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Two holes were cored at outer shelf Site U1351. Hole U1351A was drilled to 28 m core depth below seafloor (CSF-A; unless otherwise noted, all depths in this section are reported in m CSF-A), and Hole U1351B was drilled to 1031 m. Holes U1351A and U1351B were APC cored to 95 m. The XCB was then used to drill to total depth, with occasional use of the APC in attempts to improve core recovery.

Core recovery in lithologic Unit I in Hole U1351B was moderate (average = 62%); recovery was higher with APC drilling and lower with XCB drilling (86% and 48%, respectively). Recovery in Unit II was very poor, averaging ~19% (Figs. F3, F4).

Although the sedimentary succession at Site U1351 can be divided into two lithostratigraphic end-member units based on lithology and sedimentary structures, the exact boundary location is uncertain (see "Unit I/II boundary," below and Fig. F5) and lies somewhere within the interval between Cores 317-U1351B-30X and 36X (247–300 m). We tentatively place the unit boundary at the single lithologic boundary recovered within this depth range, at Section 317-U1351B-31X-4, 68 cm (262 m). The transitional interval also contains the break between logging Units 1 and 2 (see "Downhole logging") and roughly corresponds to a change in macro- and microfauna, indicating a shift from inner-middle to middle shelf settings (see "Biostratigraphy"). Unit I is Holocene–Pleistocene to early Pliocene in age, whereas Unit II is early Pliocene to late Miocene in age (Table T2).

Cores recovered from Holes U1351A and U1351B show a fairly gradual downhole transition from a heterolithic section with abrupt contacts at the top to a more featureless sedimentary section at the base, suggesting progressive changes in sedimentary styles as the margin evolved (Figs. F3, F4). This gradational change is also reflected in sediment physical properties (see "Physical properties").

Description of lithologic units

Unit I

  • Intervals: Cores 317-U1351A-1H through 6H and 317-U1351B-1H through Section 317-U1351B-31X-4, 70 cm

  • Depths: Hole U1351A: 0–28 m (total depth) and Hole U1351B: 0–262.0 m

  • Age: Holocene–Pleistocene

Holes U1351A and U1351B are offset ~20 m from each other (Figs. F6, F7) and exhibit slightly different stratigraphies (e.g., slump in Core 317-U1351A-4H; Fig. F8). The following lithologic summary of Unit I is largely based on Hole U1351B because a significant number of whole-round samples were taken from Hole U1351A for geochemical, microbiological, and geotechnical analyses and thus were not available for sedimentologic description. Although this resulted in a less complete stratigraphy in Hole U1351A, key surfaces and associated sediments (abrupt-based sand beds) allow for correlation between holes (dashed lines in Fig. F6). These key lithologic surfaces are designated U1351A-S1, U1351A-S2, U1351B-S1, and U1351B-S2 and are more fully described in "Description of lithologic surfaces and associated sediment facies."

Drilling disturbance increased downhole with the XCB method, becoming more prominent in Cores 317-U1351B-21X and below, where pronounced biscuiting was observed (Fig. F9). In many instances, intervals of shell hash or sand at the tops of cores were considered to represent downhole contamination by caved sediment (Fig. F10). This interpretation is supported by caliper logs that show the upper section of Hole U1351B to be washed out (see "Downhole logging"). In addition, intervals of suspected shell-hash cavings are also commonly associated with anomalously high magnetic remanence values (see "Paleomagnetism").

Lithologies include dark gray and dark greenish gray to olive-gray fossiliferous mud and sandy mud, dark gray very fine to medium well-sorted sand, olive-gray muddy sand, gray mud (high clay content), and shell hash. Bioclastic material includes gastropods, bivalves, barnacle plates, and bryozoans, which in places form centimeter-thick shelly beds. These lithologies are illustrated in Figures F11, F12, and F13.

As partly observed with a hand lens and verified by smear slide analysis (see "Site U1351 core descriptions"), sand- and silt-sized grains are dominated by quartz and feldspar, with common to rare mica (biotite and muscovite), chlorite, ferromagnesian minerals (various amphiboles), other dense minerals (zircon, clinozoisite, epidote, tourmaline, and others), and glauconite. Clay minerals are also present; however, the clay-sized fraction contains not only clay but a significant proportion of nonclay mineral grains. Rock fragments common in the sand-sized fraction are mainly low-grade metamorphic fragments (e.g., phyllite/slate or semi-opaque, unidentifiable polycrystalline fragments; see Fig. F14). Authigenic minerals are mainly opaque minerals (e.g., pyrite occurring as framboids and as infill in microfossils) and carbonate. A variable biogenic component consisting of bioclasts (mollusk and barnacle fragments) as well as more readily distinguishable foraminifers (whole and fragmented), echinoderm spicules and plate fragments, calcareous and siliceous spicules, and bryozoa fragments is also present. The biogenic silt- to clay-sized fraction includes nannofossils and other spicules.

