The succession cored at Site 1115 consisted of 802.5 m of sediments, including an inferred late Miocene-Pleistocene rift succession and underlying Miocene forearc unit. Twelve lithostratigraphic units are recognized on the basis of sediment or rock type, grain size, sedimentary structures, color, smear slides, thin sections, bulk mineralogy (by X-ray diffraction [XRD]), and calcium carbonate content (Fig. F1). In certain areas in which grain size varies markedly over short intervals (Fig. F2) the mineral composition as determined by XRD in fine-grained sediments proved useful in helping to define unit boundaries. In addition, geophysical logs including FMS data were used to help reconstruct parts of the succession that were poorly recovered, especially conglomeratic intervals.
The uppermost 4.4 m of the unit were cored in Hole 1115A. In addition, the whole of Unit I was recovered from Hole 1115B.
Unit I is composed of the following main types of unconsolidated sediment: nannofossil ooze, nannofossil-rich silty clay, calcareous silty clay/claystone, and volcanic ash. Volcanic ash layers are present throughout the whole of Unit I. Details of these different lithologies are as follows:
The nannofossil ooze is light yellowish brown, with numerous planktonic foraminifers, scattered colorless glass shards (e.g., Section 180-1115A-1H-1), wood fragments (1H-2), rare small pumice clasts, numerous shell fragments (1 cm × 0.5 cm × 0.5 cm; e.g., Section 180-1115A-1H-4, 80 cm) and occasional pteropods. In some intervals slightly more clay-rich laminae (i.e., nannofossil-rich clay) are present within clay-rich nannofossil ooze (e.g, Section 180-1115B-2H-3), and some partings within these intervals are graded.
Discontinuous lamination is commonly defined by concentrations of planktonic foraminifers (e.g., Section 180-1115A-1H-4). Individual laminae exhibit sharp bases and diffuse tops (e.g., Cores 1H and 2H). Occasionally, the foraminifer-rich intervals represent distinct beds with graded bases and diffuse tops (e.g., interval 180-1115B-1H-CC, 0-10 cm). Sulfide mottling is commonly present. From ~12 mbsf downward the sediment tends to be slightly more clay rich and is termed nannofossil-rich silty clay.
Smear slides indicate that the component grains are quartz, feldspar, volcanic glass, calcite, nannofossils, foraminifers, bioclasts, sponge spicules, and radiolarians (see "Site 1115 Smear Slides"). In addition, XRD analysis shows that calcite predominates with minor quartz and aragonite (Table T3). In agreement, calcium carbonate values range from 62 to 77 wt% (see "Organic Geochemistry").
Volcanic ash is present as multiple thin beds over narrow, discrete intervals and as isolated laminae and thin beds. The main types are as follows:
Seven thin beds/laminae are present within interval 180-1115A-1H-4, 89-105 cm. These are divided into three sediment types: (1) volcanic ash (silt) composed of silt-sized platy and bubble-wall shards, (2) volcanic ash (fine- to medium-grained sand), mainly composed of pipe-vesicle glass, and (3) volcanic ash (fine to very fine grained sand), made up of volcanic glass mixed with nannofossils. Elsewhere, individual glass-rich laminae are present. These exhibit sharp bases and diffuse tops (e.g., Section 180-1115A-1H-4, 85-110 cm; Fig. F3).
An ash layer of similar composition was recognized in interval 180-1115B-1H-3, 75-93 cm. Of the seven layers identified in Section 180-1115A-1H-4, four of their counterparts in Hole 1115B are much less well preserved, a result of bioturbation of the thinner ash layers.
An ash layer in interval 180-1115B-1H-6, 6-22 cm (fine-grained sand) is dominated by bubble-wall shards and rare pipe-vesicle shards, also rare detrital quartz and ferromagnesian grains. The base of the layer is sharp, whereas the top is diffuse. This ash layer was not observed in Hole 1115A.
In the interval 180-1115B-2H-3, 15-25 cm, two thin ash beds are juxtaposed. The lower bed, ~5 cm thick, grades from fine-grained sand to silt and includes organic material and detrital grains, passing up into mainly green, silt-sized glass. By contrast, the upper bed is composed of very fine silt sized glass, grading into silt.
A further volcanic ash layer comprises a lower interval of medium- to fine-grained sand, passing up into fine-grained sand. Both of these sedimentary intervals exhibit sharp bases and sharp tops. In places, "ghosts" of volcanic ash layers are present that are rarely recognizable after bioturbation. For example, within the intervals 180-1115B-2H-4, 8-10 and 18-19 cm, remnants of volcanic ash laminae are preserved.
A discrete ash lamina is present in the interval 180-1115B-2H-5, 0-1 cm.
Discrete interbeds of graded silt (with sharp bases) appear at ~12 mbsf (e.g., interval 180-1115B-2H-3, 72-70 cm). This sediment is composed of thin to medium beds of volcaniclastic silt. Individual beds exhibit sharp bases and diffuse tops, and grade from very fine silt to clay and ooze (e.g., interval 180-1115B-3H-1, 8-15 cm). The XRD analysis reveals the presence of amphibole in addition to calcite and quartz.
The volcaniclastic sands form thin to medium beds that grade from coarse-grained sand to clay. Bioturbation becomes more abundant at the top. The sand is rich in foraminifer tests and detrital grains. Smear slides reveal the presence of feldspar, rock fragments, volcanic glass, nannofossils, and planktonic foraminifers (see "Site 1115 Smear Slides").
Lithostratigraphic Unit I accumulated at upper middle bathyal depths during the Pleistocene at estimated sedimentation rates of 34-59 m/m.y. (see "Sediment Accumulation Rate"). The background pelagic sediment is dominated by biogenic material (nannofossils, planktonic foraminifers, radiolarians, and shell fragments), accounting for much of the observed mineralogy (calcite, quartz, and aragonite). Some of the fine-grained sediment also might be interpreted as mud turbidites. In addition, mainly siliceous volcanic glass is interpreted as primary air-fall tuff derived from volcanoes related to the Trobriand Arc and/or per-alkaline volcanoes of the Dawson Strait (Davies et al., 1984; Smith, 1976; Stolz et al., 1993). The volcaniclastic silts rich in amphibole and sands are interpreted as epiclastic deposits from turbidity currents.
This unit is marked by a transition from mainly ooze, to clay as the background fine-grained sediment. Volcanic ash layers (interpreted as air-fall tuff) are less abundant than in Unit I, but numerous volcanogenic layers of epiclastic origin are present. The following lithologies are present.
This greenish gray sediment is intergradational with silty nannofossil ooze (see "Silty Clayey Nannofossil Ooze"). Numerous burrows are filled with volcaniclastic sand that is locally glass rich (e.g., Section 3H-4). Sulfide mottling is widespread. Thin partings are commonly rich in foraminifers, whereas clay-rich zones (i.e., nannofossil clay) are often homogeneous with few foraminifers (e.g., interval 180-1115B-3H-CC, 0-4 cm). Scattered shell fragments, pteropods, whole or broken foraminifers, and grains of ferromagnesian minerals are commonly observed.
Smear slides reveal quartz, feldspar, volcanic glass, nannofossils, foraminifers, and sponge spicules ("Site 1115 Smear Slides"). The XRD analysis revealed calcite, quartz, and plagioclase (Table T3). Calcium carbonate values range from 35 to 64 wt% (see "CaCO3, Sulfur, Organic Carbon, and Nitrogen").
Lower in the unit the nannofossil clays become more consolidated and contain fewer nannofossils. These sediments are classified as calcareous clay (e.g., Section 8H-5). Otherwise the clay is highly burrowed, with little sign of preserved primary sedimentary structures. Smear slides of the calcareous clay contain quartz, feldspar, volcanic glass, calcite, nannofossils, foraminifers, and organic material ("Site 1115 Smear Slides"). The XRD analysis reveals calcite and minor amounts of quartz, aragonite, and plagioclase (Table T3). Calcium carbonate contents range from 50 to 65 wt%.
