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doi:10.2204/iodp.proc.317.105.2011

Lithostratigraphy

Site U1353 is located landward of the last glacial lowstand shoreline, ~20 km inboard of Site U1351. Three holes were drilled at this site to a total depth of 605 m core depth below seafloor (CSF-A; unless otherwise noted, all depths in this section are reported in m CSF-A). Holes U1353A and U1353B were drilled to 56 and 614 m, respectively, and designated logging Hole U1353C was drilled to 529 m. This location allowed for sampling shelf sediments of Holocene–Miocene age. The described succession was divided into two lithologic units.

Core recovery varied at Site U1353 as a function of drilling technique and lithology. Core recovery of Unit I in Hole U1353A averaged 101%. Core recovery in Hole U1353B was 78% for Unit I and 42% for Unit II using a combination of APC and XCB coring methods. Overall core recovery was 40%. These rates are deceptively high because a majority of the recovered sediment was downhole contamination (cave-in and flow-in; see below). Core recovery dropped considerably below Core 317-U1353B-60H when the XCB was deployed.

The description of Unit I is based on Holes U1353A and U1353B. Hole U1353A was sampled for whole-round geochemical and geotechnical analyses, but most of the core was available for sedimentologic description. The two holes are offset ~20 m from each other (Fig. F3). Throughout the uppermost 56 m they exhibit similar lithologies (Fig. F4) but slightly different stratigraphies. This observation emphasizes the fact that the shelf environment drilled at Site U1353 is dynamic and exhibits localized spatial variability in sediment deposition and preservation.

Cores recovered from Holes U1353A and U1353B show a downhole transition from a heterolithic upper section with abrupt contacts (Unit I) to a more featureless sedimentary mud–dominated section with depth (Unit II). These changes suggest progressive but gradual changes in sedimentary styles as the margin evolved (Figs. F5, F6).

An excellent Holocene–middle Pleistocene record was recovered at Site U1353. Additionally, some of the older early Pleistocene–Miocene seismic reflectors in the offshore Canterbury Basin, which at this site occur at relatively shallow depths, were penetrated. Poor core recovery from the early Pleistocene–Miocene section hindered lithostratigraphic interpretation in deeper parts of the hole.

The core-based lithologic unit descriptions below are based primarily on sedimentologic features observed in the core, including sedimentary texture and structures and smear slide and thin section observations (Table T2). We integrated these observations with X-ray diffraction (XRD) data from the dominant lithology in each core, physical property data from whole-round analyses and downhole logging (see "Physical properties" and "Downhole logging"), and paleontology (see "Biostratigraphy") and correlated them with seismic reflection profiles.

In many instances, intervals of shell hash or sand at the tops of cores were considered to represent downhole contamination by cave-in (Fig. F7; see "Lithostratigraphy" in the "Site U1351" chapter). In addition, similar shell-hash lithologies encountered at the base of many cores were also considered a form of contamination, perhaps caused by sediment being sucked into the base of the cores (flow-in). We estimate that, of the 42% core recovery in Unit II, only a third (~15%) was actually in situ sediment.

Description of lithologic units

Unit I

• Intervals: Cores 317-U1353A-1H through 8H and 317-U1353B-1H to Section 28H-2, 46 cm

• Depths: Hole U1353A: 0–56.03 m and Hole U1353B: 0–151.36 m

• Age: Holocene–early Pliocene

Unit I is heterolithic but characterized by its overall muddy character. The dominant lithology is a dark greenish gray homogeneous mud (Figs. F4A, F8A) with a few percent very fine sand. Shells are either rare and scattered or locally concentrated in layers as thick as 15 cm. Shells are dominated by the gastropod Stirocolpus and, to a lesser extent, by Tawera and other bivalves. Bioturbation is common and has an ichnofabric index of 1 (no bioturbation) to 4 (heavy bioturbation). Locally, the mud fraction can be clay rich or clay, the latter either forming centimeter-thick sharp-based beds or occurring as mottles within the relatively coarser mud. Both fining-upward (e.g., Sections 317-U1353B-9H-3 and 9H-4) and coarsening-upward (e.g., Section 19H-3) trends were observed in transitions between intervals of mud and sandy mud.

