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Deep-sea drilling has recovered rare, but areally persistent, consolidated and semiconsolidated calcareous nannofossil oozes from Tertiary strata of present ocean basins. They are ideal for studies of early phase diagenesis and lithification of carbonate and siliceous rock. Scanning electron microscope observations of hard laminae within unconsolidated Oligocene ooze sequences in the South Atlantic have revealed evidence of submarine lithification effected at bathyal depths through the chemical precipitation of calcite cement as overgrowths on organic skeletal particles and as free-growing euhedral crystals. Calcite cementation may be followed by the deposition of authigenic zeolite (clinoptilolite) and the mobilization of silica initially deposited as volcanic glass and ests of siliceous organisms. In some chalks from the Caribbean Sea and the South Atlantic, silica has been redeposited in the form of spherical aggregates about 2½-3 µ in diameter which partly fill pore spaces and may form an accessory cement. Chert stringers and lenses are also present in these chalks and the spherical aggregates may represent an early stage in the formation of deep-sea chert.
Environmental and geologic factors responsible for the lithification of Tertiary deep-sea calcareous oozes are probably not everywhere the same. Studies of Pacific chalks and oozes (Deep Sea Drilling Project Leg 18) indicate that movement of interstitial fluids in response to compaction may be important in determining the solution and lithification histories of some calcareous sediments.
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