The 622.8 m of sediment cored in two holes at Site 1207 consists largely of nannofossil ooze, clayey nannofossil ooze, chalk, limestone, and chert. Minor components throughout the sequence include foraminifers, diatoms, and radiolarians. Other minor to trace components include pyrite and silicoflagellates. Volcanic glass is a disseminated trace component and is concentrated in a few discrete ash layers in the Neogene portion of the sequence. The Neogene is characterized by pronounced color/lithologic cycles on a decimeter scale. A major unconformity/condensed interval representing the lower Miocene, Paleogene, and Maastrichtian consists of manganese oxides and clays. Chert layers are common in the lower Campanian through upper Barremian portion of the sequence. As a result of the interbedded chert, ooze, and chalk through much of the Cretaceous, recovery was poor and consisted primarily of chert (Table T3). Recovery improved somewhat near the bottom of the hole as the result of the predominance of limestone and coring without a core liner in place in the core barrel.
The sequence has been subdivided into three major lithologic units. Unit I extends from 0 to 163.8 mbsf (Hole 1207A), the apparent base of the unconformity (middle Miocene over Campanian). Unit I consists primarily of nannofossil ooze interbedded with clayey nannofossil ooze or nannofossil ooze with diatoms. These variations are expressed as decimeter scale light-dark color cycles. Unit II begins at the base of the short condensed interval and/or hiatus at 163.8 mbsf (Hole 1207A) representing the early Miocene, Paleogene, and Maastrichtian and extends from there to 335.3 mbsf (Hole 1207B), encompassing the Campanian to Turonian. It consists of mottled to homogeneous nannofossil ooze and chalk with interbedded chert or chert nodules. Unit III consists of Cenomanian(?) through Barremian nannofossil limestone and chert, including a 45-cm-thick layer of dark Corg-rich mudstone in the lower Aptian. The exact placement of the Unit II/III boundary is difficult because of the low core recovery. Downhole logs also do not show any abrupt increase in bulk density in this interval. The color of recovered chert (Table T3) changes gradually downhole, but the transition from reddish hues to dominantly gray hues occurs near the Unit II/Unit III contact in the Turonian.
Leg 198 was the first to employ the digital imaging system, which at Site 1207 worked very well. Virtually immediate access to good digital images and the color reflectance data from the Minolta 2002 System provides a powerful way to illustrate lithologic trends and events at high resolution. Carbonate data were obtained for calibration of smear slide estimates (see "Carbonate and Organic Carbon" in "Organic Geochemistry") (Fig. F5), and some X-ray diffraction (XRD) data were also collected and main X-ray peaks identified (Table T4).
Unit I has been subdivided into three subunits (Fig. F6). Subunit IA extends from 0 to 131.3 mbsf (Hole 1207A). It consists of alternating nannofossil ooze with diatoms, radiolarians, and clay in varying amounts (Fig. F7) and diatom/radiolarian or clayey nannofossil ooze, and is characterized by relatively low red/blue (680 nm/420 nm) ratios in color reflectance (Fig. F5). Periodic lithologic variations are expressed as decimeter-scale light-dark color cycles ranging in wavelength from 80 to 150 cm (Figs. F8, F9, F10). The thinner (30-50 cm) dark beds range from greenish gray (5G 6/1) to light greenish gray (5G 8/1) in color, whereas the thicker (~50-100 cm) light beds range from light olive gray (5GY 6/1) to light gray (N7). Color reflectance indicates that the cycle amplitudes are most pronounced in the upper part of the Pleistocene (Fig. F5). The contacts between the interbeds tend to be gradational from dark to light at the top, and sharp from light to dark at the bottom, overprinted by bioturbation (Figs. F9, F10).
Millimeter-scale pyrite laminae and blebs (Fig. F11) are common throughout this unit. Dark gray and green millimeter-scale "diagenetic" bands and laminae (Fig. F12) are randomly distributed, although they tend to be more frequent in the light intervals and around some lithologic contacts. Disseminated volcanic glass and discrete centimeter-thick ash bands are most frequent in the Pleistocene.
