Silica diagenesis of ODP Sites 127-795 and 127-797

The silica diagenetic boundaries, especially the transition from opal-A to opal-CT, are significant seismic and thermal horizons in the Japan Sea. Diagenesis has a marked effect on laboratory and downhole logging physical properties. Index property residues were used for X-ray diffraction and scanning electron microscopy studies that complement the physical property and logging data so that we may examine the diagenetic transitions in some detail. We identify the sharpest physical property gradients with the opal-A/opal-CT boundaries for each site; in a number of cases, the boundaries are different from those previously defined. The changes in the physical properties, especially the acoustic velocity, are distinct across the opal-A/opal-CT boundary. The thickness of the opal-A to opal-CT transition zone depends on the way in which the zone is defined. If we interpret the diagenetic transition zone to be between the point where a clear-cut change or trend starts, to the point where the changes are indistinguishable from the natural background variability, then the zone can be 10 m or more thick. The profiles of the various physical properties are often parallel, and begin to change well above the diagenetic boundary. This is consistent with the shallow occurrence of opal-CT. The opal-CT to quartz transformation, in contrast, is rapid, and the transition zone is at most 1 m thick. The physical property changes are not as distinct across the opal-CT/quartz boundary.