Site 1179 Scientific Results
Site 1179 is located at 4104.8'N, 15957.8'E on abyssal seafloor north of Shatsky Rise. The site is situated on an east-northeast-trending magnetic lineation interpreted as M8 (Figs. 4, 11), making the lithosphere at this site ~129 Ma in age (mid-Hauterivian stage) according to the Gradstein et al. (1994) time scale.
Four sedimentary units, with a total thickness of 377 m, overlie basaltic crust at Site 1179 (Fig. 12, Fig. 13). From the seafloor downward they range in age from the present to an as-yet-undetermined time in the Early Cretaceous. Unit I, at the top of the section, consists of clay- and radiolarian-bearing diatom ooze (Fig. 14). It extends from the seafloor to a depth of 223.5 m, where it is late Miocene in age. The siliceous oozes of Units I and II have three principal components: diatoms, radiolarians, and clay, with proportions that vary from core to core. Diatoms predominate in Unit I; radiolarians are common and sponge spicules and silicoflagellates contribute to the siliceous nature of the sediment. A range of greenish colors and intervals of ichnofossils and laminations suggest that the diatom ooze was deposited in a dysoxic bottom environment; neither anoxic nor fully oxic conditions prevailed for any extensive length of time. Sediment accumulated faster in Unit I than in the other units, a factor that may have contributed to that environment. The diatom ooze resembles other Neogene sections cored in the northwest Pacific (Fig. 5), with its beds of gray silicic, vitric ash of a few centimeters thickness, numerous thin, firm dark green clay layers, and contributions of illitic clay, quartz, and glass within the ooze.
The contact between Units I and II is gradational, both in color and composition, as olive-colored diatomaceous ooze gives way to yellowish brown radiolarian ooze. The top of Unit II is placed where radiolarians predominate. This clay-rich, diatom-bearing radiolarian ooze is 22.5 m thick. It extends to 246.0 mbsf, where it is of late early Miocene age. Accumulation was at a slower rate than in Unit I, and the brown coloration and virtual lack of sedimentary structures (other than mottling by burrowing) indicate an oxic environment (Fig. 15). The base of Unit II is also a gradational one, where radiolarian remains vanish downward and the clay of Unit III prevails. In time, this contact represents the return of the deposition and preservation of siliceous microfossils in the North Pacific during the Miocene.
The pelagic brown clay of Unit III is about 37.5 m thick and extends down to ~283.5 mbsf. Its age is unknown, as it is barren of fossils except for a few fish teeth and bone fragments, and there is no clear pattern of magnetic reversals except in the very top part. The clay is zeolitic, ferrugineous, mottled, and compact (Fig. 16). Core recovery was excellent (102.4%) in all of Units I, II, and III. Unit IV is chert residing in an unknown host formation that was not recovered. It extends from 283.5 to 377.15 mbsf, and its age has yet to be determined. Recovery of this 37.5-m-thick section was poor (6.7%), both as a percentage of the penetration and in the degree of fracturing. Most pieces are of vitreous chert (Fig. 17), with a wide range of colors, mottling, healed brecciation, and veins, but a few are pieces of porcellanite. A fauna of poorly preserved radiolarians in the porcellanite appear to be Early Cretaceous in age and may allow better shore-based determination of age of at least part of the cherty section.
Core 191-1179D-10R recovered chert and basalt at a depth of 377 mbsf. The actual contact between the two lithologies was not recovered. Coring of basalt continued to a TD of 475 mbsf, ending with Core 191-1179D-22R. Basement rocks recovered in Hole 1179D consist mostly of fresh aphyric basalts in massive flows (Fig. 18), pillows, and breccia with a minor amount of interpillow sediments. The section is divided into 48 igneous units, based on lithologic differences and flow and/or cooling unit boundaries (Fig. 19). All basalts are classified into three petrographic categories: olivine-poor basalt (Group I), olivine-free basalt (Group II), and olivine-rich basalt (Group III), based on mineral presence and abundance in hand samples and thin sections. The basalt of the upper eight units mostly belongs to the olivine-poor group. The basalt of the middle 16 units mostly belongs to the olivine-free group. All of the basalt from the lower 24 units belongs to the olivine-rich group. The most distinct petrologic change among the cored basalt occurs between Units 24 and 25, where the difference in content of altered olivine grains causes a distinct color change.
