Coring at Site 801 started with a mudline core from Hole 801D that was cored with the RCB from the seafloor down to 19.3 mbsf. The relationship between the depth in core to the actual depth below sea level is not clear. The zeolite-bearing brown clay from Core 185-801D-1W is attributed to lithologic Unit I as defined during Leg 129 (Shipboard Scientific Party, 1990).
The homogeneous, dark reddish brown pelagic clays with zeolites in Core 185-801D-1W are identical to the lithofacies encountered in that depth interval at Site 801 of Leg 129. The sediment is carbonate free and mostly consists of colorless to reddish brown aggregates of clay to very fine silt size. These "red brown semi-opaque objects" are common in the pelagic brown clays at other Pacific sites (Yeats, Hart, et al., 1976). The clay becomes slightly darker downcore, but no corresponding compositional change was observed in smear slides. Zeolites are rare to common, but very conspicuous components, because there are hardly any other silt-sized or larger grains. Euhedral specimens display crystal habits, common brown inclusions, and etched crystal faces that are typical for phillipsite (Kastner, 1979). Many phillipsite crystals are twinned, some showing perfect cross twinning or complex-sector twinning. Accessory components include rare ichthyoliths, Mn micronodules, and trace amounts of volcanic glass, detrital quartz, and one suspect greenish tectite particle.
No bedding planes and only one potential burrow were observed in the core. The reason for the virtual lack of sedimentary structures is not entirely clear. The presence of tracer beads in the central parts of this core may suggest that the homogeneous aspect of the sediment is due to drilling disturbance (see "Hole 801D"), but their presence may also be related to the very high porosity of the sediment as suggested by the low bulk density (see "GRAPE Density Measurements"). Except for the uppermost 25 cm of the core, which is soupy, the clays resist gentle deformation. The relatively firm nature of the clays leads us to assume that severe drilling-induced disturbance was limited to the soupy interval at the core top and that the lack of structures is not drilling related. Original absence of compositional differences seems equally unlikely because it would imply that sedimentation did not change during the >10-m.y. interval represented by the core (assuming a sediment accumulation rate of <1 m/m.y.; Shipboard Scientific Party, 1990). As an alternative, we propose that the lack of discernible bedding planes and of discrete burrows is an effect of pervasive bioturbation. This is supported by the red sediment color, which suggests that oxygenation was sufficient to allow benthic activity to be vigorous enough to disrupt any original sedimentary structures. Moreover, the correct stratigraphic order of ichthyolith (fish remains) assemblages, which consistently testifies to a younging toward the top of the core (see "Biostratigraphy"), is incompatible with pervasive drilling disturbance of the sediments.
During Leg 185, interpillow sediments were recovered from ~90 m below the lowermost interpillow sediments recovered during Leg 129. The corresponding lithologic unit during Leg 129 was labeled "interbedded basalt and silicified claystone (chert)." We describe here only those materials that contain at least some pelagic material. During Leg 185, bright-colored sedimentary interpillow material was found ~150 m below Bathonian-Callovian (middle Jurassic) dated chert in Core 129-801B-39R and ~90 m below the deepest occurrence of interpillow material in Core 129-801C-5R to a total depth of 156 m into basement. Variously brown-colored and dusky red to dusky green chert and epitaxially recrystallized radiolarian limestone, as well as siliceous, yellowish brown alteration material, were found to contain spherical and cone-shaped objects that resemble radiolarians under the binocular microscope (see "Biostratigraphy," below). Occasionally, bedding surfaces in the chert and sand-sized chert intraclasts record redeposition phenomena.