The assemblage of mineral components identified visually (e.g., quartz, feldspar, mica, chlorite, clay minerals, carbonate, pyrite, and the amphibole and epidote group minerals) compares well with data from X-ray diffraction (XRD) analysis (see XRD in "Supplementary material"). XRD data indicate that the main carbonate mineral is calcite and the main feldspar is plagioclase, with lesser orthoclase and microcline. Unfortunately, because of peak interference with the sheet silicates, it was not possible to conclusively identify glauconite in the XRD analysis.

Bedding contacts are locally abrupt to gradational, and there are a few decimeter- to meter-thick examples of coarsening-upward or fining-upward beds and color banding. Lamination and convolute bedding are rare (Figs. F8, F13). Bioturbation is common in finer grained units (ichnofabric index of 1–5) and includes discrete burrows of Chondrites and Thalassinoides. Thalassinoides burrows tend to be associated with abrupt contacts and extend as far as 50 cm below these contacts (Fig. F7).

Diagenetic changes in the lower part of Unit I are indicated by authigenic carbonate cements and limestone concretions in Cores 317-U1351B-19X, 22X, 25X, and 30X. Thin section analysis of glauconitic limestones (cemented shell-hash beds) from the core catchers of Cores 317-U1351B-19X (147 m) and 22X (175 m) shows these concretions to similarly consist of gravel-sized barnacle, bryozoa, brachiopod, bivalve, and echinoderm fragments (Fig. F15). One sample exhibits a micritic matrix; the other, more porous, sample is partly cemented by carbonate microspar. Pores within fossils have also been filled with sparry calcite. Some sand- and silt-sized quartz, feldspar, and accessory grains are also present. The moderate (10%–15%) glauconite content results mainly from the epigenetic growth of glauconite within fossil void spaces, although a few "grains" of glauconite may be allochthonous.

Within the uppermost part of Unit I, two dominant facies assemblages recur (Figs. F16, F17). The first facies assemblage (Type 1; ~5 examples in uppermost 30 m of both Holes U1351A and U1351B, as indicated in Fig. F16A) is several meters thick and consists mainly of bioturbated, very fine sandy mud (silt-dominated) with scattered common shell fragments of gastropods and bivalves (Tawera). The base of the assemblage is rich in shell fragments and contains centimeter- to decimeter-thick shell-hash beds mixed with siliciclastic materials (fine to medium sand) that fine upward into overlying sandy silt at the top of the assemblage (Fig. F17). The boundaries of each assemblage range from sharp (e.g., Core 317-U1351B-3H) to gradual (e.g., Sections 317-U1351B-4H-2 through 4H-4). However, each boundary is characterized by a concentration of shell fragments above the contact and sometimes by abundant burrows filled with shell fragments below the contact. A distinct color change from greenish gray in the shelly sandy mud above the sharp boundary grading into gray sandy mud with extensive burrowing beneath the boundary was observed only in Cores 317-U1351B-1H and 2H.

The second facies assemblage (Type 2) characterizes the middle (30–170 m) of Unit I. This facies assemblage can be as thick as ~20 m (much thicker than Type 1; see Figs. F16, F17). Examples of the Type 2 assemblage are best defined in intervals 317-U1351B-5H-1, 120 cm, through 8H-4, 5 cm (29.4–52.25 m); 8H-4, 5 cm, through 11H-2, 113 cm (52.25–72.33 m); and 19X-2, 103 cm, through 21X-CC, 26 cm (144.13–168.36 m). This assemblage consists of a greenish gray (calcareous), shelly, bioturbated mud with scattered common shell fragments of gastropods and bivalves overlying a sharp contact (Fig. F12). In Core 317-U1351B-8H, a few meter-thick shelly intervals also exist above the lower boundary. This shelly facies is followed by a transition uphole into a dark gray, micaceous very fine sandy mud that contains alternating color bands of light and dark gray clay beds (e.g., Fig. F13C) with diffuse to sharp contacts and slight upward-coarsening into interbedded very fine sand and mud. The dark gray mud is homogeneous and contains very rare shell fragments (e.g., Fig. F13B). In Core 317-U1351B-5H, this facies assemblage also contains in its uppermost interval a 2 m thick, gray, very well sorted quartz-rich very fine sand interval (Fig. F12A). Note that an alternative interpretation places this sand at the base of a Type 1 facies assemblage (Fig. F6; see "Interpretation of Unit I").

In contrast to the heterolithic middle and upper part of Unit I, a monotonous repetition of two lithologies characterizes the base of Unit I (interval 317-U1351B-27X through 31X [~218–264 m]): (1) very dark gray homogeneous clay-rich mud without shell fragments and (2) dark greenish gray sandy shelly mud (e.g., Fig. F13A). The abundant shell material in the sandy mud is dominated by barnacle plates. Contacts between lithologies are more gradational and do not appear distinctly erosional like those in the upper part of Unit I, nor are they heavily bioturbated. Furthermore, the shelly sandy mud intervals tend to be thicker (1–3.5 m) than the intervening clay-rich mud intervals (0.5–1.0 m).

