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doi:10.2204/iodp.proc.339.109.2013 LithostratigraphyDrilling at Site U1391 recovered a 670.98 m thick sedimentary section (Figs. F3, F4). The shipboard lithostratigraphic program involved detailed visual assessment of grain size, sediment color, sedimentary structures, and bioturbation intensity to describe the facies and facies associations at Site U1391. Petrographic analysis of smear slides taken regularly from Holes U1391A (n = 87) and U1391C (n = 44) were used to assess average grain size and composition of the sediments to aid in the descriptive sediment classification. Additional smear slides were taken from Hole U1391B (n = 5) at specific locations for correlation with Hole U1391A. Samples were selected from Hole U1391A (n = 37) for powder X-ray diffraction (XRD) analysis of bulk mineralogy. No samples were taken from Hole U1391C. The total carbonate content in these cores, based on shipboard analyses, ranges from 17.5 to 45.2 wt%, although shipboard analyses were only carried out for Hole U1391A (Fig. F5). These results are consistent with the abundance of biogenic carbonate and detrital carbonate estimated from smear slides, so the lithologic names determined from smear slide analyses have been used without modification through this text, the accompanying summary diagrams, and the visual core description sheets. The character of sediment physical properties, including NGR, magnetic susceptibility, color reflectance parameters, and density, records the distribution of these various lithologies and sediment components (see “Physical properties”). Characteristics of the sedimentary sequence cored at Site U1391, together with some of these additional properties, are summarized in Figure F6. The sedimentary succession at this site is divided into two lithologic units (I and II). Unit I is characterized by only contourite deposits and has been divided into two subunits (IA and IB). Subunit IA is characterized by sandy contourites and alternating reddish/brownish and greenish gray/greenish calcareous mud. Subunit IB is characterized by fewer and finer-grained contourite sequences, all without sand, and has a higher percentage of biosiliceous sediment. Color alternations in the calcareous mud sequences range from greenish gray to greenish. Unit II has a wider range of facies and features, including a debrite, contourites, a dolomitic mudstone, and microfaults. Unit II also shows less color banding than in Unit I. General descriptionTextureThe sediment is fine grained through most of Site U1391 and dominated by the clay size fraction, with sand-sized material only making a minor contribution to the textural characteristics (Fig. F4). Subunit IA has the lowest clay size fraction (54%), followed by silt (31%). Subunit IA also has the highest contribution from the sand size fraction (15%). Sandy mud and silty sands, at times with biogenic carbonate, have an average grain size from very fine to fine sand with a maximum grain size of medium sand. Silty and sandy units are generally poorly sorted, and detrital siliciclastic grains are subrounded to rounded. Detrital carbonate grains are generally subrounded to subangular, and many are abraded, indicating reworking. Subunit IB shows a slight increase in the clay size fraction to 65% and a decrease in the sand size fraction to 8%. Unit II shows similar textural characteristics to Subunit IB, with clay at 61% and sand at 9% (Table T2). Bioturbation is the most obvious secondary sedimentary structure in cores collected from Site U1391. The most common indicators of bioturbation are diffuse centimeter-scale mottling and millimeter-scale pyritic burrow fills. Black iron sulfide mottling is also common. Discrete burrows and recognizable ichnofossils are rare; those present occur in a few beds with discrete burrows of Chondrites and Zoophycos. The bioturbation index ranges from sparse to slight. CompositionAll lithologies at Site U1391 are similar in composition, with only subtle changes in abundance between Subunits IA and IB and Unit II (Fig. F4; Table T2). All lithologic units and subunits have a relatively even contribution of terrigenous components including siliciclastics (quartz, feldspars, heavy minerals, clay minerals, and volcanic glass), detrital carbonate, and biogenic components (mainly calcareous nannofossils with rare to common foraminifers). Biogenic silica components are rare but when present include sponge spicules and fragmented radiolarians. Unit II has slightly more biogenic silica than Subunits IA or IB (7% in Unit II versus 3% in Unit I; Fig. F4). No discrete ash or dust layers and no dropstones were observed. Authigenic products, such as pyrite (usually classified as opaque mineral grains) and dolomite, are also present in minor amounts throughout Unit I (<1%). Thirty-seven samples were selected from Hole U1391A for powder XRD analysis of bulk mineralogy, and nine samples were processed to show the clay mineralogy of the clay size fraction. Twenty-one of the bulk samples and six of the clay samples were from Subunit IA, and sixteen of the bulk samples and four of the clay samples were in Subunit IB (Fig. F7; Table T3). Unit/Subunit descriptionsSubunit IA
