SITE 1149 PRINCIPAL RESULTSSite 1149: Early Cretaceous Seafloor Subducting at the Izu Bonin Trench
Water depth (m): 5818
Total cored section (mbsf): 0-191.2
Core recovery (%): 91
Water depth (m): 5818
Total cored section (mbsf): 160.6-445.2
Core recovery (%): 12
Water depth (m): 5818
Total cored section (mbsf): 283.6-322 and 388.2-426.7
Core recovery (%): 8.3
Water depth (m): 5867
Total cored section (mbsf): 272.2-440.4
Core recovery (%): 17
Although previous drilling was plagued by hole stability problems in chert horizons, Site 1149 was the first to be continuously cored through sediment to basement in the Nadezhda Basin, a ~1000 km x 1000 km region seaward of the Izu-Bonin Trench. The sediment/basement contact was cored in three Holes (1149B, 1149C, and 1149D), and a total of 133 m of basement penetration was achieved in Hole 1149D. Thus, the site objectives to core and log the entire sedimentary sequence and to core the upper oxidative alteration zone in basement were successfully carried out after 20 days of operation. A presite seismic survey produced an excellent seismic stratigraphy that will enable correlation of sedimentary units at Site 1149 across the Nadezhda Basin and along the Izu-Bonin Trench.
APC coring recovered ~100% of the uppermost 160 m in Hole 1149A, whereas a combination of XCB, MDCB, and RCB systems completed coring in the 410-m-thick sedimentary section in Holes 1149A and 1149B, albeit with much lower recovery rates (<15%). An excellent set of geophysical geochemical logs from Hole 1149B will enable reconstruction of the sedimentary strata not recovered during drilling. Holes 1149C and 1149D were drilled primarily to penetrate as much basement as time allowed. Only 26 m of basement was cored in Hole 1149C before chronic sticking problems prompted abandoning the hole. A thinner sedimentary section and basement high was located on seismic line C2005 a few miles to the southeast of Holes 1149A, 1149B, and 1149C, where conditions were more promising for basement drilling. Despite some sticking and hole collapse problems, which slowed drilling and prevented logging of the basement section, 133 m of basement were successfully cored at Hole 1149D with 17% recovery. Site 1149 is one of only six other ODP/DSDP sites with significant (>100 m) penetration of basement into Mesozoic oceanic crust.
Site 1149 Geophysics
Site 1149 is located on the Pacific plate in the Nadezhda Basin southeast of Japan. It is on a slight bathymetric high ~100 km east of the Izu-Bonin Trench where the Pacific plate is flexed upward before it enters the subduction zone. Although Nakanishi et al. (1988) identified magnetic Anomaly M12 in the vicinity of Site 1149, close inspection of the lineation pattern suggests instead that Site 1149 lies on the older portion of magnetic lineation M11. This stratigraphic position has a paleontological age of late Valanginian and a radiometric age of ~132 Ma on the Channell et al. (1995) time scale. A model consistent with other portions of the magnetic lineation pattern of this age gives a constant spreading rate of 51 km/m.y. for this portion of the seafloor. These rates are comparable to the lower end of the range of spreading rates for the modern East Pacific Rise.
The seismic stratigraphy at Site 1149 is based on two 1976 vintage multichannel seismic lines and sonobuoy data obtained during cruise C2005 of the Robert Conrad, as well as a short, pre site single-channel seismic survey conducted aboard the JOIDES Resolution. Site 1149 acoustic stratigraphy conforms to that originally described by Ewing et al. (1968) for large portions of the western Pacific, consisting of (1) an upper transparent layer (weakly reflective), (2) an upper opaque layer (highly reflective or stratified), and (3) acoustic basement, or Horizon B. Early DSDP investigations (e.g., Legs 6, 7, 17, and 20) found that the upper transparent layer corresponds to a variety of lithofacies: pelagic clay with ash in the west Pacific, pelagic clay in the central Pacific, turbidite sequences in the north and east, and biogenic oozes along the equator. At Site 1149, this top acoustic layer corresponds to ash and diatom/radiolarian-bearing clay. The upper opaque seismic layer has been correlated to the uppermost abundant chert in much of the North Pacific. Drilling at Site 1149 encountered this chert at ~180 mbsf, where it corresponds to a continuous, high-amplitude reflection at ~0.2 seconds below seafloor (sbsf). Within the opaque layer, there is a change in seismic character at ~0.28 sbsf to discontinuous, chaotic to hummocky reflections, which corresponds to a change from chert/clay to chert/chalk (+ marl) at Site 1149.
