At Site 1201, sedimentary successions were recovered in four holes, 1201A, 1201B, 1201C, and 1201D. Recovery was good (88%), and a nearly complete sedimentological succession was recovered from the seafloor to the basement at 509 mbsf. Hole 1201A was abandoned after the first core because of mechanical difficulties. A very short interval (1.6 m) of pelagic sediment was recovered. Pelagic sediments and cherts were recovered in Holes 1201B and 1201C. Hole 1201B penetrated deeper (81.07 mbsf) than Hole 1201C (48.10 mbsf) by using the XCB after APC refusal and recovered interbedded sandstones and bioturbated silty claystones. Hole 1201D was drilled down to 80.40 mbsf before core recovery began and then penetrated to a depth of 591.06 mbsf. The contact between the sedimentary succession and basaltic basement was present at 509 mbsf. The sedimentary succession in Hole 1201D is characterized by interbedded volcaniclastic sandstone and bioturbated silty claystone with rare breccia intervals. A short interval of claystone immediately overlies the basement contact (Fig. F5).
The sedimentary succession at Site 1201 has been divided into two lithostratigraphic units. Unit I consists of clays, cherts, and interbedded sandstones and silty claystones that contain a significant red clay content. The uppermost portion of Unit I is characterized by pelagic clays with increasing silica diagenesis downcore, indicated by the presence of cherty intervals. The lowermost portion of Unit I is characterized by interbedded pelagic claystones, with sandstone and silty claystone turbidites. Unit II underlies Unit I and consists of interbedded sandstones and silty claystones. These sediments are composed of detrital volcaniclastic material that has been reworked by turbidity currents. The lowermost portion of Unit II consists of quiescent marine claystone. This claystone facies is present immediately above the basaltic basement contact. Interpillow material in the underlying basalts includes sedimentary material with an unclear relation to the sediments of Unit II.
Interval: Sections 195-1201A-1H-1, 0 cm, to 1H-CC, 18 cm; 195-1201B-1H-1, 0 cm, to 8X-1, 150 cm; and 195-1201C-1H-1, 0 cm, to 6H-CC, 27 cmUnit I consists of the uppermost 53 m of sediments recovered in Holes 1201A, 1201B, and 1201C. These sediments are predominantly strong brown to yellowish brown and contain a significant pelagic red clay content. Three distinct sedimentary facies can be identified. Facies IA is present in the uppermost 25 m of Site 1201 and is characterized by bioturbated and massive silty clays with manganese nodules. Facies IB consists of yellowish brown to dark brown cherts that are interbedded with the silty clays of Facies IA above and are also present as massive intervals in Cores 195-1201B-6H and 195-1201C-6H. Facies IC consists of interbedded sandstones and bioturbated silty claystone (Figs. F5, F6).
Facies IA is present in the uppermost 25 m of Site 1201 (Hole 1201A; Sections 195-1201B-1H-1, 0 cm, through 3H-6, 115 cm [0-25.35 mbsf]; and Sections 195-1201C-1H-1, 0 cm, through 3H-6, 94 cm [0-24.54 mbsf]) (Figs. F5, F6). Facies IA is characterized by strong brown to dark brown, bioturbated, massive silty clays with manganese nodules (Fig. F7). The silty clays contain a significant red clay content typical of pelagic sediments. A large (>3 cm) fragmented manganese concretion is present in intervals 195-1201A-1H-1, 64-67 cm, 195-1201B-1H-1, 28-36 cm, and 195-1201C-1H-1, 20-37 cm, and sand-sized manganese nodules are abundant throughout. Isolated rare, soft, fine-grained green nodules are present throughout Facies IA (e.g., Section 195-1201B-2H-5, 73 cm). An interval of white sand-sized fragments is present in interval 195-1201B-3H-4, 18-21 cm. These green and white fragments have been identified in smear slides and by X-ray diffraction (XRD) analysis as authigenic zeolites. A discrete interval of soft green zeolites suggests a possible hiatus surface in Holes 1201B and 1201C (intervals 195-1201B-3H-6, 56-58 cm, and 195-1201C-3H-6, 80-82 cm). This zeolite-rich hiatus surface is overlain by clay-rich sediments with abundant bioturbation and is underlain by coarser sediments (Fig. F8). The location of this hiatus surface is consistent with the middle Miocene unconformity identified at a depth of 24.8 mbsf in the age-depth plot for this site (see "Paleomagnetism" and "Biostratigraphy").