Unit I/II boundary

The Unit I/II boundary is not defined at a particular point in the succession because imperfect core recovery hampered interpretation of both the exact location of the unit boundary and its nature (gradational or abrupt). Each of the two units has a distinct lithology, and the transition interval is defined as occurring between 247 and 300 m. A range of options for the location of the Unit I/II boundary is presented in "Discussion and interpretation" (Figs. F5, F18, F19). The only lithologic contact recovered in this interval (at Section 317-U1351B-31X-4, 70 cm [262.0 m]) was selected as the unit boundary for the purpose of unit description.

Unit II

  • Interval: Sections 317-U1351B-31X-4, 70 cm, to 116X-CC, 44 cm

  • Depth: 262.0–1024.4 m

  • Age: early Pliocene to late Miocene

The main lithologies of Unit II are dark greenish gray very fine sandy mud and greenish black to dark greenish gray mud (Fig. F20), both of which may be shell bearing. A less abundant lithology is dark greenish gray very fine muddy sand. Millimeter- to centimeter-thick dark greenish gray, well-sorted very fine to fine sand occurs sporadically throughout the unit. Shells are common in the upper portion of the unit and include bivalves, gastropods, echinoid spines, and barnacle plates, but they become less frequent below Core 317-U1351B-40X and are absent below Core 101X. Many shells are fragments or partially altered fragments. Shell-hash units that occur at the tops of cores are thought to represent material derived from downhole cave-in, as also noted in Unit I. Bioturbation is not common and rarely reaches an ichnofabric index of 5; it is absent below Core 317-U1351B-113X. No distinct sedimentary structures were observed, but in higher recovery zones such structures could be masked by the locally intense biscuiting observed throughout this generally poorly recovered (recovery = ~18%) unit (Fig. F9). The terrigenous and biogenic components noted in smear slides and in XRD analyses from Unit II are similar to those listed above in the Unit I description. One exception is the presence of minor amounts of authigenic zeolite(?) cements in Core 317-U1351B-111X and deeper. The proportion of minerals and components differs slightly between units, as discussed in the following section.

More indurated intervals of clayey or sandy mud are present in many core catchers in the lower part of the hole, where drilling disturbance noticeably increased (Table T3; Fig. F21E, F21F). This induration is drilling induced and related to high temperatures generated by frictional grinding when the cutting shoe in the XCB was not properly cooled because of clogged ports in the bit (cores smelled "baked" upon splitting). However, other indurated intervals within the less indurated host sediment are in situ and a product of incipient to well-developed carbonate cementation, as shown by the presence of authigenic carbonate in smear slides and thin sections (Table T3; Fig. F21A–F21D). Such cemented intervals, defined as "nodules" when smaller than the core diameter and "concretions" when wider (truncated) than the core diameter, are present throughout the unit, particularly in Core 317-U1351B-87X and below. Petrographic examination of seven of these cemented rocks showed them to be carbonate-cemented silty sandstone to sandy marlstone. They are generally bioturbated, quartzo-feldspathic, and micaceous but variably fossiliferous and matrix rich.

No hard lithologies were recovered in the low (15%) recovery zone in Unit II between Cores 317-U1351B-42X and 87X (352–745 m), whereas the higher (27%) recovery zone between Cores 317-U1351B-87X and 116X (745–1024 m) contains more hard lithologies, which could have potentially caused poor recovery. Therefore, the low recovery rate between Cores 317-U1351B-42X and 87X must be attributed to this interval's lithology rather than to the presence of cemented or indurated beds.

This thick unit includes some gradual lithologic changes (Fig. F4). For example, the lowermost part of Unit II comprises slightly more calcareous sandy mud and is generally greenish gray in color (Fig. F20).

Downhole trends in sediment composition and mineralogy

In general, Site U1351 mineralogy and composition is fairly consistent downhole, with the relative abundances of most minerals apparently varying according to sediment texture (Fig. F22). Unit I mineralogy consists of quartz, total clays, micas (muscovite/biotite), chlorite, plagioclase, K-feldspar, and pyrite. Dolomite and siderite were occasionally observed, but peak intensities are close to the limit of detection (see XRD in "Supplementary material"). The relative intensity of the calcite peak correlates (R = 0.7, = 47) with the total calcium carbonate concentration determined by coulometry (with the exception of seven outliers, all in the uppermost 250 m). This indicates that calcite is the dominant carbonate mineral. The outliers likely represent heterogeneities in carbonate shell content between the samples used for each analysis. Overall, the depth trends in calcite peak intensity follow smear slide and bulk CaCO3 concentrations (Figs. F22, F23). There is a notable downhole decrease in the occurrence of hornblende, with a 50% decrease below ~90 m. Dense mineral percentages, as identified in smear slides, appear to be more variable (Fig. F23). The diffraction peaks for pyrite are close to the limit of detection, and the maximum intensities of these peaks show no particular downhole trend. This trend matches smear slide observations, which show fluctuating opaque mineral concentrations that lack a trend with depth. Siliceous bioclast concentrations did not exceed 1%, except for a few isolated samples in the uppermost 50 m (see "Site U1351 core descriptions").

