ABSTRACT

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. Shortly after 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 laminae (up to 1 meter in thickness) 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. Cementation as a result of calcite overgrowth did not reduce significantly the porosity of the chalk. It is possible that this mode of cementation during early diagenesis could protect the rock from compaction and pore closure during late diagenesis.

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

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

"The current enthusiasm for research on shallow water carbonates, in the Bahamas, the Persian Gulf, Bermuda and elsewhere ... has tended to divert interest from the less accessible, deeper water sediments ..." Carbonate Sediments and their Diagenesis, Robin G. C. Bathurst, 1971