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doi:10.2204/iodp.proc.344.107.2013 Lithostratigraphy and petrologySite U1413 was drilled to investigate the lithostratigraphy and structural geology of the upper slope sequence as a preliminary study for future deep riser drilling. Three holes were drilled to recover sediment and sedimentary rocks. From the seafloor to 578.84 mbsf, sediment was drilled with the APC (0–140.60 mbsf; Cores 344-U1413A-1H through 18H and 344-U1413B-1H through 3H), the XCB (140.60–186.91 mbsf; Cores 344-U1413A-19X through 26X), and the RCB (0–578.84 mbsf; Cores 344-U1413C-1R through 43R). Core recovery was excellent for Holes U1413A and U1413B, with 99% and 107% average recovery, respectively. Good recovery was achieved for Hole U1413C, with 78% recovered. Hole U1413B was drilled primarily to determine chemical pore water gradients in the uppermost sediment. Three lithostratigraphic units were distinguished in the sediment at this site, with an overall abundance of silty clay to clay (32.1%), clayey silt to sandy silt (53.3%), silty sand to sand (14.4%), and tephra (0.2%). Unit I, recovered in Holes U1413A and U1413B, is 44.60 m thick and is dominated by dark greenish gray silty clay with multiple centimeter-sized turbidite sequences of fine-sand laminae. A slump/slide event was identified in the uppermost ~3.5 m of this unit. The lithology changes into a brownish green chaotic mixture of silty clay and dark gray sand in the lowermost part of Core 344-U1413A-5H. Ten tephra layers were identified throughout Unit I. Lithostratigraphic Unit II, starting at 44.60 mbsf, is characterized by a well-consolidated light greenish gray calcareous clayey silt(stone) with occasional variations to silty clay and minor sand(stone) layers. Unit II exhibits moderate variability in the amount and extent of calcareous cementation with depth. In addition, this unit contains several horizons of lithified and reworked rounded carbonate mud clasts. Fourteen tephra layers were identified in Unit II. Near the bottom of Hole U1413A (Section 344-U1413A-20X-1), a chaotic layer of intermixed sand and calcareous clayey silt, not associated with a lithology change, most likely represents an interval composed of mass transport deposits (MTDs). Unit II continues in Hole U1413C to 366.45 mbsf with the same lithology, but heavy minerals, shell fragments, and occasional sandstone layers with sapropel and leaf fragments become more abundant with depth. The boundary between Units II and III (Section 344-U1413C-21R-3, 117 cm) is conspicuous because of the first appearance of an 18 m thick package of alternating sandstone and siltstone with common to abundant organic matter (sapropel), shell, and gastropod fragments. The matrix of the silt- and sandstone is characterized by abundant magmatic and sedimentary lithic fragments, common feldspar, and volcanic glass fragments. Foraminifers are the most abundant component of the biogenic material. After a relatively thin siltstone interval (~42 m), the remainder of the hole (426.83–578.84 mbsf) consists of massive fine- to medium-grained sandstones that contain three tephra layers. The sandstones are normally graded and range from decimeter- to meter-thick layers with occasional internal laminations, particularly in the uppermost part of the section. Some cores in Unit III are particularly rich in gastropods and reworked, well-rounded carbonate clasts. Lithostratigraphic units at Site U1413 can be well correlated to some of the units found at Sites U1378 and U1379 during Expedition 334. Description of unitsCores recovered at Site U1413 were divided into three lithostratigraphic units (Fig. F3; Table T2). Unit I (Holes U1413A and U1413B)
Unit I is 44.60 m thick and consists mainly of dark greenish gray soft silty clay. The clay is punctuated by a series of several centimeter-thick, turbiditic fining-upward sequences with millimeter-thick laminae, as well as 10 white to pinkish gray and dark gray clayey tephra layers (Fig. F4). The turbiditic fine-grained sand has sharp lower contacts, characterized by laminae composed of well-rounded sand grains that grade into the background sediment at the top. Tephra layers range in thickness from 1 to 15 cm and commonly are extremely fine grained with a clayey appearance. Only a few tephra layers exhibit sharp lower and gradational upper contacts. The main components of Unit I are terrigenous and dominated by clay. Smear slide analyses indicate that feldspar and lithic (sedimentary and magmatic) fragments are the most abundant accessory grains. Additional components include chert, opaque minerals, volcanic glass fragments, amphiboles, and rare pyroxene. Biogenic components such as nannofossils, foraminifers, and diatoms are rare. The two MTDs identified in Unit I are characterized by intermixed silty clay with sand, abrupt color change, lithology, and consolidation relative to the underlying undisturbed sediment. The first MTD is identified by color change and chaotic mixing and occurs in Sections 344-U1413A-1H-3, 34 cm, and 344-U1413B-1H-3, 34 cm (at 3.34 mbsf in both) (Fig. F5). Evidence for this MTD also appears in the pore water chemistry profiles (see “Geochemistry”). The second MTD in the lowermost part of Section 344-U1413A-5H-7 is identified by a gradational brownish green chaotic mixture of silty clay and dark gray sand (Fig. F6) before an abrupt change at 44.60 mbsf that marks the lithologic transition into Unit II. Unit II (Holes U1413A and U1413C)