The composition and mineralogy of Unit II is similar to that of Unit I, but several changes are worth noting. In addition to the less variable bulk carbonate percentage trends from smear slides and coulometry data seen in Unit II, the most useful trend for differentiating units appears to be the percentages of clay and, conversely, quartz/feldspar and glauconite. Glauconite content is highest in Unit I and lowest in Unit II, whereas clay and quartz/feldspar percentages are extremely variable in Unit I and much less so in Unit II. XRD peak intensities of quartz and plagioclase (dominantly albite) increase downhole and are significantly higher (p = 0.05, N = 60) below the Unit I/II boundary, possibly corresponding to an increase in very fine sandy mud in Unit II. Note that in the context of this discussion the clay percentage is a smear slide estimate of the clay-sized fraction (<4 µm), which probably contains a significant proportion of clay-sized quartz and other detrital grains, as suggested by thermal conductivity data (see "Heat flow").

Correlation with wireline logs

The triple combo tool string (measuring gamma ray, porosity, density, and resistivity) was run in Hole U1351B from the seafloor to the bottom of the hole at 1032 m WSF. The FMS-sonic tool string (measuring microresistivity and sonic velocity) was run from ~80 to ~486 m WSF. In addition, gamma ray and resistivity logs were acquired from 84 to ~782 m WSF in Hole U1351C (see "Downhole logging"). Three logging units were identified at Site U1351. The boundary between logging Units 1 and 2 (at 260 m wireline log matched depth below seafloor [WMSF]) corresponds within the error of core-log depth matching to the lithologic Unit I/II boundary. Although the caliper log from Hole U1351B indicates a large-diameter hole in the uppermost 600 m, comparison of wireline logs with measurements from cores from equivalent depths indicates that the gamma ray, resistivity, and density logs were not seriously affected by the enlarged hole (see "Downhole logging").

Gamma ray logs from siliciclastic settings are primarily interpreted as delineating relative changes in the amount of fine-grained sediments (clay) and thus grain size (Doveton, 1994). However, in settings such as the Canterbury Basin that have an appreciable amount of nonclay minerals in the clay-sized fraction (e.g., rock flour), increased mica and feldspar content in the silt- to sand-sized fraction, and a mixture of siliciclastic and carbonate lithologies, the downhole trends in absolute gamma ray values represent changes not only in grain size but in lithology and mineralogy. For example, low gamma ray values may be caused by carbonate-rich (coarse or fine) intervals. In addition to reflecting an increase in clay content, an increase in gamma ray values could also stem from an increase in the mica, feldspar, or dense mineral content of sands, or it may simply reflect a decrease in rock flour in the mud-sized fraction.

Standard gamma ray (SGR) and computed gamma ray (CGR) logs (Figs. F5, F18, F19) were used to examine downhole changes in lithology. SGR is the total gamma ray count from all sources, whereas CGR reflects the subtraction of the uranium contribution from the SGR (Doveton, 1994). Sandy layers of meter- and submeter-scale thickness described in the cored interval of Hole U1351B correspond to low peaks in CGR, suggesting that low gamma ray values can be used as a proxy for a coarse-grained lithology (see "Downhole logging"). However, large-scale trends of increasing gamma ray values are possibly caused by both textural and compositional changes, as discussed above and elaborated below.

The gamma ray log varies within the uppermost 300 m WMSF, having high-amplitude (~70 gAPI) swings over <10 m vertical distances. The higher amplitude swings in SGR are reflected in the CGR logs, supporting the interpretation that the variation in gamma radiation over this interval results largely from textural changes, likely intercalated sand and mud, consistent with the heterolithic record of recovered sediments from lithologic Unit I. Above 200 m WMSF within Unit I, considerable variability in the logging data is apparent in what are interpreted as alternating sand and mud intervals. The interval between 250 and 300 m WMSF reveals an overall upsection decrease in CGR, likely reflecting a coarsening-upward trend. This log can be interpreted to reflect more lithologic variability, similar to the more heterolithic log character of Unit I.