Thin to very thin beds and laminae strongly disrupted by bioturbation are present (e.g., intervals 180-1115B-3H-6, 69-70 cm, and 3H-7, 43-44 cm). They exhibit sharp bases and tops and are clearly graded from silt to clay (e.g., intervals 180-1115B-4H-3, 20-20.5 cm, and 5H-2, 97-100 cm).
This minor lithology, more typical of Unit I (see "Lithostratigraphic Unit I") is greenish gray, strongly burrowed, with few visible primary sedimentary structures. Sulfide mottling is widespread. Burrows are commonly infilled with sand (e.g., interval 180-1115B-4H-1, 10-145 cm). A number of foraminifer-rich beds grade from very fine sand to silt and then silty clay (e.g., interval 180-1115B-4H-1, 121-128 cm).
These layers are characterized by mixtures of volcanic glass, rock fragments, quartz, and feldspar (e.g., interval 180-1115B-9H-2, 0-3 cm). Unusually, a lower thin bed of volcanic ash (5 cm thick) is overlain by a second thin bed of fine- to medium-grained volcaniclastic sand (interval 180-1115B-10H-4, 48-66 cm; Fig. F4) comprising volcanic glass, mica, quartz, feldspar, and lithic fragments, based on smear-slide analysis ("Site 1115 Smear Slides").
Rare, very thin beds are present within nannofossil clay. Individual layers are sharp based and graded (e.g., interval 180-1115B-5H-7, 61-66.6 cm). In addition, three volcanic ash layers (each 1 cm thick) were observed in Sections 6H-7 and CC. The ash is mixed with detrital grains and foraminifers. Another occurrence of repeated thin volcanic ash beds is in the interval 180-1115B-10H-1, 126-129 cm. Each of three very thin beds (<2 cm in thickness) is normal graded from medium-grained sand to silty clay. Another ash bed (2 cm thick) is present in the interval 180-1115B-12H-2, 37-39 cm. Beneath Core 15H discrete volcanic ash layers were not observed. However, white acidic pumice fragments were noted in the interval 180-1115B-15H-2, 62 cm, and a thin bed of silty volcanic ash is present in the interval 180-1115B-15H-4, 142-145.5 cm. The sharp base of this ash was considerably disrupted by bioturbation. This bed contains mainly platy and bubble-wall and rare pipe-wall shards. Another similar thin bed is present in the interval 180-1115B-15H-5, 86-89 cm. Core 16H contains minor volcaniclastic sediment appearing as a graded volcaniclastic bed in the interval 180-1115B-16H-5, 10-13 cm.
Unit II accumulated in the late Pliocene-Pleistocene in upper to middle bathyal depths (500-1150 m). The average sedimentation rate was 59-79 m/m.y. (see "Sediment Accumulation Rate", for details on depth zonation). The depositional processes and provenance of Unit II are similar to those of Unit I, but for increased input of epiclastic silt and sand, interpreted as turbidites. The amount of volcanic ash of inferred air-fall origin is less, but the character and possible origins remains the same (see "Volcanic Ash").
Lithostratigraphic Unit III is characterized by the appearance of discrete thin, graded beds of volcaniclastic sand/sandstone within calcareous silty clay/claystone. It is the thickest stratigraphic unit (~240 m) at Site 1115. Calcium carbonate values in fine-grained sediments show a general decrease from >30 wt%, to around 20 wt% below 250 mbsf, with a few exceptions (see "CaCO3, Sulfur, Organic Carbon, and Nitrogen"). Volcaniclastic beds are mainly present above Core 180-1115B-22H (206.7 mbsf). The first well-indurated sediment was in Section 180-1115B-28X-CC (256.55 mbsf). Volcaniclastic sands/sandstones are present sporadically throughout the unit and are greatly affected by burrowing until near the base of the unit (below Core 180-1115C-6R). Rarely, bedding is tilted (e.g., interval 180-1115B-17H-3, 121-124 cm), and small high-angle normal faults occasionally are present (e.g., interval 180-1115B-18H-2, 40-49 cm) (see "Structural Geology").
The following lithologies are present in decreasing order of abundance:
These lithologies have similar counterparts in Units I and II (see "Lithostratigraphic Unit I" and "Lithostratigraphic Unit II"). However, the sediment becomes more consolidated downward. The sediment is highly bioturbated (Chondrites, with rare Zoophycos; e.g., interval 180-1115C-4R-2, 138 cm). Smear slides revealed quartz, feldspar, mica, calcite, nannofossils, planktonic foraminifers, sponge spicules, and volcanic glass ("Site 1115 Smear Slides"). The XRD analysis reveals major amounts of calcite, also minor quartz, plagioclase, chlorite, and illite. Foraminifer tests are locally concentrated in small burrows, also scattered small shell fragments, very rare gastropods, and rare echinoderm fragments were observed (e.g., interval 180-1115C-1R-CC, 37 cm). Sulfide mottling is common and pyrite framboids are locally concentrated in burrows.
In the lower part of Unit III, the claystone is slightly more silty with scattered angular sand-sized grains of ferromagnesian minerals, feldspar, biotite, quartz, and shell fragments (Fig. F5). Lower still, the calcareous silty claystone contains abundant detrital grains of up to granule size (e.g., intervals 180-1115B-29X-3, 12-14 cm, 47-49 cm, 106-110 cm, and 115-120 cm). Discrete shell-rich laminae rarely are near the base of the unit (interval 180-1115C-10R-4, 104 cm). The calcareous claystones include occasional dark silt- or clay-rich layers up to several cm thick (e.g., interval 180-1115B-18H-CC, 8-10 cm). Very rarely, woody, carbonaceous fragments were observed (i.e., intervals 180-1115B-29X-6, 16-18 cm, and intervals 180-1115C-7R-2, 60-61 cm, 9R-1, 81.5 cm, 10R-73 cm, and 10R-4, 64 cm).
Analysis of a small number of impregnated thin sections revealed the presence of a predominantly clay-rich and micritic matrix with variable amounts of detrital quartz, plagioclase, hornblende, biotite, lithic volcanics, organic matter, shell fragments, pyrite, and planktonic foraminifers ("Site 1115 Thin Sections"). In one thin section, shell fragments and primary micritic matrix are replaced by blocky calcite spar (interval 180-1115C-2R-1, 17-19 cm). In another, micrite is replaced by microspar (interval 180-1115C-6R-2, 17-19 cm). Foraminifers are commonly infilled with pyrite.
The XRD analyses of the silty clays reveal quite variable compositions. The typical composition is calcite, with quartz, plagioclase, illite, sporadic chlorite, and rare ankerite. More volcaniclastic-rich samples include amphibole and clay. Pyrite is consistently present in samples from below 285 mbsf. One sample of Core 180-1115B-26X contains dolomite as the major phase (Table T3).
Graded volcaniclastic sand/sandstone beds, typically <8 cm thick, mainly exhibit sharp, scoured bases and grade upward into silty clay to clay (up to 4 per section; e.g., Section 180-1115B-17H-2). The thicker beds are slightly coarser grained, especially at the base, where concentrations of detrital or volcanic-glass grains (Fig. F6), or small lithic fragments (e.g., interval 180-1115C-18R-3, 0-3 cm) are present. Parallel laminations were observed very rarely (e.g., 0.2 cm within interval 180-1115B-31X-5, 27-31 cm).
The volcaniclastic material is locally reduced to a few-mm-thick layers rich in silt-sized glass with sharp upper and lower contacts. Elsewhere, diffuse crystal-rich layers define a weak normal grading. In addition, many ghosts of volcaniclastic sandstone beds are present, marked by a concentration of sand grains, particularly within burrows (intervals 180-1115B-25X-2, 9-11 cm, and 27X-5, 84-105 cm).
Toward the base of the unit, sharp-based, graded volcaniclastic sandstones again become abundant, with less evidence of burrowing (e.g., Core 180-1115C-6R). Also, at the base of the unit, acidic extrusive rock fragments are scattered throughout (Cores 180-1115C-9R and 10R).
