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doi:10.2204/iodp.proc.324.104.2010

Sedimentology

One hole was drilled at Site U1347 on the eastern flank of Tamu Massif (water depth = 3450 meters below sea level [mbsl]). A total of 17.7 m of sediment was recovered in Cores 324-U1347A-1W through 11R over a stratigraphic interval of ~88 m before entering basaltic basement at 157.6 mbsf. The recovered sediments at Site U1347 are dominated by radiolarian-rich volcaniclastic siltstones with varying proportions of glauconite. Minor intervals of chert and claystone are also present. Bioturbation is often pervasive in the silty facies, with rip-up clasts and erosional contacts as common features suggestive of turbulent and transient depositional events. Sediments are also present as relatively thin interbeds between the massive basaltic flows and pillow basalt units within the igneous complex. These sediments are similar in character and composition (e.g., predominantly radiolarian-bearing siltstones) to the sediments above basement, although some show features consistent with thermal alteration caused by subsequent basalt emplacement.

Unit descriptions

The overall stratigraphy of the sedimentary material recovered from Site U1347, including the sedimentary beds between basaltic units, is shown in Figure F4. Three stratigraphic units were identified at Site U1347 in the sedimentary pile above the sediment/basement contact (Fig. F5).

  • Unit I: brown, black, and dark gray cherts with occasional fine-grained nannofossil chalks or porcellanites present as thin coatings (71–80.6 mbsf).

  • Unit II: glauconitic radiolarian-rich silicified limestones and calcareous sandstones, with volcaniclastics (80.6–99.8 mbsf).

  • Unit III: radiolarian-rich sandy siltstones and silty sandstones, with volcaniclastics and glauconite (99.8–157.6 mbsf).

Four additional stratigraphic units interbedded with the igneous units below 157.6 mbsf were identified. These units are divided based on their role in separating the established igneous units, rather than sedimentary-lithologic differences (Fig. F4).

  • Unit VI: radiolarian-bearing sandy siltstones (186.7–187.2 mbsf). Original bedding is disturbed.

  • Unit VIII: dark homogeneous claystones, overlying laminated radiolarian-bearing sandy siltstones (200.8–205.83 mbsf).

  • Unit XI: radiolarian-bearing sandy siltstone and volcaniclastic limestone, with interbedded vesicular basalt (258.3–258.8 mbsf).

  • Unit XIII: low-density altered volcaniclastic sandstone (278.1–278.2 mbsf).

Other small pieces of sedimentary material are present within the igneous units, mainly forming baked contact rinds at pillow surfaces. However, these fragments make up such a small proportion of the cores that they do not warrant unit divisions of their own. See "Site U1347 visual core descriptions" in "Core descriptions" for location of these sediment fragments.

Unit I

  • Interval: 324-U1347A-1W through 3R
  • Depth: 71–80.6 mbsf

Only fragments of chert with chalk coatings were recovered in Cores 324-U1347A-1W through 3R, with the exception of 9 cm of light gray pumice at the top of Core 1W. This unit is from 71 to 80.6 mbsf stratigraphically, although the majority of this unit is unrecovered, presumably because of the interbedding of well-indurated cherts with unlithified calcareous sediments. The pumice at the top of the wash core is considered to be "fall-in" from the unrecovered section above and is possibly Cenozoic in age, although no biostratigraphic age constraint is possible. The chert and chalk sequence recovered at this site on Tamu Massif appears superficially similar to stratigraphic Unit I at Site U1346 on Shirshov Massif, although little else can be deduced from the available material. The chert is very dark brown to black with circular porcellanite spots (probably recrystallized radiolarian "ghosts") and faint laminations in places. Evidence of bioturbation in the chert layers appears as indistinct burrow structures visible in some places. The minor chalk coatings are often silicified and/or recrystallized with poor preservation of the presumed original nannofossil composition.

Unit II

  • Interval: 324-U1347A-3R through 4R
  • Depth: 80.6–99.8 mbsf

The recovered portion of Unit II, which begins at interval 324-U1347A-3R, 1.13 cm, and ends at the base of Section 4R-1, spans 10.3 m. Unit II consists of silicified radiolarian-bearing glauconitic limestones and sandstones with occasional cherts. Sedimentary rocks in the lower portion of Section 3R-1 are light brownish gray heavily silicified limestone with abundant rounded glauconite grains, streaks of grey chert, and some small opaque mineral grains. The large concentration of radiolarians and degree of silicification in these limestone pieces warrants naming them porcellanites, or even radiolarites in some cases. The matrix is generally a mixture of micritic calcite and/or radiolarians with secondary silica cement.

