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doi:10.2204/iodp.proc.333.103.2012 LithologyHole C0018A was chosen to sample a series of MTDs and underlying turbidites identified on 3-D seismic data (e.g., Expedition 316 Scientists, 2009b; Moore et al., 2007; Strasser et al., 2011). Hole C0018A follows the drilling of slope deposits, forming part of Unit I, during IODP Expedition 316 (Expedition 316 Scientists, 2009a, 2009b). A total of 313.66 m of strata was drilled in Hole C0018A, with very good recovery rates (Fig. F3). Two lithologic subunits were interpreted during the examination of cores: Subunits IA (youngest) and IB (Fig. F3). The units drilled correlate with seismic Units 1a and 1b defined by Kimura et al. (2011) and by Strasser et al. (2011). As with other IODP sites (e.g., Expedition 316 Scientists, 2009b), depositional units in this report are distinguished based on variations in grain size, mineralogy, composition, and presence (and thickness) of sand and ash layers (Figs. F4, F5). Unit boundaries and intervals with evidence for MTDs were selected based on lithology, X-ray CT images, observations of structural style (see “Structural geology”), and seismic data interpretation (Kimura et al., 2011; Strasser et al., 2011). Lithologic subunits mainly comprise silty clay with variable quantities of volcanic ash. Six intervals with evidence for MTDs are observed within the cored succession, with the largest, a 61.03 m interval, located below 127.55 mbsf (Fig. F3; Table T2). The silty clay consists of clay minerals with quartz, feldspar, and abundant calcareous nannofossils, diatoms, and sponge spicules. Heavy minerals (mainly pyrite), apatite, chlorite, and biotite are present in some intervals (see Site C0018 smear slides in “Core descriptions”). In turn, micas and opaque minerals are relatively rare. From bulk powder X-ray diffraction (XRD), clay mineral content of the silty clay (averaging 47 wt% for the entire cored succession) shows some variations in depth, approaching 51 wt% on average between 21 and 58 mbsf (Fig. F5; Table T3). The relative abundances of feldspar and quartz increase steadily with depth, for an average of 20 wt% and 23 wt%, respectively. Near the bottom of Hole C0018A, feldspar reaches 23 wt% in abundance against 29 wt% abundance in quartz. Below Core 333-C0018A-13H (108.65 mbsf) the relative abundances of diatoms and spicules decrease. Below Core 333-C0018A-24T (190.65 mbsf) is a marked decrease in the relative abundance of nannofossils, diatoms, and spicules and a relative increase in the number of siliciclastic grains in smear slide samples. This change is confirmed by XRD data, which show almost complete depletion of calcite below 180 mbsf and a sharp increase in the relative abundance of quartz (Fig. F5; Table T3). Subunit IA (slope-basin facies)
Subunit IA comprises greenish gray to grayish silty clay with mostly thin (<5 cm) intercalations of volcanic ash. The subunit shows internal facies variations and can be divided into three intervals: Facies IAi, IAii, and IAiii, which are described in more detail below. Bioturbation is observed particularly in the upper part of the unit where Zoophycos and Chondrites are abundant. These ichnofossils are better observed in CT scans of cores, where they appear pervasive in some sections. Shell debris is seen in parts of the unit, particularly in Facies IAi. XRD data indicate an average content of 15 wt% calcite, 20 wt% quartz, and 47 wt% clay minerals for Subunit IA. However, variations in the relative percentage of the latter minerals are recorded at specific intervals (Fig. F5; Table T3). Six intervals with evidence for MTDs are observed within Subunit 1A and numbered from top to bottom for convenience (Fig. F3; Table T2). The upper boundary/contact is well defined for MTDs 1, 2, and 6 and is marked by a turbidite for two of them (MTDs 2 and 6) (Fig. F6). MTD 1 extends over 2.9 m of chaotic and convolute bedding in Core 333-C0018A-1H. MTD 2 comprises in its lower part several intervals of coherent bedding bounded by probable shear zones (see detailed X-ray CT description notes in Table T4). The lowermost shear zone defines the base of the