Thin sections of volcaniclastic sandstone and siltstone revealed the presence of unaltered (and also altered) plagioclase, quartz, hornblende, biotite, muscovite, plagioclase and hornblende phyric basalt, chloritized basalt, colorless glass shards, and bioclasts ("Site 1115 Thin Sections").
Rare beds of volcanic ash were observed. Smear slides show that these contain variable amounts of volcanic glass, together with quartz, feldspar, mica, rock fragments, calcite, nannofossils, and foraminifers ("Site 1115 Smear Slides"). These thin beds are similar to those of Unit II (see "Lithostratigraphic Unit II"), with sharp bases and graded tops, and commonly include concentrations of ferromagnesian minerals. These beds are strongly disrupted by burrowing (e.g., intervals 180-1115B-19H-5, 91-93 cm and 19H-6, 137-145 cm).
Examples of graded volcanic ash include the following: intervals 180-1115B-19H-7, 1-10 cm; 20H-2, 132-134 cm; 21H-6, 72-73 cm; 26X-2, 72.5-75 cm; 27X-2, 113-114 cm; and 27X-5, 37-40 cm; also in intervals 180-1115C-8R-4, 85-86 and 130-132 cm, and 12R-5, 59-61 cm. In addition, patches of volcanic ash dispersed by burrowing organisms were seen occasionally (e.g., intervals 180-1115B-25X-5, 35-41 cm; 26X-3, 34-38 and 109-109.5 cm; 29X-1, 26, 106-110, and 112-112.5 cm; 29X-2, 20-21, 40, 142, and 147 cm; and 29X-7, 26-28 cm), and fragments of pumice? were observed in the intervals 180-1115B-27X-CC, 24-26 cm, and 10R-3, 48-53 cm.
Unit III continues the same general type of sedimentation as in Units I and II. Deposition mostly took place at upper bathyal depths (150-500 m) during middle Pliocene time. Sedimentation was very rapid, at ~284 m/m.y., decreasing to ~79 m/m.y. above 190 mbsf (see "Sediment Accumulation Rate"). There is a marked decrease in magnetic susceptibility at 210 mbsf (Core 180-1115B-23H; see "Magnetic Susceptibility"). However, no obvious changes in the facies, or composition as determined by smear slides or XRD are observed here, and it is not identified as a lithostratigraphic unit boundary. In addition, the geophysical logs indicate the presence of a rather uniform muddy succession throughout Unit III, with some thin sandy intervals.
Interpretation of the photoelectric effect suggests the presence of a thin dolomite-rich layer at 291-292 mbsf (see "Lithologic Analysis"). The XRD of a sample from 292.1 mbsf (interval 180-1115C-2R-1, 9-11 cm) confirms the presence of dolomite.
The FMS images (static and dynamic normalized) indicate that the succession is relatively muddy, with occasional thin (<5 cm), bright, resistive layers that are interpreted as sands (e.g., at 154.5, 155.7, 158.3, 160.5, and 161.3 mbsf). The succession is locally more sandy at ~245 mbsf with ~5- to 10-cm-thick inferred sands occurring on average every 20-40 cm. In general, the succession becomes slightly more resistive downward, suggesting it is becoming more silty.
The core evidence shows that the graded volcaniclastic silts/siltstone and sands/sandstone, interpreted as deposits from turbidity currents, become markedly more abundant downward. However, some of these sediments could be outer shelf deposits that were extensively bioturbated in view of the inferred water depth. On the other hand, definite volcanic ash interpreted as air-fall tuff is rare. The abundance of volcaniclastic beds was probably originally greater than that visible in the cores, because many beds were so strongly bioturbated they remain only as disseminated silt or sand grains. A number of intervals are relatively rich in volcanic glass. However, this is mainly reworked with lithic and biogenic material. In Unit III we witnessed the first significant appearance of illite below 169 mbsf, which indicates a significant input of fine-grained sediment derived by continental erosion. Also, the relative abundance of pyrite suggests that subseafloor conditions were anoxic.
Lithostratigraphic Unit IV recognition is based on a slight change in grain size and increasing lithification from mainly silty claystone, to uniform dark greenish gray claystone, admixed with siltstone and claystone, termed calcareous sandy silty claystone. Shell fragments become more abundant, whereas only minor fine-grained volcaniclastic sandstone is present. The following lithologies are observed in decreasing order of abundance:
This is a poorly sorted, mixed sediment that is typically highly bioturbated with scattered volcaniclastic detrital grains and foraminifer fragments. Many burrows are infilled with fine- to medium-grained sandstone, often rich in ferromagnesian minerals. Shell fragments are locally common (e.g., interval 180-1115C-13R-3, 90-95 cm) and intact shells are also present (e.g., interval 1115C-14R-1, 124 cm). Plant debris is rarely observed (interval 180-1115C-14R-4, 58 cm). Possible Teichichnus trace fossils were noted locally.
Smear slides indicate the presence of quartz, plagioclase, volcanic glass, rare biotite, amphibole, calcite, and biogenic components including sponge spicules ("Site 1115 Smear Slides"). The XRD analysis of this interval shows that the composition is little changed from Unit III, with calcite, quartz, and plagioclase as major minerals, and illite, amphibole, and pyrite as minor minerals (Table T3). Calcium carbonate values are similar to those in the lower part of Unit III, above (i.e., ranging from ~18 to 30 wt%; see "CaCO3, Sulfur, Organic Carbon, and Nitrogen".
Thin interbeds of volcaniclastic sandstone and siltstone are rarely present in this unit (mainly in Section 12R-5). Thin sections of these rare volcaniclastics indicate the presence of quartz, plagioclase, acidic extrusive rock fragments (with hornblende and plagioclase microphenocrysts, zoned plagioclase, glassy basalt, chloritized basalt, shell fragments, benthic foraminifers, and echinoid fragments) set in a micritic matrix ("Site 1115 Thin Sections"). In addition, medium-grained lithoclasts rich in hornblende are present (Fig. F7). This unit includes pumice with abundant hornblende set in a glassy matrix (Fig. F8).
The main significance of Unit IV is that it marks a drastic decrease in the volcaniclastic sand input by turbidity currents. The geophysical logs are interpreted to indicate the presence of a mainly muddy succession (see "Downhole Measurements"). This interpretation is supported by the FMS data that indicate this unit is relatively uniform and nonresistive. Some of the sediments of Unit IV may have been deposited under the influence of currents in an outer shelf setting. However, bioturbation has extensively modified the original sedimentary structures, which makes it almost impossible to interpret depositional processes. The composition of this sand indicates a calc-alkaline-type source and remains similar to Unit III. Sedimentation continued at upper bathyal depths (150-500 m) during early Pliocene time. However, the lowermost 5 m of the unit accumulated in outer neritic water depths (50-150 m). Average sedimentation rates were ~155 m/m.y. (see "Sedimentation Accumulation Rate").
Lithostratigraphic Unit V recognition is based on an increase in grain size from mainly sandy and silty claystone to predominantly mixed fine- to medium-grained silty sandstone, as follows:
Individual beds are sharp based, with concentrations of calcium carbonate and volcaniclastic grains, and occasional parallel laminations and low-angle (<10º) cross laminations (e.g., intervals 180-1115C-15R-4, 27-28 cm; 17R-1, 4-8 cm; 17R-1, 36-40 cm; and 18R-1, 20-50 cm; Fig. F9). Individual laminations are defined by slightly coarser grains (e.g., interval 180-1115C-20R-1, 6-17 cm). The lower parts of some sandstone beds are well cemented by calcite spar, whereas the upper parts of beds are less well cemented and highly burrowed. Shell fragments are locally very abundant (e.g., intervals 180-1115C-15R-3, 87-150 cm and 16R-1, 56-60 cm). Fragments of organic matter were rarely observed (e.g., interval 180-1115C-18R-2, 52 cm).
