CHEMICAL AND MECHANICAL PROCESSES DURING BURIAL DIAGENESIS OF CHALK -AN INTERPRETATION BASED ON SPECIFIC SURFACE DATA OF DEEP-SEA SEDIMENTS

Burial diagenesis of chalk is a combination of mechanical compaction a nd chemical recrystallization as well as cementation. We have predicte d the characteristic trends in specific surface resulting from these p rocesses. The specific surface is normally measured by nitrogen adsorp tion but is here measured by image analysis of scanning electron micro graphs. This method concentrates on the micritic matrix alone. Deepsea sediments are ideally suited to the study of burial diagenesis becaus e they accumulate in a relatively conservative tectonic setting. We us ed material from the Ontong Java Plateau in the Pacific, where a > 1 k m thick package of chalk facies sediments accumulated from the Cretace ous to the present. In the upper 200-300 m the sediment is unconsolida ted carbonate ooze, throughout this depth interval compaction is the p rincipal porosity reducing agent, but recrystallization has an equal o r larger influence on the textural development. In the chalk interval below, compaction is not the only porosity reducing agent but it has a larger influence on texture than concurrent recrystallization. Below 850 m grain-bridging cementation becomes important resulting in a lith ified limestone below 1100 m. This interpretation is based on specific surface data alone, and modifies current diagenetic models.