Common sedimentary structures include burrows and indistinct mottling. Bioturbation is rare to moderate throughout Unit I. This interval was recovered by the APC, and there is no indication of deformation by drilling. We noted a distinct scour surface at interval 198-1207A-12H-5, 77-79 cm. The contact occurs in a scoured light band overlain by a dark band and ash bed (Fig. F13).
Subunit IB extends from 131.3 to 162.5 mbsf (Sections 198-1207A-15H-2 through 18H-5) and consists primarily of yellowish gray (5Y 8/1) to very pale orange (10YR 8/2) nannofossil ooze. The transition from Subunit IA is marked by a significant increase in the overall reflectance as well as in bulk density and a distinct color change from green-gray hues above to orange hues below. Carbonate content increases downhole in this unit (Fig. F5) to Core 198-1207A-17H (Fig. F14) in which clay content increases. Although radiolarians occur in this interval, siliceous microfossils generally are less abundant than in Subunit IA. The nannofossil ooze in Subunit IB is stiffer than the more siliceous nannofossil ooze above, and smear slides reveal signs of incipient dissolution and cementation of carbonate in the comparatively high carbonate content oozes. Volcanic glass is generally a trace constituent. Burrows are common to abundant throughout Subunit IB, with Zoophycos as the only recognizable trace fossil (Fig. F15).
Subunit IC is a thin interval of dark grayish brown (10YR 4/2) to light brownish gray (10YR 6/2) nannofossil clay with zeolites and manganese micronodules (Fig. F16) that extends from 162.5 to 163.8 mbsf and was recovered only in Hole 1207A (interval 198-1207A-18H-5, 25-102 cm). We attempted to recore this interval in Hole 1207B but failed to recover it. This subunit encompasses a major unconformity between the middle Miocene and the Campanian. The dark clay interval above the unconformity includes a 5-cm-thick Mn crust. Several dark brown chert nodules are present in the interval below the Mn crust. Twinned crystals of phillipsite (Fig. F17) are common. The base of the clay-rich and Mn micronodule-rich zone is intensely bioturbated and includes burrow fills of light-colored nannofossil ooze (Fig. F16). Early middle Miocene and rare Paleogene calcareous nannofossils were found below the Mn nodule layer to the base of the dark clay layer, indicating a highly condensed zone (see "Biostratigraphy"). Manganite and todorokite were identified as Mn-bearing minerals in XRD traces from this interval (Table T4).
Unit II in Site 1207 consists primarily of dusky yellow-brown (10YR 4/2) to very pale orange (10YR 8/2) to grayish orange (10YR 7/4) nannofossil ooze with chert, that was fragmented and broken during drilling, in the lower part. There are decimeter-scale cycles in color as above, but they are shorter in wavelength (20-50 cm) and have much more subtle color transitions. Common sedimentary structures include faint burrows and mottles. Bioturbation is rare to moderate throughout Unit II. Intervals of relatively chert-free ooze are characterized by good recovery (essentially 100%), whereas recovery was poor or nonexistent in cherty intervals. Rotary coring in Hole 1207B resulted in lower recovery and more core disturbance.
Lithologic Unit III consists predominantly of light gray (N7) to greenish gray (5GY 6/1) limestone and chert of varying color from pale red (10R 6/3) to olive black (5Y 2/1) (Table T3). The top of Unit III is defined on the basis of the change from friable chalk to limestone, but this designation is rather arbitrary because we recovered very little chalk. In fact, little of this sequence was recovered, except for that below ~555 mbsf. There is a general trend in chert color downhole from reddish hues near the top of Unit III to gray and black near the base (Table T3). Chert fragments often have inclusions and/or coatings of porcellanite. Although we did not recover intact bedding relationships, the FMS downhole log (see "FMS Image Logs and Cherts" in "Downhole Measurements") indicates that many of the chert horizons are relatively thin bedded (10 cm) over much of the sequence but very closely spaced.
