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

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.