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doi:10.2204/iodp.proc.339.105.2013 LithostratigraphyDrilling at Site U1387 penetrated a ~870 m thick section of sediment (Figs. F1, F2). The shipboard lithostratigraphic program at Site U1387 involved detailed visual assessment of grain size, sediment color, sedimentary structures, and bioturbation intensity to describe the facies and facies associations. Petrographic analysis of smear slides taken regularly from each hole (88 from Hole U1387A, 22 from Hole U1387B, and 124 from Hole U1387C) was used to provide detailed sediment description, identify major components, and apply a more descriptive sediment classification. Hand-drawn logs showing the recovered sediment sequence, including the distribution and structure of bedding, are included in the DRAWLOG folder in “Supplementary material.” Ninety samples were selected from Holes U1387A, U1387B, and U1387C for X-ray diffraction (XRD) analysis of powdered bulk sediment in order to gain a general indication of bulk mineralogy. The clay fraction of every fourth sample was analyzed separately. Based on shipboard analyses, total carbonate contents in these cores range from 12.9 to 36.1 wt% (average = 26.4 wt%), excluding a consolidated dolomite interval at ~460 mbsf in which carbonate content, reported as CaCO3, is 78.6 wt% (see “Geochemistry”). These results are consistent with abundances of biogenic and detrital carbonate estimated from smear slides, so the lithologic names determined from smear slide analyses have been used without modification throughout the text, the accompanying summary diagrams, and the visual core description sheets. The sediment at Site U1387 has been divided into four lithologic units (I–IV; Figs. F1, F3). Unit I is a ~450 m thick Holocene– Pleistocene sequence dominated by nannofossil mud, silty mud with biogenic carbonate, and silty sand with biogenic carbonate. These three lithologies show two distinctive stacking patterns, bi-gradational sequences (Fig. F4) and normally graded sequences (Fig. F5). Unit II is a ~145 m thick Pliocene sequence dominated by the same three lithologies as Unit I. However, Unit II is characterized by a clear cyclicity of dark- and light-colored sediments. These cycles are characterized by a downhole change from very dark greenish gray nannofossil mud to a dark greenish gray nannofossil mud to a dark greenish gray silty mud with biogenic carbonate to a basal dark greenish gray silty sand with biogenic carbonate (Fig. F6). The contacts between the lithologies within each cycle generally are gradational. However, the contact between the basal silty sand and the underlying light–dark cycle is generally sharp or erosional and, in some cases, bioturbated (Fig. F6). Biosiliceous microfossils (sponge spicules, radiolarian fragments. and rare diatom fragments) are present in some beds of very dark greenish gray nannofossil mud. Two well-consolidated massive beds (59 and 12 cm thick) of fine-grained dolomite are intercalated at the top of Unit II (Fig. F7). Unit III is a ~152 m thick Pliocene–Miocene sequence (see “Biostratigraphy”). This unit is also composed of the same three lithologies as are found in Units I and II. However, Unit III is distinguished from the overlying units by a higher proportion of the coarser-grained lithologies. In some cases, these lithologies form contorted/convoluted intervals, interpreted as slump deposits (Fig. F8). Two intervals of sandy sediments in Unit III are well lithified (Fig. F9). Unit IV is a >115 m thick Pliocene–Miocene sequence (see “Biostratigraphy”), dominated by nannofossil mud and muddy nannofossil ooze, with minor intercalations of silty mud with biogenic carbonate (Fig. F10). The character of sediment physical properties, including natural gamma radiation (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 U1387, together with some of these additional properties, are summarized in Figure F2. Unit descriptionsUnit I
Lithologies and beddingThe sediment of Unit I is composed of varying mixtures of terrigenous and biogenic components, primarily silicate minerals (quartz, feldspars, and clay minerals), nannofossils, foraminifers, and detrital carbonate (Fig. F11). The three most common lithologies in Unit I are nannofossil mud, silty mud with biogenic carbonate, and silty sand with biogenic carbonate. Nannofossil mud is dominant in Unit I, with its abundance per core varying between 30% and 100% (Fig. F3). The abundance of beds of silty mud with biogenic carbonate and silty sand with biogenic carbonate also varies throughout Unit I, with notable increases at 0–100, 170–300, and 340–450 mbsf. The large-scale variations in lithologic abundances in Unit I at Site U1387 are similar to those in Unit I at Site U1386, ~4 km northwest of this site (Fig. F12). The uppermost 100 m interval of Unit I at Site U1387 is characterized by a relatively higher proportion of silty sand with biogenic carbonate and is lithologically similar to Subunit IA at Site U1386 (0–110 mbsf). The underlying ~70 m interval at Site U1387 is mostly dominated by nannofossil mud and is lithologically similar to Subunit IB at Site U1386 (110–220 mbsf). The deepest interval of Unit I at Site U1387 (170–450 mbsf) is lithologically similar to Subunit IC at Site U1386. Structures and textureUnit I was sampled by the APC in the upper six cores, by the XCB in Holes U1387A and U1387B, and by the RCB in Hole U1387C. As a result, structures are well resolved only in the uppermost interval, although elements of the various lithologies