Lithologies and beddingThe dominant lithology is calcareous mud, alternating in places with biogenic mud. Together these lithologies account for ~90% of the sediment in this subunit. Minor lithologies include calcareous silty mud, calcareous sandy mud, nannofossil mud, and biogenic mud. Calcareous silty sand is rare (Figs. F4, F6). Structures and textureBi-gradational contouritic sequences are common throughout Subunit IA, with bioturbated and gradational lower and upper contacts and grading from silty clay to silty mud, sandy mud, or silty sand. Some of the bi-gradational sequences have a sharp to erosional contact in the middle of the sequence. In these cases, part of the underlying inversely graded sequence appears to have been removed by downward erosion of the normally graded sequence. The maximum grain size occurs directly above the erosional contact (Figs. F8, F9, F10). Two of these sequences show foraminifer-rich sand in the basal or upper part (Sections 339-U1391A-6H-4 and 6H-6; Figs. F8, F10). A few sequences with sharp bases and normal grading are also described; the maximum grain size is silty mud, sandy mud, or silty sand. A maximum of seven silty and sandy contouritic beds occur per core in Subunit IA (Fig. F4). CompositionSubunit IA is dominated by clay-sized (54%) and silt-sized (31%) sediment. Sand-sized sediment is only a minor component (15%). Compositionally, the unit has a relatively constant contribution of siliciclastics (43%), biogenic carbonate (30%), and detrital carbonate (26%) throughout. Biogenic silica makes only a minor contribution (1%), except for one specific bed in Section 339-U1391A-11H-3 in which biogenic silica is up to 50% (Fig. F4). XRD analyses of 21 bulk samples from Subunit IA record reflections from siliciclastic framework minerals (quartz, K-feldspar, plagioclase, and hornblende), clay minerals (illite, chlorite, kaolinite, and smectite), and carbonates (calcite, dolomite, and rare aragonite). Peak intensities appear fairly constant throughout the subunit. Six clay mineral analyses confirm the presence of smectite in five of the six samples. ColorAlternating greenish gray intervals and thicker reddish/brownish intervals are common. In the greenish gray intervals, colors range from dark greenish gray (10Y 4/1 and 5GY 4/1) to greenish gray (10Y 5/1 and 5GY 5/1). In the reddish/brownish intervals, colors include dark gray (2.5Y 4/1), dark grayish brown (10YR 4/1 and 2.5Y 4/2), gray (10YR 5/1 and 10Y 6/1), and grayish brown (10YR 5/2 and 2.5Y 5/2) (Fig. F11). Nannofossil mud has a lighter color range of 10Y 5/1, 5GY 5/1, 10YR 5/1, and 10Y 6/1. Biosiliceous mud is commonly greenish in color (5GY 4/1). Subunit IB
Lithologies and beddingThe dominant lithology is calcareous mud, alternating in places with biogenic mud. Together these lithologies account for ~95% of the sediment in this subunit. The only minor lithology identified is calcareous silty mud (Figs. F4, F6). Structures and textureSubunit IB has rare and widely spaced contouritic sequences with a maximum grain size of silty mud and no sand. Only bi-gradational sequences with bioturbated or gradational upper and basal contacts are observed. There is a maximum of four silty mud contouritic beds per core in Subunit IB (Fig. F4). Millimeter-scale laminations in mud and silty mud are observed at 483–485 mbsf (Sections 339-U1391C-17R-3 through 17R-CC) (Figs. F12, F13). The remainder of the unit is described as massive, and sedimentary structures are poorly preserved due to the intensity of bioturbation. CompositionSubunit IB has the highest clay size fraction (65%). Compared to Subunit IA, this subunit has a similar silt size fraction (27%) and a lower sand size fraction (8%). Compositionally, the unit has very even percentages of siliciclastics (32%), biogenic carbonate (35%), and detrital carbonate (30%). Biogenic silica makes only a minor contribution (3%), except in seven beds where it occurs with percentages from 10% to as high as 52% (Section 339-U1391A-28X-5) (Fig. F4). The 16 XRD bulk samples from Subunit IB record reflections from siliciclastic framework minerals (quartz, K-feldspar, plagioclase, and hornblende), clay minerals (illite, chlorite, kaolinite, and smectite), and carbonates (calcite, dolomite, and rare aragonite). Peak intensities appear fairly constant throughout the subunit, and are, on average, 30%–50% higher than in Subunit IA. Smectite is only present in two of the four analyses. Color: The most common dark colors are dark greenish gray (10Y 4/1) and greenish gray (10YR 5/1 and 10Y 6/1), alternating with thin greenish intervals (5GY 4/1 and 5GY 5/1) (Fig. F14). These color alternations are quite prominent throughout Subunit IB. The downhole cessation of this color alternation is one of the factors used in determining the boundary with Unit II. Unit II