Facies IB is characterized by chert (Figs. F5, F6). Chert is present as isolated dark brown to yellowish brown firm to hard intervals within the silty clays of Facies IA (Fig. F9). The first isolated cherts are present in intervals 195-1201B-3H-6, 117-120 cm, and 195-1201C-3H-6, 94-96 cm, and become increasingly abundant downcore, culminating in massive chert in Cores 195-1201B-6H and 195-1201C-6H. The chert intervals contain abundant sand-sized manganese nodules. Rare intervals contain moderate bioturbation and sedimentary structures, such as planar and cross laminations.
Facies IC consists of interbedded sandstone and bioturbated silty claystone with rare breccia. It is present in Sections 195-1201B-7X-1, 0 cm, through 8X-1, 150 cm (46.70-53.40 mbsf). The claystones contain a significant red clay content similar to the pelagic sediments of Facies IA identified above. The sediments are yellowish brown to dark brown. They are often normally graded, and the sandstones are often planar and cross laminated (Fig. F10). There is microscopic evidence to suggest that these sediments incorporate reworked material from underlying Unit II sediments (see "Smear Slide Analyses"). A pumice ash layer was identified in smear slides from an interval of sand-sized alternating black-and-white laminae (interval 195-1201B-8X-6, 76-78 cm). This material appeared to be less consolidated than the surrounding sediments. This is significant, as this was the only unreworked ash layer positively identified at Site 1201. The reworking of most of the sedimentary succession at Site 1201 likely obliterated most primary tephra layers.
Interval: Sections 195-1201B-8X-2, 0 cm, to 11X-CC, 37 cm, and 195-1201D-1R-1, 0 cm, to 45R-5, 90 cmUnit II constitutes the remaining recovered sedimentary section below Unit I at Site 1201. The transition from Unit I to Unit II is indicated by an abrupt color change from brown to greenish gray. This color change is associated with a change in the mineralogical composition of the sediments (Figs. F5, F6). In Unit I, a major component of the sediments is red clays. In Unit II, the red clays are absent and the sediments are composed primarily of reworked detrital volcaniclastic material (see "Smear Slide Analyses" and "X-Ray Diffraction Analyses"). Two distinct sedimentary facies can be identified. Facies IIA is the predominant sedimentary facies of Unit II and extends from the boundary with Unit I down to within 2.5 m of the contact with basaltic basement at 509 mbsf. Facies IIA is characterized by interbedded volcaniclastic sandstones, bioturbated silty claystones, and breccia. Facies IIB is characterized by reddish brown bioturbated claystone with some faint planar laminations. It occurs interbedded with Facies IIA near the base of the recovered sedimentary section at Site 1201 and then as a predominantly massive unit for the remaining 2.5 m immediately overlying basaltic basement (Figs. F5, F6). Sediments are present as interpillow material in the underlying basalts, but their relation to Facies IIB sediments is unclear.
Facies IIA is characterized by interbedded sandstones, bioturbated silty claystones, and breccia consisting of volcaniclastic material. Texturally, it is very similar to the overlying sediments of Facies IC (e.g., Fig. F11); however, a marked color and mineralogical change denotes the unit boundary. Facies IIA extends through the remaining recovered interval of Hole 1201B (Sections 195-1201B-8X-2, 0 cm, through 11X-CC, 37 cm [53.40-81.07 mbsf]) and through the majority of the sedimentary succession recovered from Hole 1201D (Sections 195-1201D-1R-1, 0 cm, through 45R-3, 90 cm [80.40-507.20 mbsf]). Facies IIA is predominantly dark greenish gray and becomes increasingly grayish green downhole (Figs. F5, F6). Pelagic red clays are absent. Detrital volcaniclastics and authigenic zeolites are the primary constituents of the sediment (Fig. F6).