are quite recognizable in the XCB and RCB cores. Coarser sediment (i.e., silty sand with biogenic carbonate and silty mud with biogenic carbonate) forms two distinct bedding styles, bi-gradational grading (Fig. F4) and normal grading (Fig. F5). The bi-gradational sequences are slightly more common in Unit I than in deeper units, with thicknesses varying from a few decimeters to several meters (Table T2). The most complete examples of the bi-gradational sequence coarsen upward from nannofossil mud through silty mud with biogenic carbonate to silty sand with biogenic carbonate and then fine upward through silty mud with biogenic carbonate into nannofossil mud (e.g., Fig. F4). Some of the sequences are less complete, lacking the silty sand part. The contacts between all lithologies, including between successive beds of nannofossil mud, are primarily gradational and/or bioturbated. The normally graded sequences fine upward, generally from silty sand with biogenic carbonate through silty mud with biogenic carbonate to nannofossil mud (Fig. F5). The normally graded sequences generally have sharp, erosional, or irregular bottom contacts with the underlying nannofossil mud, but their bottom contacts in places become unclear because of bioturbation. Some of these sequences are less complete, lacking the silty sand part. As an exception, a thick bed of silty sand with biogenic carbonate that shows inverse grading and a sharp upper contact is present in Section 339-U1387C-16R-3. Bioturbation and burrows are present throughout Unit I. The most common indicators 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. The bioturbation index ranges from sparse to slight, based on observation of beds with slight color changes. CompositionSmear slide observations indicate that all lithologies in Unit I are similar in composition; they are dominated by terrigenous material (siliciclastic minerals such as clay minerals, quartz, feldspars, and mica, plus detrital carbonate) (Fig. F13; Table T3). Abundances of terrigenous components, as estimated from smear slides, are 15%–75% (average = 47%) siliciclastics such as quartz, feldspars, heavy minerals, clay minerals, and volcanic glass, and 13%–40% (average = 29%) detrital carbonate. No discrete ash layers and no dropstones were observed. The biogenic fraction is primarily dominated by nannofossils, with rare to common foraminifers and rare pteropods, sponge spicules, and wood fragments. Abundances of biogenic components, as estimated from smear slides, are 10%–50% (average = 23%) biogenic carbonate (primarily nannofossils, with foraminifers for the silty sand lithology) and 0%–5% (average = 0.4%) biogenic silica (primarily diatoms and radiolarians). Siliceous microfossils exceed 2% only in the upper part of Unit I (0–30 mbsf). Macrofossil fragments and occasional nearly whole specimens of gastropods, bivalves, and echinoderms occur throughout Unit I at Site U1387. Examples of gastropod shells are illustrated in Figure F14, a coral branch in Figure F15, an Arenaria in Figure F16, bivalve shells in Figure F17, and a vermetid-like fossil in Figure F18. Total carbonate contents, as calculated by assuming all inorganic carbon to be CaCO3, range from 18.0 to 35.6 wt%, with an average of 26.4 wt% in Unit I (see “Geochemistry”). Some authigenic products, such as pyrite and dolomite (mostly recognized by its rhombic shape), are also present through the sequence but do not exceed 5% abundance. Both pyrite and dolomite generally are present in the silty sand beds. Some dolomite grains are subangular, suggesting a detrital origin. Glauconite grains are also present throughout Unit I. ColorA downhole color change is prominent in the uppermost 150 cm of the core, from yellowish brown (interval 339-U1387B-1H-1, 0–30 cm) through dark reddish gray (interval 1H-1, 30–150 cm) to dark gray. Below 15 mbsf, sediment becomes greenish gray to dark greenish gray, as shown in low a* values. In general, sediments with higher sand and/or carbonate contents have lighter colors. Bulk mineralogyThe mineral composition of 47 bulk sediment samples from Unit I was analyzed by XRD. Diffraction peaks from silicate minerals, such as quartz, plagioclase, and illite, and carbonate minerals, such as calcite and dolomite, contribute most of the total diffraction peak intensity measured (Fig. F19; Table T4). Intensities of the quartz diffraction peak in nannofossil muds and silty muds with biogenic carbonate generally vary between 18,000 and 40,000 counts in Unit I, whereas two samples of silty sand with biogenic carbonate (Samples 339-U1387A-29X-6, 137–138 cm, and 339-U1387C-14R-4, 70–71 cm) show significantly higher intensities of 50,000–65,000 counts. Peak intensity of calcite varies between 9,000 and 19,000 counts in Unit I, showing no clear trend with lithology. However, the measured inorganic carbon content and calcite + dolomite peak intensities are closely correlated. The intensity of the illite diffraction peak ranges between 3,000 and 16,000 counts, showing higher intensities in some nannofossil muds and silty muds with biogenic carbonate around 0–70 and 200–450 mbsf. Other clay minerals, such as chlorite and kaolinite, show similar trends. Plagioclase tends to be slightly more abundant in coarser sediments (i.e., silty mud with biogenic carbonate and silty sand with biogenic carbonate). Sediment between 240 and 410 mbsf contains slightly more hornblende than the other intervals of Unit I. XRD patterns of ethylene glycolated samples generally show a well-defined smectite peak for most samples in Unit I (shown in red in Fig. F20). Exceptions are a nannofossil mud (Sample 339-U1387A-37X-6W, 98–99 cm; 341.68 mbsf), a silty sand (Sample 29X-6W, 137–138 cm; 264.62 mbsf), and a second nannofossil mud (Sample 21X-6W, 92–93 cm; 188.52 mbsf). Unit II
