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doi:10.2204/iodp.proc.331.104.2011 LithostratigraphyThe drilled sequence at Site C0014 comprises interbedded, variably altered, and consolidated volcaniclastic gravel and breccia, as well as hemipelagic mud. Given the location of the site on the upper flank of an active volcanic complex, mass wasting and debris flows are likely to be important sedimentary processes, potentially leading to high rates of redeposition of both hemipelagic and volcaniclastic material. The deeper portion of the rock volume cored has been hydrothermally altered. Differing degrees and styles of hydrothermal alteration form the basis for the division of Site C0014 sediments and lithologies into lithostratigraphic units (Table T2). Unit I, the stratigraphically highest unit, shows little evidence of hydrothermal alteration and comprises a succession of coarse angular woody pumice gravel and hemipelagic mud. Unit I extends to ~10–18 mbsf across Site C0014 (Fig. F2). The contact between Unit I and Unit II is somewhat arbitrarily defined by increasing hydrothermal alteration, which is expressed in drill core by breakdown of pumice texture and mineralogy and bleaching of hemipelagic mud. The nature of the boundary varies in individual cores and may be locally defined by a high-angle sharp contact (interpreted as a fault), such as in Section 331-C0014D-2H-3, 130 cm, across which there is a distinct change in alteration intensity. However, in other cores (for example, Sections 331-C0014B-2H-6 and 331-C0014G-2H-4 through 2H-5), the increase in alteration intensity is more gradational. Unit II consists of partially consolidated hydrothermally altered mud and hydrothermally altered coarse angular pumice gravel. Clasts of pumice have been devitrified to soft clay that retains its woody texture. Unit II is 12–30 m thick with a base at 29–45 mbsf. Unit III is characterized by the occurrence of consolidated and often cemented volcanic sediments as lithoclasts within breccia or thin horizons interbedded with hydrothermally altered indurated mud. The greatest thickness of Unit III was encountered in Hole C0015G from 29 mbsf to the deepest recovered core at 128 mbsf (Fig. F2). Description of lithostratigraphic unitsUnit I
Sediment types and lithotypesHemipelagic homogeneous mudHomogeneous gray mud (silty clay to clayey silt) is characterized by abundant microfossils—dominantly intact foraminifers—typical of a hemipelagic sedimentary environment. Dark laminae (~1 mm) of humic substances and organic debris occur at the top of many sand and silt patches (1–3 cm thick) that are intermittently present. Detrital minerals are mostly quartz, feldspar, mica, and minor opaque heavy minerals. Pumiceous sedimentsAt Site C0014, coarse angular pumice gravel is predominantly composed of angular clasts of woody pumice, as at Site C0015. Woody pumice exhibits tubular structure, with tubes ~2 mm in cross section. These tubes give the pumice unique sedimentological properties, as cooling of volcanic gases within the tubes helps them to imbibe water rapidly and thus reduce the buoyancy of clasts. This mechanism probably causes woody pumice to settle near its location of effusion (Kato 1987; Fiske et al., 2001). For clast-supported gravel without a significant matrix component, the unconsolidated nature of deposits makes it difficult to distinguish sedimentary contacts between individual pumice beds. Compared with gravel at Site C0015, however, a significant matrix component is frequently present at Site C0014, which helps to delineate bedding. Matrix components are of two types: (1) hemipelagic mud and (2) silt- to grit-sized, poorly sorted angular quartz, volcanic glass, and pumice fragments (acicular volcanic glass). The bases of many pumice breccia intervals contain a coarse sand matrix of pumice fragments with a minor but significant mud fraction, whereas the top of pumice gravel typically contains a mud matrix that grades upward into an overlying mud unit. Pumiceous grit with poorly sorted clasts of angular quartz, volcanic glass, and pumice fragments (acicular volcanic glass) comprises a distinct interval in Hole C0015D (interval 331-C0015D-1H-3, 26 cm, to 1H-4, 140 cm). Lithoclasts of pumice are present but are a very minor component in this interval. Microfossils are generally absent in pumice-rich units. When they are present in gravel matrix or grit, they are typically weathered and broken, indicating that the microfossils have been reworked. Correlation within Unit ISedimentary logs were used to attempt