Although the uppermost 300 m WMSF has a variable gamma ray log over relatively short vertical distances, the interval below has more gradual trends. From 300 to 530 m WMSF, there is a steadily increasing upsection trend in the SGR record. This trend is evident in the CGR record as well, but to a lesser extent. This suggests that the increase in total gamma ray intensity over this interval is due to both textural and compositional change. An increase in total gamma ray units between 530 and 560 m WMSF corresponds to an increase in total clays seen in XRD analysis, suggesting that this interval has higher clay content. From 650 m WMSF to total depth, the SGR has interbeds of higher values interpreted as clay-rich beds and relatively thin (<5 m thick), low-value layers interpreted as beds of sand with both fining- and coarsening-upward trends. Where low gamma ray and resistivity peaks co-occur deeper in Hole U1351B, these intervals are interpreted as the result of irregular hole size rather than a change in lithology (see "Downhole logging"). The presence of hard surfaces or features in Cores 317-U1351B-87X through 116X correlates well with positive changes in the gamma ray signature and density values in the lower part of the hole (Table T3; Fig. F21).

Description of lithologic surfaces and associated sediment facies

The testing of sequence stratigraphic concepts (i.e., sequence boundaries, maximum flooding surfaces, and lowstand, transgressive, and highstand systems tracts, as defined by Posamentier et al., 1988) was a major objective of Expedition 317. As a result, the identification and description of lithologic surfaces and their associated sedimentary packages that mark transitions and discontinuities has the potential of providing information about sequence stratigraphy. These lithologic surfaces and their associated sediments are detailed in this section.

Because of time restrictions on board the 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 Ocean Drilling Program (ODP) Legs 150 and 174A, the objectives of which were also to study 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 lithologic surface numbering system used in the site chapters, tables, and summary diagrams comprises a hole-specific prefix and a surface designation (e.g., U1351A-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.

Lithologic surfaces at Site U1351 are labeled U1351A-S1 to U1351A-S2 and U1351B-S1 to U1351B-S8 (Tables T4, T5). Depositional packages were identified and characterized by their lithologic composition (sand, muddy sand, sandy mud, and mud) and the nature of their upper and lower contacts (sharp or gradational). Hole U1351A recovered only the uppermost 28 m, and only two lithologic surfaces were identified. Potential surfaces in poorly recovered or nonrecovered intervals are based on gamma log signatures. In some instances, the basal contacts of these packages are near the predicted depths of seismic sequence boundaries (Lu and Fulthorpe, 2004), allowing for a preliminary and tentative correlation of the lithology to seismically defined sequence boundaries (see "Interpretation of lithologic surfaces and associated sediment facies" and the "Expedition 317 summary" chapter).

Generally, the surfaces and/or discontinuities in Hole U1351A and the uppermost 200 m of Hole U1351B are sharp and separate coarse lithologies above the surface and fine mud beneath (Figs. F11, F12). Each contact is generally extensively bioturbated, and the burrows extend below the contacts by as much as 50 cm. Above the contacts, coarse-grained lithologies are represented by shell-rich mud, sandy mud, and fine-grained, well-sorted quartz-rich sand that contains green ferromagnesian accessory minerals and fines upward.

Surfaces U1351A-S1 and U1351B-S1

Of particular importance is the correlation of two surfaces recovered from Holes U1351A and U1351B, located ~20 m apart. Surfaces U1351A-S1 and U1351B-S1 were recovered at Sections 317-U1351A-3H-3, 77 cm, and 317-U1351B-2H-6, 65 cm, respectively, both at ~16 m (Figs. F6, F7). The lithologic expression of this surface is a sharp basal contact that is heavily burrowed and overlain by a shell bed (<30 cm thick in Hole U1351B).

Surfaces U1351A-S2 and U1351B-S2

A second pair of surfaces (U1351A-S2 and U1351B-S2) was tentatively identified as having been recovered at Sections 317-U1351A-6H-1, 26 cm (22.76 m), and 317-U1351B-5H-3, 3 cm (31.00 m), although the actual U1351A-S2 may not have been recovered. These surfaces may be overprinted and/or masked by drilling disturbance (cave-in) at the tops of Cores 317-U1351A-6H and 317-U1351B-5H (Fig. F6). The tops of the sand beds associated with these surfaces roughly align, and their compositions are similar: very fine grained, well-sorted, quartz-rich sand containing ferromagnesian accessory minerals.

Surfaces U1351B-S3, U1351B-S4, and U1351B-S5

Surface U1351B-S3 at Section 317-U1351B-8H-4, 5 cm (52.25 m), is represented by a sharp basal contact and heavy bioturbation overlain by 5 cm thick muddy gravelly sand.

The sharp boundary at Section 317-U1351B-10H-2, 45 cm (67.27 m), separating dark greenish gray sandy mud with shells from underlying gray shelly sand is described as U1351B-S4 but may be associated with caved material. The underlying shelly sand is 40 cm thick and grades downward into coarse, poorly sorted shelly sand. The bottom portions of intervals 317-U1351B-10H-2, 50–150 cm, and 10H-3, 0–121 cm, which are composed of shelly coarse sand, are also suspected to be caved material.

