ABSTRACT

The paragenetic history of the cementation observed in many sand-carbonate rocks can best be explained by understanding the complex physical-chemical changes the sediment undergoes during and shortly after burial. It is believed that these changes are sufficient to account for the precipitation and solution of cementing materials.

The distribution of beta quartz and other metastable forms of SiO₂ in nature is extensive. These forms of SiO₂ are widely distributed by sedimentary processes, and often make up a considerable volume of some sediments (up to 30%). Several sources of silica are available: (1) abrasion of siliceous sediments along beaches; (2) siliceous tests; and (3) eolian quartz dust. The SiO₂ contributed by the above sources is thought to be transformed after burial into silica cement.

Three zones of diagenesis beneath the depositional interface may be recognized--each with differing chemical and physical characteristics. The metastable forms of SiO₂ are dissolved in the upper zone because the trapped sea water is universally undersaturated with respect to these phases. In the middle zone, alpha quartz precipitates as silica cement and overgrowths when the concentration of SiO₂ of the interstitial fluid rises above 14 ppm. This is due to the higher solubility of the metastable forms of SiO₂ (up to 140 ppm) than that of alpha quartz (14 ppm). This transformation continues until all metastable forms of SiO₂ are depleted. Furthermore, this transformation is accelerated by the presence of carbon dioxide generated by bacterial ction in the middle zone.

The replacement of silica cement by carbonates is brought about by changes in the chemistry of the trapped fluids in the lower, or third zone. The concentration of carbon dioxide in this third zone is lower than in the middle zone. Increased temperature and a higher pH are characteristic of this zone. These conditions favor the precipitation of carbonates and the solution of silica. The replacement of silica by carbonate is slow, and depends upon the relative concentration of bi-carbonate ions in solution.