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Cretaceous Chert and Goo

The Cretaceous sedimentary sequence on Shatsky Rise is characterized by undercompacted nannofossil ooze to semilithified chalk that is locally partly to wholly silicified, forming porcellanite and chert, respectively (Fig. F36). The presence of chert hampered both the drilling and recovery of this section, particularly when rotary cored. The chert effectively reduced pre-Campanian recovery rates to less than 10%. Given the preferential recovery of chert and somewhat harder porcellanite over softer chalk/ooze interbeds, electric, caliper, and FMS logs through these intervals provide the best estimate of actual lithological proportions (see "Physical Properties, Downhole Measurements, and Core Logging").

The broadest temporal history of Cretaceous carbonate and biosiliceous sedimentation across the Shatsky Rise is provided by the combination of drilling results from Sites 1207 (Northern High) and 1213 (Southern High). Sediments (ooze/chalk/chert) from the Maastrichtian through the Berriasian were recovered, albeit in piecemeal fashion, with the exception of a few gaps interpreted as erosional unconformities, primarily during the Turonian–Coniacian and Barremian. The Campanian and Maastrichtian ooze was best recovered at Sites 1208, 1209, 1210, 1211, and 1212. The latter four sites record the mid-Maastrichtian Inoceramus extinction event.

High carbonate contents (up to 99 wt%) of the Cretaceous ooze/chalk suggest deposition above the CCD except during OAE1a (see "Geochemistry" in "Specialty Syntheses"). Where recoveries were higher at Site 1213, there is some indication of decimeter-scale cyclicity likely tied to variations in carbonate dissolution or production. Bioturbation was ubiquitous, with minor laminated intervals (radiolarites) associated with periodic reworking and winnowing by currents.

Prior to silicification, the Cretaceous sediments consisted of nannofossil ooze with variable amounts of radiolarians and foraminifers. The higher proportion and preservation of radiolarians down through the Cretaceous section is likely linked to the more equatorial position of the Shatsky Rise during the Early Cretaceous. At the equator, high productivity may have depressed the CCD, resulting in higher concentrations of biosiliceous material. These radiolarians are considered to be the major source of diagenetic silica in the section. Thin sections of chalk, porcellanite, and chert from Sites 1207 and 1213 verify the stages of silica diagenesis from Opal A (unaltered radiolarian tests), to Opal CT (lepispheres) to chert (microquartz and chalcedony).

Various lines of evidence suggest that silicification was rather early, prior to significant compaction of the sediment. Chert color, therefore, likely correlates with the original color of the presilicified sediment. Chert and porcellanite colors are generally either reddish brown or grayish to black. These color groupings are interpreted to reflect the prevailing redox conditions at the time of chert formation, with reddish brown indicative of more oxidizing conditions and grayish black reflecting a more reducing environment. Comparison of Leg 198 Sites 1207, 1213, and 1214, as well as Leg 32 Sites 305 and 306 reveals a very generalized secular trend of chert color (Fig. F37). It is likely that secular changes in prevailing redox conditions were driven by a combination of organic matter flux, sedimentation rate, and deep-water oxygen level.

Sedimentary Rocks of Early Aptian OAE1a

Coring at three sites recovered parts of sequences laid down during early Aptian OAE1a (Fig. F38). Very Corg-rich radiolarian claystones were recovered at Sites 1207 and 1213, whereas it appears that intervals with significant enrichment of organic carbon were either not recovered or were absent from this interval at Site 1214. The latter possibility seems unlikely given the intermediate-water depth of Site 1214 with respect to the other two sites. The lithology of the lower Aptian differs somewhat from site to site, but characteristic is the general absence of carbonate and the presence of abundant radiolarians. The Corg-rich, finely laminated radiolarian claystone at Site 1207, which is the shallowest of the three sites, was bracketed above and below by limestones. Cores at Sites 1213 and 1214 contained only radiolarites, radiolarian porcellanites, and claystones. The absence of carbonate indicates that the CCD rose to levels shallower than the paleodepth of Site 1207 but that the duration of the episode of severe carbonate dissolution was shorter there than at the two deeper sites.

