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doi:10.2204/iodp.proc.317.101.2011

Synopsis

The fundamental objective of Expedition 317 was to test the concepts of sequence stratigraphy by documenting the lithologies, ages, and depositional environments within sequences in order to determine the sedimentary processes active during sequence development and to constrain the underlying controls (both global and local) responsible for those processes. This involves testing sequence stratigraphic model (e.g., Van Wagoner et al., 1988; Vail et al., 1991) predictions of facies and lithologies associated with the development of prograding, clinoformal, continental margin sequences that are well defined by seismic interpretations. It also necessitates testing the presumption that global sea level is the primary control on sequence boundary formation (Haq et al., 1987). Other goals were coring the early Oligocene Marshall Paraconformity, constraining the early uplift and erosion history of the Southern Alps, and documenting the evolution of contourite deposition on this current-influenced margin.

Nineteen middle Miocene to Pleistocene seismic sequence boundaries (U1–U19) were defined by sequence stratigraphic interpretation of EW00-01 seismic data (Lu and Fulthorpe, 2004). Expedition 317 was designed to provide ground truth for sequences above U4 (~U5 in practice) and also to core through the Marshall Paraconformity. Prior to Expedition 317, only Leg 174A (Austin, Christie-Blick, Malone, et al., 1998) and IODP Expedition 313 (Expedition 313 Scientists, 2010) had drilled on a continental shelf for sequence stratigraphic and sea level objectives. Expedition 317, therefore, provided a rare opportunity to investigate the facies, paleoenvironments, and depositional processes associated with the sequence stratigraphic model on a prograding continental margin where large-scale depositional geometries and sequence architectures are well constrained by seismic data.

Core-seismic correlation

The Canterbury Basin formed along a passive continental margin where large volumes of sediment were derived from a tectonically and climatically evolving hinterland. Seismic sequence boundaries provide a large-scale understanding of the subsurface architecture of the margin necessary for drill site selection and for placing drilling results within a broader two-dimensional and three-dimensional context. U19–U6 were penetrated at outer shelf Site U1351 and mid-shelf Site U1353, the most landward site. U5 may also have been reached at Site U1353. U19–U8 were cored at intermediate shelf Site U1354. The slope facies of most seismic sequence boundaries (U19–U13, U11, and U9–U6), as well as the Marshall Paraconformity, were penetrated at Slope Site U1352 with the particular goal of providing age control for sequence boundaries. The slope facies of U6–U7 were also sampled at shelf Site U1351 (Figs. F13, F14).

Upper Miocene–lower Pliocene seismic sequence boundaries (~U10 and below) feature smooth, onlapped paleoshelves and rounded rollovers, or clinoform breaks, with sigmoid internal reflection geometries. In contrast, middle Pliocene to Pleistocene sequence boundaries (above U10) display eroded and incised downlapped paleoshelves and more pronounced rollovers with oblique reflection geometries. This transition reflects a background of changing paleoenvironmental conditions, against which the cored sediments can be interpreted downhole.

Similarly, the character of sequences changes horizontally landward to basinward across the transect. Sequences are not represented by a single pattern of lithofacies across the shelf and onto the slope. Shelf successions are dominated by periodic facies changes that reflect fluctuating sea level, whereas deposition of slope sediments was also influenced by north-traveling Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), and their predecessor current systems (e.g., Carter and Gammon, 2004).

Correlating seismic sequence boundaries with lithologic facies and features found in the cores will be a focus of postcruise research. However, preliminary relationships can be defined at two scales.

