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Lithostratigraphy and petrology

The objective at Site U1378 was to investigate the nature and deformation of the midslope sedimentary sequence and the uppermost portions of the basement imaged in multichannel seismic reflection data (Fig. F3). After taking LWD measurements in Hole U1378A, Hole U1378B was cored from the seafloor to 523.9 mbsf, recovering sediment and sedimentary rocks. Because of deteriorating hole conditions, the hole was terminated before reaching the basement.

Hole U1378B is dominantly composed of a monotonous sequence of silty clay to clay that alternates with widely interspersed centimeter-scale sandy layers. The interval between Core 334-U1378B-1H and 16H (~0–128 mbsf) is dominated by soft silty clay sediments. Twenty-one tephra layers and numerous thin (<5 cm) fining-upward sand sequences with erosive bases were observed in this sequence.

In the interval between Core 334-U1378B-17X and 62X (~128–513.6 mbsf), a monotonous compacted sequence of silty clay to clay alternates with widely interspersed decimeter-scale sandy layers. Sandy layers are most common in the 146 m interval between 367 mbsf (Core 45X) and 513 mbsf (Core 62X). Sandy sequences become thicker and more abundant with increasing depth within this interval. Shell fragments are widely dispersed throughout the monotonous clay sequence. Below 128 mbsf, a sequence of 61 tephra horizons is observed. The sands change character in Core 45X (~367 mbsf), gradually becoming progressively coarser and more terrigenous with depth. The lowermost core recovered from Hole U1378B (Core 63X) contains a 0.97 m thick sequence of extensively fractured silty claystones.

Description of units

The cores recovered at Site U1378 can be divided into two lithostratigraphic units (Fig. F4; Table T2). The 523.9 m sequence (Cores 334-U1378B-1H through 63X) is divided on the basis of lithologic attributes. Core recovery was extraordinary (>100%). Lithostratigraphic definitions for Site U1378 are based exclusively on core recovered from Hole U1378B.

Unit I

  • Interval: 334-U1378B-1H through 16H

  • Thickness: 127.83 m

  • Depth: 0–127.83 mbsf

  • Age: late Pleistocene? to recent

  • Lithology: fine soft silty clay sediment with fining-upward sandy sequences

Unit I consists mainly of dark greenish gray, soft silty clay sediments (Fig. F5). These clays are punctuated by a series of fining-upward <5 cm thick sequences of lithic sand and 21 tephra layers. The sandy packages have abrupt parallel bases with rounded medium-grained sand that transition to a gradational boundary at the top of the package (Fig. F5). Tephra layers range from 0.5 to 7 cm thick and commonly have abrupt bases and gradational tops.

The main components of Unit I are terrigenous, dominantly clay. Smear slides indicate that the most common accessory grains in the silt(stone) and clay(stone) in Cores 334-U1378B-1H to 5H include feldspar and lithic (sedimentary and magmatic) fragments. Components present in trace abundances include chert, opaque minerals, and volcanic glass fragments, as well as foraminifers, diatoms, and nannofossils. From Core 5H downhole, trace components in the background sediment also include chlorite, calcite, amphibole, and pyroxene. Similarly, the main components in the sandstones of Unit I change composition as a function of depth. Whereas the variability of clast and mineral phases within the small intercalated sandstone beds of the upper part is high (sedimentary and magmatic lithic fragments > feldspar, pyroxene, shell fragments, and foraminifers > amphibole and calcite > nannofossils and chlorite > volcanic glass, quartz, and biotite), the sandstones in the lower cores (334-U1378B-10H to 16H) are mainly dominated by shell fragments, foraminifers, lithic fragments (sedimentary lithic fragments >> magmatic lithic fragments), and feldspar. Additionally, amphibole pyroxene and opaque minerals and rare glass shards are present within the sandy horizons. Biogenic components also include sparse nannofossils and diatoms.

Unit II

  • Intervals: 334-U1378B-17X through 62X

  • Thickness: 385.73 m

  • Depth: 127.83–513.56 mbsf

  • Age: late? Pliocene to late Pleistocene

  • Lithology: clay(stone), silt(stone), and sand(stone)

Unit II is dominantly olive-green clayey silt(stone) and silty clay(stone) with minor layers of sand(stone), sandy silty clay(stone), clay, clayey silt(stone), and tephra (Fig. F6). The sediment is firm and well consolidated and contains tephra layers that remain unlithified to 506 mbsf (Section 334-U1378B-61X-1). In general, Unit II is massive with minor changes in the proportions of clay, silt, and sand grain sizes occurring gradually over <3 m intervals.

Within this monotonous sequence, sandy intervals become thicker and more common with depth. An upper 146.96 m thick interval of clay-rich finer sediment extends from Core 334-U1378B-17X through 44X. This sequence has a minor proportion of sand (a cumulative thickness of 26.28 m, or 11%). Small burrows are the dominant sedimentary structure preserved in this interval. In contrast, the lower section (146.96 m; Core 45X through 62X) contains two sandy intervals between Cores 45X to 49X (367–400 mbsf) and 55X to 60X (447–503 mbsf), which are superimposed on the background silty clay sedimentation. The lowermost portions of the core (depths > 367 mbsf) are composed of 59% sand-sized sediment. Coarse to very coarse loose sand layers are limited to two narrow (<8 m) intervals between 391–399 mbsf (Core 49X) and 468–474 mbsf (Sections 334-U1378B-57X-2 to 57X-7).