Lithologies and beddingUnit II is composed of the same sediment types as those of Unit I (i.e., nannofossil mud, silty mud with biogenic carbonate, and silty sand with biogenic carbonate). Twelve beds in Unit II have contourite characteristics similar to those of beds in Unit I, such as gradational and bioturbated basal contacts and basal zones with inverse grading (examples are located in Sections 339-U1387C-23R-4, 25R-3, and 27R-4). However, Unit II is clearly distinguished from Unit I based on a distinctive cyclicity of dark and light colors (Fig. F6). Approximately 50 cycles are recognized in Unit II, with thicknesses varying from 1 to 5 m. Typically, one cycle is composed of the following lithologies from top to bottom: very dark greenish gray nannofossil mud, overlying greenish gray to dark greenish gray nannofossil mud, overlying greenish gray to dark greenish gray silty mud with biogenic carbonate, overlying greenish gray to dark greenish gray silty sand with biogenic carbonate (Fig. F6). In general, the dark–light nannofossil mud facies forms >80% of each cycle. Lithologic contacts within a cycle are gradational or bioturbated, whereas the basal contact of the silty sand with biogenic carbonate generally is sharp or erosional. In some cases, however, the basal contact is bioturbated (Fig. F6). The very dark greenish gray nannofossil muds also contain trace amounts of siliceous microfossils (e.g., fragmented and rare whole radiolarians, diatoms, and sponge spicules). The upper boundary of Unit II is defined at 454.06 mbsf (interval 339-U1387C-19R-1, 106 cm), which is the top of the shallowest bed of very dark greenish gray nannofossil mud >1 m thick and represents the start of the distinctive sediment cycles. This depth is also near the Pliocene/Pleistocene boundary. Two dolostone beds are present near the upper boundary of Unit II, at 457.3–458.0 (Sections 339-U1387C-19R-3 and 19R-4) and 462.7–462.8 mbsf (Section 20R-1)(Fig. F7). Discussion during initial core description considered whether the lower dolostone material might have fallen into the hole between Cores 339-U1387C-19R and 20R, especially because the drill bit was raised ~10 m between cores. However, the logging results (see “Downhole measurements”) clearly show two high-resistivity beds at this depth, an upper bed ~70 cm thick separated by ~120 cm from a ~40 cm thick lower bed. These results indicate that the uppermost part of Core 339-U1387C-20R (i.e., the lower dolostone bed) represents material recovered in place. These dolostone layers are composed of almost pure dolomite (3–10 µm dolomite grains) (Fig. F7) but contain a few quartz grains, opaques, and ghosts of siliceous microfossils such as radiolarians and diatoms, some of which are replaced by opaque minerals and silica. Based on petrographic observations, we speculate that the dolostones were originally fine-grained sediments (mud or silty mud) that contained siliceous microfossils. Nannofossils are rare in the very dark greenish gray muds immediately above the upper dolostone bed (interval 339-U1387C-19R-3, 66–136 cm) and are absent below that bed (interval 19R-4, 48–100 cm) (Fig. F7). The lowermost boundary of Unit II is defined at 599.10 mbsf (interval 339-U1387C-34R-2, 90 cm), at the top of the shallowest of the thick, contorted beds that characterize Unit III (see “Unit III”). Structures and textureThe distinctive cycles in Unit II display normal grading, fining up from silty sand with biogenic carbonate through silty mud with biogenic carbonate to nannofossil mud. The basal contacts of these silty sand beds generally are sharp or erosional, although some are bioturbated (Fig. F6). Burrows filled with silty sand with biogenic carbonate are common, extending downward from the silty sand with biogenic carbonate at the base of one cycle into the underlying very dark greenish gray nannofossil muds at the top of the previous cycle. In some cases, these sand-filled burrows extend several decimeters below the base of the overlying silty sand layer. Bioturbation is also visible around the contacts between very dark greenish gray nannofossil mud and dark greenish gray nannofossil mud (Fig. F6). Some nannofossil mud beds near the base of Unit II show parallel laminations (Sections 339-U1387C-33R-6 to 34R-2), apparently defined by color variations. Bioturbation is present throughout Unit II but is more visible below lithologic boundaries (e.g., below greenish gray silty sand with biogenic carbonate; Fig. F6). Characteristics of the bioturbation are similar to those of the bioturbation in Unit I. The bioturbation index in Unit II ranges from sparse to slight. CompositionSimilar to Unit I, all lithologies in Unit II are dominated by terrigenous materials, including siliciclastic components (clay minerals, quartz, feldspars, mica, and volcanic glass), which form 20% to 68% (average = 45%) of the grains observed in smear slides, and detrital carbonate, with abundances of 15%–35% (average = 28%) (Fig. F13; Table T3). The biogenic fraction is dominated by nannofossils, with rare to common foraminifers and rare pteropods and sponge spicules in the silty muds and silty sands. Abundances of biogenic components are 15%–50% (average = 26%) biogenic carbonate and 0%–5% (average = 0.7%) biogenic silica (primarily diatoms and radiolarians). Two samples immediately above and below the dolostone (Samples 339-U1387C-19R-3, 134 cm, and 19R-4, 50 cm) have rare biogenic carbonate (1%–2%) but slightly more biogenic silica (5%–7%) (Table T3). The very dark greenish gray nannofossil muds generally contain more siliciclastic minerals, opaques (pyrite), and rare siliceous microfossils, whereas the dark greenish gray nannofossil muds contain more nannofossils but no siliceous microfossils. Excluding the dolostones, total carbonate contents in Unit II, as calculated by assuming all inorganic carbon to be CaCO3, range from 12.9 to 34.1 wt% (average = 25.0 wt%). The carbonate content of the dolostone is 78.6 wt% when reported as CaCO3 (see “Geochemistry”). Other minerals, such as pyrite, dolomite, and glauconite, are also present throughout Unit II. Abundances of these minerals, as estimated from smear slides, are <5% for pyrite (usually classified as opaque grains, and associated with burrows), <5% for dolomite (found as rhombic crystals), and <5% for glauconite. Glauconite and dolomite are abundant locally in the silty sand beds. No discrete ash layers or dropstones were observed. Sediment compositions generally are similar for Unit I and Unit II, with the exception of the dolostones at the upper boundary of Unit II. Thin sections of these dolostones indicate that the sediment is composed almost entirely of dolomite grains, which presumably were formed during diagenesis, with trace silicate minerals (3%–5%) and a few ghosts of siliceous microfossils, such as radiolarians and diatoms (Fig. F7B). Macrofossil fragments distributed through most of Unit II include fragments of bivalves, echinoderms, corals, and Arenaria. The few specimens found include recognizable coral fragments (Sample 339-U1387C-28R-4, 31 cm; Fig. F21). ColorThe principal colors of the lithologies in Unit II range from greenish gray to very dark greenish gray. In general, sediments with higher quartz and/or carbonate contents are lighter colors, whereas sediments containing more clay minerals and opaque grains are darker colors. Bulk mineralogyThe mineral composition of 17 bulk sediment samples in Unit II was analyzed by XRD. Diffraction peaks from quartz, calcite, illite, and dolomite account for most of the total diffraction peak intensities measured (Fig. F19; Table T4). Quartz peak intensity ranges between 18,000 and 30,000 counts, which is slightly lower than its intensity in samples from Unit I. Calcite peak intensity is approximately 17,000–20,000 counts, slightly higher than in Unit I. Clay minerals such as illite, kaolinite, and chlorite have lower peak intensities in Unit II than in Unit I, whereas smectite has a slightly higher peak intensity in Unit II. XRD patterns of ethylene glycolated samples show a well-defined smectite peak for each sample from Unit II (Fig. F20). A dark greenish gray nannofossil mud near the lower boundary of Unit II shows the highest peak intensities for smectite, chlorite, kaolinite, and illite (Sample 339-U1387C-32R-6, 90–91 cm). Hornblende and plagioclase are less abundant in Unit II than in Unit I. The dolostone sample (339-U1387C-19R-4, 39–41 cm) shows clear peaks of dolomite and quartz, but no other minerals were identified. Unit III
Lithologies and beddingUnit III is composed mainly of the same sediment types as are present in Unit II: greenish gray to very dark greenish gray nannofossil mud, dark greenish gray silty mud with biogenic carbonate, and dark greenish gray silty sand with biogenic carbonate. In contrast to Unit II, in which a clear cyclicity of these lithologies is expressed as color changes, these lithologies are not stacked as regularly in Unit III. For example, nannofossil mud is missing in some intervals (e.g., from Section 339-U1387C-40R-6 to 42R-6). Also, in contrast to the undeformed strata in Unit II, a ~4.7 m thick bed of nannofossil mud at 599.10–603.82 mbsf (interval 339-U1387C-34R-2, 90 cm, to 34R-CC, 23 cm) is highly contorted, including recumbent fold limbs as thick as 50 cm (Fig. F8). This bed is interpreted as a slump deposit, and the top of this deformed bed defines the top of Unit III. Similar deformed beds, which are recognizable by recumbent folds and/or inclined laminations, are observed throughout Unit III. Beds of very dark greenish gray silty sand with biogenic carbonate, ~30–120 cm thick, also characterize Unit III (Fig. F22). These beds contain abundant shell fragments, as well as a few granules that generally are rounded and mainly composed of quartzite, with trace abundances of metamorphic rock fragments. One of the very dark greenish gray silty sand beds with biogenic carbonate (interval 339-U1387C-39R-3, 76–110 cm) shows a clear coarsening-upward trend (inverse grading) and a sharp inclined contact with the overlying bed. Sediment in intervals 339-U1387C-44R-1, 0 cm, through 47R-1, 47 cm (692.90–722.09 mbsf) and 49R-CC, 0 cm, through 50R-1, 52 cm (748.10–750.92 mbsf) is dominated by well-cemented gray medium sandstone with biogenic carbonate (Fig. F9), which contains rare very coarse sand- to granule-size grains. As recovered, these gray medium sandstones are interbedded locally with dark greenish gray silty/muddy fine sandstone with biogenic carbonate. Because recovery is poor in this interval, however, with only 21.0 m of material recovered in 69.3 m of coring from Cores 339-U1387C-43R through 50R, the true stratigraphic relationships between the well-cemented gray medium sandstone and the dark greenish gray silty/muddy fine sandstone are