1 cm in diameter are limited to the middle of the subunit (Table T2). Additionally, volcanic glass is a common trace component throughout Subunit IA.
The lithostratigraphy at Site 1208 is based on the recovered portion of the 392.3 m of pelagic sediment and sedimentary rocks cored in Hole 1208A. First APC and then XCB coring techniques were used, with excellent to good recovery rates, respectively.
The section at Hole 1208A has been subdivided into two major lithologic units (Table T2; Fig. F5). Unit I extends from 0.0 to 328.15 mbsf. Overall recovery of this unit was excellent (95%). This unit is of Cenozoic age and is characterized by rhythmically alternating intervals of nannofossil ooze/chalk (light) and nannofossil clay/claystone (dark) with diatoms and radiolarians that are punctuated by occasional volcanic ash layers. The average carbonate content is 53 wt% (Fig. F5) (see "Carbonate" in "Organic Geochemistry"). The base of the unit is defined by an unconformity where Paleocene to Miocene claystone (base of Unit I) overlies Campanian nannofossil ooze (top of Unit II). Unit II extends from 328.15 to 392.3 mbsf and consists primarily of Campanian nannofossil ooze with interbedded chert (Fig. F5). In contrast to Unit I, the average carbonate content of Unit II is 96 wt% (Fig. F5). XCB coring of this largely unconsolidated unit resulted in much poorer recovery rates (54%). Drilling was terminated at this site when a significant cherty horizon effectively reduced the recovery rate to 0.25%. Scrapings made from the chalk/porcellanite rims on the chert fragments in Section 198-1208A-42X-CC are of middle Albian age, which suggests that the drilled interval for this terminal core spans a major Cretaceous unconformity (see "Biostratigraphy").
Unit I at Site 1208 is composed of alternating intervals of nannofossil clay and nannofossil ooze with diatoms and radiolarians that become more lithified downsection, passing into nannofossil claystone and nannofossil chalk. This transition from ooze to chalk can be seen in Core 198-1208A-24X (209.8-219.4 mbsf), where interbeds are alternately lithified and unlithified. However, intervals of ooze are present below this interval, indicating that lithification at this site is variable and not simply a function of burial depth.
The rhythmic beds that characterize Unit I change in nature and composition down through this interval. This is illustrated in Figure F5, in which carbonate values and reflectance both increase downhole through Unit I. The abundance of siliceous microfossils (Fig. F6) and terrigenous silt also decreases downsection (Fig. F6). These changes, although somewhat gradational, allow for the subdivision of Unit I into three subunits (Fig. F5). Specifically, Subunit IA is predominantly greenish gray (5GY 6/1) in color and characterized by higher clay content, moderate content of siliceous microfossils, color banding, pyrite, and ash layers, whereas Subunit IB, although similar in composition, is distinctly more yellowish orange in color with common Zoophycos, Chondrites, and Planolites burrows. The Subunit IA/IB contact is placed at the top of the first pale yellowish brown (10YR 6/2) nannofossil claystone in Section 198-1208A-28X-3 at 50 cm. The Subunit IA/IB color change also marks the downhole disappearance of ash beds and a decrease in pyrite in the sediments. Zoophycos burrows, although present near the base of Subunit IA (Table T2), are more common (or perhaps just more visible) below Core 198-1208A-28X in Subunit IB. The proportion of darker nannofossil claystone is highest in Subunit IC, progressing to claystone at the base of the subunit (Unit I/II boundary). Siliceous microfossils are only trace components of this subunit. The Subunit IB/IC contact is placed at the top of the first of a series of meter-thick nannofossil claystone units at 5 cm in Section 198-1208A-34X-4. The nature and the origin of the subunit lithologic changes are discussed further in the interpretation section.
Subunit IA extends from 0.0 to 251.6 mbsf. The higher than expected (average 100.1%) recovery rate in this interval can be attributed to slight core expansion. Where this interval was recovered by APC (0.0-185.2 mbsf; Cores 198-1208A-1H through 20H), there is no indication of drilling deformation, but below this interval (185.2-190.4 mbsf; Cores 198-1208A-21X through 28X), recovery decreases, and the core is progressively more disturbed, exhibiting biscuits and fractures.
