentage is almost evenly divided between crystalline illite, relatively low-iron chlorite, and kaolinite, all of which reflect detrital input from the Chinese mainland.
Site 449, on the eastern ridge flank, yielded a thick pelagic clay sequence ranging from an undated cover down through middle Miocene to upper Oligocene clays. An increase in smectite content during the late middle Miocene resulted from ash deposition due to volcanism on the West Mariana Ridge. Below this zone, in older middle Miocene clays, the content of chlorite, illite, and kaolinite increases considerably relative to smectite. The mineralogic change is associated with radiolarian-rich deposits of the same age. This change could mark a northward shift in the equatorial counter-current associated with increased boundary-current circulation that occurred at this time.
Pelagic clays from the other sites in the Parece Vela Basin, Mariana Trough, and Mariana Trench are smectite-rich owing to the ash content of the sediment. However, stratigraphic interpretation of the Mariana Trough
and Trench sediment is limited because the pelagic clay units consist only of thin Quaternary veneers.
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