A 70 cm thick, very fine grained, partly cemented muddy sand was recovered in interval 317-U1531B-12H-1, 50–120 cm (76.2–76.9 m). The sand is similar in character to that recovered above U1351B-S2 in that it is quartz rich, fine grained, and well sorted; however, it is partly cemented and greenish to bluish gray as opposed to the very dark gray color that occurs in Core 317-U1351B-5H. Surface U1351B-S5 is placed at the presumed base of the recovered sand interval. A lithologic break is implied by the gamma ray logs at 87 m WSF.

Surfaces U1351B-S6 and U1351B-S7

Surface U1351B-S6 is a sharp basal contact at Section 317-U1351B-19X-2, 104 cm (144.14 m), between a 1.15 m thick shelly sand bed and an underlying mud with scattered shell fragments. Surface U1351B-S7 is a heavily burrowed, sharp contact separating clayey mud below from a 1 m thick shelly sandy mud above, located at Section 317-U1351B-22X-1, 102 cm (171.4 m).

Surface U1351B-S8

Surface U1351B-S8 is a sharp, heavily bioturbated contact at Section 317-U1351B-30X-5, 80 cm (253.9 m), separating mud from an overlying shelly, muddy, fine sand bed 6.7 m thick. This bed contains a bimodal population of shells, with some shell fragments of millimeter to 1 cm thickness and bivalves that are several centimeters long and have both valves preserved.

Additionally, nodules/concretions and cemented beds that could be potential sources of seismic reflectivity were recovered from Cores 317-U1351B-9H through 116X (Table T3).

Discussion and interpretation

Interpretation of Unit I

Upper to middle part of Unit I (Cores 317-U1351B-1H through 22X)

A depositional environmental interpretation of Unit I is best accomplished in the framework of the facies assemblages (Types 1 and 2) described above, starting with the thicker, more complete Type 2. Starting at a sharp basal contact, the Type 2 facies assemblage contains upward-fining shelly sandy mud transitioning into mud that, in turn, coarsens upward into sandy mud. The formation of a shelly lag deposit, followed by a fining-upward sandy mud just above an erosional contact in the lowermost part of the facies assemblage (Fig. F17), indicates a transgressive systems tract passing up into highstand mud. Subsequent regressive facies deposition, namely the coarsening-upward sandy mud, can be attributed to shoreline progradation during periods of limited accommodation. The diverse facies assemblage of the Type 2 assemblage includes decimeter- to centimeter-thick dark to light gray clay beds (e.g., Fig. F13C). Similar clay-rich beds are observed on modern muddy continental shelves and are formed by rapid deposition following flood events (Suter, 2006; Wheatcroft et al., 1997, 2006). The preservation of such beds and the lack of subsequent bioturbation indicate relatively high sediment accumulation rates and/or frequent (subdecadal) flood events, which are typical near sediment-rich fluvial systems (Bentley et al., 2006). The upper boundary of the Type 2 facies assemblage generally ends at a truncated surface within sandy mud, except in Core 317-U1351B-5H, where the sandy mud grades upward into a very well sorted, quartz-rich very fine sand bed; in this case, the facies assemblage boundary is placed at a truncated surface above the sand, as depicted in Figure F17, rather than at the gradational base of this sand. This sand indicates a possible inner shelf, high-energy coastal environment, suggesting that this part of the facies assemblage may represent the offshore progradation of the coastline.

The components and depositional processes of the Type 1 facies assemblage are similar to those of the lower part of the Type 2 facies assemblage, although they lack the well-developed clay beds discussed above. The stacking pattern of the Type 1 assemblage indicates frequent erosion during marine transgression, indicated by the deposition of shell fragments and bioturbated mud (Fig. F16). This likely represents a transgressive-lag deposit, followed by a maximum flooding surface or the earliest phases of regression (e.g., Clifton, 2006; Suter, 2006; Saul et al., 1999). Both Type 1 and Type 2 assemblages are similar to Pliocene–Pleistocene shelf facies from the Wanganui Basin (Abbott and Carter, 1999; Saul et al., 1999; Abbott et al., 2005; Naish et al., 2005). The greenish gray color indicates more calcareous sand content, as determined by smear slide observations.

Evidence for reworking comes from thin section petrography of the limestone pieces recovered from Cores 317-U1351B-19X and 22X. Many of the rounded fossil fragments from these cores exhibit narrow borings that are generally filled with glauconite, whereas other components (foraminifers, small brachiopod shells, and bryozoa fragments) are never bored (Fig. F15). This may imply that the bored fragments were transported from some distance, whereas the other fragments represent a population originating much closer to the environment of deposition. The few "grains" of glauconite present may also have been transported. Shell beds are likely to be lag deposits, a concentration of material from several different ages, presumably created by a winnowing process of some sort.