Subordinate lithologies include

1. Centimeter- to decimeter-thick beds of greenish gray and dark greenish gray very fine sandy shelly mud shell layers to shell hash (Figs. F4B–F4C, F8B–F8C) mixed with siliciclastic materials (fine to medium sand) and abundant Stirocolpus, Tawera, echinoid spines, barnacle plates, bivalves, gastropods, and rare pebbles of graywacke sandstone as large as 1.5 cm in diameter. The sandy shelly mud and shell hash often occur as centimeter- to decimeter-thick beds at the top of the core, where they may represent cave-in;

2. Very dark greenish gray, well-sorted, very fine to fine (mostly very fine), highly micaceous sand with rare broken shells (Fig. F4D). The sand typically has a soft and soupy appearance in the core and may have been partially fluidized during drilling. Bedding relationships are not always preserved, and the limited number of upper contacts makes bed thickness estimates difficult; and

3. Sandy marl (Fig. F4E) that occurs as a rather thick, presumably Holocene record in the uppermost part of the hole (Cores 317-U1353A-1H and 2H and 317-U1353B-1H and 2H). The sandy marl consists of dark greenish gray, micaceous, homogeneous very fine to fine sandy marl as thick as 8 m and a similar greenish gray, moderately sorted very fine to fine sandy marl to calcareous sandy mud with common to rare shell fragments that occurs below Core 317-U1353B-8H (Fig. F8C).

A considerable amount of potential caved material was recovered in Hole U1353B below Section 317-U1353B-5H-1 (27.7 m). Cores are typically capped by a shell-hash lithology in the top few decimeters before transitioning, often through a soupy, mud-rich interval, into what is considered to be in situ sediment. Despite cave-in and flow-in, a discernible stratigraphy of mud with subordinate clay, sand, sandy marl, and sandy mud predominates.

Muddy shell hash or shelly mud cave-in first occurs at the top of Cores 317-U1353B-5H, 6H, and 7H and again in Core 12H. These occurrences may represent in situ sediments. Below this depth, the first example of definitely caved, washed, mud-free shell material occurs at the top of Core 317-U1353B-17H (Fig. F7B). The composition of the washed shell-hash intervals changes downhole. Species of the family Turritelidae are abundant in the upper intervals, whereas below Core 317-U1353B-26H the shells appear coarser and are dominated by bivalve fragments. Graywacke and quartz pebbles are often present in the shell-hash intervals (Fig. F7), as are irregular pieces of cemented shelly sandstone and limestone.

The mineralogy and component types of the sediments, as estimated from smear slides (see "Site U1353 core descriptions"), are similar to those observed at Site U1351, mainly quartz and feldspar, with lesser dense minerals (e.g., epidote group, amphibole group, zircon, sheet silicates [biotite, muscovite, and chlorite], and rock fragments) (Fig. F9). The exception is that the rock fragments in the detrital fraction include metasedimentary as well as low-grade metamorphic fragments (phyllite) to schist. During the course of routine foraminiferal examination of core catcher samples, sand-sized garnet grains were observed and collected in Sample 317-U1353A-2H-CC, but these were not encountered in core examinations.

The authigenic components include pyrite and other opaque minerals that commonly occur as microscopic spheroids (e.g., mainly foraminifers, nannofossils, and sponge spicules, both siliceous and calcareous). The biogenic components are the same as those observed at Site U1351, including didemnid ascidian spicules, which are very common in the top part of the succession and have a highly variable abundance below Core 317-U1353B-28H.

Mineralogy determined from XRD analyses generally agrees with smear slide composition, especially for quartz, clays, hornblende, and calcite (Fig. F10). Unit I XRD mineralogy is highly variable depending on the lithology, which is consistent with the heterolithic nature of the unit. Quartz, calcite, total clays, micas (muscovite/biotite), chlorite, plagioclase, alkali feldspars, hornblende, and pyrite occur throughout Unit I, but their concentrations are quite variable. Dolomite and siderite are possibly present in a few of the analyzed samples, but the peak heights for these samples are generally close to the limit of detection. Total clays are highest overall in homogeneous mud and lowest in calcite-rich intervals and calcareous sandy mud and sandy marl. Quartz content is highest in well-sorted sand and siltier mud. Calcite content is inversely related to total clays, with the highest content in calcareous mud/marl and the lowest in homogeneous mud. Micas are highly correlated with each other and weakly correlated with total clays and tend to follow the same lithologic associations as total clays. Hornblende and plagioclase are well correlated and roughly follow the same trend and lithologic association as quartz. Pyrite content is low (many peaks are near detection limits) and highly variable, with a weak positive correlation with calcite and an inverse correlation with quartz and clays.