Lithologic units

At the largest scale, the preferred boundaries between lithologic Units I and II at outer shelf Site U1351, mid-shelf Site U1354, and inboard shelf Site U1353 are lithologic boundaries that separate the overlying heterolithic facies of Unit I from more uniform facies below in Unit II (Figs. F13, F14, F15). The unit definitions for each Expedition 317 site are based on observed variations in lithology in the cores, particularly in the occurrence and frequency of calcareous units (generally green, shelly, and sandy). In general, Unit I is heterogeneous, containing a wide variety of facies, including interbedded terrigenous lithologies and many green marl or calcareous beds with sharp (or bioturbated) bases (Fig. F15). Conversely, Unit II is more homogeneous. On the shelf, Unit II is dominated by mud or muddy sand, with lower percentages of carbonate components and less frequent greenish calcareous beds, whereas on the slope this unit is represented by homogeneous sandy marlstone (Figs. F15, F16). However, at each site the location of the unit boundary is difficult to identify precisely because the transition between units of this type is inevitably gradational and also because of low recovery near some unit boundaries (Fig. F17). At slope Site U1352, the homogeneous sandy marlstone of Unit II changes abruptly into foraminifer-bearing nannofossil limestone of early Oligocene to late Eocene age. This hemipelagic to pelagic unit is correlative with the onshore Amuri Limestone.

The Unit I/II boundary generally conforms to the predicted depths of seismic sequence boundaries between U9 and U12 (late Pliocene) in a zone between high-amplitude seismic reflections related to clinoforms with sharp shelf-slope transitions above and clinoforms with rounded shelf-slope transitions below. The exact location is not identical at each site, in part because of the gradational nature of the transition noted above.

At Site U1353, Unit I is thought to correspond to strata between U12 and the seafloor (Fig. F13); however, a transition zone was identified between the heterolithic Unit I and the more homogeneous Unit II, reflecting the gradational evolution of the sedimentary margin. This transition zone extends between ~137 and 178 m or between U12 and U11 of Lu and Fulthorpe (2004) (Fig. F13). Heterogeneity in lithology and composition continues to ~260 m, where an abrupt change in mineralogy was observed. The uppermost transitional part of Unit II corresponds to the sequences between U9 and U12. Below 260 m, the homogeneous muddy sediments identified as Unit II correlate to strata between U9 and U5 (Fig. F13).

At Site U1354, the heterolithic upper Subunit IA corresponds to the high-amplitude seismic reflectors between U12 and the seafloor, which correlates to the same stratigraphic package as Unit I at Site U1353. Subunit IB lithology contains regular alternations of muddy strata and extends from just above the predicted depth of U9 to U12, equivalent to the transitional zone between Units I and II identified at Sites U1353 and U1351. Unit II corresponds to the clinoformal strata below U9, with the hole terminating between U8 and U7.

At Site U1351, Unit I is defined as extending from the seafloor to 262 m, although a transition zone was identified between ~247 and 300 m. This unit was distinguished by the high variability in lithofacies, which, not surprisingly, corresponds to the high-amplitude reflectors between U9 (predicted depth of 312 m) and the seafloor. All of Unit II lies within the broadly clinoformal succession between U4 and U9. The transition zone identified in the cores between Units I and II corresponds to the zone between U9 and U10, the lowermost unit of the upper high-amplitude succession and the one most similar in shape to the underlying rounded clinoforms.

For Site U1352 the boundary between Subunits IA and IB correlates to a subsequence boundary between U18 and U19 (Fig. F14), whereas the boundary between Subunits IB and IC correlates to U14, and the boundary between Units I and II correlates to a subsequence boundary at the base of the irregularly shaped high-amplitude reflectors between U13 and U11. Subunit IB corresponds to high-amplitude reflectors, and Subunit IC correlates to the irregular (possibly drift-related) reflectors below U14. In contrast to Sites U1351 and U1354, the Unit I/II boundary at Site U1352 occurs at a much higher seismic stratigraphic level (between U11 and U13), similar to the Unit I/II boundary at Site U1353. At all sites, Unit I is differentiated from Unit II by a higher degree of lithofacies heterogeneity. Subunit IIA at Site U1352 is remarkably homogeneous, although it corresponds in age and updip correlation to the basal part of Unit I at Sites U1351 and U1354.