In many intervals only vague parallel laminations, or low-angle inclined laminations, are visible (interval 180-1115C-15R-6, 100-116 cm) in the lower part of the beds. Where bioturbation is very extensive, no primary sedimentary structures were observed (e.g., Section 17R-1). Lower in the unit (Section 18R-2) burrowing is less intense. As in Units III and IV, some intervals exhibit concentrations of detrital grains that could have originated as discrete beds (e.g., interval 180-1115C-13R-2, 90-100 cm).
Many thin beds are graded, with sharp bases rich in planktonic foraminifers. As in other units (i.e., Units I-IV), these beds grade upward from medium- to fine-grained sandstone to calcareous silty claystone (Fig. F10). A thin section shows that these sandstones are rich in both benthic and planktonic foraminifers, subrounded basic and intermediate igneous rock fragments, ferromagnesian minerals, quartz, and feldspar (Fig. F11; "Site 1115 Thin Sections").
Minor amounts of siltstone and claystone are present, commonly near the top of graded beds. In smear slides quartz, plagioclase, rare biotite, rock fragments, volcanic glass, amphibole, bioclasts, and pyrite are present ("Site 1115 Smear Slides"). The XRD analysis revealed a relative decrease in calcite compared to Unit III (see "Lithostratigraphic Unit III") with plagioclase and quartz being the major minerals (Table T3). Calcite, illite, pyrite, and minor amphibole, aragonite, and chlorite are also present. Calcium carbonate contents in the fine-grained sediments range from 14 to 52 wt%. A thin section siltstone (see "Site 1115 Thin Sections") was seen to comprise a recrystallized microspar calcite cement with planktonic foraminifers, shell fragments, and with common detrital grains of felsic volcanics, hornblende, and biotite, also some brown altered basaltic glass (palagonite), and coarse plagioclase of possible intrusive origin.
Several thin beds of limestone are present. A thin section revealed that one is a packstone dominated by planktonic foraminifers, with well-preserved shell fragments, quartz, acidic volcanic rock, glass shards, minor recrystallized quartz (polycrystalline), rare aphyric basalt, and rare coarse plagioclase (of possible intrusive origin) ("Site 1115 Thin Sections").
Lithostratigraphic Unit V accumulated in outer neritic depths (50-150 m) in early Pliocene time. Average sedimentation rates were initially ~155 m/m.y., as in the base of Unit IV, but greatly decreased downward to ~45 m/m.y. (see "Biostratigraphy" for details of depth zonation). Unit V is largely made up of detrital material from a volcanic source. Acidic volcanic rock is abundant in this unit and a minor amount of plutonic igneous rock fragments may be present in addition.
The natural gamma-ray logs indicate that the K/Th ratio is relatively suggestive of the presence of a volcaniclastic component of distinctive composition (see "Downhole Measurements"). Otherwise, the log response is indicative of a mixed sandy and silty succession, becoming more clay rich near the base. The FMS images reveal a sharp change to more resistive, inferred more silty and sandy near the top of this unit at 415.0 mbsf. Occasional, thin very bright layers at 424.0, 430.4, 434.1, and 436.2 mbsf are interpreted as sandstones <15 cm thick. Near the base (467.0-472.0 mbsf), a less resistant, inferred more muddy unit is present.
Despite the limited core recovery in this unit (<20%) the facies recovered show a transition from poorly sorted, highly bioturbated sandstone in the lower part of the unit to a cross-bedded, parallel- and wavy-laminated sandstone in the middle part of this unit. Recovery in the upper part is very poor. The transition in sedimentary structures suggests an increase in current effects through time.
The laminated and cross-bedded, fine-grained sandstone may represent part of a shelf, or bar, or ridge that was formed under the influence of wave or current action. Tidal currents may also have played a role, although there is little sedimentary evidence of this (e.g., herringbone cross lamination). Also, a storm influence cannot be ruled out for sediments that may have accumulated at shallow-water depths.
Lithostratigraphic Unit VI is distinguished by a decrease in grain size from mainly mixed silty sandstone to more heterogeneous mixed sediment. Calcium carbonate values range from 10 to 25 wt% (see "CaCO3, Sulfur, Organic Carbon, and Nitrogen"). A number of related lithologies are interbedded to produce a heterogeneous succession as follows:
The lithology comprises structureless, bioturbated foraminifer-rich siltstone with abundant shell fragments (e.g., intervals 180-1115C-21R-4, 23 and 44.5 cm) and volcaniclastic grains. Smear slides reveal the presence of quartz, feldspar, rare plagioclase, biotite, rare amphibole and pyroxene, and volcanic glass ("Site 1115 Smear Slides"). Nannofossils are abundant, and planktonic foraminifers are rare to common. The XRD analysis additionally shows the presence of calcite, plagioclase, quartz, and illite, with minor chlorite and amphibole (Table T3).
This contains fewer planktonic foraminifers. Vague traces of inclined bedding are locally visible (e.g., interval 180-1115C-21R-3, 123 cm). Shell fragments are locally abundant (e.g., interval 180-1115C-21R-3, 82-95 cm). The clayey siltstone contains scattered small shell fragments, detrital grains, and planktonic foraminifers.
This facies, lower in the unit (Section 22R-1) contains abundant shell fragments and detritus (including disarticulated bivalves and gastropods), mainly ferromagnesian grains, foraminifers, and rare carbonaceous grains. Bioturbation is extensive. Shells are rarely aligned, imparting a very weak bedding. Thin sections indicate that this unit is rich in plagioclase, hornblende, biotite, benthic and planktonic foraminifers, and subrounded basalt rock fragments (Fig. F12; "Site 1115 Thin Sections").
This is a mixed deposit composed of bioturbated sediment, rich in foraminifers, showing parallel to wavy lamination and including large thin-shelled bivalves (4 cm × 2 cm in size; e.g., interval 180-1115C-23R-1, 86-110 cm; Fig. F13). The XRD analysis revealed the same composition as within the siltstones (see "Calcareous Siltstone with Foraminifers," "Calcareous and Clayey Siltstone," and "Sandy Siltstone"), with the addition of aragonite and pyrite. In addition, several thin sections revealed sandstones and siltstones to contain mixed bioclastic grains (i.e., shell fragments, benthic foraminifers, calcareous algae, echinoderm plates and spines, and rare planktonic foraminifers), and detrital grains (i.e., quartz, plagioclase, hornblende, basalt, and acidic volcanics), micritic and chloritic grains, rare microcline, and also pyrite. The grains of basalt are rounded to well rounded ("Site 1115 Thin Sections")
The whole of Unit VI accumulated in an inner neritic setting (<50 m) at an estimated sedimentation rate of 45 m/m.y. (see "Sediment Accumulation Rate"). The unit is dated as latest Miocene to earliest Pliocene on the basis of nannofossils. A shallow-water setting is supported by the abundance of reworked neritic carbonate, including calcareous algae. High uranium log values are consistent with the presence of abundant reduced organic matter. Otherwise, the geophysical logs indicate the succession is mainly sandy (based on the gamma ray, neutron porosity, and density logs; see "Downhole Measurements"). This is confirmed by the FMS data that show a sharp increase in resistivity near the boundary with the overlying unit (at 472.2 mbsf).
The frequent marked changes in grain size and sedimentary structures, including rarely preserved wavy as well as parallel laminations, suggest the unit accumulated in a coastal or shoal setting, affected to some extent by wave or current activity. Although less intense than in open, unprotected coastal areas, wave activity can also play an important role in more restricted (e.g., lagoonal) deposits, especially in transport of fine-grained sediments. The very well rounded basaltic grains possibly originated in a high-energy coastal, or fluvial setting, and were then reworked offshore by waves or currents, and interbedded with finer grained sediments containing planktonic foraminifers and nannofossils. Provenance from mainly volcanic-related setting(s) remained largely unchanged from Unit V, with a minor contribution from plutonic, as well as volcanic rocks.
