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doi:10.2204/iodp.proc.324.105.2010

Sedimentology

One hole was drilled at Site U1348 on the northern flank of Tamu Massif, Shatsky Rise, in a water depth of 3264 meters below sea level (mbsl). A thick sequence (~120 m stratigraphically) of volcaniclastic sediments topped with shallow-water calcareous sandstones, greenish clays, nannofossil ooze, and chert was recovered. Despite drilling to nearly 325 meters below seafloor (mbsf), basaltic basement was not reached at Site U1348, and a unique sequence of marine volcaniclastic rocks was recovered in Cores 324-U1348A-14R through 26R. The uppermost cores contained red chert interbedded with a remarkably well preserved section of Cenozoic/Late Cretaceous nannofossil ooze >1 m long. Yellow brecciated cherts were also recovered stratigraphically below the red cherts and above highly silicified altered sandstones. Below this, a sequence of shallow-water bioclastic sandstones with volcanic clasts was found. This sequence includes an interval of bright green zeolitic clays. Highly altered volcaniclastic sequences comprise most of the lowermost 120 m of the hole. Based on the marine fossil content and bedding structures, these sequences are interpreted to represent a mixture of in situ and redeposited volcanic materials erupted in a submarine environment.

Unit descriptions

The lithostratigraphy of Site U1348 is shown in Figure F5.

• Unit I: cherts, chalks, and nannofossil ooze (84.2–161.1 mbsf).

• Unit II: sandstones and clays (161.1–199.5 mbsf). This unit is predominantly bioclastic sandstones with volcaniclastics cemented by calcite, quartz, or silica. The sequence is altered and becomes more zeolitic downhole, including a ~60 cm interval composed almost entirely of zeolites.

• Unit III: layered granular hyaloclastite (199.5–242.0 mbsf). This unit is divided into three subunits based on degree and style of bedding and the relative proportion of bioclastic components present.

• Subunit IIIa: altered granular hyaloclastite (199.5–203.5 mbsf).

• Subunit IIIb: calcite-cemented volcaniclastic sandstone (203.5–209.0 mbsf).

• Subunit IIIc: altered granular hyaloclastite to fine hyaloclastite (209.0–242.0 mbsf).

• Unit IV: volcaniclastic sandstone with fossils (242.0–262.8 mbsf).

• Unit V: layered granular hyaloclastite (262.8–287.0 mbsf).

• Unit VI: structureless vesicular hyaloclastite breccia (below 287 mbsf).

Unit I

• Interval: 324-U1348A-1W through 10R-1, 21.5 cm
• Depth: 84.2–161.1 mbsf

Cores 324-U1348A-1W through 9R contain a mixture of red cherts, calcareous nannofossil ooze, and yellow brecciated chert beds. The recovered material from this unit is predominantly composed of small red chert pieces with patches of porcellanite and thin foraminifer-bearing chalk coatings. The chert samples vary from dark brownish red to pinkish red with pale patches and streaks of pinkish cream porcellanite. Dendritic, opaque mineral growths (possibly manganese oxide) are associated with these cherts. The growths are concentrated along siliceous veins but can also be found within the reddish siliceous matrix (Fig. F6). A well-preserved, partially laminated section of light-colored nannofossil ooze was recovered in Section 324-U1348A-2R-1 (Fig. F7). The ooze is pale yellow in color, soupy, and composed almost entirely of calcareous nannofossils with the exception of a 6 cm section of pale gray ooze at the top of Section 324-U1348A-2R-1. Although still soft, the gray ooze is slightly more consolidated and contains up to 5% radiolarians, diatoms, and silicoflagellates. The gray ooze and cherts in Core 324-U1348A-1W, stratigraphically above, are mid-Cenozoic in age, whereas the pale yellow ooze below is Late Cretaceous (Santonian–Campanian) in age (see "Paleontology"). The gray ooze and contents of the wash core could simply reflect drilling-related "fall-in" or may represent a genuine hiatus in deposition between the Late Cretaceous and mid-Cenozoic. In Core 324-U1348A-9R, the cherts become bright yellow. Some pieces can be termed "jasper," as they are opaque and display complex vein networks and silica-cemented brecciation (Fig. F8). Another interval of white to pale yellow nannofossil ooze is present in Section 324-U1348A-10R-1 between 0 and 21.5 cm. Both planktonic and benthic foraminifers are present in this ooze (see "Paleontology"). A few pieces of red chert are mixed in with the ooze at the top of this interval, and most of the sediment is intensely disturbed by drilling. A few cross-bedded horizons are visible in interval 324-U1348A-10R-1, 14–17 cm, and consist of thin lenses of coarse brownish green sand within the nannofossil ooze matrix.