correlation of pumice horizons within Unit I. All holes at Site C0014 are located in a hollow with ~2 m of relief from the highest Hole C0014B to the lowest Holes C0014F and C0014G. The bases of debris flows and mud units were used as correlative horizons to assess continuity of pumice layers across the hollow. A general thickening of individual pumice gravel units traceable between holes corresponds to a deepening of the hollow and a downslope location of the drill holes. The slope is a transient depocenter, and the thickest pumice gravel deposits likely accumulated through the amalgamation of successive debris flows, screes, and talus deposits. Resedimenting of pumice and mud is highly likely, and presumably sediments were deposited farther downslope. Unit II
Sediment types and lithotypesHydrothermally altered mudHydrothermally altered mud from Unit II is light to medium gray and frequently mottled. Most mud layers are interbedded with hydrothermally altered coarse angular pumice gravel over depth intervals of ~30 cm. Mottled mud is generally stiff and somewhat sticky. Illite, quartz, and Mg chlorite are the predominant minerals, along with widely distributed mica flakes. Silt, sand, and gravel, both angular and poorly sorted, are present and, in a few instances, form small beds as thick as 10 cm. In one instance, a thin bed of sand has loaded into underlying mud (interval 331-C0014G-3H-3, 26–27 cm; 21 mbsf). Toward the base of Unit II, it becomes increasingly difficult to visually distinguish hydrothermally altered mud from hydrothermally altered pumice gravel, although this is not always the case. Hydrothermally altered pumice gravel and grit are whitish to medium gray and comprise clasts of variably devitrified pumice, often with a woody fibrous structure preserved (Fig. F3). Toward the base of Unit II, primary clastic textures are increasingly lost, replaced by white in gray or dark gray in light gray mottling. Devitrified pumice clasts are no longer rigid and deform easily, even when structures are well preserved. Some units lack matrix support and comprise horizons of loosely consolidated clasts of mud (Fig. F3). More typically, hydrothermally altered pumice-breccia is matrix supported in hydrothermally altered mud, and a progression can be seen from the top of the unit at ~10 mbsf to the base of the unit at ~30 mbsf, where relict pumice clasts are replaced by intense oblate mottling and unconsolidated horizons are scarce. Unit III
Sediment types and lithotypesHydrothermally altered mud and volcaniclastic sedimentsThe predominant lithologies in Unit III are gray, mottled, hydrothermally altered mud as described for Unit I, with horizons of well-cemented clasts of volcanic breccia. The breccia includes volcanic clasts other than woody pumice; both amygdaloidal vesicular lavas and devitrified lapilli in a silica-rich matrix were observed in addition to cemented pumice breccia (Fig. F4). Clasts of pumice breccia in this lithology comprise fragments typically <1 cm across, as opposed to the large pumice clasts observed in Unit I. These components probably derive from a source that has gone through different sedimentation cycles than the pumice in Units I and II, where the pumice was broken into smaller fragments. Some clasts of volcanic rock were observed in interpreted “right way up” positions interbedded with 5–10 cm thick layers of hydrothermally altered mud, whereas others were found floating in a hydrothermal mud matrix. How much of this difference is a result of drilling disruption is not always obvious, although it is clear that both poorly consolidated mud and well-cemented volcaniclastic units were cored. Mineral cements were identified by X-ray diffraction (XRD) as quartz and minor anhydrite (Fig. F5), but cementing is intermittent and absent from some mud layers, many of which were disrupted by drilling. Cemented volcaniclastic breccia horizons were encountered below 30 mbsf in Holes C0015E, C0015B, and C0015G. Anhydrite appears as a rare vein filling and cementing phase below 60 mbsf, but cementing is still intermittent and in many instances is absent from mud units even below the onset of anhydrite vein formation. Lithology and velocity structureFigure F2 correlates units between holes for Site C0014. A wide range of P-wave velocities are observed at Site C0014, ranging from values typical of unconsolidated sediment to velocities closer to those of igneous and crystalline rocks. A general trend of subtly increasing velocity downhole is seen, consistent with the increasing degree of