Olivine-poor and olivine-free basalts are fine grained with subophitic texture, whereas the olivine-rich basalts have medium grain sizes with ophitic fabric in the thicker massive lava flows. Glass rims from chilled margins on pillows and massive lava flows are mostly palagonitized; near-border parts are now hyalopilitic or cryptocrystalline to microcrystalline. The groundmass of the basalts consists dominantly of lathlike plagioclases and clinopyroxenes (Ti-augites) and almost completely recrystallized glass/palagonite as mesostasis. Very fine-grained magnetite is concentrated in the altered glass; Cr-spinel, apatite, and very rare zircon are accessories. Olivine in the matrix of Group III basalts is nearly totally changed to iddingsite, as are the olivine phenocrysts in Groups I and III. Plagioclase phenocrysts are mostly fresh but in part corroded or replaced. The primary mineralogy of the basalt from Site 1179 is consistent with midocean ridge basalt rather than ocean island (alkali) basalt. Secondary mineralogy embraces calcite, celadonite/saponite, smectite, and zeolites, filling fissures, veins, and vesicles. Alteration of the basalt is surprisingly low and belongs to the low-grade zeolite facies in a possible temperature range of 10°-30°C.
Because Site 1179 has been beneath the CCD for tens of millions of years, biostratigraphy for this site relies mainly on the identification of siliceous microfossils and palynomorphs. Siliceous microfossils (radiolarians, diatoms, and silcoflagellates) are common and generally well preserved in lithologic Units I and II at Site 1179. They were absent from the red pelagic clays of lithologic Unit III and generally poorly preserved (recrystallized) in the cherts of lithologic Unit IV. Numerous radiolarian datums were identified, and upper Miocene to upper Pleistocene sediments can be assigned to established radiolarian zones, providing good biostratigraphic resolution. Two radiolarian species are identifiable in a single chert sample near the basaltic basement, indicating an Early Cretaceous age for the oldest sediments at this site.
The only core-catcher sample that contains calcareous microfossils (calcareous nannofossils and planktonic and benthic foraminifers) is Sample 191-1179B-4H-CC (36.02 mbsf). The foraminifers are not biostratigraphically useful, but calcareous nannofossils provide an early Quaternary age for this sample. There is no explanation at this time for the preservation of calcareous microfossils in this sample and in several other samples within cores of late Pliocene to early Pleistocene age, where routine inorganic geochemical analysis and palynological processing identified anomalously high CaCO3 content.
Agglutinated foraminifers are present in some upper Miocene to upper Pliocene samples, and the presence of the finely agglutinated taxon Spirosigmoilinella compressa constrains the 229.77-181.94 mbsf interval to middle to late Miocene. No calcareous nannofossils or foraminifers are found in lithologic Units III or IV. Terrestrial spores and pollen and marine dinocysts and acritarchs are present and moderately to well preserved in all samples examined in the upper ~144 m of lithologic Unit I. All of these samples are of late Pliocene to late Pleistocene age, except for the lowermost palynomorph-bearing sample examined (Sample 191-1179C-11H-CC; 143.84 mbsf), which is of late early Pliocene age. All other sediments in lithologic Unit I and all of lithologic Units II, III, and IV are barren of palynomorphs. A number of dinocyst datums can be identified, providing good stratigraphic resolution in sediments of late Pliocene to Pleistocene age. Terrestrial palynomorphs (pollen and spores) are most abundant in sediments of Pleistocene age, where they sometimes outnumber dinocysts, and in those upper Pliocene sediments with anomalously high CaCO3 content.
Sediment ages derived from biostratigraphic datums (radiolarians, dinoflagellate cysts, benthic foraminifers, and calcareous nannofossils) in the late Miocene to Holocene sequence at Site 1179 are plotted against depth in Figure 20. Biostratigraphic resolution is highest in Pleistocene sediments and lowest in the upper Miocene-lower Pliocene sediments. The shaded area encompasses the minimum and maximum ages for each sample, constraining sediment accumulation rates. Sedimentation rates were high and relatively constant throughout the late Cenozoic, averaging ~40 m/m.y. during the Pliocene-Pleistocene and ~30 m/m.y. during the late Miocene.
Sediments of Units I and II contain an excellent magnetostratigraphic sequence. In the six cores of Hole 1179B, the Brunhes (C1n) and Matuyama (C1r) magnetic chrons and the Jaramillo (C1r.1n) magnetic subchron are found, as well as the Cobb Mountain (C1r.2r1n) Subchron. The sedimentation rate for the Brunhes Chron, based on 31 m over 0.78 m.y., is 39.7 m/m.y. (Fig. 21). The first core of Hole 1179C contains the mudline and is in the Brunhes Chron. The hole was deepened to the approximate end of Hole 1179B before coring resumed. Cores 191-1179C-2H through 18H record the Matuyama (C1r), Olduvai (C2n), Reunion (C2r.1n), Gauss (C2An), Gilbert (C2Ar, C3n, C3r) and C3An magnetic chrons. The sedimentation rate between the Olduvai and Gauss Chrons is 42.8 m/m.y., and in the Gauss Chron it is 28.0 m/m.y. (Fig. 21). Magnetic polarity chrons were identified in Cores 191-1179C-19H through 23H, extending the magnetic stratigraphy of Hole 1179C from C3A back to the young end of C5Dr. The magnetic stratigraphy for Site 1179 thus consists of a complete record of virtually all the recognized polarity chrons from the mid-Miocene forward (Fig. 21).
