| IODP Proceedings Volume contents Search | |||
 | |||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.346.109.2015 LithostratigraphyDrilling at Site U1428 penetrated to a maximum subbottom depth of 211.56 m in Hole U1428A, recovering a total of 178.86 m of sediment for a recovery rate of 84.5%. The low recovery rate in Hole U1428A was due to high sand content in the lower part, which required drilling six intervals totaling 37.6 m (see “Operations”). Otherwise, the recovery rate in the upper part (Cores 346-U1428-1H to 15H; 0–137.13 m CSF-A was 104%. Drilling at Site U1429 penetrated to a maximum subbottom depth of 188.3 m in Hole U1429A, recovering a total of 190.3 m of sediment for a recovery rate of 103%. The shipboard lithostratigraphic program involved detailed visual assessment of sediment composition, color, sedimentary structures, and bioturbation intensity, supplemented by petrographic analysis of smear slides (29 from Hole U1428A, 15 from Hole U1428B, 33 from Hole U1429A, and 6 from Hole U1429B) and bulk mineralogic analysis by X-ray diffraction (XRD) (14 from Hole U1428A, 20 from Hole U1429A, and 3 from the sand unit in Hole U1428A). These were used to describe and define major and minor lithologies, define facies and facies associations, and divide the stratigraphic section into major lithostratigraphic units (Figs. F3, F4, F5, F6, F7). The sedimentary succession recovered at Sites U1428 and U1429 is divided into two major lithologic units (A and B) distinguished on the basis of sediment composition, referring particularly to the abundance of biogenic components and terrigenous fractions such as clay and sand. Unit A is dominated by calcareous nannofossil ooze and calcareous nannofossil–rich clay with diatoms and foraminifers throughout. Meter-scale downhole color changes, ranging from olive-gray to greenish gray and light greenish gray are visible but rather subtle. The color changes are better captured by the physical property “color reflectance.” When correlated with downhole variations of carbonate content, all these changes exhibit glacial–interglacial style variability. Moderate to heavy bioturbation is observed throughout the unit. Discrete tephra (i.e., volcanic ash) layers, with thicknesses ranging from decimeters to >0.5 m, occur throughout Unit A at a relatively regular frequency, and dispersed volcaniclastic material represents a minor component throughout the succession. Unit B is dominated by fine- to medium-grained, rounded to subrounded, massive sand. Quartz is the predominant mineral in this mostly unconsolidated unit. Mica is abundant, whereas feldspar, plagioclase, calcite, and hornblende are also commonly present. Physical property measurements, including natural gamma radiation (NGR), magnetic susceptibility, color reflectance, and dry bulk density profiles, reflect the various lithologies and unit boundaries (see “Physical properties”). The major characteristics of the sedimentary sequences at Sites U1428 and U1429, together with some of the physical properties, are summarized in Figures F3, F4, F5, F6, and F7. Hole-to-hole correlation of the lithostratigraphic units between the two sites is shown in Figure F8. Unit A
Lithologies and structuresUnit A consists of olive-gray, greenish gray, and light greenish gray nannofossil ooze and nannofossil-rich clay with diatoms and foraminifers. The sediment is generally moderately to heavily bioturbated with a fairly homogeneous, structureless appearance. Some intervals have subhorizonal burrows, many of which group together (Fig. F9). Well-preserved gastropods, scaphopods, and bivalves are observed throughout the unit (Fig. F10). The major lithology is interbedded with 11 tephra layers with thicknesses ranging from decimeter to 0.5 m, which can be reliably correlated between holes and sites. A thick tephra layer in the uppermost part of the sequence (interval 346-U1428A-1H-2, 73–118 cm) corresponds to the Kikai-Akahoya (K-Ah) tephra with a 14C age of ~6.82 k.y. before present (BP) (Fukusawa, 1995), whereas a second tephra covering the interval 2H-6, 100–108 cm, corresponds to the Aira-Tanzawa tephra with a 14C age of ~25.9 k.y. BP (Kitagawa and van der Plicht, 1998), suggesting the same tephrostratigraphy as that of Ijiri et al. (2005). Two different types of vitric tephra are observed: one is dark colored and coarse grained, containing glass as well as minor quartz and other minerals, and the other is whitish colored and very fine grained, containing pure volcanic glass (Fig. F11). Downhole color changes are rather subtle. Meter-scale variations of colors ranging from olive-gray to greenish gray and light greenish gray are observed and reflect changes in carbonate content based on smear slide observations (Fig. F12). This is further confirmed by XRD results (Figs. F13, F14). In general, light greenish gray colors reflect a higher nannofossil content. Although the color contrast is low and the transitions are gradual, these alternations of lithologies are better reflected in records of NGR, magnetic susceptibility, and color reflectance (see “Physical properties”). CompositionThe principal components of lithologies in Unit A are mainly biogenic with minor amounts of terrigenous and volcanic material (see Site U1428/U1429 smear slides in “Core descriptions”). Calcareous nannofossils dominate the biogenic fraction, foraminifers are commonly present, and diatoms represent a relatively low component (Fig. F15). Among the biosiliceous components, radiolarians and silicoflagellates (usually rare because they are too fragile) are also easily found. The terrigenous fraction in this unit is dominated by clay and fine silty clay and occasionally by fine-grained sand fractions. Volcanic glass and pumice account for nearly 100% of the discrete tephra layers. Variable abundances of calcareous nannofossils within Unit A are observed, but a quantitative estimate of the amplitude of their variations cannot be performed based solely on smear slide analyses. However, the most abundant nannofossil contents clearly match the lighter sediment colors (light greenish gray) and vice versa. In Unit A, the terrigenous component is low and mostly consists of quartz grains and rare clay minerals. Authigenic pyrite and dolomite represent occasionally important lithologic accessories. The fine-grained sand