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

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.硅质成岩边界，尤其是蛋白石-A（opal-A）向蛋白石-CT（opal-CT）的转变界面，是日本海内重要的地震与热学层位。成岩作用对实验室测试及井下测井获得的物理性质具有显著影响。我们选取指标性质残留样品开展X射线衍射（X-ray diffraction）与扫描电子显微镜（scanning electron microscopy）分析，以此补充物理性质与测井数据，从而能够更为细致地探究成岩转变过程。我们在每个站位均识别出蛋白石-A/蛋白石-CT界面处最为显著的物理性质梯度；在多数情况下，该界面与此前划定的界面并不一致。穿过蛋白石-A/蛋白石-CT界面时，物理性质尤其是声波速度的变化特征极为明显。蛋白石-A向蛋白石-CT转变带的厚度取决于该带的划定方式。若将成岩转变带定义为从明确出现变化或变化趋势的点位，到变化与自然背景变异性难以区分的点位之间的区间，则该转变带厚度可达10米甚至更厚。各类物理性质的剖面曲线往往呈现平行态势，且早在成岩界面之上就已开始发生变化。这一现象与蛋白石-CT埋藏较浅的产出特征相符。与之相反，蛋白石-CT向石英（quartz）的转变过程极为迅速，其转变带厚度最大仅为1米。穿过蛋白石-CT/石英界面时，物理性质的变化特征并不如前者那般显著。