Subunit IA extends from the mudline at the top of Core 198-1208A-1H down to Section 28X-3 at 50 cm. Below a 35-cm surface layer of dark yellowish brown clay (10YR 4/2) with nannofossils is an interval of mainly olive gray (5Y 4/1) nannofossil clay to clayey nannofossil ooze (Section 198-1208-1H to Section 3H-3) with a few abrupt, high-angle bedding contacts (Fig. F7; Table T2). The remainder of the subunit (Cores 198-1208A-4H to 28X) is characterized by alternating light and dark layers of nannofossil ooze and nannofossil clay, locally with diatoms, radiolarians, and foraminifers. These lithologies exhibit gradational to thoroughly bioturbated upper and lower contacts (Fig. F8). Lithologies often grade incrementally from extreme light/dark end-members within a core interval, making the "gradation" the dominant lithology (e.g., clayey nannofossil ooze). At certain intervals, the rhythms are outlined by starkly contrasting lithologic changes (e.g., Core 198-1208A-8H), whereas elsewhere the cycle variations are more subtly manifested (e.g., Core 198-1208A-19H). The lighter lithologies are generally light gray in color, whereas the darker lithologies are dominantly shades of greenish to olive gray (Table T2). Downsection trends in sediment color are outlined in Table T2. Cycle thickness (light/dark interval) varies on a decimeter to meter scale so that rhythm character is best seen in the whole-core photographs. Core-scale cyclicity is displayed in Figure F9 using a combination of digital images, color reflectance, and bulk density data.
Darker and lighter intervals generally are defined by differing proportions of biocalcareous, detrital clay, and biosiliceous material, in decreasing order of importance. Smear slide estimates of composition (see "Site 1208 Smear Slides") show that the biocalcareous components are dominated by nannofossils (averaging 54% and ranging up to 88%) with a minor percentage of foraminifers (averaging 4% and ranging up to 12%). In addition to clay minerals (averaging 25% and ranging up to 56%), the siliceous mud fraction includes detrital quartz and feldspar silt (see Fig. F6) as well as trace amounts of volcanic glass and associated accessory minerals. Biosiliceous components (see Fig. F6 for distribution) are dominantly diatoms (averaging 6% and ranging up to 20%) and radiolarians (averaging 5% and ranging up to 15%), with lesser (averaging 1%-2%) silicoflagellates and sponge spicules. Other minor components include inorganic carbonate, opaque minerals (pyrite), and zeolites.
Vitric ash layers and laminae are found throughout but are most abundant in the middle of Subunit IA (Table T2; Cores 198-1208A-5H and 13H). They range in thickness from a few millimeters to 10 cm and in color from lighter to darker shades of gray. Where thick, they are graded and locally exhibit parallel lamination. Most exhibit abrupt to scoured bases (Fig. F10). Some have been disrupted and redistributed by burrowing organisms. Isolated, well-rounded pumice clasts 1 cm in diameter are limited to the middle of the subunit (Table T2). Additionally, volcanic glass is a common trace component throughout Subunit IA.
The cores in Subunit IA are mostly moderately bioturbated, with intervals showing rare to common bioturbation, including burrow mottling. Specific ichnofauna could be identified only in a few intervals. These trace fossils show a crude downhole progression (Table T2) from Chondrites (Cores 198-1208A-2H to 4H), to vertical burrows (Fig. F11; Trichichnus?; Cores 2H, 13H, and 18H), to Zoophycos (Cores 27X and 28X) to composite burrows (Core 28X).
The sediment in Subunit IA is characterized by diagenetic features, including diffuse color banding and pyrite. Color bands are generally subhorizontal, but locally are circular to oblate where centered on discrete burrows. The color bands are present in both light and dark lithologies but are particularly visible where they contrast with lighter nannofossil ooze (Fig. F8). The bands range in color from greenish gray to grayish green (5GY 4/1, 5GY 6/1, and 10G 4/2) to dark gray (N3) and grayish purple (5P 4/2). They become less prominent near the base of the subunit (Table T2). Distinct, green millimeter- to centimeter-thick clay-rich laminae are locally associated with the color bands.
Another unique feature of Subunit IA is the presence of pyrite in various forms. Pyrite most commonly occurs within millimeter- to centimeter-scale burrow fills (blebs/pods and bands) and forms hard concretions where better developed/crystallized (Fig. F8).
Subunit IB extends from 251.6 to 311.65 mbsf. Recovery decreases to 78% in this subunit, in part because of variable induration (e.g., intervals of ooze and chalk) and XCB coring. The core is moderately disturbed with common biscuiting and fracturing.