An interesting sequence of lower Aptian dark greenish gray (5GY 5/1) limestones was recovered in Section 198-1207B-43R-1. The limestones are highly bioturbated but intercalated with several thin (1-5 cm thick) beds of silt- and sand-sized material. These thin beds have sharp, erosive basal contacts and are only slightly bioturbated. Cross-lamination was observed in one bed. The silt- and sand-sized grains consist of pyritized radiolarians and, possibly, altered volcanic glass. These characteristics suggest possible episodic traction currents.
A spectacular aspect of the recovered interval is that of ~45-cm of dark-colored, Corg-rich claystone (interval 198-1207B-44R-1, 60-105 cm; Fig. F13) in the lower Aptian. Shipboard analyses indicate Corg contents ranging between 1.7 and 34.7 wt% (Fig. F18). This claystone is finely laminated throughout, although the lamination is very faint. Intervals of radiolarian silt occur at the base and in the upper part of the claystone. Contacts with units above and below were not recovered, but the interval recovered is intact. This unit is the apparent equivalent of the Livello Selli in Italy and other similar units worldwide that represent the deposits of so-called OAE1a (e.g. Sliter, 1989; Bralower et al., 1994).
The limestones in the lower Aptian and Barremian of Unit III are typically highly bioturbated and contain pyritized radiolarians and small nodules of pyrite. Compactional flattening of burrows is common, giving the appearance of streaks and laminae of darker color (typically shades of gray). Planolites was the only trace fossil identified. Chert is generally recovered as 1- to 5-cm-thick fragments with colors ranging from dark brownish red (10R 3/4) to grayish green (10G 4/2) and dark gray (N3) to black (N1) (Table T3). Some of the cherts are grainy and appear to represent chertified radiolarites, whereas others clearly represent silicification of limestones.
Several thin sections were prepared from selected lithologies recovered at Site 1207 (see "Site 1207 Thin Sections"). A thin section of the manganese crust in Section 198-1207A-18H-5 at 44-47 cm is mostly composed of isotropic Fe-Mn minerals but in some areas shows fine-scale structures, including microstromatolitic banding and columns (Fig. F19). Locally, spherulitic clusters of zoned phillipsite crystals are included within the opaque groundmass that constitutes the majority of the Fe-Mn crust (Fig. F19).
The remainder of the thin sections were selected to document the nature of lithified sediments within the Cretaceous section. Most of the recovery in the pre-Campanian section consisted only of fragments of chert with adhering porcellanite and chalk. The nannofossil chalk, partly silicified nannofossil chalk (porcellanite) and chert, all contain varying amounts of foraminifers and radiolarians (see "Site 1207 Thin Sections"). Chert fragments in Campanian soupy ooze (Fig. F20) represent the first lithified sediment in the Cretaceous section. Minor chalk/limestone intervals show some silica cementation (Fig. F21). The transitions between chalk, porcellanite, and chert are illustrated in Figures F22, F23, F24, and F25. All phases of silica were noted. Opal occurs as unaltered radiolarian tests and as opaline cements, locally with lepisphere textures. The chalcedony and microquartz occur as replacements of radiolarian and foraminiferal tests and nannofossils, as well as pore-filling cements within microfossils and the matrix.