Pelagic brown clays cored in Hole 801D are barren with respect to calcareous and siliceous microfossils but contain fairly well preserved ichthyolith assemblages. The >250-µm fraction contains only few ichthyoliths, whereas the >64-µm fraction contains common to abundant ichthyoliths. The core catcher and three additional samples were studied to obtain biostratigraphical constraints on these otherwise unfossiliferous clays. More than 40 species were recognized in the well-preserved assemblages. Many long-ranging species were recorded in the four studied samples, such as Rectangular saw-toothed, Triangle with triangular projection, Triangle with high inline apex, Triangle with base angle, and Five peaks irregular base. The occurrence of Narrow triangle ragged base in Sample 185-801D-1W-2, 32-40 cm, points to a late Miocene or younger age. In Sample 185-801D-1W-4, 80-88 cm, ichthyoliths are abundant; the last downhole occurrence of Circular with line across is recorded, indicating a middle Miocene age for this sample. The assemblage in Sample 185-801D-1W-6, 50-58 cm, records the last downhole occurrence of Three narrow peaks and the first downhole occurrence of Plain ellipse, suggesting an early Miocene age. Sample 185-801D-1W-CC contains Skewed four or five peaks, Polygonal cavity, Polygonal cavity long rays, Flexed-triangle shallow inbase >120, and Five peaks flared base. Co-occurrence of these species indicate a late Oligocene to early Miocene age for this sample, according to Doyle and Riedel (1985) and Firth and Hull (1993). Many undescribed morphotypes occur in the studied samples, together with several specimens that resemble undescribed morphotypes of possible Cretaceous samples. If this interpretation is confirmed by shore-based studies, the clays in Core 185-801D-1W could show reworking of Cretaceous sediments during the middle Miocene.
Interpillow sediment from Leg 129 was used for preliminary radiolarian biostratigraphic analysis. Precise dating of the basal sediments at Site 801, where the oldest (middle Jurassic) in situ oceanic crust has been recovered, is extremely important (1) in calibrating the oldest M-sequence magnetic anomalies, (2) to developing a Jurassic time scale for estimating seafloor spreading rates for the Pacific, and (3) improving plate tectonic models for the Pacific and other areas. During the transit from Hong Kong to Site 801, radiolarites from Cores 129-801B-37R, 39R, and 40R were resampled. The main objectives were to (1) revise the data of Matsuoka (1991, 1992) in light of the new data on radiolarian occurrence, systematics, and biochronology and (2) obtain a more precise age of the basal sedimentary unit (Unit V) and of the chert interbedded with basalts (Unit VI).
Unit V consists of alternations of red radiolarite and claystones; Unit VI is composed of interbedded basalt and silicified claystone (chert) (Shipboard Scientific Party, 1990). Matsuoka (1991) studied in detail two samples from the lower part of Unit V (Samples 129-801B-37R-1, 16-20 cm, and 35R-3, 24-26 cm). The faunal assemblage characterizes the middle part of the Tricolocapsa conexa Zone (Matsuoka, 1983; Matsuoka and Yao, 1986). Most of the species found have also been reported from the central Atlantic (Baumgartner, 1984; Yamamoto et al., 1985) and western Tethys regions (Baumgartner, 1984, 1987). Based on the correlation with the zonation proposed by Baumgartner (1984, 1987) for the Tethyan radiolarites, Matsuoka (1991, 1992) assigned a Bathonian-Callovian age to the Tricolocapsa conexa Zone. According to the most recent radiolarian biozonation of the INTERRAD Jurassic-Cretaceous Working Group (1995) Sample 129-801B-35R-3, 24-26 cm, can be attributed to the UAZ 6 (mid-Bathonian) and Sample 129-801B-37R-1, 16-20 cm, to the UAZ 5-6 (latest Bajocian-mid-Bathonian) (Matsuoka, 1995).