It was not until Legs 129 and 185 that material below "Horizon" B was sampled in the Mariana, Pigafetta, and Nadezhda Basins. Horizon B in large areas of the Nauru, Pigafetta, and East Mariana Basins correlates to middle Cretaceous volcanic material (sills, flows, volcanogenic turbidites), and it was not clear if this material would be encountered at Site 1149 below Horizon B at ~0.42 sbsf, or whether this would correspond to true oceanic basement. Weak and discontinuous reflections at ~0.7 sbsf in the original multiseismic measurement (MSM) air-gun records at preliminary site BON-10 also prompted concern that basement could lie as deep as 700 mbsf. Drilling in Holes 1149B and 1149C, however, encountered oceanic basement at 410 and 401 mbsf, respectively, thus demonstrating that Horizon B in the Nadezhda Basin corresponds to true oceanic crust. Based on the few records available, the seismic stratigraphy of the Nadezhda Basin is laterally continuous and correlates well with the lithostratigraphy developed for Site 1149.
Site 1149 Sediments
The sedimentary section (between 0 and ~410 mbsf) recovered at Site 1149 consists of carbonate-free clays with variable admixtures of volcanic ash and siliceous microfossils, cherts porcellanites, and calcareous nannofossil chalks or marls. On the basis of the distribution of these lithologies, the sedimentary column above the basaltic crust has been divided from top to bottom into five lithologic units (Fig. 14):
Unit I (0-118.20 mbsf) consists of carbonate-free ash and diatom/radiolarian-bearing clay. Volcanic ash is present as both discrete ash layers and disseminated throughout the clay sequences. Thickness of the discrete ash layers typically varies from a few millimeters to 5 cm; thicker layers (20-45 cm), however, are also present. This unit has been dated late Miocene (i.e., 6.5 Ma) to late Pleistocene, based on an excellent magnetostratigraphic record (Fig. 15). Sedimentation rates during deposition of Unit I as derived from the shipboard magnetostratigraphy were on the order of 18 m/m.y. (Fig. 16). The abundant assemblage of siliceous plankton, mainly diatoms, silicoflagellates, and radiolarians yield a preliminary age of Pliocene for Core 185-1149A-9H, in agreement with the paleomagnetic data.
Unit II (118.20-179.1 mbsf in Hole 1149A and 160.6-180 mbsf in Hole 1149B) consists of undated dark brown pelagic clay with several discrete ash layers present only in the upper 30 m of the unit (Unit IIa) and notable palygorskite in the lower half (Unit IIb). Unit II clays are also noted by a change in porosity, pore water chemistry, and bulk sediment chemistry. Clays from Unit II are barren of siliceous or calcareous microfossils, and thus their ages at this point remain undetermined. These clays, however, contain ichthyolith assemblages that increase in abundance downhole, probably as a result of a decreased sedimentation rate. Shore-based analyses of the ichthyolith assemblages should allow for relative ages for this unit to be determined. Clays in Units I and II indicate pelagic deposition below the CCD.
Unit III (180-191.2 mbsf in Hole 1149A; 180-282.30 mbsf in Hole 1149B). The top of this unit is marked by the first occurrence of indurated siliceous lithologies (i.e., chert and porcellanites). This unit is characterized by very low recoveries (<5%). The recovered sediments consist of radiolarian chert, porcellanite, and zeolite-bearing clay, whose age is undetermined at this point. These sediments are typical of a predominantly siliceous depositional environment.
