Silicified Sediments and Silica Diagenesis in the Goban Spur Area of the Northeast Atlantic, Leg 80

Abstract

X-ray powder diffraction and optical and scanning-electron microscope analyses of sediment samples taken from four sites drilled in the Goban Spur area of the northeast Atlantic show variable diagenetic silicification of sediments at several stratigraphic horizons. The results are as follows:1. The silicified sediments are middle Eocene at Site 548, Paleocene to lower Albian at Site 549, upper to lower Paleocene at Site 550, and lower Turanian at Site 551.2. There are three types of these silicified sediments: nodular type in carbonate-rich host sediments, bedded type in clayey host sediments, and a type transitional between the other two.3. Silica diagenesis is considered to progress as follows: dissolution of siliceous fossils; precipitation of opal CT in pore spaces and transformation of biogenic silica (opal A) to opal CT, development of opal CT cement; chalcedonic quartz precipitation in pore spaces and replacement of foraminiferal tests by chalcedonic quartz; and finally, transformation of opal CT to quartz, and cementation. But the strong influence of host-sediment types on diagenetic silica fades is recognized. Bedded-type silicified sediments in a clayey environment indicate a lower grade of silica diagenesis. Only very weak chalcedonic quartz formation is recognized, and there is no opal CT cementation, even in Lower Cretaceous bedded-type clayey silicified sediments.4. The rf(101) spacing of opal CT shows two distinct trends of ordering or decrease with burial depth; one is a rapid change, in the case of nodular silicified sediments, and the other is a more gentle shift, found in bedded silicified sediments.5. Diagenetic silica facies of the nodular type develop as irregular concentric zones around some nodule nuclei. Also, quartz-chert nodule formation occurs at rather shallower horizons, and is discordant with the trend of decreasing d(101) spacing in opal CT.6. Silicified sediments at Site 551 are shallower than at the other sites. The diagenetic silica facies suggest the probable erosion of 300 m or more of sediment at this site.7. The zeolites clinoptilolite and phillipsite were found in the sediment samples recovered on Leg 80. Clinoptilolite occurs from the shallower levels to the deepest horizons of diagenetically silicified zones, suggesting that clinoptilolite formation is related to diagenesis of biogenic silica. Phillipsite at Site 551 (Section 551-5-2) may originate from volcanogenie material. INTRODUCTIONDetailed studies of silicified sediments from DSDP samples have clarified the worldwide distribution of cherts and other silicified sediments in ocean basins. These studies have yielded many new insights regarding the occurrence and the nature of various siliciffed sediments. They have also revealed very diverse features of silica diagenesis and the formation of cherts and other siliceous sediments, such as porcellanites and diatomites (Heath, 1973;Garrison et al., 1975;Keene, 1975;Kelts, 1976;Kagami, 1979;Pisciotto, 1980;Riech, 1980;Grehin et al., 1981;Hein et al., 1981; Riech, ...

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

“…With progressive burial, the aging process goes on, via opal-CT to the final quartz stage in mature quartz cherts. Apparently this maturation is very regular in clayey sediments, though highly retarded, whereas in calcareous host rocks it is a more rapid and less depth-dependent process (Otsuka, 1985).…”

“…suggest that the progressive increase of the opal-CT crystallinity with time and depth is a dissolution-reprecipitation mechanism, with tridymite layers preferentially dissolving and cristobalite layers preferentially precipitating. Otsuka (1985) proposed for the silicified sediments of Leg 80 (Northeast Atlantic) that erosion of several hundred meters of sediment at some sites might explain the nearsurface occurrence of silicification and inconsistencies with the expected progressive maturation sequence-an argument that hardly can be applied to the silica diagenesis of the New Jersey Transect drilling sites.…”

The Cenomanian/Turonian Boundary Event (CTBE) at Hole 641A, ODP Leg 103 (Compared with the CTBE Interval at Site 398)

“…It is common in Lower Cretaceous strata with com parably high sedimentation rates, as well as in Upper Creta ceous strata with very low sedimentation rates. This relation is also valid for several coeval and lithologically comparable sites in the North Atlantic (Melieres, 1979;Otsuka, 1985). Further more, it is obvious from SEM, thin section, and washed residue studies that zeolite is associated more commonly with partially dissolved siliceous remains (chiefly radiolarians) than with volcanogenic parent material.…”

Authigenic zeolites and their relation to silica diagenesis in ODP Site 661 sediments (Leg 108, Eastern Equatorial Atlantic)

“…In contrast, a comprehensive accepted (1987); whereas other authors mentioned that the formation of clinoptilolite and biogenic silica alteration are independent phenomena (RmCH, 1981;OTSUKA, 1985).…”

“…6B) show a more or less dense and amorphous surface. As replacements of radiolarian skeletons by clinoptilolite are often reported, especially from North Atlantic sites (MI~LIERE, 1979;RIECH & VON RAD, 1979;OTSUKA, 1985;THUROW, 1988) we have investigated the radiolarian skeletons in detail. In all cases however, we only found opal-CT replacements.…”