The primary objective at Site 1201 was to drill a cased reentry hole into basement and install a broadband seismic observatory with a battery assembly and data-recording unit. There were also numerous ancillary scientific goals related to cores or wireline logs obtained at the proposed drill site. We hoped to address the following topics: geochemistry, age and isotopic characteristics of the upper ocean crust, Pacific plate paleolatitude and tectonic drift, and Tertiary climate, sedimentation, and ash deposition. The sediment column and igneous basement at Site 1201 were cored successfully, and the borehole seismometer was emplaced as planned. Consequently, it should be possible to accomplish most of the scientific objectives related to coring and placement of the seismic observatory; many were accomplished during the leg itself. The logging program, however, was seriously curtailed owing to technical difficulties and the collapse of the borehole before the logging program in the lower part of the borehole could be completed.
Drilling at Site 1201 yielded a composite 600-m-thick sedimentary and basaltic section through Miocene to Eocene strata with three major unconformities, between 0 and 5 Ma, 14.8 and 24.1 Ma, and in the top section of Biozone NP24. Holes 1201A, 1201B, and 1201C were cored to a maximum depth of 90 meters below seafloor (mbsf) with the advanced piston corer/extended core barrel (APC/XCB) system and recovered soft pelagic sediments. Hole 1201D was cored with the rotary core barrel (RCB) to 600 mbsf with excellent recovery (78%). Hole 1201E consists of a cased reentry hole with the seismometer instrumentation cemented into the basaltic basement.
The sedimentary section at Site 1201 consists of two lithostratigraphic units. The uppermost unit (0-53 mbsf) consists of clays, cherts, and interbedded sandstones and silty claystones that contain a significant red clay content. The underlying unit (53-509 mbsf) is composed of interbedded dark gray and dark green to gray-green sandstone and silty claystone with breccia and claystone. These sediments are composed of detrital volcaniclastic material from the Palau-Kyushu Ridge that has been reworked by turbidity currents. Turbidites exhibit a downhole trend from high-energy to low-energy turbidites. Changes in sediment supply, basin slope gradient, proximity to the source arc, tectonics (uplift and volcanism), erosion, and sea level could be responsible, in whole or part, for the change in the energetics of the turbidity currents. The diagenetic history is documented by changes in clay mineral and zeolite assemblages with depth that are consistent with changes in pore water composition. A 2.5-m-thick marine claystone is present immediately above the basaltic basement contact at 509 mbsf.
Basement rocks consist of highly altered basaltic pillow lavas. Glassy pillow margin rims are observed in the core, along with hyaloclastite intervals. The rocks are aphyric to moderately plagioclase phyric in discrete intervals. Hypocrystalline rocks are most abundant in the upper part of the core but decrease with depth, correlating with the degree of alteration, which is very high in the upper tens of meters and gradually decreases in the lower part of the core. Alteration products are mainly clay minerals and zeolites in addition to minor carbonate. The original mineralogy consists of plagioclase, clinopyroxene in variable amounts increasing with depth, olivine, and opaque minerals. Preliminary geochemical data suggest a strong arc affinity.
The topmost (0-29 mbsf) and the lowermost (462-509 mbsf) sections are barren of nannofossils. Moderately to poorly preserved nannofossils in the middle section allowed us to recognize six biozones spanning Zones NP19/NP20 to NP25. The turbidites between 53 and 462 mbsf represent an expanded sequence of late Eocene to late Oligocene age. Separated by a short hiatus and lying on top of the turbidites is a 25-m-thick sequence of upper Oligocene (NP25) red claystone. The age of the sediments directly above the basement contact is unknown. Compared to Deep Sea Drilling Project (DSDP) drilling results at Sites 290 and 470, the Eocene sediments (>34.3 Ma) recovered at this site are the oldest so far identified on the sedimentary apron of the Palau-Kyushu Ridge.
Preliminary interpretation of the magnetic inclination record identified 64 reversals of the geomagnetic polarity timescale (GPTS). The top 29 mbsf provides a complete record from the Thvera Subchron (C3n.4n) through late and middle Miocene polarity intervals to Subchron C5Bn.1n, close to the base of the middle Miocene. Strata from the last 5 m.y. are apparently missing at Site 1201. The magnetic inclination record in the RCB section of the turbidite interval between 100 and 500 mbsf defines several long normal and reversed polarity chrons (C12n-C16n.2n) that are well constrained by biostratigraphic ages. Sedimentation rates decreased through time from the late Eocene-Oligocene (100 m/m.y.) to the Miocene-early Pliocene (3 m/m.y.) and reflect the tectonic and environmental history of the West Philippine Basin in the vicinity of the Palau-Kyushu Ridge. Magnetic inclinations in the basaltic basement are shallow and indicate a position of the Philippine plate near the equator at ~7° paleolatitude during the middle Eocene.
The composition of interstitial water at Site 1201 is quite unusual for deep-sea sediments and reflects profound diagenesis of the reworked volcaniclastic material that comprises the lower 350 m of the section. Most of the volcaniclastics have been replaced by clays and zeolites (smectite, heulandite/clinoptilolite, analcime/wairakite, erionite, and chabazite). The most unusual feature is an extremely large increase in pH, Ca, and chlorinity; whereas seawater is mainly a sodium chloride solution, this altered seawater contains mainly calcium chloride. Calcium increases to 270 mmol/kg, 27 times the concentration in seawater, by leaching from the volcaniclastic material. Chlorinity increases to 645 mmol/kg, 20% higher than the seawater value, by loss of water of hydration to the altered tuffs. The gain in Ca is balanced by a loss of nearly all the Mg and K from the seawater, but mainly by a 70% decrease in Na to 140 mmol/kg. Sulfate decreases as well, from 28 to 15 mmol/kg, by 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 precipitation of authigenic minerals. The rise in pH to 10.0 from the seawater value of ~8.1 likewise reflects extreme alteration at the in situ temperatures of 10°C or less.
The results of the multisensor track (MST) measurements showed significant variation in the physical properties of the three main lithostratigraphic units. These measurements show a correlation to individual sequences of turbidites and may provide scaling laws that could be used to characterize the turbidites and constrain depositional environments.
Hole 1201D was logged with the triple combination (triple combo) tool from 80 mbsf to total depth (TD) and with the Formation MicroScanner (FMS)/sonic tool from 80 to 366 mbsf. During the first run, hole conditions appeared to be good in the basement and sedimentary section with the exception of a few tight spots below bit size. Two logging units that correspond to the turbidite sequence and the basaltic basement can be distinguished. The logs show no apparent trends with depth other than a significant step at the contact between the sediment and the basaltic basement. Within the turbiditic sedimentary sequence, fining-upward cycles, which correlate well with the sedimentological description of the cores, can be discerned. It is an important result for the seismometer installation that the basement section is relatively homogenous and shows a slight increase in resistivity and a decrease in gamma ray values with depth, which may indicate a trend toward slightly less altered material.
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