Unit II is dominated by light greenish gray calcareous clayey silt(stone), with smaller variations in grain size to silty clay(stone) and minor sand(stone) layers. The sediment is well consolidated to lithified in the lower part of the unit and contains 14 tephra layers that remain unlithified down to 188 mbsf (Section 344-U1413C-3R-1) (Fig. F7). The lithology of Unit II shows variable amounts of calcareous cementation, as well as occasional horizons where lithified and reworked rounded carbonate mud clasts are common. In general, Unit II is massive, with minor changes in the proportions of clay, silt, and sand. Grain sizes and abundances gradually change over several meter-long intervals. Sandstone layers become more abundant toward the base of the unit and contain increasing amounts of organic matter (sapropel and leaf fragments) and shell fragments. Framboidal pyrite was observed macroscopically and in many smear slides from Unit II. Bimodal distribution in organic material is evident by the sparse presence of shell fragments, diatoms, and nannofossils throughout the unit, whereas foraminifers are a major constituent of the sand-sized fraction of the sediment. The main component of Unit II is terrigenous clay. Smear slides indicate that the most abundant accessory grains in the silt(stone) and clay(stone) include feldspar and lithic (sedimentary and magmatic) fragments. Components present only in trace abundances include amphibole, calcite, biotite, chlorite, and glass shards. A horizon of chaotically mixed sand and calcareous clayey silt near the bottom of Hole U1413A at 151 mbsf (Section 344-U1413A-20X-1) is not associated with a lithologic boundary but instead may represent an MTD interface (see “Paleontology and biostratigraphy,” “Geochemistry,” and “Physical properties”). Unit III (Hole U1413C)
Unit III consists of olive-green, meter-scale silty sand and sandstone sequences with erosive lower contacts. The two packages of predominantly sandy deposits are separated by a ~42 m thick siltstone sequence and represent ~80% of the total unit thickness. The two sandy intervals are between 366.45 and 384.61 mbsf (interval 344-U1413C-21R-3, 117 cm, to 23R-3, 9 cm) and between 426.83 and 578.84 mbsf (interval 27R-5, 55 cm, to 43R-CC, 20 cm). The sandstone to conglomerate beds are mostly decimeter thick but can reach up to a meter in thickness and are normally graded (Fig. F8). Relatively large shell fragments (as much as 2 cm in diameter) and millimeter- to centimeter-sized, well-rounded to subrounded sedimentary lithic clasts make up the coarser fractions of the sandstone and the matrix-supported conglomerates (Fig. F9). Rip-up clasts, millimeter-scale laminations at the top of the fining-upward sequences, and chaotic mixing are also observed throughout the coarser sequences. Tephra layers are rare in this unit, with only three horizons found within the upper part of Unit III. Framboidal pyrite occurs throughout the unit. Bioturbation is variable throughout the unit but generally decreases with depth. Foraminifers, shells, and shell fragments are conspicuous within the sandstone, along with disseminated sapropel and plant debris that is also recognized throughout the coarser horizons (Figs. F9, F10) of Unit III. Smear slide observations indicate that the sandstone is dominated by terrigenous lithic clasts that are primarily composed of magmatic rock fragments and feldspar minerals. Glauconite commonly appears in discrete enriched horizons (Fig. F11). Chlorite is the most abundant accessory mineral, followed by volcanic glass, opaque minerals, and amphibole. Trace components include calcite, pyroxene, and quartz. Biogenic components observed from smear slides are rare but include nannofossils and diatoms and foraminifer fragments. X-ray diffraction analysesPreliminary X-ray diffraction analyses of Site U1413 sediment suggest that there is little compositional variation in lithostratigraphic Units I–III. X-ray diffractograms indicate that the major mineral components are phyllosilicates, including smectite and chlorite, as well as plagioclase, quartz, and calcite (Fig. F12). Zeolites are ubiquitous in Unit I as heulandite and laumontite, but in Unit II, the peak intensity of laumontite decreases, whereas the peak intensity of heulandite increases. The only zeolite present in Unit III is heulandite. Amphibole (hornblende) and analcime peaks are also recognized in Unit I, Unit II, and, at a weaker intensity, in the shallower part of Unit III. Pyrite peaks are present in all units. Depositional environment and correlation to Sites U1379 and U1378 (Expedition 334)The cover sequence recovered from Site U1413 is a terrestrially sourced upper slope sequence consistent with high sediment accumulation rates throughout the depositional interval (see “Paleontology and biostratigraphy”). Recognition of organic debris and thin (<5 cm) normally graded sand beds with sharp erosional bases within Unit I of Holes U1413A and U1413B is consistent with depositional features of a distal clastic turbidite facies and correlates well with Hole U1378B. The clay-rich deposits of Unit II at Site U1413 are probably lithostratigraphically similar to those observed in Unit II in Hole U1379C but lack the carbonate-rich Subunit IIB. A large proportion of the sandy Subunit IIC at Site U1379 was not recovered at Site U1413. Finally, Unit III of Site U1413 can also be well correlated with Unit III of Site U1379. In summary, our observations indicate that the drilled interval at Site U1413 is a condensed section of Site U1379. |
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