Sediment of Unit I was deposited under oxic conditions as indicated by its homogeneous to highly bioturbated nature. There is evidence of variations in productivity and/or carbonate dissolution in the prominent carbonate cycles that characterize most of the unit. The darker-colored intervals generally contain higher amounts of very well preserved biosiliceous material and probably represent intervals of higher surface water productivity. It is also possible that these intervals represent periods of higher carbonate dissolution. Based on average sedimentation rates of 18.4 m/my over the last 8 m.y. (see Fig. F32), the cycle frequency appears to be similar to that of glacial-interglacial cycles, with the dark beds representing glacials and the light beds representing interglacials. A preliminary analysis of the frequency of the dark-light cycles using color reflectance data (Fig. F26) suggests that the dominant period of the cycles corresponds to obliquity (41 k.y.) for the period from 0.6 to 2.5 Ma. The cold-water nature of the nannofossil assemblages (see "Neogene" in "Calcareous Nannofossils," in "Biostratigraphy") in the dark layers suggests that they represent "glacial" intervals, which in the Quaternary Pacific Ocean, were generally times of higher productivity with lower rates of carbonate dissolution. Thus, the prominent color and compositional cycles are similar to those observed elsewhere in the North Pacific (Haug et al., 1995). This character developed in Core 198-1207A-15H (Fig. F14) at a depth of ~132 mbsf (late Miocene; ~8.3 Ma) and continued through the Pleistocene. Peaks in abundance of biosiliceous material based on smear slide determinations (Fig. F7) occur at depths of 25-35 mbsf (latest Pliocene), 65-75 mbsf (early Pliocene), and 95-110 mbsf (latest Miocene-early Pliocene). These intervals are marked by significant decreases in bulk density that indicate zones of higher porosity because of the relatively low compressibility of biosiliceous-rich sediment (e.g. Core 198-1207A-5H; Fig. F10). The large changes in bulk density over intervals of a few meters give rise to prominent reflectors in the seismic reflection profiles over the site, creating a highly reflective Neogene-Quaternary sequence.
Below 132 mbsf (Subunit IB), the sediment is more oxidized, characterized by shades of pale orange and gray orange and an increase in the red/blue reflectance ratios. The carbonate content increases, giving rise to higher bulk density (Core 198-1207A-15H; Fig. F27), although biosiliceous material is present to a depth of 156 mbsf. Cyclic dark and light intervals are present from 132 to 156 mbsf, but the average thickness of the cycles is less than those found above 132 mbsf. The average sedimentation rate of Subunit IB is <2 m/m.y., which, when taken with the sediment composition, suggests that the oxidized character is a product of lower surface water productivity and slower sediment accumulation.
The character of seismic reflection profiles over the Northern High of Shatsky Rise suggests that the Neogene-Quaternary sequence is a drift deposit in which there is a systematic change in the thickness of that seismic unit (see "Background and Objectives"). However, there is no direct evidence in the cores for influence of deep currents sweeping the top of the North High during the Miocene-Pleistocene. We did not observe any winnowed, coarse-grained layers or laminae suggesting traction currents. Sharp contacts, several of which could be attributed to current erosion, were observed in several cores, most notably Core 198-1207A-5H (Fig. F13); however, there is no evidence of missing time at that level based on preliminary biostratigraphy. A sharp contact at the top of a dark layer in Core 198-1207A-17H (Fig. F14), at ~150 mbsf, may represent an unconformity or highly condensed interval based on several missing planktonic foraminiferal zones (N15 over N11). However, there is no obvious erosional feature in this interval, and carbonate contents remain relatively high (Fig. F5), although there is a concentration of sand-sized mineral grains and larger discoasters near the contact.
The character of Subunit IC suggests an unconformity and/or condensed zone that caps the Cretaceous sequence and is, in turn, overlain by Miocene strata. A manganese crust formed near the top of the contact, probably as the result of episodic or long-term seafloor exposure. It is not clear whether this interval represents several episodes of sedimentation followed by sediment removal by currents or a single episode of slumping followed by short-term exposure. Seismic reflection profiles to the north and south of the drill site exhibit somewhat chaotic reflections below the reflector that is interpreted as representing the top of the Cretaceous. Reflectors representing Upper Cretaceous horizons terminate against the overlying unconformity in all directions away from the site, suggesting an erosional origin for the unconformity. Unit I reflectors appear to onlap the unconformity to the east and west of Site 1207.
It is possible that the undermining of the flanks of Shatsky Rise occurred as the result of a prolonged rise in the lysocline and CCD during the post-Maastrichtian, causing slumping of these units. However, the overburden on the Upper Cretaceous does not appear to have been too thick at any time as indicated by the unlithified oozes. It is more likely, however, that the unconformity represents one or more periods of erosion by strong bottom currents sweeping over the top of the rise. Away from Site 1207, the seismic reflection records indicate considerable truncation of underlying reflectors in the upper Cretaceous sequence against the reflector that represents the unconformity surface (Fig. F1). The downcutting appears to be greatest on the flanks of the Northern High.