Much of the succession is normally graded at multiple scales (Figs. F11, F12). In these fining-upward sequences, bioturbated silty claystones overlie sandstones that often exhibit planar, cross, wavy, and rare trough cross-laminations. Basal contacts are sharp and erosional. Fining-upward sequences become increasingly finer grained downhole, with fewer intervals of massive sandstone and no breccia intervals (Fig. F5). Rare inversely graded intervals are present (e.g., interval 195-1201B-14R-1, 2-53 cm). Calcareous intervals in silty claystone and calcareous sand-sized fragments in sandstone (overcalcified foraminifers) (see "Biostratigraphy") are present throughout Facies IIA.
Rounded green clasts of zeolite-cemented sandstone are present in Section 195-1201D-1R-2 as rare isolated clasts and increase in abundance downhole. Fine-grained bright green zeolite-cemented intervals are first observed in Section 195-1201D-17R-3 (Fig. F13). These isolated intervals (with a maximum thickness of 42 cm in interval 195-1201D-356R-2, 52-94 cm) increase in abundance downhole. Thin (<5 cm) white gypsum-bearing intervals are rare and are first observed in Section 195-1201D-32R-6. Thin (<5 mm thick) white zeolite veins oriented oblique to laminae are rare and are first observed in interval 195-1201D-14R-5, 99-103 cm. They are encountered again in Cores 195-1201D-24R, 30R, and 37R. Lastly, unusual thick laminae first occur in Section 195-1201D-31R-5. Sand-sized grains within the laminae are "inflated" by alteration. The laminae are 2 to 3 mm thick and have sharp upper and lower contacts that are wavy. The primary planar and cross laminations of the sediment are still identifiable, but the altered thick, wavy laminae make the structures appear "fuzzy" or out of focus (Fig. F14) (see "Smear Slide Analyses" and "X-Ray Diffraction Analyses").
Facies IIB consists of reddish brown bioturbated claystone with rare faint planar laminations. It occurs interbedded with Facies IIA in Sections 195-1201D-43R-1, 0 cm, through 44R-7, 71 cm (484.10-503.41 mbsf), then as a predominantly massive unit in Sections 195-1201D-45R-4, 0 cm, through 45R-5, 90 cm (507.36-509.76 mbsf), with the exception of faint planar laminations in the uppermost portion of Section 45R-4. A thin (<5 mm thick) white quartz layer is present in interval 195-1201D-45R-5, 63-63.5 cm (Fig. F15). The lowermost massive interval of Facies IIB is reddish brown and becomes dark brown toward the basement contact in Section 195-1201B-45R-5, 90 cm (Fig. F16). Similar fine-grained sediments are present as interpillow material in the underlying basalts. These sediments are mustard yellow, strongly altered, and include encrusted radiolarians. No age assignments or genetic relationships with the overlying Facies IIB sediments could be determined at this stage of investigation.
Besides visual core description and XRD analyses, the textural and compositional characteristics of the unconsolidated to semiconsolidated sediments of Unit I and uppermost Unit II in Holes 1201A and 1201B were inferred from smear slide analyses (see "Site 1201 Smear Slides"). The reddish to brownish silty clays generally have high clay concentrations, which mostly vary between 80% and 95%. Higher concentrations of silt-sized (15%-30%) and sand-sized (1%-5%) quartz, feldspar, and heavy-mineral grains in the reddish to brownish silty clays are confined to the uppermost 2 m of the recovered sediment succession. Down to ~10 mbsf, the red silty clays include small but ubiquitous portions of volcanic glass shards, which disappear downhole. Authigenic phillipsite crystals become common at the expense of volcanic glass. This distribution pattern might indicate the possible dissolution of volcanic glass and a related neoformation of zeolite promoted by low sedimentation rates.