Unit I/II boundary

A pronounced burrowed contact at Section 317-U1353B-28H-2, 46 cm (151.36 m), is taken to represent the base of Unit I (Fig. F11A). Burrows at this surface (Option A) are filled with sandy marl from the overlying lithology and extend 20 cm into the underlying mud. Sharp-based clay beds occur for a few meters below this to Section 317-U1353B-29H-4, 58 cm (155.46 m), but for convenience and correlation purposes we consider the contact at 151.36 m as the most appropriate surface to use for defining the base of Unit I. Other potential basal contacts include the following:

• Option B (Section 317-U1353B-24H-2, 121 cm [137.07 m]), where a silty mud with common shells occurs above a bioturbated mud (Fig. F11B). Marl and clay beds, however, occur below this interval, and, because this surface is not as pronounced as the contact at 151.36 m, we prefer to retain the latter as the most appropriate boundary (Fig. F11A); and

• Option C (Section 317-U1353B-34H-2, 32 cm [178.82 m]), where a folded, clay-rich muddy interval rests sharply on a muddy shelly fine sand (Fig. F11C).

Unit II

• Interval: Sections 317-U1353B-28H-2, 46 cm, through 98X-CC
• Depth: 151.36–604.65 m
• Age: early Pliocene–middle to early Miocene

Although Unit II is also characterized by its muddy nature, it is differentiated from Unit I by its general lithologic uniformity. It lacks the clay interbeds and greenish sandy mud and marl lithotypes present in Unit I, and beds of sand (or sandstone) are not common. The few occurrences of sand in Unit II could potentially be caved artifacts.

The dominant lithology of Unit II consists of two similar, rather homogeneous muddy sediment types (Fig. F12A–F12B). One is a dark greenish gray, micaceous very fine sandy mud. In this lithology, shells may be present within centimeter-thick diffuse shelly layers. Burrows are common and typically occur as dark gray fine sand–filled structures of a few millimeters to a centimeter in diameter. A second lithology consists of dark greenish gray, gray, and greenish gray mud with minor amounts of very fine to fine (mostly very fine) sand. Both types of sediment have no bioturbation to complete bioturbation (ichnofabric indexes of 1–5). Rare graywacke sandstone pebbles as large as 1.5 cm in diameter occur scattered throughout the sandy mud and mud (Fig. F12C). Where present, shells are represented by partially dissolved specimens and dominated by bivalves and gastropods, including Ostrea, Dosinia, Maoricolpus, and rare scaphopods. No distinct sedimentary structures were observed, but biscuiting throughout this generally poorly recovered unit could have masked them. Rare soft-sediment folding occurs as open folds in interval 317-U1353B-34H-1, 60–85 cm (177.6–177.85 m).

Calcareous sandy cemented zones with abundant shells first appear with cave-in or flow-in material in Sections 317-U1353B-31X-CC, 33H-3, 34H-1, 34H-4, and 34H-CC. Sands with abundant shells were also recovered in situ (Fig. F11C). Thin sections show that these samples consist of sandy limestone with little evidence of bioturbation and contain randomly oriented shell clasts and abundant residual intraparticle porosity. Both of these examples contain glauconite grains and authigenic glauconite infilling foraminifer chambers and borings in shell fragments. Although pyrite infilling of foraminifers is commonly present in the muddy sediments, no examples were observed in thin section. Concretions lower in the section (e.g., Core 317-U1353B-88X) are similar to those found at Site U1351, being bioturbated, matrix rich, sandy, slightly fossiliferous marlstone.

Sand occurs sporadically throughout Unit II as a minor component and consists of very dark greenish gray shelly muddy fine sand, often with calcareous concretions as large as 4 cm in diameter. They form beds as thick as 15 cm.

Shell-hash units that occur at the tops of cores are thought to represent downhole cave-in, as noted in Unit I. These layers occur commonly as deep as Core 317-U1353B-60H but are less common in deeper intervals.

All of the minerals observed in Unit I are present in Unit II, but hornblende and green ferromagnesian mineral concentrations are substantially lower in the uppermost part of Unit II and are not observed below 365 m in XRD data (Figs. F9, F10). Micrite occurs in many samples below Core 317-U1353B-29H, and well-crystallized rhombs of carbonate are very common in occasional samples (e.g., Cores 47H and 76X).

