At various times during the Oligocene, profuse blooms of the planktonic calcareous algae, Braarudosphaera rosa, contributed large numbers of braarudosphaerid pentaliths to deep ocean sediments of the South Atlantic Ocean basin. During such intervals the carbonate compensation level for B. rosa was considerably depressed; nevertheless, many of the pentaliths were disaggregated into wedge-shaped segments as a result of solution. Following deposition, skeletal calcite liberated by dissolution was reprecipitated as low magnesium calcite overgrowths on discoasters, coccoliths, pentalith segments, and minute particles of skeletal debris. As shown by scanning electron micrographs, extensive development of the secondary overgrowths led to the formation of low magnesium chalk lami ae within the otherwise unconsolidated Oligocene ooze sequence. Paleo-oceanographic conditions rather than absolute sediment age or depth of burial were responsible for the submarine lithification of the chalk laminae.

At one locality (Rio Grande Rise, SDSP Site 22, Sample 22/4/1), calcite cementation was followed by the deposition of silica derived from the dissolution of siliceous microfossils and volcanic glass. The silica was reprecipitated as spherules (3-5 microns in diameter) of alpha-cristobalite which partly filled the interstices of the rock and are responsible for the weakly silicified condition of part of the chalk. Clinoptilolite, a second authigenic silicate in the chalk, is readily distinguished in scanning electron micrographs from the detrital quartz and mica also present in insoluble residues of the chalk.

Abundant cristobalite spherules are also present in weakly silicified chalk lenses sampled within the Eocene "Horizon A" radiolarian chert sequence at DSDP Site 29B. These results indicate that cristobalite spherules represent the initial stage of silicification of carbonate rock in the deep-sea environment.

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