The calcareous intervals of Unit I, encountered in Sections 195-1201B-4H-3, 5H-4, 9X-CC, and 10X-1, are mainly composed of calcareous nannofossils. The cherts of Facies IB include abundant opaline particles, which partly originate in encrusted radiolarian tests. The XRD results show no indication of the transformation of amorphous opal to a higher degree of crystal ordering. The cherts, therefore, actually represent protocherts or so-called "porcellanites." Furthermore, phillipsite lathes appear to be concentrated and amalgamated in the opal layers.
The interbedded sandstone-silty claystone succession of Facies IC again includes volcanic glass shards. The sandstones may bear detrital zeolite grains, which apparently were reworked from diagenetic cements of the underlying sandstones of Unit II (Fig. F17). One distinct laminated ash layer with pumice glass shards was identified in Section 195-1201B-7X-2 (Fig. F17).
Microscopic investigations of thin sections were carried out to verify the textural and compositional variations of the lithified sedimentary rocks of Unit II and to gain insights into their diagenetic alteration (see "Site 1201 Thin Sections").
From a sediment-petrographic view, the immature sandstones and fine-grained breccia of Unit II can be classified as lithic arenites with almost no quartz, high abundances of feldspar, and dominant rock fragments (lithoclasts). The detrital material is mostly derived from volcanic sources and was redeposited by sedimentary processes. This is demonstrated by current bedding and the admixture of volcanic clasts of different origins in a given sample, which is not consistent with pure tephra that should show a more homogenous composition. Furthermore, exotic clasts, such as light gray clasts of fossiliferous shallow-water limestone, are occasionally associated with the volcaniclastic material (Figs. F18, F19). Some of the samples include foraminifers and calcareous nannofossils that indicate open-marine conditions. Their scarcity can be related to high sedimentation rates of detrital material that dilutes the pelagic rain of biogenic matter.
The composition of the lithic arenites is consistent with the provenance and detrital input from an undissected magmatic arc source (Fig. F20) (Dickinson, 1985). An undissected arc represents a juvenile magmatic arc that was not eroded and unroofed to its roots where coarse-grained intrusive rocks prevail. Sediment supply from such a source provides a large amount of fine crystalline, sand-sized volcanic rock fragments. Alternatively, the erosion of plutonic rocks from a dissected arc produces more sand-sized quartz and feldspar.
The volcanic clasts of Unit II are diverse in texture and appear to be of similar basaltic to andesitic composition. They comprise more or less vesicular porphyries and vitrophyres that include plagioclase, clinopyroxene, and, to a lesser extent, hornblende and orthopyroxene phenocrysts embedded in a glassy to hypocrystalline groundmass (Figs. F18, F19). The plagioclase crystals are typically zoned with sodium-rich margins and calcic cores, pointing to hypabyssal crystallization of the phenocrysts (Fig. F21). Tentatively, four volcanic rock varieties were distinguished, although transitional types are also present. The most abundant and variable type is black to grayish brown plagioclase-augite porphyry/vitrophyre with relatively few vesicles and a dark tachylitic groundmass. This rock type also appears as an oxidized reddish scoria, which represents a minor but ubiquitous component in the arenites of Unit II (Fig. F19). Another subordinate rock variety of more dacitic composition is made up of aligned plagioclase laths in a dark, fine groundmass (pilotaxitic "trachytic" texture). The most conspicuous rock type consists of vitric fragments of sideromelane with abundant bubble-shaped vesicles separated by wide glassy bubble walls and low concentrations of crystalline phenocrysts (Fig. F18). Fragments of phenocrysts from these diverse rock types are also present as single detrital grains in the sandstones, particularly in the fine-grained sandstones. The siltstones consist almost exclusively of fine detrital glass shards of sideromelane.
