Next Section | Table of Contents


Principal Results

The principal objective at Site 1201 was to install a long-term borehole seismic observatory in the middle of the Philippine plate. Although this was successfully accomplished, the observatory will not be activated until its brains are installed during an ROV visit in the fall of 2001 and no data will be recovered until it is revisited in 2002. In the meantime, the core recovered during the course of preparing the hole for the observatory produced striking and, in some cases, unexpected results.

Drilling at Site 1201 yielded a composite 600-m-thick section consisting of 510 m of Miocene through late Eocene sediments and 90 m of basalt. The sedimentary section consists of two lithostratigraphic units (Fig. F20). The uppermost unit (0-53 mbsf) consists of soft pelagic clays, cherts, and interbedded sandstones and silty claystones that contain significant amounts of red clay. The underlying unit (53-512 m) is composed of a thick section of interbedded turbidites composed of detrital volcaniclastic material and traces of reef detritus from the Palau-Kyushu Ridge, which range in size from coarse sandstones and breccias through silty claystone to claystone. The individual turbidite layers range from tens of meters to a few millimeters in thickness and tend to decrease in thickness and grain size downsection (Figs. F21, F22), reflecting a gradual change from high-energy to low-energy deposition. The basal 20-30 m of the unit consists of interbedded turbidites and reddish tan to chocolate brown claystones deposited in a quiet marine environment. One of the most striking features of the entire sediment section at Site 1201 is the color of the turbidites, which range from dark gray to dark greenish gray in the upper 240 m of the unit, where the volcaniclastics are (relatively) fresh, and then range from deep green to gray-green to the base of the unit. Thin section and XRD analyses show that these changes are related to progressive alteration with depth, including the devitrification of glass, the replacement of the calcic cores of plagioclase by clays, and the infilling of voids and vesicles by clays and zeolites in the upper part of the unit and the wholesale replacement of volcaniclastic material in the lower part of the unit by smectite, chlorite, and zeolites (chabazite, erionite, heulandite/clinoptilolite, and analcime/wairakite) during diagenesis.

The composition of the interstitial water at Site 1201 is very unusual for deep-sea sediments and reflects the profound diagenesis that has occurred in the turbidites in the lower part of the section. The most striking feature is an extremely large increase in pH, Ca, and chlorinity with depth in the pore water; whereas seawater is mainly a sodium chloride solution, the altered seawater near the base of the sediments is mainly a calcium chloride solution (Fig. F23). Calcium increases to 270 mmol/kg, 27 times the concentration in seawater, by leaching from the volcaniclastic material. Similarly, chlorinity increases to 645 mmol/kg, 20% higher than seawater values, due to the removal of water during the formation of hydrous minerals such as clays and zeolites. The gain in Ca is balanced by the removal of 70% of the Na (to 140 mmol/kg) and the loss of nearly all of the Mg and K from the seawater during the formation of clay, smectite, and zeolites. Sulfate decreases as well, from 28 to 15 mmol/kg, by the precipitation of gypsum in response to the elevated Ca concentration. Alkalinity falls from the seawater value of 2.4 to <1 meq/kg as it is consumed by the precipitation of authigenic minerals. The rise in pH to 10.0 from the seawater value of 8.1 also reflects extreme alteration. Many of the pore water gradients in the top of the turbidite section can only be supported by ongoing reactions, which is consistent with the fact that the volcaniclastics at this level are not yet completely altered. Deeper in the section, however, many of the geochemical gradients approach zero, implying that equilibrium has been achieved and that the geochemistry observed is that of "fossil" pore water.

To reconstruct the geological history of the site and determine the timing of diagenesis, it is necessary to look at the microfossil and paleomagnetic record. The topmost (0-29 mbsf) and lowermost (462-509 mbsf) sections are barren of nannofossils, but moderately to poorly preserved nannofossils in the middle section allowed us to recognize six biozones spanning NP19/NP20 to NP25 (Fig. F24). The turbidites between 53 and 462 mbsf represent an expanded sequence of late Eocene to early Oligocene age. Separated by a short hiatus and lying on top of the turbidites is a 25-m sequence of upper Oligocene (NP25) red claystone. Compared to DSDP drilling results at Sites 290 and 447 (Karig, Ingle, et al., 1975; Kroenke, Scott, et al., 1981), the upper Eocene sediments (>34.3 Ma) recovered at this site are the oldest so far identified on the sedimentary apron of the Palau-Kyushu Ridge. Because the 47-m interval overlying basement at the site could not be dated on board ship, this is clearly a minimum age; dating of this critical interval must await the results of shore-based radiolarian studies.

