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doi:10.2204/iodp.proc.344.105.2013 Lithostratigraphy and petrologySite U1412 was drilled to investigate the lithostratigraphy and structural geology of the slope-toe sequence and the uppermost portions of the basement, as interpreted in multichannel seismic reflection data. The primary goal of this site was to penetrate the décollement and investigate the fluid flow regime within the sediment and the oceanic crust. Four holes were cored at Site U1412, from 0 to 108.20 mbsf with the APC (Cores 344-U1412A-1H through 15H), from 108.20 to 304.34 mbsf with the XCB (Cores 344-U1412A-16X through 25X and 344-U1412B-3X through 20X), from 300.00 to 387.00 mbsf with the RCB (Cores 344-U1412C-1R through 10R), and finally, further downslope, from 350.40 to 363.98 mbsf with the RCB (Cores 344-U1412D-2R and 3R). Overall core recovery at this site was moderate (average = 59%) but became poor (<20%) toward the bottom of Holes U1412B–U1412D. Three units were distinguished in the sedimentary rocks (Fig. F3; Table T2). Unit I is predominantly dark greenish to greenish gray clay (50%) with small variations in grain size to silt (24%) and silty clay (26%) and contains eight tephra layers. This unit is terrigenous-influenced sediment. Unit I shows strong indications for gas expansion and gas hydrate disassociation. Unit II is a light to dark grayish brown calcareous nannofossil ooze with varying amounts of diatoms and sponge spicules. Primary sedimentary structures were not observed because of very strong drilling disturbances. Unit III is a sequence of grayish green clayey siltstone with minor interlayered sandstone and contains five dark grayish tephra layers. Its dominant matrix composition is terrigenous (clay) and contains nannofossils and radiolarian and diatom fragments. The sediment in Hole U1412D is slightly more clayey, is highly bioturbated, and shows a decrease in biogenic components. Description of unitsThe 387 m long combined sedimentary sequence of Hole U1412A (Cores 344-U1412A-1H through 25X), Hole U1412B (Cores 344-U1412B-1H through 20X), Hole U1412C (Cores 344-U1412C-1R through 10R), and Hole U1412D (Cores 344-U1412D-2R through 3R) is assigned to three lithostratigraphic units (Fig. F3; Table T2). Unit I (Holes U1412A and U1412B)
Unit I is massive dark greenish gray clay intercalated with silty clay horizons that become more calcareous rich toward the bottom of Hole U1412A (Fig. F4). Unit I contains eight tephra layers and a glauconite-rich layer. The clay sediment is slightly bioturbated and moderately well cemented by calcite. Silty clay horizons are dispersed throughout the length of the unit, with only a few silty-sandy horizons confined to two sections in the middle of Hole U1412A (Cores 344-U1412A-13H and 14H). Abundant sponge spicule–filled pods ranging in size from a few millimeters to ~1 cm are disseminated throughout the sediment, but their occurrence decreases toward the bottom of Hole U1412A, becoming rare in Hole U1412B. The tephra layers in Unit I range in thickness from 2 to 7 cm and are characterized by a dark or light greenish gray color, normal grading, sharp to gradational upper contacts, and sharp lower contacts (Fig. F5). Ash pods, a few millimeters thick, are disseminated throughout both holes. The glauconite-rich layer is ~10 cm thick and has contacts that are sharp to gradational (Fig. F6). Common centimeter-sized pods of glauconite also occur in the uppermost 10 m of both holes. The main matrix components of Unit I are terrigenous and dominated by clay. Smear slides indicate that the most common accessory grains in the silty clay include feldspar and lithic (sedimentary and magmatic) fragments. Trace components include amphibole, calcite, biotite, quartz, chlorite, glass shards, pyroxene, and opaque minerals. Biogenic components include nannofossils, diatoms, and radiolarian and foraminifer fragments. The nannofossil component becomes dominant with depth. Unit I is moderately to highly disturbed by gas hydrate in Hole U1412A and fractured by drilling in Hole U1412B. Unit II (Holes U1412B and U1412C)
