Implications of Discontinuous Burial or Uplift for Diagenesis, Cementation and Formation of Secondary Porosity in Siliceous Sediments

Behl Richard J.; Deason, Charlotte T.; and Kassa, Tesfalidet
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A history of discontinuous burial or tectonic inversion has important ramifications for localized loss or gain of porosity in highly siliceous sediments. Diagenetic changes in silica phase, lithology, and porosity are usually considered under conditions of progressive burial in which phase transformation kinetics are driven by continuous increases in temperature and overburden. Yet, every one of the classic study locations and most oil fields have experienced a different history than this, where initial burial was followed by tectonic uplift or a pause in burial. When burial is halted, the transformation zone between opal-A and opal-CT or opal-CT and quartz phase rocks becomes a site of enhanced, localized diffusional transport of silica from higher to lower solubility phases between and along strata and across larger stratigraphic intervals. Horizons can be sufficiently silicified to create seismically resolvable reflections that are unrelated to the current burial depth of active prograde diagenetic transformation. These horizons can be “frozen” or “fossil” reaction fronts that have been uplifted above the zone of active transformation, or ‘paleo” reaction fronts that continued to be buried and no longer separate silica phase zones but are still discernible by excess silicification during pauses in burial. Uplifted successions that had barely reached the initial transformation of opal-CT to quartz can develop substantial secondary porosity by scavenging of opal-CT for precipitation of quartz in “late-formed” cherts. Diagenetically enhanced porosity in opal-CT rocks pure enough to normally form chert can reach >50% – typical of much more shallowly buried opal-A diatomite. Silica derived from opal-CT dissolution is reprecipitated as pore- and fracture-filling quartz to form thickened beds or nodules of chert and cemented chert breccias.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013