The downhole distribution of these diverse rock fragments shows high variability. From the analysis of a few thin section samples, there seems to be a crude trend toward more abundant porphyries/vitrophyres in the upper part of Unit II and larger amounts of sideromelane in the deeper part (see "Site 1201 Thin Sections"). This trend might be attributed to an uphole grain size coarsening, as porphyries/vitrophyres tend to be enriched in the coarse sand- to granule-sized fraction. According to the criteria proposed by Fisher and Schmincke (1984), this might indicate a change in the overall mode of volcanism. Subaqueous and subaerial explosive eruptions at an early stage are documented by the sideromelane fragments. A higher contribution of eroded subaerial tephra and effusive rocks during a later stage are indicated by the tachylitic porphyries/vitrophyres. The presence of red oxidized scoria throughout the section, however, indicates that subaerial volcanics were also produced during the early stage. This type of scoria forms around terrestrial volcanic vents, where lithified lava is maintained at high temperatures for extended periods while in contact with the atmosphere.
Another important aspect of Unit II is a high degree of diagenetic alteration, which shows a depth-dependent pattern, especially in the paragenesis of zeolite minerals. Although the exact identification of zeolite and clay minerals awaits further confirmation by shore-based studies, some general downhole trends seem to be evident on the basis of shipboard findings from thin sections and XRD results. The upper clastic rocks of Unit II are relatively fresh and grade into strongly altered sequences farther downhole. Throughout the unit, pore-lining clay rims around the detrital grains, which also form the coatings of vesicles in the sideromelane clasts, likely initiated the cementation and induration of the sediments (Fig. F21). Authigenic phillipsite crystals, which characterize the red silty clays of Unit I, also appear in the sedimentary rocks of upper Unit II, where they are present as prismatic lathes in the pore spaces (Fig. F21). Below ~100 mbsf, zeolite minerals in the pore spaces occasionally appear as pore-filling fibrous spherolites composed of phillipsite and, possibly, clinoptilolite and analcime (Fig. F22). In some samples, carbonate patches replace detrital grains or appear as pore fillings.
Intergranular to poikilotopic cementation of pore spaces and the replacement of glass in the sideromelane clasts by sparry zeolites is evident below ~180 mbsf and becomes pervasive below 250 mbsf (Fig. F23). The remaining lower part of Unit II, with its bright green layers, shows increasing palagonitization of silt-sized glass shards to zeolites and green clay minerals. Larger grains in many of the sandstones are also affected by strong alteration. They were replaced by zeolites and only show the ghosts of their former detrital precursors, which likely consisted of unstable vitric clasts (Fig. F24). Since the plagioclases and augites are not affected by alteration at this depth, the high abundance of altered sand grains gives further evidence for compositional variations of source materials through time, with a higher contribution of pure volcanic glass during an earlier stage of volcanism. Below 400 mbsf, several generations of zeolite minerals (chabazite and erionite) are associated with sulfate minerals (Fig. F25).
Results from semiquantitative XRD analyses on the composition of the sediments and sedimentary rocks of Units I and II confirm the findings from the other applied methods, with additional detail (Table T2; Figs. F26, F27). The XRD results provide further insight into the downhole distribution of quartz, which is difficult to identify in the smear slides and thin sections. Higher quartz abundances are confined to the uppermost silty clays of Unit I down to 15-20 mbsf and decline progressively toward the upper part of Unit II at ~80 mbsf. In the upper 5 m, quartz is associated with illite, which is not present in the remaining part of the section, pointing to a higher proportion of terrigenous dust in the uppermost sediments. In most clastic components of Unit II, quartz ranges near the limit of detection, consistent with sediment provenance from basic to intermediate volcanic sources. Slightly higher quartz concentrations are again present in the lowermost red claystones of Unit II.
The mineralogical patterns inferred from the XRD results nicely illustrate the downhole change of diagenetic assemblages with an increase of gypsum and Ca-rich zeolites such as chabazite (CaAl2Si4O12·6H2O) and erionite ([Ca,K2,Na2]2Al4Si14O36·15H2O) at the expense of phillipsite ([K,Na,Ca]1-2[Si,Al]8O16·6H2O), analcime (NaAlSi2O6·H2O), and clinoptilolite ([Na,K,Ca]6[Si,Al]36O72·20H2O) below 250 mbsf. At this depth, a marked change in clay mineralogy is evident with the first appearance of well-crystallized smectite downhole. Conversely, the upper portion of the section is dominated by various types of expandable and poorly crystallized clay minerals. The changes in diagenetic minerals are consistent with pore water profiles at Site 1201, which exhibit the presence of extremely CaCl-rich interstitial waters below 250 mbsf (see "Geochemistry"). Similar diagenetic patterns and pore water profiles were reported from lithic arenites of the Izu-Bonin forearc sedimentary basin (Egeberg et al., 1990; Marsaglia and Tazaki, 1992).