Unit II is light to dark grayish brown silt-sized calcareous ooze with abundant biogenic silica (i.e., diatoms and sponge spicules) (Fig. F7). The massive upper part of the calcareous ooze becomes more lithified downhole and grades from very conspicuous white and laminated, 50 cm thick diatomaceous ooze into grayish brown nannofossil ooze (Fig. F8). Because of the high degree of drilling disturbance, no observations could be made regarding bioturbation. Good recovery and less disturbance in the lowermost portion of the unit allow for the identification of lamination and bedding structures, as well as reworked material. The main matrix components of Unit II are calcareous (nannofossils and foraminifers) and silicic (sponge spicules, diatoms, and radiolarians) biogenic components. Additionally, microscopic observations of smear slides indicate that the most common accessory grains in the ooze include feldspar, glass, and lithic (sedimentary and magmatic) fragments. Trace components include amphibole, calcite, chlorite, and opaque minerals. The uppermost 30 m of recovered sediment has a chaotic mousse-like structure due to the extensive drilling disturbance. Downhole, progressively coarse sand and pebble- to cobble-sized drill cuttings appear, often intermixed with fall-in material from the upper part of the hole. Unit III (Holes U1412C and U1412D)
Unit III is clayey siltstone with minor interlayered sandstone (Fig. F9) and contains five dark grayish tephra layers (two in Hole U1412C and three in Hole U1412D) that are as thick as 10 cm (Fig. F10). Sediment is dark grayish green clay with silt in Hole U1412D. Within core sections that experienced the least drilling disturbance, bioturbation appears to be quite abundant, with common burrows. The slightly calcareous clayey siltstone layers are well sorted and mostly massive. Rare laminations appear in both the siltstone and sandstone, with the latter occasionally showing reworked sedimentary lithic lenses (Fig. F9). The abundance of sand horizons increases gradually toward the base of the hole and becomes most abundant in Core 344-U1412C-8R. The sandstone beds do not show clear contacts with the underlying siltstones. Organic material (e.g., sapropel) in the sediment is generally sporadic but appears to increase in abundance toward the bottom of the hole. Siltstone and sandstone smear slides indicate that the dominant matrix composition is clay with abundant feldspar and sedimentary rock fragments, rare to common opaque minerals, calcite, quartz, hornblende, and glauconite, along with rare biotite, pyroxene, and volcanic glass. Biogenic components identified in smear slides include common nannofossils and fragments of foraminifers, radiolarians, and diatoms. Biogenic components are much diminished in Hole U1412D. X-ray diffraction analysesPreliminary X-ray diffraction (XRD) analyses of sediment from Holes U1214A–U1214C suggest that there is little variation in composition within the site (Fig. F11). Unit IThe major mineral components of Unit I are phyllosilicates, including chlorite and smectite, quartz, plagioclase, and calcite. Zeolites (laumontite and heulandite), analcime, hornblende, and pyrite are also present in all samples. Small variations in the proportion of calcite and hornblende are observed. Biotite was only observed in three samples. Unit IIThe major mineral component of Unit II is calcite. Other identified minerals include pyrite and dolomite (Fig. F11). Dolomite is present in various proportions until it becomes nearly absent from Section 344-U1412B-8X-CC to the bottom of Unit II. Based on the background XRD patterns, we can conclude that Unit II samples contain a large amount of amorphous material. Unit IIITwo Unit III samples show that the major mineral components are phyllosilicates, including chlorite and smectite, quartz, calcite, and plagioclase (Fig. F11). Zeolites and hornblende are present in small amounts. The presence of amorphous material in Unit III could not be established. Depositional environmentThe lithology of Site U1412 represents typical lower slope sediments that are dominated by terrigenous sediment. In the lower part of the hole (204 mbsf) the lithology (Unit II) changes abruptly to Miocene calcareous ooze similar to that found at Sites U1381 and U1414, before changing to Pleistocene hemipelagic sediment again. This alternating sequence probably reflects accretion of large slivers from the incoming frontal prism. |
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