A sharp contact exists between the first porcellanite and the last black chert in Section 324-U1347-3R-1. One black chert nodule was recovered at the top of Section 4R-1. The remaining sedimentary rocks in Section 4R-1 are less heavily silicified, greenish gray glauconitic sandstones with volcaniclastics. Radiolarians are very common throughout and are likely the source of the secondary silica cement found in this unit. These sandstones also contain some dark clasts, many of which are subangular, that appear volcanic in origin and probably represent weathered basaltic fragments. Green glauconite clasts are a major constituent and comprise the majority of the grains in the sandstones. These grains often have a rounded appearance, because of the replacement of circular radiolarian tests, and in some cases display yellowish limonitic alteration halos. Where the glauconite has replaced radiolarians, fine netlike biogenic structures are often preserved (Fig. F6). These glauconitic grains are mostly composed of very well preserved whole radiolarian tests, but a few are fragmented or fractured and show possible evidence of reworking. Secondary pyrite, often as clusters concentrated in voids, is common throughout. Much of the sandstone shows cross-bedding (Fig. F7). A large (~2 cm) scalloped shell fragment is present at interval 324-U1347A-4R-1, 36 cm.

Unit III

  • Interval: 324-U1347A-5R through 11R
  • Depth: 99.8–157.6 mbsf

From the top of Core 324-U1347A-5R at 99.8 mbsf to the middle of Core 11R at 157.6 mbsf, a thick sequence of indurated, dark gray, radiolarian-rich sandy siltstones with volcaniclastics and glauconite was recovered. The bulk of these sediments is composed of dark brown clay minerals. The clay composition, as determined by X-ray diffraction (XRD) analysis, is a mixture of altered zeolitic clays (heulandite, fedotovite, and phillipsite) with montmorillonite in some samples (Table T2). As expected, calcite is present in all samples, along with orthoclase, glauconite, albite, talc, and pseudomalachite in certain intervals.

The siltstones have a large radiolarian component. Preservation of the microfossils is variable, from very good to poor (Fig. F8). In some of the coarser intervals, many radiolarians are replaced by secondary calcite leaving only circular "ghosts" in the silty matrix. Radiolarians are, however, well preserved in other horizons (Fig. F9). Other rare biogenic constituents such as fish teeth can also be observed in certain intervals, especially in smear slide preparations. Altered plagioclase laths, small basaltic clasts, and volcanic glass are also present throughout the cores and occur in especially high abundance in Cores 324-U1347A-6R through 10R (Fig. F10). The volcanic origin of much of the fine material is evidenced by the dominance of certain zeolites that are commonly associated with the breakdown of mafic volcanic material.

The majority of the material in Unit III is fine grained, ranging from clay to silt, although coarser intervals are present. Sediments generally coarsen upward from the base of the unit to the top. Clay-sized material dominates in Cores 324-U1347A-9R through 11R near the base of the unit, and sandy siltstones with coarse sand interbeds become more common in Cores 5R through 6R at the top of the unit. Two thin, black, coarse sandstone beds consisting of calcite-cemented volcanic (basaltic) clasts and glass were found at intervals 324-U1347A-5R-1, 27.5–41 cm, and 6R-1, 73–77 cm. These beds were within the radiolarian-bearing sandy siltstone intervals but were recovered as isolated pieces because of drilling disturbance. Interestingly, adjacent to the sandstone bed in Section 324-U1347A-6R-1, a small but very coarse bed of glassy volcaniclastics containing very dark gray spherical grains between 0.5 and 7 mm in diameter was recovered (Fig. F11). Some of these glass spherules have a core of coarser material with a finer grained rim. No concentric rings are observed within this rim. Large pieces of brown volcanic glass are also present outside of the spherules.

Although the overall trend within this unit is coarsening upward, we observe many smaller graded and fining-upward sequences. These are often marked by sandy siltstones at the base followed by siltstones, with laminated claystones at the top. In some cases, the fine clay at the top of these sequences is bioturbated with Chondrites ichnofossils and then truncated by an erosional contact with a coarser horizon (e.g., interval 324-U1347A-8R-2, 41–48 cm). Structures such as scour marks, sharp erosive contacts, lenses of coarser material, churned strata, and rip-up clasts are common throughout in the upper part of the unit, especially in Cores 324-U1347A-6R through 8R (Fig. F12).