Preliminary interpretation of the magnetic inclination record identified 64 reversals of the geomagnetic timescale in the sediment section. Although the Pliocene-Pleistocene section (0-5 m.y.) is apparently missing, the barren pelagic sediments in the top 29 mbsf provided an excellent record from the Thvera Subchron (C3n.4n) through the late and middle Miocene polarity intervals to Subchron C5Bn.1n or close to the base of the middle Miocene (Fig. F25). Major unconformities are present between 14.8 and 24.1 Ma and in the top section of Biozone NP24 at ~25-30 Ma. Surprisingly, the magnetic inclination record in the turbidites between 100 and 500 mbsf defines several long normal and reversed polarity chrons (C12n-C16n.2n) that are well constrained by biostratigraphic ages. The combined biostratigraphic and paleomagnetic results show that the sedimentation rates were moderate (35 m/m.y.) in the late Eocene, then very high (109 m/m.y.) in late Eocene-early Oligocene time, when the turbidites were being deposited, and then decreased to very low values (3 m/m.y.) during the Miocene, when the pelagic sediments at the top of the section were being deposited (Fig. F26).

Although the age of the basement could not be determined aboard ship, its composition and provenance are clear. The 90 m of basement drilled at Site 1201 consists of altered pillow basalts having a composition that is transitional between that of arc tholeiites and MORB and backarc basin basalts (Fig. F27). Geochemical and thin section analysis shows that the basalts have been strongly weathered, especially at the contact with the overlying sediments, where they show significant Na uptake and depletion in Ca. Hyaloclastites in the section have been palagonitized and altered to smectite, and interpillow sediments recovered from within the upper 10 m of basement contain marine microfossils, indicating eruption in a marine environment. Magnetic inclinations in the basaltic basement are shallow and indicate a position of the Philippine plate near the equator, at ~7° paleolatitude, during the Eocene.

From the data provided above, it is evident that the basement at Site 1201 formed near the equator by submarine eruption during the Eocene before 34.3 Ma. The composition of the basalts, which are transitional between island arc tholeiites and MORB or backarc basin basalts, suggests they erupted in an arc or backarc setting. The absence of calcareous nannofossils and the presence of siliceous microfossils in the interpillow sediments and pelagic sediments immediately overlying the basement suggests that the basement formed in a deep water environment below the carbonate compensation depth (CCD) (Fig. F28).

Beginning in the late Eocene and continuing into the early Oligocene (from ~35 to 30 Ma.), pelagic sedimentation at the site became mixed with, and was finally overwhelmed by, increasingly thick, coarse, and energetic turbidites composed of arc-derived volcaniclastics and reef detritus. The composition and timing of the turbidites is consistent with a source to the east in the Palau-Kyushu Ridge, which was an active arc from ~48 to 35 Ma (Arculus et al., 1995) and only began to subside at ~28 Ma (Klein and Kobayashi, 1980). The presence of scoria and rounded lithic clasts in the volcaniclastic breccias at Site 1201 is consistent with subaerial erosion, but the absence of plutonic fragments indicates that the ridge remained undissected (Dickinson, 1985; Valloni, 1985). The upward coarsening of the turbidites can be attributed to many possible causes, including changes in arc elevation and erosion, sediment supply, proximity to source, tectonics, sea level, and slope gradient. Alteration of the volcaniclastics would have commenced immediately after deposition and is continuing to the present in the upper part of the section, but diagenesis would only have begun when the turbidite section became sufficiently thick for the temperature in the lower part of the section to reach 85° to 125°C (Fisher and Schminke, 1984).

Between the late Oligocene and early Pliocene, the deposition of turbidites came to an end and pelagic sedimentation resumed at Site 1201 as subduction and volcanism moved to the Marianas in response to plate reorganization and the Palau-Kyushu Ridge subsided. Finally, even pelagic sedimentation ceased at ~5 Ma, presumably in response to bottom currents caused by a change in bottom-water circulation.

Next Section | Table of Contents