Diagenetic evolution of porosity microfabric in biosiliceous sediments, Monterey Formation, California, United States

Biosiliceous sediments have a unique diagenesis with abrupt transformations in porosity, density, sonic velocity and rock strength. With time or increased temperature, biosiliceous sediments undergo two transitions in silica phase. Based on the depositional fabric, the diagenetic siliceous rocks develop different microfabrics of intercrystalline micro- and nanoporosity with distinct pore shapes and sizes. A history of discontinuous burial also influences the evolution of secondary porosity and cementation. Diatomaceous or radiolarian sediments are composed of microfossil tests formed of hydrous, X-ray-amorphous opal-A. These microfossil skeletons form both intra- and interparticle porosity, commonly within the range of 55%–80% of the total rock volume. Burial compaction is primarily mechanical, and high porosity is preserved until the first silica-phase change. At this transition, precipitation of lower solubility opal-CT in radial clusters termed “lepispheres” creates a new silica sink that increases dissolution of the diatom-supported framework that then collapses under the lithostatic load. Bulk porosity decreases to ∼20–45% in porcelanite with chert porosity decreasing to <5% by pore-filling cementation. Pores in the opal-CT stage are entirely intercrystalline with morphologies defined by equant to bladed crystals and their intersections. In detrital-poor porcelanites (>75% diagenetic silica), porosity consists of separate zones of nanopores and micropores associated with the lepispheric structure. In less-pure rocks (50%–75% silica), pore shapes are more irregular, with patchy or homogeneous distribution without a lepispheric zonal pore structure. With continued burial or time, opal-CT converts to quartz and porosity in porcelanite generally decreases to ∼10–30% while the mean pore size and pore throats increase tenfold and pore shapes become more equant. Original diatomite sedimentary fabric continues to influence the distribution of pores through two diagenetic steps of dissolution and reprecipitation. When burial is continuous, minimal silica mobility causes the phase transformation of strata to be compositionally conservative and sedimentary microfabrics are remarkably preserved in opal-CT and quartz porcelanite. When burial is discontinuous, the paused transition zones keep silica phases of different solubility in proximity long enough to allow transfer of silica from strata of the higher-solubility to the lower-solubility phase, e.g., from opal-A to opal-CT or from opal-CT to quartz.