Unit II

• Interval: 324-U1348A-10R-1, 21.5 cm, through 13R
• Depth: 161.1–199.5 mbsf

Below the chert-ooze sequence is a thick succession of sandstones with a few beds of zeolite or clays extending ~40 m stratigraphically. Unit II extends from interval 324-U1348A-10R-1, 21.5 cm, to the base of Core 13R. Core 10R is predominantly altered, quartz-cemented, dark yellow sandstone with light gray streaks of chert and a few altered volcanic clasts. Across interval 324-U1348A-10R-1, 48–50 cm, the original sandstone has been broken into ~1 cm sized pieces and recemented with white/clear chert, creating a silica-sandstone breccia, which was recovered as one biscuited piece (Fig. F9). Fractures are common in Core 10R, as are small cavities (vugs) filled with quartz crystals ~1 mm in size. Recovery in Core 11R was especially poor. Only 9 cm of coarse, loosely calcite-cemented bioclastic sandstone, preserved as four small nuggets (biscuited by drilling), was recovered. These sandstones are bright yellow in color and moderately to well sorted with subrounded grains. The majority of the grains are tubular coral fragments <2 mm long, with additional bioclastic material including bryozoan and shell fragments (Fig. F10). Small pyrite clusters also make up a minor component of the sandstone. Beneath the yellow sandstone biscuits, a sequence of poorly lithified brownish orange sandstone is interbedded with two soft layers of greenish yellow and olive-green clays (Fig. F11). The sandstone is calcite cemented, crumbly, and contains subrounded quartz crystals and biogenic fragments as well as a few opaque mineral grains. The first clay layer is pale greenish yellow and spans ~20 cm of Section 324-U1348A-12R-1 (13.5–33 cm). Within the clay layer is a thin (~1 cm) hard, white, silty band composed of small needlelike grains. The second soft clay layer, in interval 324-U1348A-12R-CC, 4–12 cm, is greener in color and of similar texture but grades into yellow siltier material toward its base (11–12 cm). X-ray diffraction (XRD) analysis reveals the clay layers are composed of different minerals. The lighter yellow-green layer is mainly composed of zeolite, whereas the greener clay is predominantly composed of celadonite (see "Alteration and metamorphic petrology"). This interval also corresponds to a spike in potassium, as seen in the natural gamma ray (NGR) data (see "Downhole logging"). The high potassium content is also confirmed by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) analysis of the green clays (see "Geochemistry"). The base of Core 324-U1348A-12R is soft pale yellow silt and clay. Stratigraphically below, in Core 13R, is a light yellow-brown carbonate-cemented coarse to very coarse sandstone with abundant bioclasts and altered volcaniclastics. The bioclastic materials include bivalve shells (some as large as ~2–3 cm), crinoid ossicles, and fragments of echinoderm plates (Fig. F12). The altered volcanic clasts are larger, granule or pebble sized, and most are pale green in color and vesicular, although a few darker, less altered basaltic clasts are present. Interval 324-U1348A-13R-1, 0–135 cm, is structureless and unbedded. Two small lenses of darker red-brown material are present at 125–126.5 cm, indicating erosive scouring. A 2 cm interval of brownish orange clay is present beneath the coarse sandstone. This layer is soft, containing tiny brownish shards of altered vitric glass, iron oxides, and calcite, and sits atop finely laminated fine- to medium-grained sandstone. The sandy grains are angular with a volcanic ash and clay component. Within this sandstone, orange-red, strong brown, and grayish green color banding seems to track slight lithologic changes in grain size. The greener layers appear to be more clay rich. Smear slides reveal that the volcanic component is primarily fibrous isotropic zeolite and altered glass with iron oxides and occasional feldspar laths.