Subunit IB consists of alternating lighter and darker lithologies that exhibit gradational to bioturbated contacts. These lithologies are variably indurated, exhibiting alternations of ooze/chalk and clay/claystone within the core. The overall average carbonate content calculated for Subunit IB is 63 wt% (Fig. F5), reflecting a mixture of nannofossil ooze/chalk and nannofossil clay/claystone. Again, as in Subunit IA, the gradational and bioturbatively mixed lithologies (clayey nannofossil ooze/chalk and nannofossil ooze/chalk with clay) are often the dominant lithology. The colors of the end member lithologies are more strongly distinct than those of Subunit IA, reflecting contrasting differences in carbonate and clay content. The lighter-colored lithologies range from very light gray (N8) to moderate yellowish brown (10YR 5/4) and very pale orange (10YR 8/2). Measured carbonate values for these lighter units range up to 89 wt%, whereas the darker units range as low as 28 wt%. Darker intervals range in hue from dusky yellow (5Y 6/4) to moderate yellowish brown (10YR 5/4). Figure F8 shows that the darker beds (carbonate minima) in Subunit IB are somewhat more calcareous (>20 wt%) than those at the base (<20 wt%) of Subunit IA.
Bioturbation ranges from rare to abundant (Figs. F12, F13, F14). The color contrast between light and dark layers serves to highlight burrow structures. The trace fossil assemblage includes common Zoophycos (Fig. F12), Chondrites (Fig. F12), Planolites (Fig. F13), and composite burrows (Fig. F14). Pyrite is present in some burrows in Core 198-1208A-28X and in Sections 29X-1 and 29X-2 but was not noted in the remainder of the subunit.
Smear slide analyses indicate that nannofossils are the dominant component of the lighter intervals, with generally lesser quantities of clay minerals (7%-25%), foraminifers (3%-5%), carbonate (3%-6%), biosiliceous material (1%-8%; diatoms, radiolarians, sponge spicules, and silicoflagellates), volcanic glass (trace), and quartz (trace). In comparison, the darker intervals generally contain higher percentages of biosiliceous material (5%-17%) and clay minerals (20%-45%).
Subunit IC extends from 311.65 to 327.66 mbsf. As in Subunit IB, the core exhibits moderate drilling disturbance (biscuiting), but core recovery rates were slightly higher (86%) in Subunit IC.
This subunit consists of alternating meter- to decimeter-thick intervals of (1) dark yellowish brown to grayish brown nannofossil clay/claystone, and/or claystone with nannofossils, and/or claystone (at the base of the subunit), and (2) lighter grayish orange to yellowish brown nannofossil ooze/chalk. A limited number of carbonate analyses from this unit indicate a range of values (36-83 wt%) similar to those of overlying Subunit IB. However, in Subunit IC there is a greater proportion (~66%) of thicker, darker, carbonate-poor beds. Smear slide estimates of composition (see "Site 1208 Smear Slides") indicate that the darker beds are relatively clay rich (50%-93%) and carbonate poor (0%-50%), with traces of volcanic glass and up to a few percent radiolarians. Lighter beds contain less clay (5%-30%) and more nannofossils (60%-90%). Both light and dark layers contain trace to minor amounts of zeolites (phillipsite?), foraminifers (trace-10%), and opaque minerals, including iron oxides.
Subunit IC is characterized by variable bioturbation, from abundant to rare. Discrete trace fossils include Zoophycos, Planolites, and composite burrows. Downsection, contacts between lighter and darker intervals become less bioturbated and sharper. In addition, the darker lithologies become increasingly depleted in carbonate, culminating in barren claystone at the base of Section 198-1208A-36X-CC (see Fig. F17), just above the unconformity that also serves as the Unit I/II boundary (Section 198-1208A-36X-CC, 25 cm). Biostratigraphic data indicate that this is a condensed section (see "Biostratigraphy"). A digital image across the unconformity and into the base of a condensed section is shown in Figure F15, along with the magnetic susceptibility and reflectance records.
Unit II consists of relatively pure nannofossil ooze with minor (<1%) chert. The overall recovery rate is 54%, with better core recovery in chert-free intervals. The chert hampered drilling rates and reduced the downhole recovery rate to 0.03%. Generally the core in Unit II appeared undisturbed except in areas where chert had been fragmented (drilling breccia).
The ooze has an average carbonate content of 96 wt%, making this a relatively pure pelagic unit. In addition to the dominant nannofossil component, smear slide estimates of sediment composition (see "Site 1208 Smear Slides") list minor (trace-5%) percentages of foraminifers, micrite, radiolarians, chert, and clay. The nannofossil ooze is dominantly very pale orange (10YR 8/2) but varies to pinkish gray (5YR 8/1) at the base. The chert fragments in the core range from yellowish to reddish brown in the Campanian section but are dominantly moderate reddish brown (5YR 5/4) in the Albian section (Core 198-1208A-42X). The only chalk in the working half of the core encrusted the surface of chert fragments, but in addition to chert fragments, the micropaleontology sample from the core catcher included 5 cm of whitish chalk.