unknown. The gray medium sandstone with biogenic carbonate is moderately sorted and shows no evidence for bioturbation. The dark greenish gray silty/muddy fine sandstone with biogenic carbonate is moderately to poorly sorted and also shows no evidence of bioturbation. The petrographic observation of a thin section from the dark greenish gray silty sandstone (interval 339-U1387C-44R-1, 9–14 cm) indicates that fine sand grains are supported by a matrix of silt-sized carbonate, which is partly replaced by calcite cement (Fig. F9). The silty matrix accounts for ~30% of each field of view, with ~40% siliciclastic grains (mostly quartz with trace feldspars, heavy minerals, and mica) and ~30% biogenic carbonate grains (foraminifers and shell fragments). Most of the quartz grains are angular, although some grains of rounded quartz, polycrystalline quartz, or quartzite are also present. This variety of grain shapes and compositions indicates multiple sources for these quartzose grains. Well-rounded grains of green glauconite are present, indicating reworking of sediments from the shelf margin. Some benthic foraminifers are also present. Petrographic analysis of a thin section of gray medium sandstone (interval 339-U1387C-45R-1, 74–77 cm) reveals a grain-supported medium sand with calcite cement filling the pore spaces (Fig. F9). The calcite cement accounts for ~20% of each field of view, with ~75% siliciclastic grains (mostly plagioclase, quartz, and K-feldspar, with trace mica and rock fragments) and ~5% foraminifers. This sample has a high percentage of feldspars, almost reaching the 25% level necessary for its description as an arkose. A thin (~4 cm) bed of black carbonaceous sediment present in interval 339-U1387C-42R-5, 63–67 cm (680.13–680.17 mbsf), contains interlaminated silty sand and terrestrial organic debris (Figs. F23, F24). This bed is overlain by a normally graded medium bed of silty sand with biogenic carbonate. The lower boundary of Unit III is at 750.92 mbsf (interval 339-U1387C-50R-1, 52 cm), which is the deepest occurrence of a bed of sandy sediment (silty sand, sand, and sandstone) thicker than 2 cm. Five very thin interbeds of silty sand with biogenic carbonate are present between 750.92 mbsf and Section 339-U1387C-54R-3, but all five are 2 cm thick or less. Excluding these very thin sandy beds, the underlying sediment is a continuous succession of the nannofossil mud and muddy nannofossil ooze that characterizes Unit IV. In summary, Unit III is distinguished from Units I and II by:
Structures and textureSimilar to the lowermost part of Unit II, parallel laminations are visible throughout Unit III, particularly in fine-grained sediments such as nannofossil mud and silty mud with biogenic carbonate. Some coarser sediment (i.e., silty mud with biogenic carbonate) also shows subtle parallel laminations. In some cases, these laminations form recumbent folds and/or inclined laminations, indicating slump deposits. Most of the beds of dark greenish gray silty sand with biogenic carbonate display normal grading, fining upward from a sharp, irregular, or erosional basal contact. However, in some cases the nature of the basal contact is unclear because of bioturbation. In contrast, the very dark greenish gray silty sands with biogenic carbonate in Cores 339-U1387C-39R and 40R contain abundant shell fragments but show no clear evidence of normal grading; in fact, three of these silty sand layers are inversely graded (Sections 339-U1387C-39R-3, 40R-2, and 41R-2). Unit III contains a higher proportion of coarser sediment, such as silty sand with biogenic carbonate, than Units I and II, and has an average grain size of silt to fine sand and a maximum grain size of medium sand to granule. This contrasts with Units I and II, which contain a higher proportion of fine-grained sediment and have an average grain size of clay to silt. Bioturbation is the most obvious secondary sedimentary structure in Unit III and is present throughout the observed section. Characteristics of the bioturbation are similar to those of Units I and II, with a bioturbation index ranging from sparse to slight. In some parts of Unit III, burrows are filled with fine pyrite grains or have been pyritized. Some vertical veins of pyrite, 5–11 cm long, and a microfault in Section 339-U1387C-40R-4 are additional secondary structures. CompositionAs is true for Units I and II, all lithologies in Unit III are dominated by terrigenous material. The abundance of siliciclastic components, such as clay minerals, quartz, feldspars, and mica, ranges from 30% to 62% (average = 46%), and the abundance of detrital carbonate ranges from 15% to 35% (average = 27%) (Fig. F13; Table T3). Abundances of biogenic components range from 20% to 50% (average = 28%), and the biogenic fraction is composed entirely of biogenic carbonate (primarily nannofossils, with rare to common foraminifers in the silty sand and silty mud lithologies). Biosiliceous microfossils are not observed in Unit III. Total carbonate contents range from 18.2 to 38.2 wt% (average = 29.7 wt%) in Unit III, excluding two sandstone samples (339-U1387C-44R-1, 37–38 cm, and 45R-1, 76–77 cm) that contain 71.7 and 37.2 wt% carbonate, respectively, as CaCO3 (see “Geochemistry”). Macrofossil debris is distributed through most of Unit III as recognizable fragments, including bivalves, echinoderms, and corals. Shell fragments are particularly abundant in some beds of the very dark greenish gray silty sand with biogenic carbonate. Authigenic components are dominated by pyrite (<5%), usually classified as opaque grains and associated with burrows, dolomite (<5%), found as rhombic crystals, and glauconite (<5%). Glauconite and dolomite are common in some of the silty sand beds. These results are similar to the corresponding component abundances in Units I and II, except for the absence of biosiliceous microfossils. ColorThe principal colors of the lithologies in Unit III range from greenish gray to very dark greenish gray. The sandstones in intervals 339-U1387C-44R-1, 0 cm, to 47R-1, 47 cm (692.90–722.09 mbsf), and 49R-CC, 0 cm, to 50R-1, 52 cm (748.10–750.92 mbsf), are gray to dark greenish gray. The thin (~4 cm) bed of carbonaceous sediment in interval 339-U1387C-42R-5, 63–67 cm (680.13–680.17 mbsf) is black (Fig. F23). In general, sediments with higher sand and/or carbonate contents are lighter colors. Bulk mineralogyFifteen bulk sediment samples from Unit III were analyzed by XRD. Diffraction peak intensities for quartz, calcite, and illite are the major contributors to the total peak intensities identified for each sample (Fig. F19; Table T4). Quartz peak intensity ranges between 17,000 and 46,000 counts, with an exceptionally high intensity of ~84,000 counts in a sample of the medium sandstone (Sample 339-U1387C-45R-1, 76–77 cm). A muddy fine sandstone sample (339-U1387C-44R-1, 37–38 cm) shows a much lower peak intensity for quartz, indicating a range of mineral compositions in the sandstones. Calcite peak intensity ranges between 16,000 and 22,000 counts, excluding the two sandstone samples. These results are similar to those of Unit II. In the two sandstone samples (339-U1387C-44R-1, 37–38 cm, and 45R-1, 76–77 cm), calcite XRD peak intensities are ~39,000 and ~91,000 counts, respectively, which are much higher than the calcite intensities for other samples. The relative sizes of these calcite peak intensities are apparently inconsistent with the geochemical analysis of carbonate contents in these two samples, which gave values of 71.7 and 37.2 wt%, respectively (see “Geochemistry”). We attribute the exceptionally high intensity of the calcite peak in the gray medium sandstone (Sample 339-U1387C-45R-1, 76–77 cm) to the very highly crystalline nature of its calcite cement, compared with the cement of the silty fine sandstone sample (44R-1, 37–38 cm). Support for this interpretation comes from petrographic observations of thin sections from the two sandstones, which indicate that the calcite cement in the gray medium sandstone is very highly crystalline compared with the cements of the dark greenish gray silty fine sandstone (Fig. F9). The gray medium sandstone also shows the highest peak intensity for plagioclase and K-feldspar, which is consistent with the observation that some of the rocks analyzed by thin section had high percentages of feldspar, almost reaching the 25% level necessary to describe the sample as an arkose. A nannofossil mud sample at 619.7 mbsf (Sample 339-U1387C-36R-3, 78–79 cm) shows the highest peak intensities in Unit III for clay minerals such as illite, kaolinite, and chlorite. The remaining intervals in Unit III are characterized by low abundances of clay minerals. Smectite is an exception to this statement, in that it appears to have a higher average intensity count in Unit III than in the other lithologic units. However, the XRD patterns of three ethylene glycolated samples from Unit III (Samples 339-U1387C-36R-3W, 78–79 cm; 40R-5W, 87–89 cm; and 44R-1W, 37–38 cm) do not show a clear smectite peak (Fig. F20), suggesting difficulties in using the automated mineral identification software to identify smectite on diffractograms of unglycolated samples. Further analysis after glycolation is necessary to confirm the presence of smectite in each sample. Unit IV
Lithologies and beddingThe dominant lithologies in Unit IV are nannofossil mud and muddy/clayey nannofossil ooze (Fig. F3). In the upper ~50 m of this unit (interval 339-U1387C-50R-1, 52 cm, to 55R-4, 26 cm), very thin (2 cm) beds of dark greenish gray silty sand with biogenic carbonate and thick beds of dark greenish gray silty mud with biogenic carbonate (<1.2 m thick) are interbedded with thick (>1 m) beds of dark to very dark greenish gray nannofossil mud. Some dark–light cycles, similar to those in Unit II, are present in the upper ~20 m of Unit IV (Sections 339-U1387C-50R-1 to 52R-3). The deepest occurrence of a thick (~80 cm) bed of silty mud is in Sections 339-U1387C-55R-3 and 55R-4. The middle part of Unit IV (interval 339-U1387C-55R-4, 26 cm, through 58R-CC, 28 cm; 803.16–834.38 mbsf) is dominated by dark greenish gray nannofossil mud, and the lower part of Unit IV (interval 339-U1387C-59R-1, 0 cm, through 61R-CC, 16 cm; 836.80–865.85 mbsf) is dominated by dark greenish gray clayey/muddy nannofossil ooze (Fig. F10). Structures and textureSediment in Unit IV locally shows subtle parallel laminae, which are the only primary sedimentary structure visible. Bioturbation is common throughout this unit and is particularly apparent near color contacts between beds. Bioturbation intensity generally is slight. Some ichnofauna such as Chondrites appear throughout Unit IV. In addition, diffuse centimeter-scale mottling and millimeter-scale pyritic burrow fills are also present throughout the unit. In some cores disturbed during drilling, sedimentary structures are poorly preserved (e.g., Core 339-U1387C-57R). The sediment grain size in Unit IV is uniformly fine, with an average grain size of clay. The maximum grain size is medium sand because of the presence of