Site 1201 is located just west of the Palau-Kyushu Ridge axis along the eastern edge of the western Philippine Basin. Deposits similar to those encountered at Site 1201 have been described from earlier drilling in the West Philippine Basin during DSDP Leg 31 (Site 290) (Karig, Ingle, et al., 1975), DSDP Leg 59 (Site 447) (Kroenke, Scott, et al., 1981), and in the Izu-Bonin forearc sedimentary basin during ODP Leg 126 (Sites 792 and 793) (Shipboard Scientific Party, 1990a, 1990b). The paleoenvironmental setting for the sedimentary succession recovered at Site 1201 is summarized in Figure F28. Our model of sedimentation is similar to others constructed for basins in the West Pacific (e.g., Marsaglia et al., 1995 [their fig. 20]; Clift et al., 1994; Rodolfo, 1980). The remainder of this section is discussed within the context of the stages of sedimentation outlined in Figure F28, starting with the oldest sediments recovered at Site 1201. The final paragraph addresses the history of diagenesis at Site 1201.
The sedimentary succession at Site 1201 begins immediately above the contact with basaltic basement at 509 mbsf (Fig. F16). Claystones of Facies IIB were deposited in a quiescent marine environment with a possible eolian component (Figs. F6, F15, F28). Facies IIB claystones contain quartz and clay minerals and are derived from the erosion of a source that is different from that of the overlying turbidite facies, which is composed of detrital volcaniclastic material. Facies IIB sediments are fine grained and largely structureless. The brown color of the sediments can be attributed to many possible factors. The claystones may have originated as terrigenous sediments that were oxidized subaerially and subsequently redeposited in the marine environment. Alternatively, slow sedimentation rates may have enabled oxidation of the sediments to occur with circulating bottom waters. The brown color may also be the consequence of the influence of hot pore fluid from the basement interacting with the claystones. Geochemical gradients suggest that the latter scenario is likely the case for the lowermost sediments (see "Geochemistry"). Although the claystone includes a high proportion of iron, which might indicate metalliferous sediments, the high aluminum concentration is inconsistent with this assertion and could reflect the high concentration of pelagic clay minerals in the sediment. The cause of the high concentration of manganese in the claystones (up to 6%) was not determined on board ship and will be investigated in postcruise studies.
The majority of the sedimentary succession at Site 1201 records deposition of detrital volcaniclastic (Unit II, Facies IIA), and pelagic (Unit I, Facies IC) sediments by turbidity currents (Fig. F28). In Figure F28, subaqueous and subaereal volcanism is proposed as the source of the volcaniclastic material and a fringing reef/shelf environment is a likely source of the reworked carbonate material observed throughout Facies IIA. This is consistent with the timing of the growth and maturity of the Palau-Kyushu magmatic arc proposed in studies from the West Pacific (e.g., Bloomer et al., 1995 [their fig. 4]).
Eruption-driven turbidity current sequences were continuously pumped into basins surrounding the arc as described by Underwood et al. (1995 [their fig. 1]). The petrographic composition of Facies IIA sediments with almost no quartz and abundant plagioclase feldspars and lithic fragments (Fig. F20) is consistent with a nearby sediment supply from an undissected arc environment (Dickinson, 1985; Valloni, 1985). Sediment supply from a relatively young magmatic arc is also inferred by the composition of feldspars and lithoclasts that indicate basaltic to andesitic volcanism.
The turbidites in the sedimentary succession at Site 1201 exhibit an uphole trend from low-energy turbidites to high-energy turbidites (Figs. F5, F6). This uphole trend can be attributed to many possible factors; these are indicated in Figure F28. Changes in sediment supply, slope gradient, proximity to source, tectonics, erosion, and sea level could all be responsible, in whole or in part, for the energetics of the turbidity currents.