An additional intriguing feature of the rocks at Site 1214 is the presence of at least four thin, gray layers of altered tuff interbedded with radiolarian claystones. Altered tuffaceous material was present in association with the Corg-rich layers recovered at Sites 1207 and 1213, but no discrete layers were noted. The presence of altered volcanic ash on Shatsky Rise reinforces the association of volcanism with OAE1a in the Pacific Basin (e.g., Larson, 1991a; Larson and Erba, 1999).

Cenozoic Carbonate Deposition

Sediment deposition for much of the Cenozoic occurred above the CCD and the lysocline. During the Paleocene and Eocene, the CCD maintained a position below Sites 1209, 1210, 1211, and 1212 resulting in the deposition of carbonate rich (>95%) oozes at those localities, and clays at deeper localities (i.e., Site 1208). Periodic lysoclinal shoaling, however, produced low-frequency and low-amplitude oscillations in carbonate deposition with a dominant cycle frequency close to that of the 100-k.y. eccentricity cycle. Carbonate deposition was further perturbed by episodic dissolution "events." One of the most prominent lies at the Paleocene/Eocene boundary (~55 Ma) as represented by a sharp, thin basal Eocene clay-rich layer at each site. These dissolution horizons probably resulted from a rapid shoaling of the lysocline and CCD brought about by the massive dissociation of methane hydrate and its subsequent oxidation to CO2 in bottom waters (Dickens et al., 1997). From the uppermost Paleocene clays, carbonate content gradually recovers to levels that may exceed pre-event carbonate levels (Fig. F39). This suggests the lysocline overshot its original depth, consistent with the predicted silicate weathering feedback as the primary sink for the CO2 (Dickens, 2000).

The middle Eocene is marked by a shoaling of the lysocline over the entire rise as inferred by a systematic decrease in carbonate content at Sites 1209, 1210, and 1211. Recovery occurs at the Eocene to Oligocene transition, which shows a gradual rise in carbonate content that reflects a deepening of the lysocline and CCD over the Shatsky Rise (Fig. F28). This CCD transition, which was global in extent (van Andel, 1975), initiated in the latest Eocene and peaked just above the boundary in the earliest Oligocene. At its peak, the CCD was sufficiently deep to allow carbonate to accumulate at Site 1208 (today at a depth of 3.3 km). In the mid-Oligocene, the lysocline and CCD began to shoal and, for extended intervals in the early and middle Miocene, were located above the Shatsky Rise, resulting in regional deposition of several prominent clay-rich condensed intervals. By the late middle Miocene, carbonate deposition resumed across the entire Shatsky Rise as the CCD deepened close to its present-day levels. From that point to the present, orbitally driven variations in the lysocline together with carbonate production and clay fluxes have created distinct lithologic cycles across the entire rise.

Neogene–Quaternary Sediment Drifts and Biosiliceous Sedimentation

Based on the seismic character of the Late Neogene–Quaternary sections across the Shatsky Rise, thick sections at Sites 1207 (Northern High) and 1208 (Central High) are drift deposits, whereas those at the Southern High are thinner pelagic drape deposits. Although the drift deposits at Sites 1207 and 1208 show no distinct sedimentological evidence for current reworking, the higher than expected sediment accumulation rates at these sites are in part attributable to drift deposition. We believe that current reworking served to amplify regional variations in input of wind-borne volcanic ash and eolian terrigenous material, as well as production of biosiliceous material at these sites. The greater abundance of diatoms, radiolarians, and silicoflagellates in the upper Neogene–Quaternary sections at Sites 1207 and 1208, illustrated in Figure F40, can be attributed, in part, to a northerly increase in sea-surface productivity.

Clay Mineral Authigenesis

Green diagenetic laminae are prevalent in Quaternary and Neogene sediment at Sites 1207–1211. Similar features have been described at Ontong Java Plateau and Lord Howe Rise and were interpreted as altered layers of volcanic ash. Our observations have led us to conclude that the green laminae encountered at Shatsky Rise are diagenetic features composed primarily of saponite, a smectite group clay (see "Lithostratigraphy" in the "Site 1210" chapter). Clay was observed in discrete bands, along the edges of burrows filled with volcanic glass grains, but also as an authigenic product infilling foraminifer tests (Fig. F41). Alteration of disseminated grains of volcanic glass in the matrix and background saponite in XRD scans of bulk sediment suggest that saponite may be forming throughout the bulk sediment. Formation of this Mg and Ca bearing clay is also consistent with pore water profiles (see "Geochemistry" in "Specialty Syntheses").

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