foraminifers. CompositionThe major lithologies in Unit IV are nannofossil mud and clayey/silty nannofossil ooze. The relative proportion of terrigenous components decreases compared to their abundances in Unit III, whereas the relative proportion of biogenic carbonate (especially present as nannofossils) increases. Siliciclastic abundances in Unit IV are 20%–70% (average = 42% for clay minerals, quartz, feldspars, and mica) and detrital carbonate abundances are 15%–30% (average = 19%) (Fig. F13). The increase in biogenic carbonate is most prominent in the lower part of Unit IV. Total carbonate contents, calculated as CaCO3, range from 27.6 to 35.9 wt% (average = 30.3 wt%) in Unit IV (see “Geochemistry”), which is slightly higher than the total carbonate contents of other lithologic units. Fragments of shallow-water shells are rare in Unit IV. ColorThe principal color of the lithologies in Unit IV is very dark greenish gray to greenish gray, with subtle changes in color caused by bioturbation. Several color bands have sharp contacts (e.g., interval 339-U1387C-59R-3, 24 cm). Bulk mineralogyEleven bulk samples from Unit IV were analyzed by XRD. The peak intensities for quartz decrease downcore, whereas the peak intensities for calcite are variable but increase slightly downcore (Fig. F19; Table T4). XRD patterns for three ethylene glycolated samples from Unit IV show well-defined smectite peaks and indicate variable smectite/illite ratios among these samples (Fig. F20). DiscussionGeneral observationsContourites and turbidites in Unit IGiven the setting of Site U1387, several lines of evidence support the interpretation of Unit I as a sequence mainly composed of contourite deposits, providing evidence for current transport and changing flow speeds. Among these lines of evidence are
Three intervals in Unit I (0–100, 170–300, and 340–450 mbsf) are relatively typical examples of a mixed sandy/muddy contourite succession as defined by Gonthier et al. (1984) (Fig. F3), whereas the other parts of Unit I (122–170 and 300–340 mbsf) are relatively typical examples of a muddy/silty contourite bed succession. Unit I also contains beds that show normal grading and sharp or erosional basal contacts. Some of these may be true turbidites, whereas others we interpret as base-cut-out contourites. We identified at least 65 bi-gradational sequences with sandy contourites and 31 normally graded beds in Unit I (Figs. F2, F23). As described above, Unit I at Site U1387 can be correlated to Unit I at nearby Site U1386 (located ~4 km northwest of Site U1387). Compared with the sedimentary succession in Unit I at Site U1386, which is dominated by sandy contourites and muddy contourites and has only a few turbidites, Unit I at Site U1387 contains more beds that have been interpreted as turbidite deposits (Fig. F24). This difference in depositional processes between Sites U1386 and U1387 could be related to the fact that Site U1387 is located closer to a bathymetric channel, previously described by Llave et al. (2001, 2007a) and Hernández-Molina et al. (2006). Cycles in Unit IIUnit II is dominated by cyclic variations in color and lithology, 1–5 m thick. The base of each cycle generally is composed of light-colored silty sand with biogenic carbonate that grades upward into
The basal light-colored silty sand shows normal grading with sharp/erosional basal contacts (Fig. F6). The characteristics of the basal silty sand (i.e., normal grading and a sharp bottom contact) suggest that the basal silty sand bed represents a turbidite; however, no diagnostic stratification styles or sedimentary structures can be recognized within these silty sands. An alternative interpretation is that the basal silty sand layer was formed by bottom current flow. The upper dark-colored nannofossil mud in each cycle is characterized by an increase in bioturbation and burrows. This muddy portion of the cycle is interpreted as a muddy contourite. The burrows are generally filled with silty sand, whose grain size and composition are almost identical to those of the overlying basal silty sand. This indicates that the burrows formed during or soon after deposition of the silty sand layer at the base of the overlying unit. If the basal silty sand was deposited by a turbidity current, then we suggest that the upper part of the light-colored nannofossil mud was deposited as fine-grained sediment originally supplied by the turbidity current was reworked and deposited by bottom water currents. In this scenario, the gradual color change from the lighter to the darker nannofossil mud within a cycle records a gradual change in depositional processes from a downslope turbidity current to alongslope bottom water currents. In other words, these sediments can be regarded as a combination of turbidite and contourite. Interpretation of the normally graded silty sand beds in Unit I (Fig. F5), which are not associated with similar changes in their overlying muds, requires further work. In contrast, the cycles of Unit II likely formed by the interplay of turbidity currents and bottom currents (Figs. F2, F24). Based on the shipboard age-depth model (see “Biostratigraphy”), the ages for Sections 339-U1387C-19R-CC (458.6 mbsf) and 29R-CC (558.6 mbsf) are 3.19 and 3.8 Ma, respectively. Within that interval of ~0.6 m.y., ~30 cycles were deposited in Unit II, for an average duration of ~20 k.y. per cycle. Coarse shelly sand layers in Unit IIIAt least three thick to very thick beds of dark to very dark greenish gray silty sand, which contain abundant shell fragments and sparse