In this study, two new samples (Samples 129-801B-37R-1, 0-6 cm, from Unit V and 39R-1, 8-11 cm, from Unit VI) were examined in detail. Radiolarians are abundant, but preservation is generally poor. With careful hand picking it was possible to identify several specimens to the species level. Table T3 lists radiolarian species from the two samples. Thirty-five species are identified in Sample 129-801B-37R-1, 0-6 cm, and ten species in Sample 129-801B-39R-1, 8-11 cm. The difference in species diversity between the two samples may be because of the difference in preservation. In Sample 129-801B-37R-1, 0-6 cm, the co-occurrence of Orbiculiforma (?) sp. X, Hexasaturnalis tetraspinus, Tricolocapsa (?) sp. aff. T. fusiformis, and Spongocapsula perampla characterizes the UAZ 6 (mid-Bathonian). The faunal combination in Sample 129-801B-39R-1, 8-11 cm, of Guexella nudata, Stichocapsa robusta, Ristola (?) turpicula, Orbiculiforma (?) sp. X, and Tricolocapsa (?) sp. aff. T. fusiformis indicates the UAZ 5-6 (latest Bajocian-mid-Bathonian).
Based on the new work described here, and the new biochronology since Leg 129, we propose a revised age for the basal sediments at Hole 801C to latest Bajocian-mid-Bathonian. Shore-based studies may allow refinement of this age assignment.
Moreover, thin sections from Hole 801C interpillow material, which occurs ~170 m below these dated sediments, have revealed spherical and cone-shaped radiolarian morphotypes that belong to the spumellarian and nassellarian suborders (Fig. F4), as well as common fecal pellets.
The pelagic brown clays recovered at Hole 801D belong to a widespread depositional facies of the Cenozoic to Holocene Pacific. These clays are characterized by a deficiency in biogenic, volcanogenic, and terrigenous particles (e.g., Murray and Renard, 1891; Arrhenius, 1963). The lack of calcareous microfossils points to a deposition well below the Cenozoic open-ocean calcite compensation depth (CCD) (Berger, 1970). In addition, the absence of siliceous microfossils suggests that the clays were deposited below oligotrophic waters, similar to modern sediments from below the North Pacific Gyre (Davies and Gorsline, 1976). The rarity of volcanic and terrigenous detritus attests to distal position of the drill site. The very low accumulation rates on the order of <1 m/m.y., the relative abundance of ichthyoliths, and the presence of a suspect tectite fragment corroborate this interpretation.
Interpillow cherts and limestones recovered in Core 185-801C-13R and below contain replaced radiolarian tests as well as fecal pellets. These sediments were trapped between pillows and sheet flows, and the elevated temperatures associated with pillows and flows are likely to be responsible for the partial recrystallization of the sediment (see "Basement Alteration"). The presence of microfossils and metabolic products of organisms that thrive in the photic zone of the ocean suggests that normal pelagic sedimentation contributed, at least partly, to the interpillow material. Scarce nannofossils found during Leg 129 and the carbonate-bearing lithologies may either indicate deposition above the Middle Jurassic CCD or special conditions to enable the preservation of carbonate (Shipboard Scientific Party, 1990). The absence of carbonate in the sediments overlying the volcanic basement suggests that Site 801 had subsided to below the CCD by the end of volcanic eruptions.
The sedimentary section at Site 801 had already been drilled during Leg 129, and recovery of sediments during Leg 185 was limited to a wash core (Hole 801D) and to the interpillow sediment (Hole 801C) that is interbedded with basalts below 594 mbsf. Pelagic brown clays from the wash core contain very little terrigenous material and testify to the distal position of Site 801. The clays are barren with respect to calcareous and siliceous microfossils, but they contain rich ichthyolith assemblages that indicate a late Miocene to late Oligocene age for this core. Accumulation rates were on the order of <1 m/m.y., a value that is characteristic of the distal Pacific, away from volcanic arcs, riverine terrigenous input, the Asian dust plumes, or the oceanic zones of high biological productivity. The interpillow sediment consists of dusky red and dusky green cherts and recrystallized radiolarian limestone and is found more than 150 m below the sediment/basement contact; bedding surfaces and radiolarian ghosts are occasionally preserved.