Unit III

• Interval: 324-U1348A-14R through 18R
• Depth: 199.5–242.0 mbsf

Subunit IIIa

This subunit is mainly composed of granular hyaloclastites. The hyaloclasts are dark colored, subangular to very angular in shape, and scattered within a finer grained matrix (Fig. F13). Most of the hyaloclasts are coarse sand to lapilli sized and can be divided into two types: dark angular clasts and dark to light gray vesicle-rich grains. The angular clasts are probably highly altered aphyric basalts. One large angular clast, 3 cm long, spans interval 324-U1348A-14R-3, 57–60 cm (Fig. F13A). The vesicle-rich clasts could be spherules of altered hyaloclastics (see "Petrology and igneous petrology"). The matrix materials are finer hyaloclasts cemented by calcite. This unit is mainly structureless and poorly sorted, but interval 324-U1348A-14R-1, 24–57 cm, is slightly laminated in places.

Subunit IIIb

The top of Subunit IIIb spans interval 324-U1348A-14R-CC, 4 cm, to 15R-1, 7 cm, and comprises cream to light gray calcite-cemented sandstone (Fig. F13B). Weak calcite-rich horizontal bands are found within the subunit. Coarse, rounded volcaniclastics are scattered throughout but are concentrated into horizontal bands. One large bivalve shell is present in Section 324-U1348A-14R-CC. Although a dark reddish brown chert pebble is present at the top of Section 15R-1, it is interpreted as fall-in from the upper part of the hole and is unlikely to be in situ.

Subunit IIIc

This subunit is mostly dark greenish gray poorly sorted granular hyaloclastite, hyaloclastite sandstone, and fine hyaloclastite (Figs. F14, F15). Grading is found in some parts of the subunit. Some beds are normally graded, but reverse grading is more common (Fig. F15). Beds are horizontally laminated in the upper section of this subunit (Sections 324-U1348A-15R-1 through 15R-4, 16R-1, and 17R-3 through 17R-5) and slightly inclined in the lower part of the subunit (Sections 18R-1 through 18R-2). The dominant sedimentary facies of Subunit IIIc is structureless, but in places successions of fine- to medium-bedded granular hyaloclastites, hyaloclastite sandstones, and fine hyaloclasts are present (Fig. F16). The structureless and bedded parts generally show a layered repetitive structure. Large, dark clasts (>0.5 cm) with subangular to very angular shape are concentrated into some intervals (e.g., interval 324-U1348A-17R-5, 30–33 cm; Fig. F15B). At these intervals, sediments often show clast-supported structure. Among the clasts, large (>1 cm), dark gray vesicular basalts are present.

Unit IV

• Interval: 324-U1348A-18R-CC through 20R-4
• Depth: 242.0–262.8 mbsf

Unit IV is composed of bioclastic sandstone and volcaniclastic sediments cemented by gray to light gray calcite and zeolite. The upper sections of the unit (interval 324-U1348A-18R-CC, 0 cm, to 19R-2, 39 cm) are structureless and dominated by volcaniclastics. Gray to dark gray volcanic clasts are scattered throughout the unit. Most clasts are subangular to subrounded, but some are rounded. Some bedding is present in places, although clasts within these beds are generally poorly sorted. Unit IV is characterized by a concentration of biogenic materials such as shell fragments, crinoid ossicles, gastropods, and a large ammonite (Fig. F17). In addition, glauconite and small, dark biogenic clasts are found throughout Sections 324-U1348A-18R-CC and 19R-1. Bioturbation structures, such as infilled burrows, are common throughout this unit. The lower part of the unit (interval 324-U1348A-19R-2, 39 cm, through 20R-4, 150 cm) is mainly composed of a succession of thin- to medium-bedded volcaniclastic silt to coarse sands and coarse volcaniclastic sandstones. Reddish scoria clasts are present throughout the unit (Fig. F18). The base of the unit is placed at the bottom of an interbedded silty sand and medium sandstone section.