induration and lithification seen within Unit III at Site C0014. Grain size distributionMean grain sizes of matrix sediment from Holes C0014A and C0014B vary from 3.35Φ to 7.83Φ and from 2.64Φ to 7.78Φ, respectively. Hole C0014A sediments comprising mud or pumiceous lithotypes have distinctly different grain sizes, with a clear boundary occurring at ~4.3 mbsf (Fig. F6A; Table T3). Hemipelagic mud in Unit I is characterized by fine-grained sediments in which the clay fraction is >25% and the sand fraction ranges from 0% to ~10%. In pumiceous units, even the matrix consists mostly of sandy sediment, with a clay fraction <15% and a sand fraction >50%. Similarly, sediments in Hole C0014B show large downhole variations in grain size patterns between different depositional units (Fig. F6B). The hemipelagic mud has a similar grain size distribution to that of Hole C0014B, with a clay proportion >25% and a sand fraction <1%. The sand and silt units in pumiceous horizons are composed primarily of silty sand, with a clay fraction of <10% and a sand fraction of 20%–74%. The standard deviation of the grain sizes reveals that measured sediments from Hole C0014A are poorly sorted to very poorly sorted, whereas those from Hole C0014B are very well sorted to very poorly sorted. Depositional units in both holes show variable skewness, among which all hemipelagic mud skews coarse and all pumiceous and hydrothermally altered units skew fine. Kurtosis measurements suggest that grain size patterns of hemipelagic mud in both holes are platykurtic, whereas pumiceous units are mesokurtic in Hole C0014A and mesokurtic to leptokurtic in Hole C0014B. On the classic ternary diagram of Folk and Ward (1957) for grain size discrimination, sediments in both holes are classified mostly as silt within the hemipelagic mud lithotype and silty sand to sandy silt in the pumiceous and hydrothermally altered sediments (Fig. F7). Grain size distribution patterns further discriminate different sediment origins (Fig. F8), showing three significantly different types. The uppermost part of Unit I contains hemipelagic mud that exhibits similar grain size patterns in Holes C0013F, C0014A, and C0014B, with one dominant peak at ~7.5Φ, which is probably diagnostic of the deposition of normal hemipelagic terrigenous sediment in the central Okinawa Trough. Hydrothermal grit (hydrothermally altered sand and gravel in Hole C0014B) has distinctly different grain size patterns from hemipelagic mud: it shows very fine skew, mostly mesokurtic to leptokurtic, and has major peaks at 1Φ to ~2.5Φ. In comparison, hydrothermal mud with pumiceous clasts or mud-supported pumiceous gravel generally have variable grain size patterns with several dominant peak percentages. The grain size patterns of these sediments vary between typical hemipelagic mud and hydrothermal grit (Fig. F8). Although we used the term mud for operational purposes during visual core description, it is clear that grain size analysis has considerable potential for discriminating the provenance of fine-grained sediments. Comparison of grain size distributions between Holes C0013F, C0014A, C0014B, and piston Core DGKS9604, which were all taken from the central Okinawa Trough, shows that the uppermost hemipelagic mud in each hole has a similar mean grain size of ~7Φ (Fig. F9). Nevertheless, it is impossible at present to make a detailed correlation between these sediments because of the lack of chronostratigraphy for Holes C0013F, C0014A, and C0014B coupled with variable sediment accumulation rates in the central Okinawa Trough. The chronostratigraphy of Core DGKS9604 is well established by 11 14C dates and oxygen isotope measurements of planktonic and benthic foraminifers, which indicate that 2.05 m of mud has been deposited during the Holocene at this location (Yu et al., 2009; Dou et al., 2010). Hemipelagic mud is 4.3 to 8.0 m thick in Holes C0014A and C0014B but <1.0 m thick in Hole C0013F (Fig. F9), suggesting much higher Holocene sedimentation rates in Holes C0014A and C0014B. The depths of the seafloor at Holes C0014A (1059.5 meters below sea level [mbsl]) and C0014B (1059.0 mbsl) are almost the same as at Hole C0013F (1059.8 mbsl) and are much deeper than at Core DGKS9604 (766 mbsl). Different sediment accumulation rates during the Holocene at these locations could readily be caused by variation in sediment supply, particularly local volcaniclastic input, local topography, and ocean circulation patterns, and deserves more in depth investigation. |
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