Correlation with wireline logs

Gamma ray and resistivity logs were acquired in Hole U1353C, a designated logging hole, prior to hole abandonment. Caliper data from Hole U1353C indicate that the hole was enlarged, but the caliper made contact throughout the entire section and the logging data are likely of acceptable quality (see "Downhole logging").

The computed gamma ray (CGR) log from Hole U1353C was used to examine downhole changes in lithology and was compared with the CGR log from Hole U1351B (Fig. F13). As noted in the "Site U1351" chapter, gamma ray values for Canterbury Basin sites are likely influenced by changes in both clay mineral content and bulk mineralogy (e.g., feldspar, mica, and chlorite) content. As in Hole U1351B, sandy layers of meter-scale thickness described in the cored interval of Hole U1353B correspond to low peaks in CGR (e.g., Sections 317-U1353B-28H-1, 130 cm, to 28H-2, 150 cm), indicating that low gamma ray values (low clay mineral content) can be used as a proxy for coarser grained sediment.

Within the uppermost 100–250 m in Hole U1353C (~100–300 m in Hole U1351B), large-scale increases and decreases in gamma ray values are interpreted as broad-scale fining- and coarsening-upward intervals (Fig. F13). The variable character of the gamma ray log in this interval is similar to that seen in Hole U1351B, with high-amplitude (~70 gAPI) swings over <10 m vertical distances. This variability can likely be attributed to textural changes (probably layer thickness of intercalated sand and mud) and is consistent with the heterolithic nature of Unit I.

Although the uppermost 250 m in Hole U1353C has a variable gamma log over relatively short vertical distances, the interval below has more gradual trends that are very similar to those in Hole U1351B below 300 m (equivalent to lithologic Unit II at both sites). The characteristics of the gamma ray log are consistent with the limited lithologic variability of Unit II in adjacent Hole U1353B. A number of distinctive increasing–decreasing (likely fining–coarsening) trends in gamma ray values (e.g., 370–480 m in Hole U1353C) can be correlated between the two sites, which suggests that both sites experienced comparable large-scale changes in sedimentation influenced by eustasy, climate, or sediment supply. Notable differences between the two sites within this depth interval include the lack in Hole U1353C of a slight overall fining-upward trend observed between 300 and 550 m in Hole U1351B and generally lower mean gamma ray values in Hole U1353C (~50 gAPI versus ~60 gAPI in Hole U1351B), which likely indicate lower clay mineral contents at the more proximal shallow-water location of Site U1353.

Downhole trends in sediment composition and mineralogy

Mineralogical and compositional trends are gradational from Unit I to Unit II, but each unit has a notably different character. Unit I mineralogy is highly variable depending on lithology, which is consistent with the heterolithic nature of the unit. Quartz, calcite, total clays, micas (muscovite/biotite), chlorite, plagioclase, alkali feldspars, hornblende, and pyrite occur throughout Unit I, but their concentrations are quite variable (Figs. F9, F10). Dolomite and siderite were occasionally observed, but peak intensities are close to the limit of detection (see XRD in "Supplementary material"). Siliceous bioclast concentrations do not exceed 1%.

Although no discernible downhole trends in mineralogy are apparent in Unit I, recognizable changes in mineral content occur in Unit II below 260 m. Quartz and total clay content are generally inversely correlated, with the lowest quartz (highest clay) content between ~151 and 260 m. Total clay content based on XRD analyses is highest in homogeneous mud, especially in the uppermost 50 m of this unit. Quartz peak intensities are highest in cores that are predominantly very fine, well-sorted sandy mud and muddy sand. Calcite is again inversely correlated with total clays and quartz, and abundance is highest in shell-rich muddy sand and muddy sandstone. As at other sites, the depth trends in calcite follow smear slide and bulk CaCO₃ concentrations. Micas are highly correlated with each other and generally follow the same depth trend as plagioclase.