The unit is generally moderately bioturbated, with burrow mottling but no identifiable trace fossils. Larger white blebs of nannofossil ooze may be burrow fills or diagenetic features. Two intervals in Core 198-1208A-41X contain fragments of Inoceramus: one interval is unconsolidated (fragmented during drilling?), and the other is cemented.
Unit II is relatively unconsolidated ooze that has undergone little cementation. However, authigenic carbonate (silt-sized, rod-shaped crystals) identified in smear slides may be incipient cement.
The homogeneous to moderately bioturbated nature of the Subunit IA sediment implies that it was deposited under oxic conditions. The prominent color and compositional cycles in Subunit IA are similar to those observed in Subunit IA at Site 1207. As at Site 1207, the prominent cyclicity within this unit is likely tied to variations in productivity and/or carbonate dissolution. However, at Site 1208 the cycles appear to be somewhat less dependent on variations in biosiliceous material and more dependent on the interplay between clay and calcareous nannofossil fluxes. The darker-colored intervals generally do contain somewhat greater amounts of reasonably well-preserved biosiliceous material than the light-colored intervals; thus, the former may represent periods of higher surface water productivity and higher clay input from zonal winds.
The cycle frequency in Unit I appears to be similar to that of glacial-interglacial cycles, with the dark beds representing glacial periods and the light beds representing interglacials. A preliminary analysis of the frequency of the dark-light cycles using color reflectance data (Fig. F16) suggests that the dominant period of the cycles corresponds to eccentricity (100 k.y.) from 0 to 0.8 Ma and obliquity (41 k.y.) for the period from 0.8 to 2.7 Ma, but a trend that represents the long period of eccentricity is superimposed on the latter cycles.
Regional studies (e.g., Natland, 1993) of ash distribution across the Shatsky Rise suggest that ash beds present in Subunit IA are wind-borne sediments that likely were carried to the site from volcanic eruptions along the Japan and/or Kurile magmatic arc systems. The maxima and subsequent waning of ash input expressed in the frequency of ash beds in Table T2 could be linked to changes in volcanic activity along the arc systems or to changes in wind patterns over time. Isolated pumice clasts were most likely rafted to the site by surface currents (Kuroshio) that pass across the submerged, pumice-producing calderas of the Izu-Bonin magmatic arc to the west (described in Taylor et al., 1990) and flow directly over Shatsky Rise.
As in Subunit IB at Site 1207, the sediment of Subunit IB at Site 1208 is more oxidized, as indicated by the yellow to orange hues and an increase in the red/blue reflectance ratios. Cyclic dark and light intervals are present throughout the subunit, but the average thickness of the cycles (decimeter scale) is less than those in Subunit IA (meter scale). The low average sedimentation rate of 5.9 m/m.y. (Fig. F23) in combination with sediment color and composition suggests that, as at Site 1207, these oxidized sediments are products of lower surface water productivity, slower sediment accumulation, and/or carbonate dissolution. Note that the estimated sedimentation rate does not take into account the effects of sediment compaction, and thus these rates may have been slightly higher.
The condensed zone within Subunit IC (Fig. F17) probably reflects the influence of strong currents sweeping the top of the Central High and the effects of carbonate dissolution. These conditions probably prevailed during much of the Paleogene, preventing the accumulation of significant amounts of sediment. The CCD is estimated to have been at 3-3.3 km in the Pacific through much of the Paleogene and early Miocene, with the exception of the Oligocene when it deepened by 1 km (van Andel, 1977; Rea and Leinen, 1985; Rea et al., 1995). The condensed interval is slightly thicker at Site 1208 than at Site 1207, suggesting either less erosion or dissolution at Site 1208. Higher sedimentation rates at Site 1208 deterred the growth of authigenic phases such as manganese oxides (no macroscopic concretions), phillipsite (only minor amounts in smear slide estimates of composition), and chert (none observed in Unit I). The alternating carbonate- and clay-rich lithologies suggest that the process(es) responsible for slow sedimentation was somewhat cyclic, perhaps related to fluctuating CCD levels (discussed above) and/or intensity of erosional currents. Furthermore, the transitional nature of Subunits IC and IB suggests that the intensity of this process(es) gradually waned over time.
Unit II is a pelagic unit, deposited in open ocean conditions above the CCD.