Subunit IIA correlates with the prograding clinoforms between U8 and the base of the aforementioned erosional features between U11 and U13. The bottommost, more calcareous, part of this subunit occurs in a set of down- and onlapping reflectors directly overlying U8. Subunit IIB correlates with the more irregular, possibly channelized, reflectors between U8 and U4. The portion of this subunit that contains mud beds is located above U6, and the current-dominated, wavy-laminated calcareous lithofacies in this subunit occurs below U6. The very calcareous Subunit IIC correlates with the parallel seismic reflections between U4 and the Marshall Paraconformity at this site. Unit III lies below the high-amplitude Marshall Paraconformity reflector.

One objective of drilling on the shelf was to test the hypothesis that these contrasting characteristics of sequence boundaries and seismic facies above and below ~U9 occur because the paleoshelves of U9 and the underlying sequence boundaries were not subaerially exposed at lowstand, whereas those above U9 may have been exposed, probably because of increasing eustatic amplitudes during the Pleistocene. The lithologic contrasts between Units I and II at shelf Sites U1351, U1354, and U1353 appear to broadly support this hypothesis, with the heterolithic sediments of Unit I representing deposition under conditions of fluctuating sea level since the mid-Pliocene. The fact that the transition is gradational also implies that the change in processes operating to produce these sediments is also gradational and not an abrupt change from one type of process that produces rounded shelf-slope breaks to one that produces sharp shelf-slope breaks.

The change in lithology between units is mirrored in the percentage of carbonate components, as estimated from smear slides and thin sections and measured using X-ray diffraction (XRD) and coulometry. The carbonate percentage in Unit I is high and variable (Fig. F16). The carbonate percentage in Unit II at shelf sites is low and only variable at the top of the unit, whereas it is moderate and less variable at slope Site U1352 (Fig. F16). At shelf Site U1351, the carbonate percentage begins to increase downhole, which is a reflection of the shift of depositional environment toward the slope with depth at this site. At slope Site U1352, the carbonate percentage also increases downhole, reflecting a rise in oceanicity (planktonic foraminiferal abundance) of the depositional environment (Fig. F16). Subunit IIC and Unit III are both limestones, deposited in truly oceanic environments, with little to no terrigenous sediment input.

Correlation to Site 1119

Site 1119 is located ~35 km northeast of Site U1352 in slightly deeper water (~400 m) but essentially along strike and in a similar upper slope position, ~5 km from the modern shelf-slope break (Shipboard Scientific Party, 1999b). The stratigraphic succession at Site 1119 was divided by shipboard scientists into three lithologic units that correspond to units defined on seismic profiles across the site. Unit I, interpreted as slope-drape deposits, extends from 0 to 88 m and ranges in age from 0 to 0.42 Ma. It comprises beds of silty sand and hemipelagic silty clay (mud) deposited on the slope. The underlying Unit II comprises drift deposits of silty sand and glacial silty clay. Unit II is divided into three subunits according to the nature of the drift deposits (composition and sedimentary structures) and their correlation with three seismically defined drift packages. The last major sandy interval at ~400 m (~2.5 Ma) and the underlying noncalcareous mudstone marks the boundary between Units II and III, the former being more sand rich and more calcareous. This boundary roughly corresponds to the top of a series of strong reflectors that marks the top of another drift sequence.

In thickness, lithology, and seismic character, Unit I at Site 1119 roughly corresponds to Subunit IA at Site U1352. Seismic facies mapping between the sites suggests that the drift packages composing Unit II at Site 1119 are different from those at Site U1352 (Lu and Fulthorpe, 2004) and that there is a significant difference in the thicknesses of the seismic sequences between the sites. Subunit IB is interpreted as a transitional slope/drift facies, becoming more drift dominated into Subunit IC.

The major units at both sites are roughly defined by carbonate content, with Unit I at Site 1119 and Subunit IA at Site U1352 exhibiting generally lower minimum and lower maximum carbonate values than the lower units at each site (cf. fig. F24 in Shipboard Scientific Party, 1999b; Fig. F18). Comparison suggests that Units II and III at Site 1119 roughly correspond with Subunit IB and the top of Subunit IC at Site U1352. A distinctly low carbonate interval at ~430 m at Site 1119 corresponds to a similar low-carbonate interval occurring at ~620 m at Site U1352.