Lithostratigraphic Unit VII is recognized on the basis of a change from relatively coarse grained heterogeneous sediments, to still mixed, but finer grained, mainly silty facies. In contrast to Unit VI, in which calcite is predominant, plagioclase and aragonite are important in Unit VII, based on XRD data (Table T3). Calcium carbonate values range from ~10 to 41 wt%, with only one sample being very depleted (0.3 wt%; see "CaCO3, Sulfur, Organic Carbon, and Nitrogen"). In common with Unit VI, a number of lithologies are interbedded, as follows:
This is dark greenish gray and has abundant shell fragments and foraminifers, bivalves, and gastropods (disarticulated). Shells are rarely aligned, imparting a weak bedding. Traces of organic matter are locally present. Rare articulated bivalves and gastropods are present. Several types of burrows were observed (including Zoophycos), some filled with detrital sand grains, others with silty clay. The burrows are commonly rendered visible by traces of black monosulfide.
These intervals are very similar to the sandy siltstone beds, but are coarser, up to granule size (e.g., Section 28R-1), with common shelly fragments, foraminifers, and sand-sized detrital grains (i.e., ferromagnesian minerals, biotite, plagioclase, and rock fragments; Fig. F14). Burrows are infilled with detrital sand grains and small shell fragments. Rare concentrations of detrital grains define vague intervals between 13 and 20 cm thick with weakly defined top and bottom contacts to the laminae. Rare carbonaceous detritus and pyrite are also present.
Thin-section study revealed the sandstone to contain quartz, plagioclase (fresh and altered), zoned plagioclase, biotite, pyroxene, hornblende, palagonite, altered basaltic grains, plagioclase and hornblende phyric acidic volcanics, and microcline set in a calcareous and micritic matrix (Fig. F15; "Site 1115 Thin Sections"). The sandstone contains the following bioclasts: shells, calcareous algae, shell fragments, and echinoderm fragments, in addition to the detrital grains mentioned above. Many of the grains are moderately to well rounded.
This forms a very minor component of Unit VII as thin beds or isolated laminae. In addition, some calcareous silty claystones, which are structureless and rich in shell fragments and foraminifers, are present.
Smear slides reveal the presence of quartz, plagioclase, rare volcanic glass, accessory minerals (amphibole), calcite, and pyrite ("Site 1115 Smear Slides"). Nannofossils were only noted in a few samples; planktonic foraminifers are rare or absent. The XRD analysis revealed the presence of plagioclase, calcite, quartz, illite, chlorite, aragonite, and pyrite (Table T3).
Unit VII is dated as late Miocene on the basis of calcareous nannofossils. A relatively low sedimentation rate of >13 m/m/y. is inferred (see "Sediment Accumulation Rate"). In addition, foraminifers indicate inner neritic water depths (0-50 m). In common with Unit VI, Unit VII accumulated in a shallow-water setting. The increase in shelly bioclasts and volcaniclastic material, the decrease in average grain size, and the decrease in observed presence of planktonic foraminifers are all consistent with a relatively low energy setting, possibly a marine lagoon. However, some shell-rich beds may represent shell lags that resulted from reworking from wave or tidal activity in a lagoonal setting. The high gamma-ray logs are again suggestive of reduced organic matter in the upper part of the unit (above 526 mbsf) as in Unit VI (see "Lithostratigraphic Unit VI"). Beneath this, hole conditions were poor and the low gamma-ray response is suggestive of the presence of abundant soft nonresistive material, possibly organic-rich (coaly?) material, as inferred for Site 1109 (see "Downhole Measurements"). The FMS images reveal a mainly muddy succession with rare, thin-bedded (~10 cm) resistive inferred sandstones (e.g., at 528.6 mbsf). In addition, the mainly volcanic-related provenance remained unchanged from Unit VI, again with some input of plutonic rock. In addition, a few grains of possible metamorphic rock were also noted.
This unit is distinguished by the appearance of darker, organic-rich, finer grained sedimentary rocks and bioclastic limestone. Calcium carbonate values vary drastically from 0.3 wt% in organic-rich sediment (with 3.84% carbon), to ~53 wt% in limestone (see "CaCO3, Sulfur, Organic Carbon, and Nitrogen"). Lithologies present are as follows:
Organic-rich silty claystone is present either as massive, finely disseminated material; scattered particles (0.5-0.7 mm in size; interval 180-1115C-30R-3, 108-137 cm); or discrete thin laminae (coal like; e.g., interval 180-1115C-30R-3, 0-20 cm; Fig. F16). Dark greenish gray, less organic rich claystone is present near the base of the unit. Smear slides revealed typical detrital fragments, as in the overlying Unit VII. Bioclasts are restricted to rare planktonic foraminifers ("Site 1115 Smear Slides"). In addition, XRD analysis allowed the recognition of plagioclase, quartz, K-feldspar, pyrite, amphibole, and probably smectite in different samples (Table T3).
Rare occurrences of this lithology were sampled for study in thin sections (Table T3). A medium-grained sandstone is texturally mature with well-rounded grains cemented by sparry calcite (Sample 180-1115C-29R-1, 72-73 cm). The grains include acidic volcanics, altered basalt (locally chloritized), plagioclase, biotite, pyroxene, hornblende, polycrystalline quartz, and rare microcline (Fig. F17). A fine-grained siltstone contains a similar range of detrital grains, together with benthic foraminifers and shell fragments set in a micritic matrix (interval 180-1115C-29R-1, 82-83 cm).
Rarely, near the top of the unit, several isolated clasts of bluish gray limestone were recovered. These contain shell fragments, burrows infilled with black volcaniclastic sediment, and veins of calcite (intervals 180-1115C-29R-2, 28-32 and 62-65 cm; and 29R-3, 32-34 cm; and Section 29R-CC).
Sediments of Unit VIII accumulated in a marine, inner neritic setting (<50 m) during late Miocene time. The sedimentation rate is estimated as >13 m/m.y. (see "Sediment Accumulation Rate"). The abundance of plant-rich material, including coaly laminae, suggests accumulation in a quiet-water, lagoonal setting adjacent to a vegetated land area. This is again supported by the low response of the gamma-ray log that is interpreted to suggest the presence of coaly layers up to 2 m thick (see "Downhole Measurements"). The FMS images of most of this unit are difficult to interpret as a result of numerous washouts. In addition, micritic carbonate is a common constituent of marine lagoons, in which the source of calcium carbonate could be detrital and/or biogenic (e.g., from breakdown of calcareous algae, or even direct precipitation).
This unit is distinguished by a marked increase in grain size, with alternations of sandstone, siltstone, and conglomerate. A single calcium carbonate value within rare claystone yielded a value of ~41 wt% (see "CaCO3, Sulfur, Organic Carbon, and Nitrogen"). Details of the lithologies are as follows:
Conglomerate is made up of well-rounded clasts (up to 4.0 to 0.3 cm), mainly basalt (e.g., intervals 180-1115C-31R-1, 8-87 cm, and 30R-5, 45-70 cm; Fig. F18). Individual clasts exhibit varying extents of alteration with onion-skin weathering observed. A matrix between the clasts is composed of coarse-grained sand and granules of the same material, in turn cemented by calcite spar. A possible vertical root trace (0.4 cm wide × 4 cm deep) was observed in one instance (interval 180-1115C-30R-CC, 6-9 cm). Where matrix is abundant, the fabric is more matrix supported than clast supported (interval 180-1115C-31R-1, 8-87 cm).
The sandstone is structureless, medium grained, poorly sorted, and well cemented (by calcite), with scattered subrounded sand grains. Greenish gray clay-rich material is present between grains. Small (1.5 cm) radiating veinlets are filled with calcite in one case (interval 180-1115C-30R-5, 28 cm). A single sample of siltstone contains only calcite and plagioclase, by XRD study (Table T3).