A notable decrease in clay and an increase in quartz and sheet silicates (especially chlorite) occurs from the top of the unit to ~260 m, below which the mean values remain relatively constant, with variance that is less than that observed in Unit I. An abrupt change in mineralogy, as observed in smear slides and by XRD analyses, occurs at ~260 m between Cores 317-U1353B-61X and 62X, the point at which layers with high carbonate concentrations no longer occur (apart from outliers, which are all related to diagenetic effects—e.g., rhombs observed in Core 317-U1353B-76X and concretions). In addition to low carbonate concentrations, the sediments below this interval, with the exception of lithified intervals, also have extremely low ferromagnesian mineral concentrations as well as higher dense mineral concentrations. Nannofossil and planktonic foraminifer abundances also are extremely rare below this depth (see "Biostratigraphy").

Description of lithologic surfaces and associated sediment facies

The description of lithologic surfaces and their associated packages as Types A, B, and C follows that of Site U1352 (see "Lithostratigraphy" in the "Site U1352" chapter). Because of time restrictions on board ship, surfaces were only examined close to the predicted depths of seismic sequence boundaries; therefore, the lithologic surfaces identified here are implicitly linked to the predicted occurrences of sequence boundaries identified on the seismic (Lu and Fulthorpe, 2004). A similar approach was used on board ODP Legs 150 and 174A, the objectives of which were also the study of sea level changes. Postcruise study will attempt to clarify the exact relationship of all lithologic surfaces and facies associations to sea level changes and seismic stratigraphy. The numbering system used in the site chapters, tables, and summary diagrams comprises a hole-specific prefix and a surface designation (e.g., U1353A-S1) that links each surface to a seismic sequence boundary; therefore, these lithologic surfaces and associated sediments are thought to be correlative between sites across the transect. The lithologic surfaces and facies associations identified at this site are designated U1353A-S1 and U1353A-S2 and U1353B-S1 to U1353B-S10 (Table T2).

Surfaces U1353A-S1 and U1353B-S1

Both recovered intervals show a ~4 m thick, very dark gray, micaceous, well-sorted very fine grained sand bed. The sand is present from Sections 317-U1353A-3H-4 through 3H-6 (13.80–18.05 m) and from Sections 317-U1353B-2H-4, 45 cm, through 2H-7 (12.95–16.79 m). Surfaces U1353A-S1 and U1353B-S1 are tentatively placed at the top of the sand. Surfaces U1353A-S1 and U1353B-S1 separate the sand from the dark greenish gray calcareous sandy mud above, comprising a Type A contact and associated facies. The sediment beneath the sand in both holes is gray mud with rare shells and shell fragments.

Surfaces U1353A-S2 and U1353B-S2

Holes U1353A and U1353B contain 6 and 8 m thick fine sand beds of Type A, respectively. The intervals are Sections 317-U1353A-5H-2 through 5H-CC (27.97–34.15 m) and 317-U1353B-4H-1, 95 cm, through 5H-6 (26.95–35.58 m). The upper contact of the sand bed is sharp in Hole U1353B, but the lower contact was not recovered in either hole. Surfaces U1353A-S2 and U1353B-S2 are tentatively placed at the top of the sand at ~27 m (Table T2). The sand is similar to that associated with U1351A-S1 and U1351B-S1, but the sediment above contains abundant shells (Chlamys delicatula and Stirocolpus) and broken bivalves and gastropods. The gray mud beneath contains rare shells and shell fragments (mostly Stirocolpus).

Surface U1353B-S3

Surface U1353B-S3 is positioned at Section 317-U1353B-9H-3, 88 cm (60.68 m), between green calcareous mud and gray mud. The sediments beneath in Cores 9H and 10H (62–67.56 m) are also typical of a Type A contact and facies association. The repetitive stratal relations (gray mud beneath green calcareous shelly mud) found in Cores 9H and 10H are similar to those found in Hole U1351B, where several depositional events are present within a 10 m interval.

Surface U1353B-S6

Surface U1353B-S6 is possibly located at Section 317-U1353B-23H-2, 140 cm (133.10 m), at the base of a series of shelly mud beds with gradational upper and lower contacts. This identification is largely motivated by biostratigraphic results that indicate a hiatus in Core 23H between the Pleistocene (1.69–1.73 Ma) and Pliocene (>2.78 Ma) between 121.16 and 135.71 m (see "Biostratigraphy").

Surface U1353B-S7

A typical Type A contact and facies association defines U1353B-S7 at Section 317-U1353B-28H-2, 40 cm (151.3 m). This is the Unit I/II boundary (Fig. F11A).