MTD 2 interval (Fig. F7). MTD 3 comprises an interval with visual evidence for remobilization near its top. Examination of the CT scan and structural data (see detailed X-ray CT description notes in Table T4) led us to consider this interval as part of a thicker MTD zone. MTD 4 is a relatively thin interval (50 cm) associated with a fluidized ash layer (Fig. F8). MTD 5 extends over cores that were also disturbed by the coring process. A zone of remobilization is identified based on visual evidence and CT scan in Core 333-C0018A-9H, although this core was damaged during extraction from the core barrel. CT scan images provide evidence for a shear zone with a sharp lower boundary that defines the base of MTD 5 (see detailed X-ray CT description notes in Table T4). It is yet unclear whether this MTD interval corresponds to a single event deposit. MTD 6 is a ~61 m interval between 127.55 and 188.57 mbsf and corresponds to the main MTD body identified in the seismic data (Strasser et al., 2011). As already noted, a turbidite deposit is found immediately above its upper boundary. Chaotic and convolute bedding (Fig. F9) and mixing of ash with hemipelagite deposits are observed in the cores, but other intervals remain coherently bedded. Several shear zones are identified from CT scans of the lower part of MTD 6 (Table T4), but none could be positively identified as the basal surface. The base of MTD 6 was therefore defined at the top of the Pink volcanic ash bed (see below), which forms a distinct thick, fine-grained, sand-sized volcanic ash layer separating disturbed landslide material from undisturbed material with original horizontal bedding below. Because the core containing this coarse sand-sized ash layer experienced severe disturbance because of gas expansion upon core retrieval, deformation and structures indicating shearing at the base of the landslide cannot be conclusively identified within this layer. The deposition of Subunit IA occurred on a continental slope dominated by hemipelagic settling, submarine landslides, and minor contributions of volcanic ash. The onset of the hemipelagic-dominated paleoenvironment occurs at ~190 mbsf. The Subunit IA/IB boundary is defined at the base of a >1 m thick, likely reworked, coarse, sand-sized volcanic ash at 190.65 mbsf (Section 333-C0018A-23H-CC). Based on characteristic microscopic features observed in smear slides (i.e., bubble wall type glass shards and abundant hornblendes as heavy minerals [Fig. F8]), this sand bed can be tentatively correlated with the Pink volcanic ash bed with the age of ~1.05 Ma (Hayashida et al., 1996) (see Site C0018 smear slides in “Core descriptions”). Turbidity current contributions to deposition from a mixed volcaniclastic and siliciclastic source substantially increase below this latter boundary (Fig. F3; see “Subunit IB (sand-rich slope basin)”). Facies IAi
The dominant lithology in Facies IAi is greenish gray silty clay with minor contributions of volcanic ash. Ash layers are <2 cm thick, fine grained, light to dark gray, and at least partly laminated, with sharp lower and diffuse upper contacts. These ash layers consist mainly of fresh glass with minor pumice, quartz, feldspar, mica, and calcareous nannofossils. Clay mineral content ranges from 40 to 47 wt% and averages 44 wt%, quartz content ranges from 18 to 26 wt% and averages 21 wt%, and feldspar ranges from 15 to 22 wt% and averages 17 wt%. Calcite content ranges from 11 to 26 wt% and averages 18 wt% (Fig. F5; Table T3). An ~3 m thick interval with evidence for MTDs is present in Core 333-C0018A-1H in the form of convolute tilted strata and intervals of remobilized mud clasts (debrite) overlying tilted strata. Facies IAii