Two thin sections of sandstone contain the following detrital components: basalt (with feldspar microphenocrysts), chloritized basalt, variolitic glassy basalt, altered acidic volcanic, schist, plagioclase, hornblende, perthitic feldspar (mermikitic), microcline, pyroxene, minor chalcedonic quartz, as well as various bioclasts (including coral), all set in calcite spar cement. Many of the lithic grains are well rounded (Fig. F19; "Site 1115 Thin Sections").
The well-rounded nature of the conglomerate clasts is suggestive of prolonged sediment transport or reworking in a high-energy setting (i.e., littoral, beach, or fluvial). The geophysical logs are interpreted to indicate the presence of conglomerate between 574 and 567 mbsf. The elevated photoelectric effect is explicable by the presence of a calcite spar cement, as observed in the cores (see "Downhole Measurements"). The FMS images indicate the presence of a very bright resistive unit from 570 to 576 mbsf. In detail, discrete resistive beds 10-30 cm thick are separated by less resistive (muddy) intervals, individually 20-30 cm thick. Several factors favor a fluvial setting: (1) marine fossils are absent; (2) the presence of interstitial sand and granules within the conglomerate is consistent with a fluvial setting; (3) the clasts show alteration suggestive of subaerial weathering; (4) a probable root trace was observed; and (5) a calcium carbonate nodule, possibly incipient caliche is present. Diagenetic processes include precipitation of minor amounts of (spherulitic) chalcedonic quartz and extensive calcite spar. The porous and permeable poorly sorted, texturally mature sands probably acted as conduits for fluid flow after deposition.
This unit is recognized by the appearance of well-graded intervals, including sandstone, siltstone, and minor granule conglomerate. Levels of calcium carbonate in fine-grained sediment are generally low (<2 wt%), with the exception of a single more calcareous sample with about 9 wt% calcium carbonate (see "CaCO3, Sulfur, Organic Carbon, and Nitrogen"). Details of the component lithologies are as follows:
Many of the sandstones are reddish brown, in contrast to the mainly greenish and grayish colors of the entire overlying succession. A few sandstones exhibit sharp, scoured bases (e.g., interval 180-1115C-32R-3, 54-68 cm; Fig. F20). Normal grading is common, and in one case inverse-to-normal grading was observed (Section 32R-1). Several relatively thick (0.35 m), normal-graded beds also exhibit local inverse grading in their basal few centimeters (e.g., interval 180-1115C-32R-3, 75-96 cm). Parallel lamination is also well developed, but largely obliterated by burrowing. In one case a massive sandstone bed includes a large mud clast (8 cm wide). A poorly sorted, medium- to coarse-grained sandstone with abundant rounded mud clasts was observed locally (Fig. F20). Small shell fragments are rarely present (interval 180-1115C-33R-2, 127-140 cm). Shell fragments and calcium carbonate material become more abundant lower in the unit (e.g., interval 180-1115C-34R-1, 19-80 cm). Near the base of the unit, fine sand-sized shelly material forms faint thin layers (0.2-0.3 cm). Bedding is mainly flat, but inclined bedding planes were rarely observed (interval 180-1115C-34R-1, 80-135 cm).
Thin sections of the sandstone reveal grains of pyroxene phyric basalt, glassy basalt, palagonite, and flow-banded well-crystallized basalt, together with fresh pyroxene and subordinate plagioclase, all set in a minor calcite spar cement (Fig. F21; "Site 1115 Thin Sections").
A small number of thick beds up to 50-80 cm thick have lower parts composed of granule conglomerate that is usually <10 cm thick that graded upward to medium-grained sand with siltstone or claystone rip-up clasts comprising irregularly laminated siltstone and burrowed claystone (i.e., intervals 180-1115C-32R-3, 0-58.5 cm, and 33R-1, 32 cm). In addition, a few thin beds were noted with sharp bases grading from very fine grained sandstone with parallel lamination (interval 180-1115C-32R-1, 12-22 cm), or convolute lamination, passing into siltstone and claystone. Furthermore, in several sections contorted and irregular beds of calcareous sandstone interfinger with clay-rich sandstone (Fig. F22).
This siltstone is greenish gray, weakly indurated, mainly structureless, poorly sorted, homogeneous, and weakly burrowed, with scattered sandy grains. In places, the siltstone is highly calcareous and very well cemented (interval 180-1115C-31R-1, 91-108 cm; "Site 1115 Thin Sections"). The siltstone is clay rich and rarely exhibits convolute lamination (e.g., interval 180-1115C-32R-1, 8-12 cm), or parallel lamination (e.g., Section 32R-1). In addition, siltstone also constitutes the upper parts of individual graded beds (of which the bases were probably not recovered).
The XRD analysis of fine-grained sediment revealed an unusual mineralogy, with pyroxene as a major mineral, and minor constituents including amphibole, hematite, and illite (Table T3).
Unit X is interpreted to have accumulated in middle Miocene (Serravallian) time in an outer neritic setting, based on the combined presence of neritic fossils and planktonic microfossils. The sedimentation rate is poorly defined, based on limited biostratigraphical control, but could be similar to or slightly less than the unit below (>135 m/m.y.; see "Biostratigraphy"). The geophysical logs, including FMS, suggest the unit as a whole is relatively sandy with thin- to medium-bedded sandstones <15 cm thick on the average of every 0.5-1.0 m. The neritic shells were derived from a shallow-water setting, possibly a calcium carbonate-influenced coastline or shoal. Also, the reddish color may suggest a well-oxidized depositional setting. The fine-grained clastic sediments clearly accumulated largely from suspension. In view of the thick-graded units, including large rip-up clasts, the coarser grained sediments either accumulated entirely from turbidity currents, or in part as storm deposits (i.e., tempestites). For example, the shallow-water carbonate was possibly swept offshore by storm currents. An origin mainly as high-density turbidites, is, however, favored by the absence of obvious ripple lamination typical of storm deposits. Also, there is little evidence of current reworking as in the two overlying units (Units VIII and IX).
The provenance was mainly from pyroxene phyric basalt, or basaltic andesite. The relative uniformity of the source lithology without strong alteration is suggestive of a relatively local source. In view of the abundance of altered basalt present, it is likely that the unusual zeolite is detrital, possibly following low-grade alteration of a volcanic rock.
This unit is distinguished by the presence of sandstones and packstones, interbedded with siltstones and claystones and minor conglomerate. Calcium carbonate was determined as <2 wt% in two samples of claystone (see "CaCO3, Sulfur, Organic Carbon, and Nitrogen"). However, routine testing of core samples with dilute HCl and the results of thin-section analysis show that some lithologies (e.g., packstone) are much more calcareous than this.
This lithology is coarse-grained sandstone, with abundant shell fragments (Fig. F23). Upper and lower bed contacts are sharp (e.g., interval 180-1115C-35R-1, 133-135 cm). Coarse-grained beds are pebbly at the base, with shell fragments, mud, and silty rip-up clasts (3-5 cm × 2-3 cm), also some carbonate clasts. In one normal-graded bed there is a decrease in the size of clasts from small pebbles (0.5-1.0 cm) to granules at the top. Otherwise, no fabric is seen in this bed (interval 180-1115C-35R-3, 93-145 cm). In another bed (which lacks a preserved base) large elongate clasts (4 cm × 2 cm) are present (interval 180-1115C-35R-2, 16-18 cm). Some individual siltstone and claystone clasts exceed the core diameter (intervals 180-1115C-35R-3, 20-30 and 85-90 cm). Rarely, these clasts are well rounded. In addition, a single, very thick graded bed (>2 m) spanning several sections was observed (Sections 34R-3 and 34R-CC). This bed grades from coarse-grained sand to silt. The lower interval contains rip-up clasts of silty clay and a variety of volcanic rock fragments.