A higher relative sea level farther down in the section is consistent with a backarc basin environment experiencing rapid initial rifting and subsidence. Subsidence associated with rifting followed by uplift associated with the growth of the magmatic arc could greatly increase the slope gradient over time and would result in high-energy turbidites characterized by coarser sediments higher in the section. This interpretation may be supported by preliminary observations of seismic profiles that suggest a graben structure underlying Site 1201. Shore-based studies of seismic profiles are required to further substantiate this observation. Coarser sediments could also be produced by an increase in volcanic activity or an increase in erosion of the source volcanic rocks, as indicated in Figure F28.
Unit I sediments are indicative of a pelagic environment. Pelagic sediments are incorporated into the uppermost turbidite sequences. Over time, however, the sediment supply from the volcanic arc ceased. The lack of sediment feeding the depositional system caused the turbiditic environment to be replaced by one that is characterized by slow pelagic sedimentation with an eolian component (Fig. F28). The increase in terrigenous quartz and illite in Unit I might be an indication of stronger dust supply from Eurasia during late Cenozoic climate deterioration.
According to the age-depth plot from Site 1201, the sediments at the top of the sedimentary succession are early Pliocene in age but the rest of the section is missing (see "Biostratigraphy" and "Paleomagnetism"). To determine whether bottom currents at Site 1201 might be responsible for this hiatus, the position of a sonar beacon was tracked during its release and ascent from the seafloor on two occasions using the ship's DP system. On one occasion, the beacon rose straight to the surface, but on a second, it encountered an 8-cm/s current bearing 200° south-southwest throughout the bottom 1000 m of the water column. A bottom-water current with this velocity could hold pelagic sedimentation in suspension, thereby depleting the site of pelagic sediments younger than the onset of the current. In any case, the absence of pelagic sedimentation since the early Pliocene suggests a significant change in the paleoceanographic regime in the region at that time.
Unit I sediments underwent increasing levels of silica diagenesis downcore, with firm to hard cherty intervals high in opal content (Fig. F9) culminating in massive chert (porcellanite) intervals in Cores 195-1201B-6H and 195-1201C-6H.
Many features of the Facies IIA sediments are indicative of increased diagenetic alteration of the sediments with depth. The general color trend downhole from dark greenish gray to grayish green indicates increased zeolite content with depth. Green sandstone clasts cemented with zeolites and fine-grained bright green zeolite-cemented intervals increase in abundance downhole. White zeolite veins and gypsum-bearing intervals also increase in abundance, although they remain rare. Unusual inflated laminae in which primary planar and cross-laminar structures are distorted by sand-sized grains that are inflated by diagenetic alteration are first observed in Core 195-1201D-31R and increase in abundance downhole. These "zeolitized" sediments indicate increasing alteration with depth.
Three possible causes might explain the diagenetic pattern at Site 1201: (1) fluids derived from seawater and hydrothermal fluids from the underlying basalts could meet and interact with solid phases in the sediment pile, (2) burial diagenesis may promote the alteration of unstable glass shards with depth, and (3) the degree of diagenetic alteration may be related to compositional and textural changes, with stronger alteration in the deeper section, where abundant silt-sized glass shards favor chemical attack because of their larger particle surface area. A combination of the second and third processes seems most likely. The composition of the pore water appears to be the result of the diagenetic reactions and indicates a present state of equilibrium between the interstitial fluids and the solid phases. The rough estimate of the modern geothermal gradient of 12.5°C/100 m at Site 1201 (see "Physical Properties") supports the assumption of burial diagenesis under moderately elevated temperatures. Accordingly, the thermal exposure of the strongly altered material reached at least 50°C. The observed paragenetic assemblage, on the other hand, suggests burial temperatures of 85° to 125°C (see Fisher and Schmincke, 1984, and references therein). This difference suggests that the geothermal gradient at Site 1201 may have been higher in the past, for instance, during a time when this location was a focus of magmatic arc development during the Eocene to Oligocene.