Unit I/II boundary

Lower part of Unit I to Unit II transition (Cores 317-U1351B-29X through 36X)

The character of lithofacies assemblages—from more heterogeneous lithologies toward the top of Unit I to interbedded clayey mud and sandy shelly mud at the base—can be related to changing environmental conditions. This change occurs at ~200 m and is primarily revealed by lithology and substantiated by biostratigraphy, gamma ray logs, and seismic reflection profiles. These results reveal downhole changes in margin sedimentation processes and paleoenvironments from an inner middle shelf setting to an outer shelf–upper slope setting.

As mentioned in "Description of lithologic units," a range of options exists for a more precise definition of the Unit I/II boundary, based on both lithologic variations and interpretation of the available downhole gamma logs (Figs. F5, F18, F19). These potential boundary options are as follows:

  • Option A, 247 m: An abrupt increase in downhole total gamma ray values corresponds to a change in lithology in the recovered cores from homogeneous mud at the base of Section 317-U1351B-29X-CC (247.37 m) to muddy sand at the top of Section 30X-1 (247.1 m). Although the actual boundary was not recovered, the abrupt change in gamma ray values suggests a sharp contact.

  • Option B, 262 m: A sharp lithologic boundary occurs at the base of a thick, clay-rich mud (Section 317-U1351B-31X-4, 68 cm). Below this recovered interval, boundaries between green sandy lithologies and gray muddy lithologies are gradational or unclear, whereas clear, sharp, or slightly bioturbated boundaries, as well as a more heterolithic assemblage of lithologies, exist above this interval. This option is used in the descriptions of Units I and II, above.

  • Option C, 264 m WSF: A rapid decrease in downhole gamma readings correlates to the interval from Cores 317-U1351B-31X through 32X, within the depth uncertainty corresponding to a change from a more uniform gamma profile below to a more serrate profile above.

  • Option D, 280 m WSF: The top of an overall increasing trend in CGR and SGR values and the base of a more variable pattern above can be correlated to the interval from Cores 317-U1351B-33X through 34X, within depth scale uncertainties.

  • Option E, 300 m WSF: The top of a trend of gradually increasing SGR and CGR values that change above this point to a more variable log pattern with smaller scale increasing uphole trends can be correlated to the interval from Cores 317-U1351B-35X through 36X, within depth scale uncertainties.

Interpretation of Unit II

Poor core recovery and moderate to intense drilling disturbance in Unit II prevented detailed interpretation of the depositional environment within the generally texturally uniform sandy mud and muddy sand lithologies of Unit II.

The gradual downhole increase of planktonic foraminifers from ~600 m to the base of Unit II and the downhole transition from an inner neritic planktonic and middle shelf benthic foraminiferal assemblage at the top of Unit II to an extraneritic planktonic and outer shelf to uppermost bathyal benthic foraminiferal assemblage at the base of Unit II demonstrate a transition within Unit II from a continental shelf environment to more open ocean, continental slope conditions, as proposed by Lu and Fulthorpe (2004) and Carter (2007). The presence of reworked barnacle plates and bivalve and gastropod shell fragments in the uppermost intervals of Unit II (Cores 317-U1351B-33X through 49X) also attest to a higher energy shelf-slope environment than that which existed during the early stages of Unit II deposition (see "Biostratigraphy").

Site U1351 falls within the Canterbury Drift succession of Carter (2007), which is defined as sediment drifts that underlie the eastern South Island coastal plain, shelf, and slope, ranging in age from latest Oligocene–early Miocene on shore to Holocene on the continental slope. On shore, these strata are exposed as the Tokama Siltstone (Field and Browne, 1989), which is also called the Bluecliffs Formation (Carter, 2007). Offshore, the Canterbury Drift succession is found within Units II–III (0.28–3.92 Ma) of ODP Site 1119 (Carter and Gammon, 2004). By combining textural results from both the Tokama Siltstone and Site 1119, Carter (2007) contended that the Canterbury Drift succession formed by along-strike northward sediment transport. An overall fining-upward succession within the Canterbury Drift succession is hypothesized to result from the progressive increase in sediment transport distance from the source, coupled with a tectonic/climatic overprint that led to an increase in mud deposition over sand. These more proximal and distal equivalents provide additional insight into the depositional environment of Unit II at Site U1351.

The gross lithology of Unit II is similar to that observed at both the Tokama Siltstone and Site 1119, which, in combination with well logging results and biostratigraphy, suggests that Unit II is likely an equivalent sedimentary package. Texturally, the generally uniform nature of the sandy mud and muddy sand lithology of Unit II is similar to the massive silty mudstone of the Tokama Siltstone and Site 1119. The higher calcareous content below 600 m and the increasing occurrence of cemented concretions and nodules at depth are similar to observed lithologies from the Tokama Siltstone. Furthermore, downhole gamma ray logs from Site U1351 reveal an overall fining-upward trend above ~725 m that may be similar to the temporal textural trends observed by Carter (2007) for the Canterbury Drift succession.

