Silica Diagenesis in Continental Slope and Rise Sediments off Eastern North America (Sites 603 and 605, Leg 93; Sites 612 and 613, Leg 95)

Abstract

Eocene to Cretaceous siliceous sediments of four Deep Sea Drilling Project sites on the continental margin off eastern North America (Sites 603, 605-Leg 93; Sites 612, 613-Leg 95) were analyzed for their major geochemical and mineralogical composition. Under different environmental conditions (burial depth, in situ temperature, host-rock fades) of the upper slope to lower rise, conversion of biogenous opal-A to opal-CT took place in biosiliceous chalks and marls older than early middle Eocene. Dense sampling revealed the sharp character of the diagenetic silicification front (seismic reflector A c), which demonstrates a widespread, more or less synchronous (51-53 Ma), rapid change of silicoplankton fertility and preservation, which later led to the marked boundary between an opal-A and an opal-CT diagenetic facies. In clay-rich sediments on the lower rise (Site 603), the opal-A opal-CT transformation is largely retarded, whereas clinoptilolite formation is enhanced. Host-rock composition and the, consequent interstitial water chemistry, apparently controlled by palaoclimatology and paleoceanography, appear to be the most important factors influencing the rate and mode of silica diagenesis. Diagenetic alteration of the primary siliceous oozes to porcellanitic chalks and porcellanites took place only in a narrow equilibrium range of the complex geochemical sediment-pore water system. These ideal conditions were realized in our case and at most sites in the Atlantic Ocean in sediments older that 51 m.y.

Biogenic Silica at ODP Site 647 in the Southern Labrador Sea: Occurrence, Diagenesis, and Paleoceanographic Implications

“…The occurrence of high biogenic silica contents in the Labrador Sea during Eocene time corresponds well with other studies of opal accumulation in the North Atlantic (e.g., Riech and von Rad, 1979;Fenner, 1981;Ehrmann and Thiede, 1985;Thein and von Rad, 1987). The production of biogenic opal reached a maximum during (late) early and (early) middle Eocene around 52 Ma (Riech and von Rad, 1979;Boersma et al, 1987).…”

“…The result was strong dissolution of siliceous parti-cles. Zeolite precipitation (clinoptilolite) was also found in Core 105-647A-13R; the silicon could be derived from dissolved opal (see Fenner, 1981;Stein, 1985;Thein and von Rad, 1987).…”

“…The maximum of opal-A to opal-CT transformation is developed in a depth of about 300 mbsf, where no more opal-A was found. It is well known from numerous DSDP sites (e.g., Riech and von Rad, 1979;Hein et al, 1978;Thein and von Rad, 1987) that silica diagenesis proceeds as a maturation from opal-A -> opal-CT -*■ quartz. This diagenetic transformation from unstable to stable mineral phases is influenced by time, burial depth, temperature, and host rock facies (Riech and von Rad, 1979).…”

“…4 and 5). Because the formation of opal-CT is possible only when considerable amounts of biogenic opal have been buried in the sediment (Riech and von Rad, 1979;Thein and von Rad. 1987), we can assume that higher amounts of biogenic silica must have been deposited in the late Eocene (Fig.…”

“…The opal-CT section is characterized by carbonate-rich sediments (40%-50%), and the level where fragments of biogenic opal-A still exist consists of carbonate poor clayey sediments. Clays are known to retard the opal-A -> opal-CT transformations (Williams et al;Thein and von Rad, 1987).…”