Much of this unit contains evidence of bioturbation. Well-defined trace fossils (ichnofossils) are present in some cores. The character, pervasiveness, and ichnofacies change from the base of the unit to the top. In Cores 324-U1347A-5R through 6R, mottled textures in some intervals suggest minor to moderate bioturbation including some Planolites-like structures, occasionally with reduction halos. The degree of bioturbation increases through Cores 7R and 8R, reaching moderate to intense levels. In Core 8R, the bioturbation reaches a peak. Large, U-shaped, vertical burrows (Teichichnus?) can be seen in Section 324-U1347A-8R-1, and Chondrites is present in the clay-rich parts of Sections 8R-2 and 8R-3 (Fig. F13).

Whereas the unit is mostly devoid of bioclasts, occasional calcitic shelly fossils are present, such as the small bivalve fragments found in Sections 324-U1347A-8R-CC and 6R-1 to 6R-2. A poorly defined small (1.5 cm) ammonite impression was found in interval 324-U1347A-8R-5, 92 cm.

Basalt was first encountered in Core 11R at 157.6 mbsf. The location and summary of these beds can be seen in Figure F4 (see "Igneous petrology" for a more detailed account of the igneous units). Description of the sedimentary rocks, which interbed with the basaltic flow and pillow units, follows below.

Unit VI

  • Interval: 324-U1347A-15R-1
  • Depth: 186.7–187.2 mbsf

Within Section 324-U1347A-15R-1, a 50 cm long section of sandy siltstones separates two massive basalt flow units. Logging data suggest the actual thickness of this unit is ~4.5 m. These sediments are composed of two distinct lithologies with differing coloration: a greenish gray radiolarian-bearing sandy siltstone and a very dark gray sandy siltstone.

Traces of original bedding can be seen, although the bedding is inclined and there are signs of soft-sediment deformation. The siltstones are well indurated, brittle, and display hackly fractures. At interval 324-U1347A-15R-1, 50–57 cm, a feature interpreted as a gas or fluid escape pipe can be seen cutting across remnant bedding planes. Sedimentary interbeds are also present in Core 13R within the Unit IV basaltic flow, although they are not extensive enough to warrant a unit division of their own. One piece, at interval 324-U1347A-13R-7, 19.5–21.5 cm, consists of a sandy siltstone with a diverse, well-preserved radiolarian assemblage.

Unit VIII

  • Interval: 324-U1347A-16R-5 through 17R-1
  • Depth: 200.8–205.83 mbsf

In Sections 324-U1347A-16R-5 and 17R-1, ~2.2 m of sediments were recovered, which are thought to be part of a 4.5 m thick sedimentary package between two massive basalt flows. In Section 16R-5, 72 cm of black homogeneous silty claystones was recovered. In Section 17R-1, 143 cm of dark greenish gray radiolarian-bearing sandy siltstones are located stratigraphically above the massive basalt of Unit IX. Sediments from both of these intervals are brittle, with hackly fractures and waxy textures.

Unit XI

  • Interval: 324-U1347A-22R-5
  • Depth: 258.3–258.8 mbsf

Between the upper and lower pillow lavas in Section 324-U1347A-22R-5, small pieces of sedimentary material spanning ~50 cm in the core were found. Logging data suggest these pieces represent a largely unrecovered, ~5 m (logging Unit VIIIb) sedimentary interbed. A piece of dark gray silty claystone with some color banding was recovered between 18 and 39 cm. Heavily recrystallized radiolarians are visible in this piece as well. Between 39 and 68 cm a piece of fine-grained basalt with a chilled margin on the upper surface separates the claystone from a small piece of sandy volcaniclastic-bearing limestone, 5 cm in length.

Unit XIII

  • Interval: 324-U1347A-24R-5
  • Depth: 278.1–278.2 mbsf

A pale gray low-density altered volcaniclastic sandstone is found in Section 324-U1347A-24R-5, between the upper and lower pillow lavas (stratigraphic Units XII and XIV). This small rock has a medium-grained sandstone texture and is noncalcareous and porous. XRD data reveals that it is composed solely of quartz and cristobalite (see Table T2). As both the upper and lower contacts were not recovered, it is difficult to determine whether this piece represents a highly altered in situ volcanic rock or redeposited sedimentary material of volcanogenic origin.