The most calcium carbonate-rich beds were observed in Section 35R-4, as a very coarse grained packstone composed of bioclastic carbonate, with centimeter-sized intraclasts of greenish gray clayey siltstone (interval 180-1115C-35R-4, 0-20 cm). Slightly lower in the unit, sporadic packstones are composed of well-sorted, slightly normal graded bioclastic calcarenite. In thin section, the packstone contains benthic foraminifers, echinoderm debris, planktonic foraminifers (filled with pyrite), calcareous algae, bivalve shell fragments, bryozoans, and coral fragments (Fig. F24). In addition, detrital grains include pyroxene, hornblende, quartz, plagioclase, pyroxene, and rarely, olivine phyric basalt, acidic volcanics, limestone, very rare chloritic schist, micaceous calc-schist, and iron oxide.
Greenish clayey siltstone contains scattered small bioclastic grains (e.g., interval 180-1115C-35R-4, 38-40 cm). Locally, lamination (including fine convolute lamination) is inclined at up to 50º and the rock is cut by anastomosing cracks (interval 180-1115C-35R-4, 50-76 cm). In places the siltstone changes to a reddish brown color (interval 180-1115C-35R-4, 86-105 cm). These red siltstone intervals exhibit dipping laminae, small-scale convolute lamination, synsedimentary microfaults, dark gray clayey siltstone rip-up clasts, and occasional rhodoliths. Elsewhere, rhodoliths are again observed (interval 180-1115C-35R-4, 10-12 cm). Some individual fragments are rich in microfossils cemented by calcite (Section 39R-CC).
Interbeds are parallel laminated, with sharp basal and upper contacts (interval 180-1115C-35R-1, 106-135 cm). Locally, the siltstone is dark gray, parallel laminated, lacks bioturbation, and appears to be rich in organic matter (e.g., Fig. F25).
There is a single interval of reddish brown clayey siltstone, with subrounded pebbles of dark gray siltstone and bioclastic pebbles (i.e, algal oncolites). A single clast of derived conglomerate was noted in interval 180-1115C-35R-4, 86-105 cm.
In addition, a single conglomerate clast was recovered from the lower part of the unit. This is composed mainly of small (0.5-1.5 cm), subangular to rounded calcium carbonate or limestone pebbles, also very well rounded siltstone and claystone clasts (interval 180-1115C-37R-CC, 3-6 cm).
Unit XI accumulated rapidly in an upper bathyal to outer neritic setting (~150 m) during middle Miocene time at an estimated sedimentation rate of >135 m/m.y. (see "Sediment Accumulation Rate"). The carbonate rock (packstone) and associated volcaniclastic sediment were redeposited from a neritic setting, presumably by high-density turbidity currents. The source area included pyroxene phyric basalt, as in the overlying unit. A continental slope setting is inferred from the abundance of sedimentary structures indicative of sediment instability, including soft-sediment deformation, evidence of slumping or sliding, and dewatering. Specifically, one internally deformed unit is interpreted as part of a small slide block that partially broke up during transport. It is likely that slope instability characterized a wide area of the inferred slope environment. However, it is also possible that the site lay within a submarine channel, with only very localized slumping of soft sediment from the banks of the channel. In addition, the isolated clast was possibly transported within a debris flow. A relatively uniform sandy and silty succession is present, based on the gamma-ray, resistivity, neutron porosity, and density logs (see "Downhole Measurements"). The FMS data further indicate that a particularly resistive layer is present near the top of the unit from 607 to 614 m that may correspond to the presence of calcite-cemented sandstone in the cores. Conversely, one of several thin, relatively muddy intervals is present from 614.2 to 614.5 mbsf. The FMS data further suggest that unusual lithologies are present between 640 and 650 mbsf, an interval of minimal recovery.
This lowest unit in Hole 1115C is marked by a change from heterogeneous, variably calcareous sedimentary rocks of the overlying unit to a much more homogeneous, finer grained, less calcareous succession of sandy siltstone alternating with silty sandstone and rare silty claystone. Levels of calcium carbonate vary within a narrow range, 5-10 wt% (see "CaCO3, Sulfur, Organic Carbon, and Nitrogen").
Dark gray, bioturbated, calcareous sandstone (with mainly Chondrites and rare Zoophycos), is present mainly near the top of the unit, with scattered fragments of shells and planktonic foraminifers. Planar lamination and cross lamination are locally preserved (e.g., intervals 180-1115C-41R-2, 17-18 and 29-31 cm; and 41R-4, 35-45 cm). Bioturbation varies in different beds from minor to intense. Darker, apparently more organic-rich beds are generally less bioturbated. In places, burrowing is so intense that primary lamination is largely obscured. Below 670 mbsf sandstone largely disappears other than as rare intercalations. Exceptionally, in the interval 180-1115C-47R-2, 0-15 cm, a sandstone grades into siltstone, marked by a very irregular base, parallel or inclined lamination, and unusual thin (mm) clay-rich veins.
In thin sections, the sandstones are compositionally quite constant ("Site 1115 Thin Sections"). Samples studied are fine-grained arkosic sandstones, with plagioclase, quartz, planktonic foraminifers (pyrite filled), acidic volcanic fragments, and rare chloritic grains. In general, there is a high ratio of individual mineral grains to rock fragments (>80%). Also, organic matter is common.
From 670 mbsf downward, there is a slight decrease in average grain size from silty sandstone to dark gray and pale gray sandy siltstone, with occasional silty sandstone interbeds (e.g., interval 180-1115C-41R-CC, 12-22 cm). Otherwise, the composition and sedimentary structures remain unchanged from above. Individual intervals of sandy siltstone (0.1-0.4 cm thick) are calcareous and very highly bioturbated, with parallel or slightly wavy lamination (Fig. F26). Sandy siltstones are present near the base of the hole, in Section 54R-5, as thick laminae to thin beds with sharp bases and tops, which are commonly erosional based. Small bioclastic fragments include possible echinoderms. Locally, a small (<0.5 cm) lensoidal structure is infilled with sandstone. Small, synsedimentary mud clasts are present between 65 and 75 cm.
From 710 mbsf downward, the sand content further decreases and the lithology is calcareous siltstone. Otherwise, there is little change. The rocks are burrowed throughout, with scattered planktonic foraminifers and rare sand-sized shelly fragments. Thin-section study indicates the presence of abundant biotite, feldspars, clinopyroxene, rare quartz, and subrounded basalt fragments (Fig. F27; "Site 1115 Thin Sections"). Parallel and convolute lamination are vaguely preserved. In places, thin (3-5 cm) beds grade from very fine grained sandstone to siltstone at the base, to uniform silty clay at the top (e.g., five such intervals are present in Section 46R-1, and another in Section 47R-1; Fig. F28). In other cases similar very thin beds exhibit both sharp upper and lower boundaries (e.g., interval 180-1115C-46R-2, 10-107 cm). Inverse-to-normal grading is also locally visible on a scale of several centimeters (e.g., Section 48R-1).
The graded beds are commonly less burrowed than the adjacent lithologies, and some intervals are more or less devoid of burrowing (e.g., Section 47R-7). Commonly, bioturbation is concentrated in 3- to 5-cm-wide zones (e.g., Section 47R-1). Small shell fragments are locally present (e.g., interval 180-1115C-46R-5, 124-127 cm). In this lower part, only rare intervals of fine, calcareous sandstone are present, with scattered planktonic foraminifer tests and very rare benthic foraminifers (e.g., interval 180-1115C-47R-7, 26 cm).
The XRD analysis of the fine-grained sedimentary rocks revealed a relatively constant composition with plagioclase, and sometimes calcite, pyroxene, and quartz as major constituents, and smectite? and zeolite (clinoptilolite?) as minor components (Table T3).
Downward in the unit finer grained silty claystone begins to appear, first as thin laminae (Section 42R-5), then as discrete very thin beds, 1-2 cm thick (e.g., intervals 180-1115C-45R-1, 10, 75, and 105 cm). Several such thin claystone intercalations are seen in intervals 180-1115C-45R-4, 44-46 and 100-107 cm. Rare dewatering structures are evident (i.e., small subvertical dewatering conduits infilled with sand; interval 180-1115C-45R-5, 0-10 cm).