Surfaces U1353B-S8 and U1353B-S9

These surfaces are positioned at the base of a sandy marl bed (Section 317-U1353B-34H-2, 50 cm [179.0 m]) and a slump and shelly mud interval (base of Section 41H-CC [202.84 m]). Downhole gamma ray logs contribute to this interpretation by revealing a dramatic increase from 20 to 65 counts per second (cps) at 180 m that correlates to a decrease in the resistivity logs and a similar increase at 205 m (see "Downhole logging"). These trends suggest that the surfaces occur near a transition between sandy and muddy lithologies and potentially represent either a Type A or B contact and facies association.

Surface U1353B-S10

This surface is tentatively positioned at 252.6 m at the top of gray sandy mud that grades upward to a shelly muddy sand bed (Section 317-U1353B-57H-1, 100 cm [252.6 m]). This designation is largely motivated by biostratigraphic results that indicate a potential highstand in this interval. A sand bed is also present in Section 56H-1 (250.2–251.6 m), which contains abundant fragments of bivalves, scaphopods, and barnacles. This interval is unusual in that it also contains a graywacke pebble and large fragments of Crassostrea?.

Discussion and interpretation

Interpretation of Unit I

Unit I represents a heterolithic assemblage that was likely influenced by fluctuating sea level during deposition. The Type 1 and Type 2 cycles, as described in "Lithostratigraphy" in the "Site U1351" chapter, may also exist at Site U1353 but are not as prominent. Such cycles occur in the uppermost 30 m of Holes U1353A and U1353B, but poor recovery below hampers further definition of them below this depth.

The dominant homogeneous mud is interpreted as a shelf facies. Given its uniform nature and lack of sedimentary structures, it is not easy to interpret purely from a sedimentologic perspective. The macrofauna from these muds suggests deposition in fluctuating inner to outer shelf settings. Benthic foraminifers indicate that water depth fluctuated throughout deposition, ranging from subtidal to outer shelf (see "Biostratigraphy"). The muddy character suggests that the site was largely removed from major sources of sediment. Interbedded gray clay beds may represent episodic fluxes of fine-grained sediment derived from fluvial or glacial discharges. These conclusions are similar to our interpretation of the muds of Unit I at Site U1351 (see "Lithostratigraphy" in the "Site U1351" chapter).

Similarly, the sandy intervals probably represent transgressive or regressive periods. The green shelly mud may represent periods of late highstand and regression, when more calcareous sediment was deposited and perhaps when terrigenous sediment diminished. The gray sandy intervals are not particularly well recovered, and their top and bottom contact relationships are not preserved in core, being either soupy or nonrecovered. They contain scattered shells, are well sorted, and have a composition consistent with derivation from Torlesse graywacke. We suggest that they represent either transgressive shoreface deposits or sediment deposited immediately after the transgression period (see Browne and Naish, 2003). Shell-hash intervals were likely formed in a shoreface setting through to beach and were probably also a transgressive shoreface deposit.

Interpretation of Unit I/II boundary

The three boundary options (A–C) described in "Unit I/II boundary" may be equivalent to some of the four boundary options described in "Lithostratigraphy" in the "Site U1351" chapter; however, our ability to further test this hypothesis is limited because of poor core recovery at Site U1353.

Smear slide and coulometry data show a peak in carbonate percentage from 140 to 150 m associated with the sediments overlying the Unit I/II boundary. In Unit I, homogeneous gray mud has a variable carbonate composition, but below the boundary the mud only rarely contains calcareous material. The variability in carbonate content in the uppermost 100 m of the succession (Figs. F9, F10) is associated with other lithologies, such as greenish gray sand or sandy mud or shell beds. Smear slide and XRD data also indicate a reduction in ferromagnesian minerals at ~152 m (Figs. F9, F10).

Placing the boundary at Section 317-U1353B-28H-2, 46 cm (151.36 m), would be consistent with a similar occurrence of clay beds below the Unit I boundary and within Unit II at Site U1351 (see "Lithostratigraphy" in the "Site U1351" chapter).