Relatively pure nannofossil oozes and interbedded cherts indicate pelagic sedimentation on the northern part of Shatsky Rise during the Late Cretaceous as it moved northward on the Pacific plate away from a broad equatorial belt of high productivity. The input of biosiliceous material waned in the Campanian as indicated by the absence of chert layers in the upper Campanian oozes.
A highlight of coring is the recovery of a 45-cm-thick dark brown, finely laminated, Corg-rich claystone of early Aptian age (Section 198-1207B-44R-1) (Fig. F18). This bed records deposition and preservation of Corg during part of OAE1a. The fine laminations suggest that anaerobic conditions prevailed at or above the seafloor on the Northern High of Shatsky Rise, which was located ~5°N at 120 Ma (see "Background and Objectives"), the approximate time of OAE1a. It is possible that an expanded oxygen-minimum zone that developed within the equatorial divergence impinged on Shatsky Rise. The paleodepth of this part of Shatsky Rise was most likely shallower than 3 km at that time. Similar Corg-rich layers were recovered elsewhere in the Pacific Ocean at Site 463, on the Mid-Pacific Mountains, nearly 20° south of Site 1207, but possibly still within an expanded equatorial belt of high productivity. Moderate to abundant bioturbation throughout the remainder of the recovered section suggests oxygenated conditions prevailed during deposition.
In Section 198-1207B-43R-1, several thin (1-5 cm) layers of silt- and sand-sized material are interbedded with greenish gray (5GY 5/1) limestones. These layers have sharp, erosive basal contacts and a suggestion of cross-lamination. They contain altered glass, pyritized radiolarians, and other grains that appear to have been winnowed and transported by traction currents. These features might suggest that Shatsky Rise was swept by bottom currents during the Aptian following OAE1a.
Sediment of Unit I is unlithified but has experienced some compaction with depth, on the basis of trends in bulk density (see Fig. F45). The major feature of Unit I is its dominant green gray color and the relative abundance of pyrite. The low redox conditions probably result from a combination of relatively high productivity in surface waters and consequent high Corg flux to the seafloor with initial preservation under high average sediment accumulation rates. The sulfate concentration in interstitial waters decreases, and alkalinity increases somewhat downcore through Unit I (see Fig. F41), indicating that sulfate reduction has occurred as indicated by the observation of pyrite blebs and laminae in the sediments (Fig. F18). The low redox conditions and pyrite formation is accompanied by a pronounced decrease downhole in bulk magnetic susceptibility (Fig. F44) and magnetic intensity (Fig. F28), probably as a result of the dissolution of magnetic minerals. Another interesting redox-related feature is the common green to dark gray laminae and bands (Fig. F13) that have been termed "diagenetic laminae." These probably represent sites of precipitation of reduced iron that has migrated from darker, clay-rich intervals into lighter-colored, less clay-rich intervals. They usually occur within a few centimeters of the contact between intervals of darker and lighter colors. Such diagenetic laminae are also seen locally as halos around dark-colored burrows.
Another intriguing aspect of diagenesis is the lack of cementation/lithification of nannofossil ooze of Late Cretaceous age. One would expect that the diagenetic potential in these oozes is relatively high because of the abundance of well-preserved calcareous nannofossils in them. The lack of lithification suggests that burial depths were never much greater than at present. This could be explained by frequent slumping and or current erosion of sediment deposited between the Campanian and middle Miocene. Although cementation did not occur to any great extent, rhombohedral micritic carbonate grains were observed in smear slides of the Campanian oozes. The presence of chert interbeds up to 10 cm thick indicates an active driver for silica diagenesis in the absence of significant carbonate diagenesis.
The top of this unit marks the change to limestone. Strongly flattened burrows in much of the recovered limestone below 550 mbsf indicate considerable compaction prior to cementation. The gray to green colors and common pyrite and pyritized radiolarians indicate that low redox conditions prevailed after deposition.