that is found in thinly bedded layers appears well sorted and made of quartz and abundant mica and occurs only intermittently. Bulk mineralogyThe XRD analysis results are listed in Tables T3 and T4. In general, the Middle Pleistocene–Holocene sediment at these two sites is composed mainly of quartz, plagioclase, clay minerals (including smectite, illite, and kaolinite and/or chlorite), and calcite, as well as minor amounts of halite and pyrite. Dolomite is sparsely present between 50 and 100 m CSF-A in Hole U1428A and between 60 and 150 m CSF-A in Hole U1429A. There may be other minor minerals that were not identified by the XRD analysis of bulk samples. Calcite is mostly derived from nannofossils and foraminifers, both of which are abundant in the calcareous nannofossil ooze throughout the unit at the two sites. Figures F13 and F14 show the downhole variations in peak intensity of the identified minerals at Sites U1428 and U1429. In general, the peak intensities of quartz, plagioclase, K-feldspar, and clay minerals (smectite, illite, kaolinite and/or chlorite) in Unit A show four cycles. For example, quartz shows relatively higher peak intensity at 7.3, 52.8, 93.0, and 113.6 m CSF-A at Site U1428 and at 7.6, 90.8, 128.3, and 157.2 m CSF-A at Site U1429. In contrast, the calcite peak intensities display opposing variations from the detrital minerals. Cyclic variations in mineral composition are also reflected by color and physical properties, as discussed above. Pyrite shows higher intensity peaks between 70 and 110 m CSF-A at Site U1428 and between 130 and 170 m CSF-A at Site U1429. In addition, halite shows a generally increased downhole trend at both sites. Unit B
Lithologies and structuresThe principal lithology of Unit B consists of unconsolidated fine- to medium-grained, well-sorted, rounded to subrounded, massive sand (Fig. F16). No sedimentary structures can be observed because of heavy drilling disturbance and very poor recovery. Between 159.50 and 167.25 CSF-A in Hole U1428A (Sections 346-U1428A-23H-1A to 23H-6A), a nannofossil-rich clay unit with a thickness of 7.7 m is observed. CompositionUnit B is composed predominantly of fine- to medium-grained sand. The mineralogical composition consists of abundant quartz and mica. Other minerals include feldspar, plagioclase, calcite, hornblende, zircon with minor clay minerals, and volcanic glass. Shell fragments and foraminifers are also present (Fig. F16). Bulk mineralogyThe results of XRD analyses conducted on sand from Hole U1428B are listed in Table T5. The sand consists mainly of quartz, K-feldspar, plagioclase, detrital calcite, muscovite with minor hornblende, pyroxene, dolomite, smectite, and chlorite (Fig. F17). Zircon was also observed in smear slides but not detected by XRD, possibly because of very low zircon content in the bulk sand. In addition, some opaque black mineral/rock fragments are observed by microscope but their identity remains unknown. These black minerals are likely to be weathered volcanic rock (with smectite coating). However, exact identification requires further onshore observation. Most of the minerals/rock fragments have very similar grain size (60–120 µm) and are characterized by very rounded to subrounded shape, suggesting a fairly long transport distance, during which the grains were sorted. Summary and discussionThe sedimentary successions at Sites U1428 and U1429 record the paleoceanographic history of the northwestern end of the Okinawa Trough from the Middle Pleistocene to Holocene. The lithostratigraphic characteristics appear to act as a sensitive recorder of regional oceanographic changes, with far-reaching linkages to global climatic variations. Furthermore, owing to their targeted locations, the sedimentary successions at these two sites bear great potential for studying sediment source and sediment transport dynamics, with paleoenvironmental implications closely associated with the Asian continent. The lithology of Sites U1428 and U1429 consists of two units (A and B) based on biocalcareous and terrigenous content including clay and sand. Unit A is composed mainly of calcareous nannofossil ooze and calcareous nannofossil–rich clay and is the predominant unit at both sites. Subtle color changes are observed downhole, presumably as a result of variations in nannofossil content. This inference is further supported by variations of calcite content measured through XRD analysis. The meter-scale color changes through the unit imply, in general, that the paleoceanographic conditions responded to glacial–interglacial variations of global climate during the last 400 k.y. Of more region-specific relevance to eventual interpretation of the sequence will be sea level changes and the associated repeated emergence and exposure of wide continental shelves, and the evolution of the Kuroshio Current. Terrigenous sediment flux to the sites is expected to closely respond to the emergence and exposure of the continental shelves on glacial–interglacial timescales. However, shipboard observations did not provide strong lithologic and sedimentological evidence. Further onshore investigation is highly needed in this regard. Unit B is a sand unit, consisting of fine- to medium-grained, rounded to subrounded, massive sand with a thickness reaching 74 m in Hole U1428A. The upper age limit of the sand unit is ~0.4 Ma, which is equivalent to the bottom age of the overlying Unit A (see “Biostratigraphy”). Drilling at neither site penetrated through this unit and therefore did not allow determination of the bottom age of the sand unit. Little in the way of sedimentary structure is preserved because of drilling disturbance and low recovery of the mostly unconsolidated sand and facies or facies associations could not be classified. However, given that the sand is well sorted, rounded to subrounded in grain shape, and contains a high concentration of mica, it strongly suggests that the unit formed by upward accumulation of sand, which had been transported a long distance by currents (e.g., bottom currents), as opposed to gravity (debris) flow, such as in the case of the formation of turbidites. Further mineralogical and geochemical investigations should lead to a better understanding of the provenance of the sand, together with the transportation dynamics and paleoceanographic implications. |
|||