Smear slides of the finer grained sediment indicate the presence of common quartz and feldspar, coupled with sporadic biotite, accessory minerals (pyroxene), rock fragments, calcite, and pyrite. Bioclasts present are nannofossils, planktonic foraminifers, and rare sponge spicules ("Site 1115 Smear Slides"). The XRD analysis of the fine-grained sediments of Unit IX indicates an abundance of plagioclase, with subordinate calcite, quartz, pyroxene, smectite? and zeolite (Table T3).
A medium- to coarse-grained ash-rich volcaniclastic layer with sharp upper and lower contacts was observed in the interval 180-1115C-45R-3, 107-108 cm.
Unit XII accumulated in the middle Miocene at an upper bathyal depth (150-500 m) at a rapid average sedimentation rate of >135 m/m.y., unchanged from Unit XI above (see "Biostratigraphy"). However, interpretation of conventional logs shows that the succession is made up of relatively constant alternations of sandstone and siltstone, based on the gamma-ray, resistivity, neutron porosity, and density logs (see "Downhole Measurements"). The FMS images further reveal a very homogenous succession, with occasional thin (<5 cm), more resistant, inferred sandstone layers. Possible cross bedding is imaged between 677 and 679 mbsf.
The sandy siltstone and silty sandstone probably underwent extensive bioturbation of original discrete beds. The predominance of bioturbation points to well-oxygenated bottom conditions, although the abundance of organic matter particles seen in thin sections suggests that there was a high input of terrestrial organic matter. The siltstones and sandstones were largely deposited from suspension and are interpreted as deposits from turbidity currents in view of the water depth below wave base, as inferred from paleontological evidence. Relative to Unit XI above, there is less evidence of sediment instability, which is consistent with a relatively stable, tectonic setting. After lithification, the succession was locally faulted.
The provenance of Unit XII was from a calc-alkaline magmatic suite, assumed to be from the Trobriand volcanic arc to the southwest. During the middle-late Miocene, Site 1115 is inferred to have formed part of a submerged forearc ~70 km northeast of the front of the Trobriand Arc (see "Hanging Wall and Northern Margin Sites" in "Thematic Overview" in the "Leg 180 Summary" chapter).
The depositional history at Site 1115 can be divided into three sequences: (1) a mainly deep-water forearc succession culminating in a relative regression; separated by an erosional unconformity from (2) a nonmarine to fluvial, lagoonal, coastal succession; and (3) a shallow- to deep-marine succession related to rifting of the Woodlark Basin.
Unit XII accumulated mainly by deposition of turbidity currents during the middle Miocene in an upper bathyal (150-500 m) forearc basin to the northeast of the Miocene calc-alkaline Trobriand Arc (Davies et al., 1984; Johnson et al., 1978; Lock et al., 1987; Smith and Milsom, 1984; and Stolz et al., 1993). The source material included distinctive pyroxene phyric basic extrusives. Comparison with the seismic stratigraphic record suggests that the cored interval represents only the top of a very thick (>2 km) forearc succession. Unit XI, also late Miocene in age, represents a similar provenance and shoaling water depths. The presence of redeposited neritic carbonate indicates additional input of shallow-water deposits. Marked sediment instability is recorded, related either to local channeling or regional tectonic movements. Subsequently, Unit X records a more shallow-water setting (50-150 m), with accumulation at a similar or possibly somewhat reduced sedimentation rate with substantial input of shallow-water carbonate. Alternative explanations for the shallowing are that the forearc basin simply overfilled and became emergent, or, more probably, that the basin was tectonically uplifted, possibly as a result of regional compression. Sequence 1 culminated in emergence with a resulting uncomformity and a hiatus.
Based on the biostratigraphy of Site 1115 and the Nubian-1 well, the time span of this hiatus is shorter than 5.54-9.93 Ma (see "Site 1115"). Units IX to VII accumulated during part of this time, after 8.6 Ma. A subaerial to fluvial environment (Unit IX) was transgressed by the sea, giving rise to a low-energy inner lagoonal setting with a high organic-matter content (Unit VIII), then to a more open-marine lagoonal setting (Unit VII).
Since 5.54 Ma the area of Site 1115 subsided. A shallow-marine setting (Unit VI) became an open-shelf setting influenced by traction currents (Unit V). Sediment deposition accelerated, but subsidence was even faster: turbidity currents deposited Unit IV during the early Pliocene at ~155 m/m.y. in water depths of ~150 m. The provenance of volcaniclastic sandstone was from a calc-alkaline arc source that remained little changed throughout the Pliocene. During middle and earliest late Pliocene time (Unit III), rapid deposition (393 m/m.y. until 3.11 Ma) was followed by reduced accumulation (79 m/m.y.) all in an upper bathyal setting (150-500 m). Calcium carbonate increases generally upward, which correlates with a general increase in pelagic carbonate component. Graded volcaniclastic sandstones deposited by turbidity currents are common, whereas volcanic glass-rich units are interpreted as air-fall tuff (see "Record of Ash Eruption"). The basin at Site 1115 deepened during late Pliocene- Pleistocene time to upper middle bathyal depths (Unit II). The relative amount of volcaniclastic sand deposited by turbidity currents decreased (Unit II), and also the overall sedimentation rate decreased to 63 m/m.y. Pleistocene deposition (Unit I) was dominated by nannofossil ooze with volcanic ash (see paragraph below), and sedimentation slowed further (34 m/m.y.), post 0.5 Ma.
The sedimentary succession recovered at Site 1115 contains 161 volcanogenic ash layers. They total 474 cm in thickness and, therefore, represent about 0.5% of the 802.5 m section. The thickness of individual layers ranges from 0.5 cm to 18 cm. The stratigraphic distribution and number of volcanogenic ash layers observed per core, as well as the total thickness of ash observed per core, are shown in Figure F29. The highest frequency of layers per core is 19, constituting up to 5.6% of the total recovered thickness of the single core (Core 180-1115B-29X).
The distribution of volcanic ash layers through time is as follows:
Only one volcaniclastic ash layer was recovered in the middle Miocene section (>13.6 Ma; see "Biostratigraphy"). No volcanogenic ash layers were recovered from the uppermost middle Miocene to lowermost Pliocene. Beginning in the middle Pliocene sediments at ~385 mbsf (3.58 Ma; see "Paleomagnetism") and extending to 150 mbsf (~2.55 Ma; see "Paleomagnetism" and "Biostratigraphy") the amount of volcanogenic material dramatically increased. Within this interval, volcanogenic material is dominantly in the form of volcaniclastic sandstone layers. Several volcanic ash layers (i.e., ash fallout) happened during this interval, but its volumetric contribution was minor compared with that supplied by volcaniclastic turbidites.
Volcaniclastic ash layers are relatively rare in the uppermost Pliocene and in the Quaternary (<1.95 Ma; see "Biostratigraphy"). The influx of volcanogenic material was delivered predominantly by ash fallout. Some of the ash fallout layers are associated with volcaniclastic sand beds that, together, are interpreted as primary pyroclastic material (e.g., Carey and Sigurdsson, 1980; Fisher and Schmincke, 1984).
The discovery of ash fallout layers and associated volcaniclastic turbidites at Site 1115 has important implications for the volcanic-tectonic evolution of the Trobriand Arc and eastern Papua (Smith, 1976; Davies et al., l984; Johnson et al., 1978; Lock et al., 1987; Smith and Milsom, 1976; and Stolz et al., 1993).
Taken together, the above types of volcanogenic deposits indicate a relatively local, at least partly subaerial, volcanic source. The stratigraphic distribution of volcanogenic ash layers indicates that local volcanism extends at least to the early Pliocene (3.58 Ma). Given the proximity of the Amphletts Islands, Egum Atoll, Fergusson Island, and Goodenough Island, it seems likely that the record of volcanogenic deposits reflect regional volcanic-tectonic activity. For example, volcanic rocks from these islands have been dated at 6.3 Ma to Holocene (see Smith and Milsom, 1984, and references therein).