Unit IV (282.30-416.40 mbsf) consists of radiolarian chert, porcellanite, marlstone, and chalk with intercalated calcareous lithologies. Shipboard biostratigraphy of carbonates recovered in Cores 185-1149B-16R to 29R yielded well to poorly preserved calcareous nannofossil assemblages with average high diversity and good preservation. Preliminary ages are assigned from the Hauterivian Lithraphidites bolli Zone from 311 to 340 mbsf (Core 185-1149B-18R to 21R). Downhole assemblages are dominated by Watznaueria barnesae, Cruciellipsis cuvillieri, and Tubodiscus sp. The first occurrence of T. verenae is in Core 185-1149B-24R and indicates the lowermost Hauterivian-uppermost Valanginian. Rucinolithus wisei, a species that may be restricted to the Valanginian, is present from Core 185-1149B-25R downhole. Persistence of T. verenae downhole to Core 185-1149B-29R confirms a Valanginian age for the basement/sediment contact, consistent with the assigned M11 seafloor magnetic anomaly (132 Ma). The sedimentation rates during this time interval (i.e., 125.8-~132 Ma), derived from calcareous microfossil biostratigraphy, is ~20 m/m.y. (Fig. 16). Sediments from this lower unit were likely deposited when the site reached subequatorial paleolatitudes, which were characterized by high primary productivity.
The relatively simple stratigraphy and sedimentological changes at Site 1149 provide an exceptional natural laboratory to examine diagenetic processes operating over a long time scale (~135 m.y.) in a sequence bounded by basaltic crust and the oceanic reservoir. Interstitial waters recovered in sediment from the seafloor to the basement from Holes 1149A and 1149B reflect a low supply of organic matter, alteration of volcanic ash and authigenic clay formation at mid-depths, diagenesis of biogenic opal and carbonate, and on-going basement alteration. Dissolved phosphate and ammonium have well-defined shallow maxima, although absolute concentrations are relatively low, reflecting low organic matter contents. Alkalinity remains <3 mM throughout the entire sequence, and sulfate is present at all depths (minimum value of 19 mM). The deep brown pelagic clays of Unit II define a sequence that is undergoing active authigenesis, acting as a sink for dissolved silica, strontium, and potassium, and as a source of alkalinity, ammonium, and lithium. X-ray diffraction (XRD) results and the concentration profile of dissolved silica together reflect the diagenetic transformations of opal-A to opal-CT (at ~170 mbsf) and of opal-CT to diagenetic quartz (at ~310 mbsf). Basement alteration and the relatively low diffusivity of Units IV and V lead to strong diffusive gradients in Ca and Mg, with extreme enrichments in Ca recorded (135 mM) in interstitial waters from a few meters above the basement contact (407 mbsf in Hole 1149B). Uptake of potassium and sodium, and high dissolved Cl (638 mM near basement) reflect hydration and alteration of the oceanic crust. These dramatic gradients and fluxes in the interstitial waters record ongoing alteration in the oceanic crust ~135 Ma after it formed.
Bulk chemical analyses of the sediments at Site 1149 corroborate several of the processes inferred from the interstitial-water analytical results, as well as reveal others that are better recorded in the bulk solid phase. In stratigraphic order, Site 1149 sediments preserve a well-developed metalliferous sedimentary profile in Fe/Al variations in the lower 130 m of section (280-410 mbsf), which documents clearly the decreasing influence of plume precipitation with lateral distance from the ridge. The biogenic rich sediments Units III and IV are high in Ba/Al, signaling an increase in biological productivity as the site lingers near the paleoequator. Unit II pelagic clays are highly enriched in K2O (> 4 wt%), recording the complement to the K uptake inferred from the pore waters, possibly because of authigenic formation of K-rich zeolites in these sediments. In addition to the discrete ash layers observed throughout Unit I and Subunit IIa, a depletion in Nb/Al with respect to average shales indicates a significant (25%-35%) dispersed ash component in upper sediments. Shore-based studies will help to identify the source of these ashes, most probably as either the Izu-Bonin or Japan volcanic arcs.