Interpretation of Unit II

Poor core recovery and moderate to intense drilling disturbance in Unit II at Site U1353 precludes detailed interpretation of its generally texturally uniform muddy character. Lithologies other than muds that would provide useful information regarding environments of deposition were not recovered. In addition, the generally poor preservation of fossils in Unit II (foraminifers and nannofossils) hinders detailed depositional interpretation. Planktonic foraminifers are absent below Core 317-U1353B-60X. Benthic foraminifers show a generally shallow but variable subtidal to outer shelf marine character (see "Biostratigraphy"). A trend like that in Unit II in Hole U1351B, where planktonic foraminifers show a downhole increase in abundance and a change from a continental shelf environment to a more open ocean continental slope setting, was not observed in planktonic foraminifers at Site U1353 (see "Biostratigraphy"). Nannofossils are not present in many samples (especially between Cores 317-U1353B-89X and 97X, with the exception of Core 90X). Macrofaunas indicate a range of inner shelf to outer shelf depositional settings.

These observations, together with the general mud-rich nature of the unit, suggest a relatively quiet depositional setting on a shelf below a fair-weather wave base. The lack of abundant carbonaceous debris may suggest that the setting was more likely a middle to outer shelf setting removed from major sources of sediment supply. Similar conditions occur on the modern shelf, where much of the Holocene sediment is palimpsest with little terrigenous sediment reaching the middle to outer portions of the shelf (Herzer, 1981). The relatively consistent mineralogy for this unit attests to sediment supply from a single source. Infrequent sand beds and deviations in mineralogy, as well as shelly layers within the mud and sandy mud, may represent occasional storm deposition or derivation of sand from a coastal source (beach or river). The presence of rare graywacke pebbles may also represent storm-related offshore transport of coarse material.

The gross lithology of Unit II is similar to that observed in the Tokama Siltstone on land (Carter, 2007), in Unit II at other sites, and at ODP Site 1119 and likely represents an equivalent sedimentary package. Texturally, the generally uniform nature of the sandy mud and mud lithologies in Unit II is similar to the mud/mudstone of the above-mentioned units.

We suggest that Unit II at Site U1353 represents an extraneritic and/or outer shelf to uppermost bathyal setting based largely on planktonic foraminifers. Equivalent units at Site U1351 were interpreted to have been deposited from contour currents derived from the south. Site U1353 is landward of Site U1351 and would have been in a shallower setting, but it may also have been influenced by similar northward-flowing contour parallel currents in the form of sediment drifts. However, Unit II at Site U1353 had a greater sediment input from source areas to the west, as indicated by its sandier content, discrete shelly layers, and rare graywacke pebble clasts.

Significant hiatuses are recognizable from fossil group ages (see "Biostratigraphy"), but these were not often recovered in the core, implying considerable periods of erosion and missing sections that were likely caused by sea level and/or tectonic fluctuations that affected the shelf environment.

Interpretation of lithologic surfaces and associated sediment facies

Surfaces U1353A-S1 and U1353A-S2 and U1353B-S1 to U1353B-S4

These surfaces are well-defined Type A and Type B contacts and associated facies and are tentatively correlated to seismic sequence boundaries U19, U18, U17, and U16 (Table T2). The upper two surfaces in both holes, correlated to U19 and U18, are associated with thick fine sand beds that are continuous in both holes. Similar well-sorted, fine gray sand is associated with U1351B-S2 and correlated to U18 at outer shelf Site U1351. However, the inner shelf sand beds of Site U1353 are thicker (up to 8 m) than those of the outer shelf. Surface U1353B-S3 is recognized by a sharp basal contact at 63 m and correlates with the predicted depth of U17 at 59 m. Poor recovery (Core 317-U1353B-13H included only a core catcher sample and Core 15H had no recovery) prevents lithologic correlation with U16 (72 m). This boundary may occur at the base of a calcareous very fine sandy mud at 80.8 m (Section 317-U1353B-14X-CC). Biostratigraphy shows a hiatus of ~0.8 m.y. from 0.5 to 1.26 Ma between Sections 12X-CC and 14X-CC, which is consistent with U16 near this interval.

Surfaces U1353B-S6 to U1353B-S10

Surface U1353B-S6 is tentatively correlated with U13, which has a predicted depth of 112 m, and U1353B-S7 is tentatively correlated with U12, which has a predicted depth of 151 m. A correlation to U11 cannot be established because of a no-recovery interval at its predicted depth of 171 m. Surfaces U1353B-S8 and U1353B-S9 are tentatively correlated to U10 and U9, which have predicted depths of 192 and 241 m, respectively. However, because of core recovery, sediments and surfaces below 150 m are questionable. Surface U1353B-S10 is tentatively correlated to U8, which has a predicted depth of 290 m (Fig. F14).

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