Lithologies and beddingThe dominant lithology is calcareous mud, alternating with biogenic mud. Together these lithologies account for ~80% of sediments in this unit. Minor lithologies identified include calcareous silty mud and a dolomitic mudstone (Figs. F4, F6). Structures and textureUnit II has a wider variety of sedimentary structures than does Unit I, including more silty mud contourite sequences. The contourite beds in Unit II are bi-gradational, several of them with a sharp upper contact (top-cut-out contourite). The base of a debrite (≥35 cm thick) occurs at the top of Section 339-U1391C-30R-1 (632.6–633.2 mbsf) (Fig. F15; the overlying Core 29R was not recovered). The debrite intraclasts and the matrix are both muddy. The intraclasts show indications of having been burrowed prior to emplacement, are subrounded to rounded, and are as large as a few centimeters in diameter. A 56 cm thick dolomitic mudstone bed occurs between 632.6 and 633.2 mbsf (interval 339-U1391C-32R-7, 20–76 cm). This sedimentary rock contains abundant fine dolomite grains as well as a few biosiliceous fossils, such as sponge spicules, and rare calcareous nannofossils (Fig. F16). The mudstone bed is overlain by biosiliceous mud. Millimeter-scale laminations of mud and silty mud with biogenic carbonate occur regularly between 634.5 and 650.1 mbsf (Sections 339-U1391C-33R-2 to 34R-5), some of which appear to be low-angle cross-lamination. A reverse microfault offsetting at least two burrows is present in Section 339-U1391C-34R-6 (650.1–651.14 mbsf) together with associated dewatering structures (Fig. F17). CompositionUnit II comprises a dominant clay size fraction (60%), a silt size fraction (31%), and a minor sand size fraction (9%). Compositionally, siliciclastics are the dominant component (40%), and biogenic carbonate (28%) and detrital carbonate (23%) have a relative equal contribution. Unit II has generally higher biogenic silica (7%) than Unit I. This is evident throughout Unit II, whereas in Unit I it is present only in discrete beds (Fig. F4). No XRD samples were processed from this unit. ColorUnit II displays a dominant greenish gray background color (10Y 4/1 and 10Y 5/1) that alternates with thin and less common (n = 6) greenish intervals (5GY 4/1). DiscussionTextual and compositional characteristics are very uniform throughout Site U1391. Accordingly, lithologic unit and subunit division is based on subtle variations in composition and facies characteristics. Specifically, Subunit IA is characterized by cyclic color alternation between reddish gray and greenish gray. This subunit also contains more silty mud and silty sand units than the underlying units. These silty units have been interpreted as parts of contourite sequences. Stacked contourite sequences are relatively common with top- and/or base-cut-out sequences. Erosional or sharp contacts are common within these stacked sequences, indicating changes in current velocity. Subunit IA correlates with formal and informal Subunit IA at Sites U1386, U1387, and U1389. Subunit IB has fewer distinct contourite sequences and is dominated by thick beds of calcareous muds with green mottles, some of which are rich in biogenic silica (diatoms and sponge spicules), and laminated. Subunit IB clearly represents a low-energy contourite depositional environment, and the preservation of lamination in parts of the subunit probably indicates periodically decreased oxygenation of the bottom waters resulting in less biological activity (a decrease in bioturbation). In Unit II (Pliocene), the alternating successions of reddish and green calcareous muds are less obvious and more irregular, suggesting some change in the controls on cyclicity. The dolomitic mudstone in Unit II occurs at a similar stratigraphic position as that observed at Site U1387, and we would interpret a similar formation mechanism linked with a hiatus in sedimentation. This is clear evidence for widespread strong bottom current activity linked with MOW intensification at ~3 Ma. Debrites and microfaulting are also present in Unit II, the latter perhaps indicative of slump/slide processes. Both deformation features indicate slope instability and downslope mass movement that might be related to regional tectonic activity. |
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