Burial Diagenesis of Pelitic and Carbonate Deep-Sea Sediments from the Arabian Sea

The holes completed during Leg 23A were drilled into largely pelagic carbonate sequences ranging in age from Recent to Paleocene as well as into the siliciclastic turbidites of the Indus Cone. They presented the opportunity to study carbonate and clay mineral diagenesis of sedimentary sequences that accumulated in the same basin. The clay minerals observed in the 1300-meter-deep Indus Cone borehole (Site 222) are illite, chlorite, kaolinite, 17Å and 11Å mixed-layer illite/montrnorillonite, and palygorskite. The first three of these clay minerals were brought into the basin by the Indus River, whereas the 17Å mixed layer was stripped from the continental shelf of India and redeposited by density currents into the deep sea. Palygorskite is either of eolian or authigenic origin. Under increasing overburden pressure progressive lithification of the clays takes place by compaction and expulsion of pore waters. Moreover, from 300 to 1300 meters a slight decrease of illite and a concomitant increase of 17Å mixed layers is also observed. Over the same depth range the peak width at half height of air-dried illite narrows. These changes are interpreted to indicate a breakdown of illite, a reduction of expandable layers in "open" illites, and a neoformation of 17Å mixed-layer illite/ montmorillonite. Hence, clay mineral diagenesis starts at much shallower depths than previously believed. Investigation of the nannofossil carbonates recovered at Sites 220 and 223 demonstrates that compaction only accounts for the transformation of soft to stiff ooze. Further lithification and reduction of porosity takes place by cementation which increases with depth. The top of the semilithified chalk marks the limit where the grain framework is stabilized by cementation at point contacts. It is at this stage that the fossils begin to exhibit progressive changes with depth. First, precipitation of overgrowth cement on discoasters is observed. Below about 100 meters the individual segments of coccoliths also start accreting secondary calcite. With increasing depth, accretionary growth on these segments bridges the space between plaeolith shields, finally filling it completely. Moreover, precipitation of calcite overgrowths also fuses micarb particles causing further induration of the sediment. At even greater depths the central fields of coccoliths are clogged, and calcite crystals have grown into foraminiferal chambers. Lithification of nanno oozes in the deep-sea environment is largely accomplished by dissolution of supersoluble particles, pressure solution and reprecipitation of syntaxial low magnesian calcite overgrowths on discoasters, coccoliths, and micarb grains (autolithification). It is not related to age of the sediment and is less controlled by depth than by primary carbonate content. A sudden rise of the original carbonate content subsequently causes a lithification front which also appears on the seismogram. Early silicification affected some Eocene chalks. The silica is derived from organic opal and is reprecipitated as low-cristobalite spherules. Silicification postdates precipitation of overgrowth cements in these intervals. It is concluded that lithification of clays and of pelagic carbonates is achieved through entirely different processes which operate at different rates. Generally, pelagic carbonates become lithified at much shallower depths.