Downhole measurements in Hole 1149B were made after completion of drilling. Five logging runs were performed, consisting of one pass with the geophysical tool string, two passes with the geochemical string, and two passes with the Formation MicroScanner (FMS) sonic tool string. The geophysical string provided the most physically complete logging run from ~10 m above the volcanic basement contact at 400 mbsf (Core 185-1149B-28R) to within the clay-volcanic ash section at ~65 mbsf (Core 185-1149A-8H). The subsequent logs were limited by additional fill in the bottom of the hole and/or a 20-m-thick section of tight hole in the pelagic clays at 140-160 mbsf (Cores 185-1149A-16H to 18H) just above the uppermost cherts. Overall, the logging data agree very well with the lithologic units identified from cored intervals. Geophysical properties (e.g., resistivity, density, and P-wave velocity) delineate well the clay/ash layers, and the chert/clay layers. The radioactive element data identify regions predominantly composed of pelagic clay. The geochemical log data correlate Si-rich zones with chert layers, Ca-rich zones with the presence of nannofossil marls, and Al-rich zones with clay and ash layers. Calcium from the geochemical log is the best indicator of carbonate-rich sediment, a feature that the other logs do not distinguish well. The logging data will be essential to reconstructing the sedimentary section, which suffered from 6% average recovery in Units III and IV. Over 90% of Site 1149 was logged or recovered.
In contrast to the East Mariana and Pigafetta Basin sediments subducting at the Mariana Trench, the Nadezhda Basin sediments subducting at the Izu-Bonin Trench lack a mid-Cretaceous volcaniclastic section and contain more siliceous and carbonate rich biogenic material because of its longer passage beneath zones of high biological productivity. Shore-based geochemical studies will demonstrate the extent to which these clear differences in sedimentary lithologies can be traced to the volcanic output from the two arc systems.
Site 1149 Basement
The sediment/basement boundary was recovered in three holes: at 410 mbsf in Hole 1149B, at 401 mbsf in Hole 1149C, and at 307 mbsf in Hole 1149D. Approximately 35 and 26 m of basement was drilled in Holes 1149B and 1149C, respectively, before hole conditions halted operations; a thinner sedimentary sequence in Hole 1149D may have contributed to better hole conditions there, where 133 m of basement was drilled. The contact zone between the sediments and basement in Hole 1149B is brecciated and filled with interfragment sediment. Igneous units in all three holes consist of aphyric basalt pillows, thin flows, and interpillow breccia. Cooling units are on average <50 cm thick, in contrast with Hole 801C, where the cooling units averaged >50 cm. The predominance of these thin, fractured, and brecciated units at Site 1149 may have contributed to the overall low recovery of basalt (<20%). Plagioclase and olivine phenocrysts are rare (usually <1%). Most basaltic glass has been highly altered; fresh glass exists on a few pieces (~10), most of which are in Core 185-1149D-9R. The chemical compositions of the least altered lavas in Hole 1149B are fairly primitive (>7.9 wt% MgO) and low in Fe2O3 (<10 wt%) but otherwise typical of MORBs from the modern East Pacific Rise.
The volcanic rocks at Site 1149 are spectacularly altered with a pervasive dusky red alteration, which commonly displays a light gray to brown mottling. Complex, multicolored alteration halos also are present along fractures and other surfaces that were exposed to circulating fluids. The halos are up to 2 cm wide and range in color from brown to dark green. In addition to the abundant alteration halos, another striking feature of the basalt from all three holes is an abundance of reddish fracture surfaces. In general, the alteration is more intense at Site 1149 than at Site 801. At Site 1149, alteration halos comprise 34 vol% of the recovered basalts, as opposed to ~2 vol% at Site 801. Veining is also more intense, with 35 veins/m at Site 1149 compared to ~25 veins/m at Site 801. Finally, flow breccias and hyaloclastites are a common feature in the cored interval at Site 1149, whereas unlike Hole 801C, massive flows and interpillow sediments are uncommon. The more intense alteration at Site 1149 may lead to greater overall budgets for K than at Site 801 (e.g., >0.3 wt% K2O).
Although Hole 1149D was drilled in a basement high, the recovered basalts are similar in their igneous and alteration characteristics to Holes 1149B and 1149C basalts. Thus, the basement topography near Hole 1149D is probably related to near ridge processes (abyssal high formation?) rather than off-axis magmatism.
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