Throughout Unit II, fining- and coarsening-upward decimeter-scale sequences of sand are present. In the coarser sand layers, rip-up clasts, rounded clay lenses, and abundant shell fragments are commonly observed. Framboidal pyrite was observed both macroscopically and in many of the smear slides throughout Unit II. Shell fragments, diatoms, and nannofossils are sparse but ubiquitous throughout Unit II. Foraminifers are partly enriched within the sediments and are a major constituent of the sand-sized fraction (especially the sandy silt[stones]). Wood or plant material is rarely found in Unit II, although in Core 334-U1378B-24X (193.95–199.9 mbsf) wood fragments (charcoal) and organic matter are common. Intervals of abundant calcareous concretions or lithified carbonate mud clasts are rare and concentrated within a 2 m interval of Core 27X (Sections 334-U1378B-27X-4 to 27X-CC; 222.08–224.41 mbsf).

The main components of Unit II are terrigenous, dominantly clay. Smear slides indicate that the most common accessory grains in the silt(stone) and clay(stone) include feldspar and lithic (sedimentary and magmatic) fragments. Components present in trace abundances include amphibole, calcite, biotite, chlorite, glass shards, and opaque minerals. The main components in the sandstones are lithic (sediment > magmatic) clasts and feldspar crystals. Biogenic components are rare but include nannofossils, diatoms, and foraminifers.

Tephra layers

Hole U1378B cores contain 82 tephra layers intercalated with the background sediment of Units I and II. Individual tephra layers range in thickness from 0.5 to 48 cm. In Unit I, the interval between Cores 334-U1378B-1H and 16H (0–128 mbsf), 21 tephra layers are widely distributed within the background silty clay sediment. In Unit II, 49 tephra horizons are identified in the upper silty clay-rich sediment succession, concentrated in two intervals from 134 mbsf (Section 334-U1378B-18X-2) to 166 mbsf (Section 21X-4) and 205 mbsf (Section 25X-5) to 333 mbsf (Section 40X-7), containing 9 and 38 tephra layers, respectively. Between 354 mbsf (Core 43X) and the bottom of the hole (523.9 mbsf), 14 additional tephra layers are recognized. In general, tephra is most common in the upper part of Unit II.

Compositions of the 82 identified tephra layers are variable. Of all the layers, 30 are light gray to pinkish/brownish white felsic ash, 40 are pinkish gray/brown layers, and 12 are pinkish/greenish black layers. Dark black tephra layers account for ~15% of the total tephra/bed assemblage in Hole U1378B. Tephra layers are usually soft, but those from Core 52X downward are indurated and fulfill the definition of tuff.

Unconformable and/or inclined bedding of tephra horizons is rare. In some instances, the tops of the tephra layers exhibit bioturbation. In general, the tephra layers are characterized by a sharp basal contact with the subjacent terrigenous sediment and a gradational contact with the overlying ash-bearing pelagic sediment. Many layers are normally graded and well-sorted (Fig. F7). Felsic white ash is mainly (>90 vol%) composed of clear, colorless glass shards varying from angular blocky, cuspate, flat, and Y-shaped shards with nearly no bubbles to highly vesicular, pumiceous textures with many elongated bubbles.

The transparent glass shards of the felsic tephras show no visible signs of alteration until 439 mbsf (Core 334-U1378B-60X). Grain size ranges from very fine to coarse ash (millimeter size). The mineral assemblages in the tephra consist of plagioclase, pyroxene, amphibole, and biotite but vary in abundance and composition. These variations are likely diagnostic for variations in the source areas. Smear slides indicate a decrease in mineral contents and species in the more evolved tephras from Cores 7H to 15H within Unit I, whereas felsic tephras below Core 20X are dominated by a high amount of crystals (e.g., amphibole, feldspar, and biotite). Plagioclase is the dominant phenocryst phase, but some tephras are dominated by amphibole and biotite, which typically occur in the most evolved felsic layers. Dark gray mafic ash layers consist dominantly of dark to light brown glass shards. Most of the glass shards have blocky shapes, are medium to poorly vesicular, and show evidence of extensive alteration, especially in the deeper portions of the hole. The mineral assemblages of the mafic tephras include plagioclase, pyroxene, and spinel. Relative to felsic tephras, the mafic tephras contain more crystals.

X-ray diffraction analysis

Preliminary X-ray diffraction analyses of the sediment suggest that lithostratigraphic Units I and II are compositionally similar. X-ray diffractograms indicate that the major mineral components are clay minerals including illite, smectite, kaolinite, attapulgite, and chlorite, as well as primary minerals including calcite, anorthite, and quartz. Amphibole (hornblende and richterite), chlorite, pyroxene (augite), and pyrite peaks are also recognized (Fig. F8).

Depositional environment

The cover sequence recovered from Hole U1378B is a terrestrially sourced upper slope sequence consistent with high sediment accumulation rates throughout the depositional interval (Fig. F3) (see also “Paleontology and biostratigraphy”). Recognition of wood debris and thin (<5 cm) normal-graded sands with sharp erosional bases within Unit I in Hole U1378B is consistent with deposition within the distal facies of a clastic turbidite sequence.