The similarity of the lithologic units/subunits at Sites 1207 and 1208 is paralleled by their similar diagenetic features and histories: (1) in Subunit IA, the green-gray color, presence of pyrite, and "diagenetic laminae" and (2) formation of chert and lack of cementation in Cretaceous oozes ((see "Diagenesis," in "Lithostratigraphy" in the "Site 1207" chapter for discussion of the origin of these features).
Additionally, ooze/chalk transitions at Site 1208 occurred sporadically throughout the Neogene sequence. This suggests that in a setting like the Shatsky Rise, the degree of ooze lithification (cementation) may be more tied to sediment composition (e.g., clay content and mineralogy, siliceous microfossils, organic content) and texture (degree of bioturbation, abundance of foraminifers) rather than to burial history (see "Physical Properties" in the "Site 1210" chapter for further discussion).
The lithologic units and subunit boundaries at Site 1208 roughly correlate with changes in sedimentation rate (see "Sedimentation and Accumulation Rates"). The Campanian to Holocene section recovered at Site 1208 is in many ways similar to the section encountered at Site 1207 on the Northern High of Shatsky Rise; lithologic units are based on roughly parallel changes in sediment character, which correspond to similar breaks in rates of sedimentation. Two notable exceptions are the thicker condensed section above the Paleocene/Campanian unconformity (discussed previously) and the expanded Pliocene-Pleistocene section at Site 1208. Prior to ~3 Ma, sediment accumulation rates at Site 1208 (22.3 m/m.y.) were approximately the same as those at Site 1207 (18.4 m/m.y.), but after 3 Ma, the rates at Site 1208 increased to 42.4 m/m.y. (Fig. F23). This rate of sedimentation is surprisingly high and warrants further discussion.
Site 1208 is situated on the Central High of the Shatsky Rise. A seismic reflection profile across this site shows a stratified lens of sediment that extends from a canyon, which is interpreted as an erosional trough to the west and laps onto a sediment-covered basement high to the east. The seismic character of the sedimentary section, along with the very high sedimentation rates that prevailed during the Neogene-Quaternary, strongly suggest that the stratified lens of sediment is a drift deposit formed by current redistribution of sediment that settled on the Central High.
Whereas currents likely played a part in the formation of the Campanian/Paleocene unconformity and overlying Paleogene to Miocene condensed section (Subunit IC), there is no distinct sedimentological evidence (e.g., winnowed lags and contourites) to support drift deposition in the overlying section (Subunits IB and IA) at Site 1208. One exception may be the high-angle contacts in this uppermost part of Subunit IA; similar features on the Southern High of the Shatsky Rise have been interpreted as erosional features (e.g., Storms et al., 1991). Alternatively, evidence for winnowing, such as thin foraminiferal lags, could have been subsequently destroyed (dispersed) by burrowing organisms. But significant winnowing seems unlikely because, in the same intervals that are likely to have been affected by such currents (Subunit IA), bioturbation was not of sufficient intensity to completely disperse centimeter-scale ash beds. This analogy may not completely hold because it assumes that the burrowing organisms did not eschew the ash layers.
Perhaps the more classical contour deposits are concentrated within and adjacent to the trough, and Site 1208 received the fallout from current-generated fine-grained suspensions that drifted from the furrow to the east, most likely as a nepheloid layer. Such a mechanism should then have been active throughout the history of the drift, not only in the post-3 Ma portion of Subunit IA. Interestingly, there are post-3 Ma increases in the percentages of biosiliceous (Fig. F6), terrigenous (Fig. F6), and pyroclastic (Table T2) components at Site 1208. The concomitant increase in these components helps explain the high sedimentation rates across the post-3 Ma interval, but this rise in "background" sedimentation may not fully explain the magnitude of that increase. Alternatively, this site may have been situated in a favorable position with respect to atmospheric circulation patterns to receive a higher proportion of eolian and/or ash components, as well as to preserve a higher proportion of siliceous microfossils from 3 Ma to the present.
The sediment drift deposits at Site 1208 are somewhat similar to those drilled along the Meiji Seamount on Leg 145 in that both comprise fine-grained sediment devoid of sedimentary structures except for bioturbation (Rea et al., 1993). Furthermore, the post-3 Ma sections at the Meiji sites are generally more enriched in glacially derived components (e.g., dropstones) and ash beds (Rea et al., 1995). However, these higher latitude sites experienced an earlier pulse in biosiliceous sedimentation in the Miocene to early Pliocene not recorded at Site 1208 (Rea et al., 1993, 1995); at Site 1208, this shift occurred in the late Pliocene and continued into the Pleistocene.