Sedimentary carbon content

Thirty-seven samples were taken from Cores 324-U1347A-3R and 4R in Unit II, Cores 5R through 10R in Unit III, and Cores 15R through 17R in the basaltic interbeds. Both total carbonate (CaCO3) content and total organic carbon (TOC) content in weight percent were determined (Table T3). The amount of carbonate in the samples overall is low, averaging 6.04 wt% (±14.79 wt%, 2σ). Carbonate contents range from 0.98 to 37.65 wt%. Most values, especially within the Unit III siltstones, are below 10 wt% CaCO3 with the majority clustered between 1 and 5 wt%. A silicified limestone from interval 324-U1347A-3R-1, 15–16 cm, has the highest carbonate content.

TOC percentages at Site U1347 are generally low, ranging between 0.07 and 1.28 wt%, with most samples containing <0.5 wt%. One sample from the dark homogeneous claystone in interval 324-U1348A-10R-1, 53–55 cm, has a relatively high TOC value of 1.28 wt%. The TOC from the basaltic-interbedded sediments is low, from 0.10 to 0.53 wt%, although the dark coloration of Section 324-U1347A-16R-5 hints at possible higher organic matter content prior to alteration.

Interpretation

A relatively thick sequence of sedimentary material was recovered at Site U1347 before entering basaltic basement at 157.6 mbsf. Three stratigraphic units are identified in these sediments, representing three distinct depositional environments. Unit I consists of interbedded cherts and carbonates and was probably deposited in a fairly deep pelagic environment with high calcareous nannofossil and radiolarian fluxes and little terrestrial influence. Little else can be deduced about the nature of this unit because of poor core recovery. Unit II consists of silicified limestones and sandstones with copious glauconite clasts and radiolarians and occasional shell fragments. These suggest relatively shallow water conditions in a high-productivity marine setting. The trough cross-bedding structures are suggestive of very shallow water, possibly an upper shoreface environment above wave-base. The high zeolite content and presence of basaltic volcaniclastics, including glass, suggests Unit III was deposited close to a large volcanogenic source. Abundant radiolarians in this unit suggest high-productivity conditions. The coarsening observed from the bottom of Unit III to the top, coupled with the changing style of bioturbation, suggests sequential shallowing within this unit.

The change in lithology and structural indicators between Units III and II, suggestive of shallowing, presents certain problems when considering the classic model of a subsiding volcanic edifice (e.g., Winterer and Sager, 1995) and certainly merits further investigation. It may be that the deposition of Unit III was sufficiently rapid, through the emplacement of sequential turbidites, to outstrip the rate of subsidence, thus allowing progradation of the siltstones to occur. This would account for the shallowing-upward sequence without requiring the entire massif to be uplifted. However, as neither the sedimentation rate nor paleodepth of this unit is known, it is difficult to evaluate the feasibility of this scenario. The overall change from Unit III to Unit I, however, does suggest a deepening sequence (nearshore marine to pelagic), which is supported by changes in benthic foraminifer assemblages (neritic to bathyal) across the same interval (see "Paleontology").

Unit I

The interbedded chert and chalk at this site on Tamu Massif is very similar to that recovered in stratigraphic Unit I at Site U1346 on the Shirshov Massif. The depositional environment was therefore probably very similar: a pelagic environment with deposition occurring above the carbonate compensation depth on a baythmetric high created by the subsidence of the volcanic edifice of Shatsky Rise. The age of these cherts, and indeed all the sedimentary units at Site U1347, has been constrained to the Berriasian to late Valanginian (Early Cretaceous) based on calcareous nannofossil biostratigraphy and preliminary investigation of the radiolarians. This age range places Unit I in the same broad age range as Site U1346 Unit I. The dark color of the cherts at both sites also suggests the two units may have a similar mode of deposition. Although the recovery of Unit I at Site U1347 is very poor, the dark coloration of the cherts suggest a nonoxidizing environment, possibly related to a relatively high sedimentation rate compared to a red chert facies depositional mode (e.g., Fontilea et al., 2006).