The dominant lithology of Facies IAii is greenish to dark gray silty clay, silt, and sparse fine-grained sand (Fig. F3). Most silty and sandy layers have a dominant volcaniclastic composition, but metamorphic lithic fragments with secondary quartz and feldspar are also present in variable amounts. Heavy minerals are abundant. Mica is abundant in some levels (see Site C0018 smear slides in “Core descriptions”). Individual sand and ash beds are generally <5 cm thick, showing a predominant fining-upward trend into silt and locally silty clay with indistinct boundaries between the different lithologies. However, the sand and coarse volcanic ash beds have sharp bases. Silty clay beds are typically <1 m thick with minor sandy silt and clayey silt intervals also present. Clay mineral content ranges from 34 to 74 wt% and averages 51 wt%, quartz content ranges from 5 to 24 wt% and averages 15 wt%, and feldspar ranges from 10 to 38 wt% and averages 21 wt%. Calcite in this facies varies from 1 to 31 wt% in relative abundance (Fig. F5; Table T3). The large variations in XRD values observed in this section can be partially attributed to the lithologies and textures sampled, varying from coarse volcanic ash to silty clay (Fig. F5). The relatively higher sand- and silt-size ash content of this facies suggests the deposition of volcaniclastic sediment by relatively low energy plumes of volcanic material (Division E of Bouma [1962]). In addition, a substantial portion of the finer grained volcaniclastic material in Facies IAii comprises ash fall deposits. Deposition in Facies IAii occurred on a continental slope basin dominated by hemipelagic settling with some contribution of volcanic ash and low-energy volcaniclastic turbidity currents. The Facies IAii/IAiii boundary at 57.5 mbsf (Section 333-C0018A-7H-3, 53 cm) is marked by the lowermost appearance of a volcanic ash deposit. Below this ash deposit, volcaniclastic contribution is mostly recorded as scattered laminae of silty ash in the underlying Facies IAiii. One interval with evidence for MTDs occurs between 39.45 and 46.73 mbsf (Table T2). It is shown as convoluted silty clay strata with mud clasts. The lower part of the subunit (below Section 333-C0018A-5H-8, 44 cm) comprises a homogeneous silty clay that, on X-ray CT scan data, shows convolute and tilted strata with internal shear zones (see also Table T4) (Fig. F7). Facies IAiii
The bulk of Facies IAiii comprises greenish gray silty clay intercalated with four intervals with evidence for MTDs. In contrast with Facies IAii, volcanic ash is somewhat less abundant in this succession. However, darker silty and clayey laminae are visible in the background hemipelagic silty clay. Turbidites are rare and appear associated with volcanic deposits (e.g., Section 333-C0018A-13H-9). Sand beds are virtually absent in Facies IAiii with the exception of Sections 333-C0018A-14H-7 and 14H-8 (125.40–126.45 mbsf), in which successive intervals of volcanic ash and sand are observed. The last of these intervals (125.65–126.45 mbsf) mobilized deposits that appear correlative to the Azuki volcanic ash bed (0.85 Ma; Hayashida et al., 1996), as indicated by the observation of abundant bubble wall type glass shards and obsidian grains in smear slides (Fig. F8). Clay mineral content ranges from 31 to 53 wt% and averages 46 wt%, quartz content ranges from 10 to 26 wt% and averages 21 wt%, and feldspar ranges from 11 to 35 wt% and averages 18 wt%. Calcite content ranges from 0 to 39 wt% and averages 15 wt% (Fig. F5). Deposition in Facies IAiii occurred in a setting dominated by hemipelagic settling with some contribution by air fall volcanic ash and fine-grained volcaniclastic turbidity currents. In comparison to Facies IAii, volcaniclastic contribution is sparse but increases downward. The Facies IAiii/Subunit IB boundary is marked by the appearance of redeposited volcanic ash bed at 190.65 mbsf (Section 333-C0018A-23H-CC, 21 cm), which is tentatively correlated with the Pink volcanic ash bed with the age of ~1.05 Ma (Hayashida et al., 1996) based on the abundant presence of bubble wall type glass shards and abundant hornblendes as heavy minerals (Fig. F8). Several intervals with evidence for MTDs occur in Facies IAiii, namely at 58.01–65.75 mbsf (Section 333-C0018A-7H-5), 66.44–67.93 mbsf (Section 8H-4), 75.91–94.37 mbsf (Sections 9H-3 through 11H-5), and 127.55–188.57 mbsf (Sections 15H-2 through 23H-1) (Table T2). These intervals consist of convoluted intervals of banded silty clay strata with scarce mud clasts as observed on visual core descriptions. Tilted, convolute, and sheared intervals are visible in X-ray CT scans (Table T4). The larger thicker MTD interval comprises a mixture of mud-rich debrites, remnant hemipelagic strata, and convoluted banded deposits denoting extensive deformation in their interior (Fig. F8). The base is marked by a region disturbed by drilling, probably in relation to the presence of gas and water in a (sandy and silty) volcaniclastic deposit attributed to the Pink volcanic ash bed (Fig. F3). On XRD data, the maximum contents of clay minerals and quartz are observed in the thickest MTD interval below 127.55 mbsf. In contrast, a relative reduction in the relative abundance of calcite is observed in the same interval when compared with the strata above (Fig. F5). Subunit IB (sand-rich slope basin)
Subunit IB consists of interbedded fine-grained to medium sand, silty sand, silty clay, and clay. Subhorizontally dipping, 4–20 cm thick sand layers typically show a sharp and erosional base and normal grading. Bed spacing is 20–30 cm in average, but muddy intervals up to 4 m thick comprising no intercalated sand layer occasionally occur. The sand content of this subunit is markedly higher than overlying Subunit IA (Figs. F3, F4). Silty and fine-grained silty clay and dark olive-gray sand are dominated by metamorphic and volcanic lithic grains (quartz content averaging 29 wt% by XRD) and feldspar (averaging 23 wt% by XRD). The proportion of lithic fragments decreases in very fine sand and silt layers. Carbonate content is mostly below detection by XRD, with a local maximum of 6 wt% at 307.95 mbsf. The large variations in XRD mineral abundances throughout this section (Fig. F5) derive from the varied lithologies and textures sampled (sand to silty clay; Fig. F10). Some sand beds show normal grading with indistinct upper boundaries that grade into silty clay. The silty clay is greenish gray and is only slightly bioturbated or mottled in places. The bases of sandy beds are sharp and erosional (Fig. F10). Ash and dispersed volcanic glass are widespread in this unit, and accumulations occur as light gray bands in otherwise greenish gray strata. The unit represents a sand-rich slope wedge system, fed by both siliciclastic and volcanogenic sources. Sand-silty clay alternations represent high-frequency cycles of turbidite flows sourced from proximal areas on the slope. X-ray fluorescence analysesX-ray fluorescence analyses were performed on 59 samples from Hole C0018A to estimate the bulk chemical composition of the sediments and to characterize compositional trends with depth and/or lithologic characteristics (Fig. F11; Table T5). Major element contents show several variations with depth that are compatible with the lithologic subunits as defined earlier. The silty clays of Subunit IA are characterized by low SiO2 content (60.2 wt% on average), compared to the underlying sand-rich Subunit IB (67.5 wt% on average) (Fig. F11). This difference may be explained by a higher proportion of siliciclastic grains in Subunit IB. In contrast, Subunit IA contains higher and less scattered Al2O3, Fe2O3, and MgO contents (Fig. F11), which can be related to a higher abundance of clay minerals. The significant decrease in CaO content observed in Subunit IB (2.0 wt% on average versus 8.8 wt% in Subunit IA) (Fig. F11) is consistent with the very low calcite content observed by XRD below ~190 mbsf. The large variations observed for almost all the major elements at given depths between ~50 and ~100 mbsf within Subunit IA (Fig. F11) essentially account for the sampling of specific lithologies such as volcaniclastic sand beds and volcanic ash layers. The lower part of Subunit IA, from ~100 to 190 mbsf, shows a remarkably homogeneous composition in regard to Al2O3, Fe2O3, K2O, P2O5, MgO, and TiO2 contents (Fig. F11; Table T5). The bulk composition of the thickest MTD occurring from 127.55 to 188.57 mbsf does not differ from the composition of the homogeneous overlying strata. By contrast, Subunit IB is characterized by largely scattered concentrations in almost all major elements that may be related to the heterogeneous nature of this sedimentary unit consisting of interbedded sand, silt, and clay. |
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