LAMINATION DURING SILICA DIAGENESIS - EFFECTS OF CLAY CONTENT AND OSTWALD RIPENING

As a result of silica diagenesis, silica-rich formations (the Monterey Formation in California) become highly laminated both with respect to mineralogy (quartz, opal-CT, opal-A, siliceous shale) and to physical properties (hardness, porosity, and permeability). The degree and patterns of lamination vary according to the detrital material content and to the extent of diagenesis. Several qualitative theories to interpret these processes have been proposed in the past. However, quantitative models are presently lacking. In this paper, mathematical models are developed to describe the diagenetic sequence opal-A to opal-CT (and by extension to quartz) via a dissolution-reprecipitation mechanism. The models account for dissolution, nucleation, and precipitation of the various amorphous and crystalline phases. We explore possible self-organization mechanisms that may lead to rock lamination, including clay effects and particle coarsening. Molecular diffusion in the liquid phase is assumed to regulate layering. The models predict diagenetic lamination that can be used to explain field observations.