Unit V

• Interval: 324-U1348A-20R-5 through 23R-1, 108 cm
• Depth: 262.8–287.0 mbsf

Unit V consists of fine to granular hyaloclastites. The upper half of the unit (interval 324-U1348A-20R-5, 0 cm, through 22R-1, 36 cm) is generally structureless but does show weak normal and reverse grading in places. Within the graded beds sediments are clast supported but are matrix supported in the structureless intervals. Large volcanic clasts (>5 mm) are present throughout Unit V. The clasts are classified into three types:

1. Dark to light gray vesicular hyaloclasts generally larger than 1 cm,

2. Dark gray basaltic fragments generally smaller than 1 cm, and

3. Light gray to light greenish gray altered basaltic fragments generally <1 cm in size.

The matrix, mainly composed of altered sand-sized hyaloclasts and fine hyaloclastite, is dark greenish gray in the top part of the unit (Sections 324-1348A-20R-5 through 20R-6) and dark gray in the remainder of the upper portion of the unit.

The lower half of Unit V (interval 324-U1348A-22R-1, 35 cm, through 23R-1, 108 cm) is characterized by well-defined bedding, where inclined layers are common (Fig. F19). Dips of the inclined layers are generally ~20° (Fig. F19). The dip angle gradually increases with increasing depth, especially when compared to the subhorizontal laminations seen in Unit III (see "Structural geology"). Downhole logging data (FMS) suggest that the dip of the inclined layers is predominantly toward the southeast throughout the lower half of Unit V (see "Downhole logging"). The interval includes hyaloclastite breccia and granular hyaloclastite containing vesicular hyaloclasts. The inclined layers exhibit normal grading within strata containing predominantly dense hyaloclasts and reverse grading within strata composed principally of vesicular hyaloclasts. Another noteworthy feature of the lower half of Unit V is the presence of bioturbation in some intervals.

Unit VI

• Interval: 324-U1348A-23R through 26R
• Depth: 287.0–324.1 mbsf

Unit VI is predominantly composed of dark gray hyaloclasts, many of which are heavily altered. This unit is almost exclusively clast supported and generally structureless (Fig. F20), though a few intervals contain interbedded finer materials (e.g., interval 324-U1348A-26R-1, 55–78 cm). The finer materials are composed of siltstones, sandstones, and altered vesicular hyaloclasts and are often faintly laminated. The upper portion of Unit VI shows slightly chaotic and churned features probably associated with bioturbation, but the degree of bioturbation decreases toward the middle and lower portions of the unit and is entirely absent in some sections. A small amount of fresh volcanic glass is preserved within this unit (see "Igneous petrology").

Interpretation

A strong volcanogenic influence at Site U1348 is evidenced by Units II–VI. In addition, many of the beds show evidence of alteration, which changed both the composition and textures seen in the cores at this site. In contrast, Unit I, which is composed of cherts, chalks, and oozes, appears devoid of volcanogenic components and was probably deposited in a pelagic environment. The calcareous sandstones in Unit II contain abundant remains of epifaunal and reef-building fauna, indicative of a shallow-water marine environment. The persistent volcaniclastic component (including palagonite and vesicular hyaloclasts) in Unit II indicates a volcanic source proximal to the area of carbonate deposition. The green clays in Core 324-U1348A-12R may represent a sequence of volcanic ashes, subsequently altered to zeolites and celadonite in the marine environment. The rocks in Units III–VI are almost entirely volcanogenic and are predominantly composed of various sized altered glass fragments. The depositional setting for these units is probably submarine, but modes of deposition may range from compacted primary hyaloclastite to redeposition of the volcanic material by turbidites.

Unit I

Some of the cherts in Unit I contain remnant bedding features preserved as porcellanite patches and streaks within the secondary silica matrix. High concentrations of foraminifers within some chert and porcellanite pieces indicate these rocks represent calcareous oozes subsequently silicified by remobilized silica. The silica was likely sourced from dissolved radiolarian tests. Reddish brown cherts dominate here and differ from the black cherts recovered from Sites U1346 and U1347. Cherts can be a variety of colors, including black/gray, brown, red, blue, green, and yellow. The specific color, however, is often related to factors such as

1. The redox conditions at the time of deposition,

2. The percentage of silica relative to contaminants in the chert,

3. The chemical composition of the contaminants, and

4. Any subsequent alteration processes.

Red cherts are usually enriched in oxidized iron–bearing minerals such as hematite and thus are indicative of oxic bottom water conditions. The color of cherts at Shatsky Rise has previously been related to changes in sedimentation rate (Fontilea et al., 2006), where redder cherts are associated with lower sedimentation rates. This relationship may also be applicable to the Unit I cherts at Site U1348.