Sediment composition and diagenesis

XRD peak intensities of quartz and plagioclase correlate (R = 0.7, N = 60), but there is no correlation between the combination of micas and/or total clays versus quartz and plagioclase (also observed in smear slides), suggesting that different depositional processes control the relative concentration of each mineral component within Site U1351 sediment. The abundant fine clay–sized (<4 µm) nonclay mineral fraction in these sediments can be attributed to onshore glacial processes that likely produced large quantities of glacial flour that were transported offshore by some combination of meltwater discharge, eolian processes, and marine dispersal mechanisms. This fine mineral fraction significantly affects the physical properties of the sediments (see "Physical properties").

Terrigenous minerals (quartz, feldspar, mica, chlorite, amphibole group, and epidote group minerals) and minor lithic components observed in smear slides (mainly silt to fine sand) and thin sections of sandstones from deeper in the hole are consistent with a metamorphic source. There is evidence of grain reworking (e.g., well-rounded grains), so the metamorphic detritus may be partly recycled rather than being only first-cycle material derived from more southerly schist outcrops. Evaluating potential downhole contributions from nonschist lithologies will require more detailed petrologic and mineralogic study.

The downhole decrease in percent amphibole minerals is likely a function of burial dissolution rather than a signal of provenance change because serrate (cockscomb) dissolution textures of ferromagnesian minerals were noted in smear slides. Other downhole trends include an increase in carbonate cementation, first appearing as micrite crystals and later as more pervasive cements. This lithification first affects more calcareous lithologies, and at depth there is local pervasive carbonate cementation of sandy lithologies. The minor amount of zeolite(?) identified in smear slides and thin sections at the bottom of the hole could be linked to an unconformity (long-term exposure surface) or to dissolution of trace amounts of volcanic glass components in the sediment. Unfortunately, the quantities were insufficient for precise zeolite mineral identification with the XRD technique.

Interpretation of lithologic surfaces and associated sediment facies

Previous analyses of seismic records using the EW00-01 seismic reflection grid allowed interpretation of seismic sequence boundaries U19–U4 and their predicted depths and ages (Lu et al., 2003, 2005; Lu and Fulthorpe 2004). These seismic sequence boundaries provide large-scale understanding of the margins' subsurface seismic facies. The lithologic surfaces and their associated sedimentary packages identified are in some instances near the predicted depths of seismic sequence boundaries (Lu and Fulthorpe, 2004), allowing a tentative correlation of lithology to seismically defined surfaces (Tables T4, T5).

Surfaces U1351A-S1 and U1351A-S2 and U1351B-S1 to U1351B-S6

Lithologic surfaces U1351A-S1 and U1351A-S2 and U1351B-S1 to U1351B-S6 and their associated facies are characterized by sharp basal contacts and heavy burrowing beneath (Table T4). Shelly sandy mud is present above the contact for 50 to 100 cm. These lithologic surfaces and their associated packages show pronounced lithologic variability and are easy to identify in the cores. The lithologic surfaces and their associated packages were tentatively correlated with U19–U13. The surfaces and their associated facies developed on the inner to middle shelf, and in some of these packages stratal relations suggest that erosion and/or sediment bypass are associated with the formation of the discontinuity. Additionally, the facies within the upper part of Site U1351 show considerable lithologic variability.

Surfaces U1351B-S7 and U1351B-S8

Surface U1351B-S7 is correlated with U12, which has a predicted depth of 166 m. Surface U1351B-S8 is tentatively correlated with U10, which has a predicted depth of 237 m.

Surfaces deeper than U1351B-S8

Several issues contribute to the lack of lithologic correlation to seismic sequence boundaries in the lower part of Site U1351 (~240–900 m). Primarily, recovery gaps at 308–313, 316–323, 394–400, 611–620, and 890–899 m prevented lithologic correlation to U9, U8, U7, and U6, which have predicted depths of 312, 394, 614, and 895 m, respectively. Additionally, sharp contacts and/or discontinuities are difficult to identify visually in cores from the lower part of Hole U1351B associated with U9–U6. These sediments were deposited when the position of Hole U1351B was on the middle to outer shelf (U8–U9) or slope (U6–U8), resulting in less heterolithic variability compared to the uppermost 240 m, where the position of Site U1351 was progressively farther landward of the shelf break. Finally, many nodules, concretions, and cemented beds that could be potential sources of seismic reflectivity are present in the lower part of Hole U1351B, making identification of lithologic surfaces likely to be the cause of seismic sequence boundaries more complex. Further correlation and identification of lithologic surfaces will form part of postcruise research.