21. CLAY MINERAL STRATIGRAPHY AND RELATED DIAGENESIS IN THE DEEP SEA DRILLING PROJECT LEG 60 SEDIMENTS1

X-ray diffraction studies on the <2-µm fraction of clay samples from cores collected on DSDP Leg 60 in the Mariana Trough and western Pacific plate (Sites 453, 454, and 452) were analyzed to explore the clay mineral stratigraphy of the region. Certain sites (Sites 453, 456, and 458) were used to examine the relationship between authigenic clay mineral development and variations in the calcium and magnesium concentrations in the pore water. This study is a continuation of a similar investigation on DSDP Leg 59 pelagic clays in the South Philippine Sea. These combined analyses indicate that the detrital clay components in the South Philippine Sea sediments have not changed markedly since the Neogene. Clay minerals in the < 2-µm separates consist of> 80 percent smectite with minor amounts of chlorite, illite, kaolinite, and possibly vermiculite. However, the smectite percentage is enhanced by the for-mation of authigenic smectite presumably derived from the alteration of volcanic glass in the sediments. Study of the authigenic clay components shows a general increase in smectite downhole associated with increasing calcium and decreasing magnesium in the sediment pore waters. In addition to dioctahedral smectite (montmorillonite), the authigenic study reveals traces of trioctahedral smectite (saponite), celadonite, possibly "proto-celadonite, " and wairakite in various samples