The yellow color of the chert recovered in Core 324-U1348A-9R, its opacity, and the evidence for brecciation and recementation are consistent with low-temperature hydrothermal alteration. The pervasive yellow color is likely to be derived from the presence of goethite (iron oxyhydroxide), which is often associated with hydrothermal systems. The alteration seen in the underlying units also suggests circulation of warm fluids through the sediments after deposition. As the cherts and chalks were likely deposited long after the main active phase of volcanism on Tamu Massif, the source and mechanism of this hydrothermal alteration remain enigmatic.

Although very few soft sediments were recovered in Unit I, the cherts are presumably interbedded with calcareous nannofossil ooze in the upper part of the sequence, as suggested by the material recovered in Core 324-U1348A-2R, and with more lithified chalk and porcellanite in the lower part, as suggested by material recovered in Cores 3R through 9R. The recovered oozes are almost exclusively composed of pelagic constituents, mostly calcareous nannofossils and occasional siliceous components. This suggests a relatively deep water setting far removed from a terrestrial source. In this sense, Unit I at Site U1348 is similar to Unit I at both Sites U1346 and U1347 and probably represents deposition after Tamu Massif had subsided out of the photic zone.

Unit II

Unit II sandstones are primarily composed of biogenic components such as echinoderm, coral, bivalve, and bryozoan fragments, with varying proportions of volcaniclastics. The percentage of volcanic constituents and the amount of zeolite in the cores increase toward the bottom of this unit. The uppermost sandstones in Core 324-U1348A-10R are bright yellow and composed of quartz-cemented, iron oxide–stained grains. The cementation, degree of oxidation, and proximity of the yellow cherts in Core 9R stratigraphically above suggest this part of the sequence may have been subjected to low-temperature alteration after deposition.

Interestingly, the coarse biogenic sandstones in Core 324-U1348A-11R are primarily composed of rounded coral fragments and bryozoans, suggestive of a stable, warm, very shallow marine environment. The degree of sorting seen in the rocks from this core and the rounded nature of the grains further supports a very shallow environment where wave action could move and rework the sediment. This unit also contains greenish rounded vesicular volcaniclastic grains, providing evidence for a proximal source of weathered volcanic material if not direct evidence for contemporaneous volcanic activity nearby. These rocks do not seem to have been altered to the same degree as the overlying strata in Core 10R, although this may be due to the different primary constituents in the two cores responding differently to the alteration.

The greenish yellow clays in Core 324-U1348A-12R most likely represent altered ash deposits. The angular nature of the grains suggests volcanic glass was the primary constituent, which has subsequently been altered to a mixture of zeolites or celadonite. The presence of remaining detrital feldspars and the high potassium content of these rocks support this interpretation. However, the geochemistry of these clays, as deduced from ICP-AES analysis, is not consistent with basaltic source material and more closely resembles the alteration products of continental or arc volcanism (see "Geochemistry"), thus suggesting the source of this ash may have been from a relatively proximal, hitherto undescribed, island arc source.

Biogenic sandstones in Core 324-U1348A-13R are dominated by echinoderm fragments, with a strong volcanogenic component. The amount of palagonite (an alteration product of volcanic glass) throughout the core is consistent with a prolonged input of volcaniclastic material coeval with shallow-water carbonate deposition. The reddish orange to yellow or greenish laminated sand and siltstones at the base of Core 13R are carbonate cemented, highly altered, and contain many compacted angular clasts. The dominance of zeolites with occasional altered plagioclase laths suggests the primary sediment was fine-grained volcaniclastics. The depositional environment was likely to have been a fairly shallow marine environment with a large detrital or primary source of volcanic glass. However, the sediments are undisturbed (unbioturbated), indicating fairly rapid sedimentation.