Unit II

The silicified limestones and sandstones in Unit II have a high radiolarian content. Preservation of biogenic silica in marine sediments is indicative of a high-productivity marine setting, most likely related to equatorial upwelling. The carbonate component, although now extensively recrystallized, looks to be fine grained, and therefore probably was originally sourced from calcareous marine plankton or inorganic carbonate muds. The large concentration of glauconite in these rocks is suggestive of postdepositional reducing conditions and relatively low sedimentation rates in water <500 m. The replacement of biogenic components with glauconite is a common feature and can be seen in the replacement of radiolarians in Cores 324-U1347A-3R and 4R. The glauconite-replaced microfossils do not appear to be broken or transported a great distance, suggesting in situ formation within the siliceous limestones and sandstones. The cross-bedding, which is so prevalent in this unit, suggests agitated shallow water. Although cross-bedding can also be generated in deeper settings by contour currents, the packages of cross-bedded laminations seen in these cores most closely resemble structures normally seen in depths corresponding to the offshore–shoreface transition (Davis, 1985).

Unit III

The clear volcanogenic origin of much of the material in Unit III strongly suggests that this unit represents the altered weathering products of a proximal basaltic volcanic source. Altered volcanic glass and occasional plagioclase laths are found throughout the sequence, as are zeolites, such as phillipsite and heulandites, commonly associated with the weathering and alteration of mafic igneous materials. The fine grain size and lack of large igneous clasts suggests transportation some distance from the source. This is further supported by the degree of hydraulic sorting seen in several of the cores. The presence of rip-up clasts and scour marks suggests that, at least in some cases, the emplacement of the material was by rapid lateral movement. In the lower cores (324-U1347A-9R through 11R), the dark coloration, slightly elevated TOC, fine-grained matrix, and lack of bioturbation except for Chondrites indicate a quiet dysoxic environment. In the middle to upper Cores 5R through 8R the brownish gray coloration, slightly coarser material, increased bioturbation, and large burrow structures suggest a higher energy oxic environment. The coarse band of spherical glass grains in Section 6R-1 has a clear volcanogenic origin. The lack of clear concentric rings with the rims suggests that these structures do not represent accretionary lapilli and more closely resemble glass spherules, probably related to explosive emergent volcanism in the vicinity.

One model to explain the sedimentary sequence observed in Unit III is progradation where products from a weathering basaltic volcanic edifice infill a local basin, thereby creating a shallowing-upward sequence within the siltstone complex. The basin infill is then capped by a radiolarian, carbonate-rich, shallow-water lithology with fewer volcaniclastics (Unit II). Several lines of evidence seem to support shallowing upward from Unit III to II. Eventual subsidence of the volcanic massif would lead to pelagic conditions and the deposition of chalk and chert (Unit I).

Basalt interbeds

The sedimentary beds in between the basalts are generally composed of similar material to Unit III, radiolarian-bearing siltstones, although they have clearly been altered by proximity to the basalts during emplacement. In some cases, thermal alteration of the sediments has resulted in a waxy texture. The stratigraphic thickness of these beds is difficult to determine because of fracturing during drilling and poor recovery, but logging data suggest Units VI, VIII, and XI are each 4.5–5 m thick (see "Downhole Logging"). Unit XIII is not visible on the logging data, suggesting a different origin for this interval.

If the sedimentation rate within these interbeds could be measured, some estimate of the time between basaltic emplacements at this exact location could be made, although this would not necessarily be indicative of a hiatus in eruptive activity over the entire massif. However, no shipboard estimate of biostratigraphic age was possible in these beds, making sedimentation rates impossible to constrain. It should be noted, however, that greater age constraint from radiolarian biostratigraphy may be possible with further study. The disturbed bedding in Unit VI (Core 324-U1347A-15R) suggests soft-sediment deformation, possibly related to slumping or disturbance caused by emplacement of the upper basalt flow (Unit V). If mass sediment transport occurred, the thickness of the unit gives no useful estimate for hiatus duration. In contrast, the sediment in Unit VIII has 1 m of undisturbed fine laminations throughout Section 324-U1347A-17R-1, suggesting in situ deposition of sediments and possibly a longer period between emplacements of the basaltic flows of Units IX and VII than between those of Units VII and V. The presence of a small altered limestone bed in Unit XI suggests higher carbonate flux relative to volcaniclastics during the hiatus between emplacement of basalt Units XII and IX. The origin of the silica-rich rock in Unit XIII is unknown and warrants further investigation. It may be a highly altered in situ volcanic deposit, which has developed a sedimentary texture because of high temperature thermal alteration and remobilization of silica.