well-rounded quartzite granules, are present in Unit III (e.g., Section 339-U1387C-39R-2). These beds are very poorly sorted, massive, and exhibit sharp contacts with the overlying and underlying beds. Based on these characteristics, these beds are interpreted as deposits associated with higher concentration sediment mass gravity flows and therefore are considered to be debrites. These debrites occur in association with some soft-sediment deformation and contorted bedding, interpreted to indicate slump deposits. The presence of debrites and slump deposits may suggest that downslope processes were much more influential during the deposition of Unit III, and these deposits could be correlated laterally with similar debrites and bioclastic turbidites at Site U1386 (see “Lithostratigraphy” in the “Site U1386” chapter [Expedition 339 Scientists, 2013d]). Formation of dolostoneBased on petrographic observations, we speculate that the dolostone was originally fine-grained sediment (mud or silty mud) that contained siliceous microfossils. The very dark greenish gray mud immediately above the dolostone contains very few nannofossils (interval 339-U1387C-19R-3, 66–136 cm). The very dark greenish gray mud below the dolostone (interval 19R-4, 48–100 cm) contains no nannofossils but does contain biosiliceous microfossils (radiolarians, diatoms, and sponge spicules). These observations suggest that sediments in the depth intervals now occupied by the dolostone (457.3–458.0 and 462.7–462.8 mbsf) were originally enriched in biosiliceous microfossils, which were most likely replaced by the fine-grained dolomite during diagenesis. Shipboard micropaleontological investigation identified a ~1.3 m.y. hiatus (i.e., from 3.19 to 1.9 Ma) between Sections 339-U1387C-19R-1 and 19R-CC (see “Biostratigraphy”). We speculate that formation of the dolostone is closely linked to the hiatus; extensive precipitation of dolomite might have occurred at a shallow depth below the sediment/water interface (probably <10 m below the paleoseafloor), where the sulfate reduction-methanogenesis zone was located for at least 1 m.y. Depositional historyThe overall depositional history of Site U1387 can be summarized as follows:
During the late Miocene, fine-grained sediment such as nannofossil ooze and nannofossil mud were deposited in a low-energy, likely hemipelagic, depositional environment. A lithologic change from nannofossil ooze to nannofossil mud in the lower to middle part of Unit IV (Fig. F1) indicates that the input of terrigenous siliciclastic components gradually increased through time. An increasing influence of turbidity currents during the late Miocene/early Pliocene (upper part of Unit IV) is indicated by the first occurrence of a thick bed (~80 cm) of silty mud at 803 mbsf (Sections 339-U1387C-55R-3 and 55R-4) and an increase in the number of thin silty sand beds (Fig. F24). The depositional environment changed during the late Miocene/early Pliocene from the hemipelagic mud-dominated environment (Unit IV) to one with an increasing input of sandy sediment (Unit III)., However, the details of the transition from the mud-dominated Unit IV to the sand-dominated Unit III are unclear because of poor recovery. The increase in sandy sediments in Unit III is associated with an increase in downslope transport processes, indicated by the presence of slump deposits and debris flow deposits within a sequence dominated by sandy turbidites (Figs. F2, F22, F24). Some silty sand beds in Unit III are interpreted as sandy contourites, indicating that bottom currents were active during the early Pliocene. The processes that deposited Unit II, which is characterized by interbedded turbidites and contourites and the distinctive dark–light cycles, dominated deposition until the late Pliocene, probably ~3.19 Ma. In support of this interpretation, Roque et al. (2012) also concluded that turbidite processes were important in this area during the early Pliocene. The hiatus between ~3.19 and 1.9 Ma is marked by two beds of dolostone, which are interpreted to have formed by shallow diagenetic processes during this extended time of seafloor stability. The depositional environment after the hiatus (Unit I) was dominated by alongslope processes and bottom currents, depositing muddy/silty contourites and sandy contourites with minor influence of downslope turbidity currents (Fig. F24). We recognize at least three intervals within Unit I that suggest stronger alongslope flow and/or a more proximal source of sand, as recorded by more frequent occurrences of sandy contourite beds (0–100, 170–300, and 340–450 mbsf) (Fig. F24). In these intervals the number of turbidite beds is also higher, indicating that downslope processes were also slightly more active during these times. The intervening intervals in Unit I are dominated by muddy contourites, indicating weakening of alongslope transport and/or a reduced sand supply at Site U1387. These trends are similar to those seen at Site U1386, ~4 km northwest of this site. As discussed in the Site U1386 chapter, three scenarios can explain the variation between mud-rich and sand-rich contourite intervals:
Based on the shipboard biostratigraphic information, the mud-rich interval at 100–170 mbsf at Site U1387 records deposition from ~800 to ~400 ka, slightly younger than the age of the lithologically similar Subunit IB at Site U1386. Further research will be needed to improve correlations of Pleistocene sediments between Sites U1386 and U1387. |
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