Units III–VI

The Cretaceous volcaniclastic beds (Units III–VI) at Site U1348 preserve a record of nearby submarine volcanism. Microscopic characteristics of the clasts in Units III–VI that support a dominantly hydroclastic source are presented in "Igneous petrology." Unit VI in particular is almost completely composed of altered hyaloclasts. Hyaloclastite was defined for rocks composed of glass produced by nonexplosive spalling and granulation of pillow rinds by Rittman (1962). However, the term has been expanded to all vitroclastic (i.e., glassy) tephra produced by the interaction of water and hot magma or lava (Fisher and Schmincke, 1984). Dominant hyaloclastites suggest primary hydroclastic deposition.

Units III and V are mainly composed of poorly sorted granular hyaloclastite and alternating layers of granular hyaloclastite to fine hyaloclastite. The alternating layers are often laminated and graded. These are considered to represent deposition by low-density turbidites (Bouma, 1962). Units III–VI are interpreted to have been deposited in submarine environments.

Unit III probably represents two mass flows (Subunits IIIa and IIIc) divided by sediments deposited during a quiescent period (Subunit IIIb). The sedimentary features are divided into two characteristic facies: thick clast-rich sequences and cyclic fine to coarse tuff sequences (interbedded in the thick clast-rich sequence).

The thick clast-rich sequences are mostly poorly sorted and can be structureless or exhibit graded bedding (both normal and reverse) (Figs. F13, F14). Their thickness attains a maximum of ~10 m. Clasts are mainly composed of angular vesicular hyaloclasts (often altered and white) and angular to subangular basaltic lithic clasts. The matrix is mainly altered vitric clasts and vesicular fragments. The lower part is often reverse graded, whereas the upper part often displays normal grading. We consider this interval to represent mass flow deposits, which are divided in terms of their position in the Lowe sequence (Lowe, 1982). The reverse graded part is considered to represent the R2 division (traction carpet) of the Lowe sequence, whereas the normally graded part is considered to represent the R3 division (Lowe, 1982) (Fig. F21). Such evidence suggests that these thick clast-rich sequences were deposited by high-density turbidity currents (Fig. F22). The grains are mostly primary volcaniclastics, and some thick clast-rich sequences can be interpreted as subaqueous pyroclastic flow deposits (e.g., Fiske, 1963; Fiske and Matsuda, 1964; Gibson et al., 2000) and/or hyalotuff (e.g., Honnorez and Kirst, 1975; Heiken and Wohletz, 1985; Yamagishi, 1987; Busby, et al. 2006).

Unit IV represents a quiescent period of volcanism. The concentration of biogenic materials and glauconite suggest deposition in neritic environments. The lower part of Unit IV is characterized by fine-grained sediments and abundant bioturbation, which suggest a lower rate of volcaniclastic deposition (Fig. F22). The red color of some scoria within this unit indicates oxidation in subaerial environments.

The sedimentary features of Unit V are inclined layers and foreset beds (Fig. F19), mainly composed of parallel stratified volcanic sandstone with lapilli (altered vitric and lithic clasts). Graded bedding (normal and reverse) and laminations are present throughout. Clast-supported basaltic and/or low-vesicular clasts are concentrated in layers. These deposits are considered to represent turbidites and grain flows (Bouma, 1962; Lowe 1979, 1982; Stow, 1986). The inclined layers may have been deposited as part of the "slope apron" of a volcano. The gradual increase of dip with increasing depth in the core implies decreasing slope angle through time, possibly related to the progradation of the slope apron (Fig. F22).

Unit VI is composed of structureless altered hyaloclasts, mainly clast supported, and generally does not contain fine-grained sediments. Some armored clasts are found in this unit. This facies association indicates rapid primary deposition. Most of the sedimentary features in Unit VI are consistent with a model in which materials were derived from primary deposits near the vent and then deposited on the flanks of the volcano. However, there is little evidence of gravity flow deposits (e.g., subaqueous pyroclastic flows, turbidites, subaqueous grain flows, and debris flows). The materials are not stratified (laminated), graded, or matrix supported. Therefore, it is likely that the materials represent in situ hyaloclastite and/or autobreccia. The presence of one interbedded fine-grained interval (interpreted as turbiditic) and the absence of shallow marine sedimentary structures (e.g., wave ripples) indicate deposition in submarine environments at depths below wave base (Fig. F22).

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