The magnetization of the sediments in Cores 191-1179C-24H through 26X is predominantly normal polarity and could be from the Cretaceous normal superchron (121-83 Ma). If this interpretation is correct, there may be a hiatus between Cores 191-1179C-23H and 24H. Using accepted correlations between magnetic polarity and geologic time, the Miocene/Pliocene boundary is present near the middle of Core 191-1179C-15H and the middle/late Miocene boundary is near the bottom of Core 191-1179C-22H. Core 191-1179C-23H contains the deepest interpretable record and has the early/middle Miocene boundary in Section 5. The bottom of Core 191-1179C-23H has an age of ~17.6 Ma.
Using the magnetic polarity sequence, the sedimentation rate curve can be divided into three main segments with different slopes (Fig. 21). These three sections correspond to lithologic Units I, II, and III. Cores 191-1179C-22H and 23H, which consist of pelagic brown clay, display the slowest sedimentation rate (1.5 m/m.y.) for the upper part of Unit III. Sedimentation rates increase upward. Unit II, which spans Cores 191-1179C-22H through 20H, has a sedimentation rate of 7.6 m/m.y. For Unit I, the rate is 30.4 m/m.y. The curve implies either that Site 1179 drifted into an area of increased productivity in the late Miocene and Pliocene or that oceanographic conditions changed in such a way as to lead to a 200-fold increase in sedimentation.
Samples from the basaltic basement section of Hole 1179D give mainly negative inclinations consistent with reversed magnetic polarity at a site north of the equator. Reversed polarity was expected because the site is located on magnetic Anomaly M8. A preliminary paleolatitude for the basalt samples is 4.2° ± 9.9°.
Measured physical properties of sediments recovered at Site 1179 correlate well with lithostratigraphy. P-wave velocities in the sediments are typically 1530-1550 m/s. Average densities range from 1.265 to 1.450 g/cm3, and average porosities range from 67% to 83% (Fig. 22). Average densities and velocities in the basalts are 2.745 g/cm3 and 5002 m/s, respectively. Within Unit I, bulk density, thermal conductivity, and natural gamma radiation decrease with depth in the upper 150 m of the section and porosity increases to >85%. We suspect that these seemingly paradoxical trends in density and porosity are caused by a progressive increase in the relative abundance of diatom fragments, which have low grain densities and contain large volumes of intragranular pore space. The contact between Units I and II is marked by small changes in physical properties, but the transition from Unit II to the pelagic clays of Unit III is dramatic. Densities increase, porosities fall to <70%, and there are marked increases in natural gamma radiation and magnetic susceptibility. Marked changes in natural gamma radiation and magnetic susceptibility at a depth of ~265 mbsf also suggest a compositional change within Unit III.
Wireline logging data were limited to one tool string (the triple combination [triple combo]) and to depths of <300 mbsf, owing to bridging of the hole. The caliper data from 5777 to 5820 mbrf showed hole diameters ranging from 7 to 16 in. On most logging runs, the caliper remained fully opened (16.5 in). The triple combo measurements (particularly the APS) are affected by the large hole size; furthermore, the HLDT developed a functioning problem, so the density and porosity data should be treated cautiously. The DIT data clearly recorded the lithologic change between lithologic Units II and III, marked by an increase in the electrical resistivity at 243 mbsf. The boundary between Units III and IV is marked by a strong increase in resistivity (from 0.7 to 3.5 ohm-meters). The chert layers consist of alternating high resistivity layers (nodules) and interbedded low resistivity (clay rich) layers. The data from the MGT are well correlated with total natural gamma counts from the HLDT and present much higher vertical resolution and better defined layer boundaries. Gamma-ray measurements responded to the presence of ash layers in the uppermost part of the logged section. The DSA tool was deployed successfully on Cores 191-1179C-4H and 7H. The maximum pressure recorded by the tool was ~9000 psi, and shocks did not exceed 2.7 g. Plots of acceleration data show clearly the downhole heave and all significant events during core barrel hole penetration and recovery.
Sediment samples from Holes 1179B and 1179C were collected to characterize both the chemistry and microbial activity in this environment. Microbial activity will be inferred from incubation experiments and from shore-based lipid analysis. Four whole-round cores (WRCs) were collected from different depths (5, 30, 100, and 200 mbsf) for incubation experiments. In addition, a more extensive set of WRCs were collected at depths ranging from 0.05 to 278.12 mbsf for detailed analyses of bacterial lipid changes with depth. As certain cell membrane lipids are diagnostic of particular groups of bacteria, the lipid analyses may reveal the presence of different types of bacteria throughout the sedimentary column. Interstitial pore waters were collected for chemical analyses from the same approximate depths taken for lipids in order to relate the microbial communities to the geochemical sedimentary environment. Whole rounds cut on the catwalk were also sampled on several occasions for PFT contamination tests.
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