Potential sequence boundaries in cores

Shelf sites (U1351, U1353, and U1354)

Finer scale correlations between lithology and seismic interpretations are also possible. Correlation of specific lithologic features with seismic sequence boundaries will be improved by postcruise analysis, but some intriguing relationships can be drawn based on shipboard observations. Lithologic contacts and associated sediment packages were identified near the predicted depths of seismic sequence boundaries at all sites (Figs. F19, F20). Identification was based on changes in lithologic composition, the presence of reworked sediments, and the nature of the contacts. Basal surfaces were not always recovered—for example, in zones where recovery was poor because of the (inferred) presence of sand. In such cases, the surfaces were picked based on the presence of sand (or the assumption of washed-out sands) and correlation to wireline logs. 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 profiles (Lu and Fulthorpe, 2004). A similar approach was used during 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 consists of a hole-specific prefix and a surface designation (e.g., U1351A-S1) that links each surface to a seismic sequence boundary (Figs. F19, F20); therefore, these lithologic surfaces and associated sediments are thought to be correlative between sites across the transect.

Candidate sequence boundaries are best identified in lithologic Unit I at each site, where recovery was highest (Fig. F17). For example, the lithologic surfaces of Unit I at Site U1351 are overlain by a series of lithofacies associations composed of upward-fining shelly sandy mud topped by regressive coarsening-upward sandy mud. Our working model is that the package is a transgressive wave-eroded ravinement surface overlain by a lag deposit corresponding to a transgressive systems tract. Subsequent highstand mud units are topped by the regressive coarsening-upward sediments. The facies assemblage includes decimeter- to centimeter-thick clay beds similar to those attributed to rapid deposition by flood events on modern shelves near sediment-rich fluvial systems. Truncated versions of this facies association lacking the regressive highstand phase occur in the uppermost 30 m at Site U1352, suggesting more frequent erosive episodes or nondeposition of this facies. There are more such truncated facies associations than there are seismic sequence boundaries, and it is therefore possible that some of the truncated facies associations may represent higher order cycles or autocyclicity related to migration of distributaries and/or shelf depocenters. The facies associations, both complete and truncated, are similar to those known from the Wanganui Basin, between the North and South Islands of New Zealand (Abbott and Carter, 1999; Saul et al., 1999; Abbott et al., 2005; Naish et al., 2005).

Unit I surfaces at Site U1351 may be correlative with U19–U11 (Fig. F20). The geometries of U19–U11, as well as paleoenvironmental interpretations from biostratigraphy, show that these surfaces developed on the middle to outer shelf, where eustatic fluctuations had the most significant direct impact. In contrast, surface U1351-S8 and the underlying strata formed when the position of Site U1351 was on the outermost shelf, progressively migrating (downhole) to the upper slope. Consequently, U1351-S8 is less sharp than U1351-S1 to U1351-S7, and reworked sediments are more difficult to identify below U1351-S8.

These lithologic surfaces were also identified in Unit II at Site U1351, although cemented intervals occur, some of which may correlate with seismic sequence boundaries. Diagenetic changes begin in the lower part of Unit I (at ~150 m), with authigenic carbonate cements and limestone concretions. Unit II is dark greenish gray very fine sandy mud and greenish black to dark greenish gray mud. Shells are less frequent below ~350 m and are absent below ~890 m, probably reflecting the downhole shift from shelf to slope environments. The indurated clayey or sandy mud found in some core catchers in the lower part of the unit was drilling related, caused by frictional heating and baking by the XCB cutting shoe. However, other indurated intervals represent in situ carbonate cementation. These nodules or concretions are present throughout the unit, particularly below 750 m. These have no clear relationship to intervals of low recovery.

The facies associations linked with seismic sequence boundaries at Site U1351 may also be present in Unit I at Site U1353, but they are not as prominent. The uppermost three, and possibly four, surfaces identified at Site U1353 correlate well with U19–U16 (Fig. F20). Surface U1353-S4 was not recognized in cores because of low recovery, but a biostratigraphic hiatus that is potentially correlative with U16 was recognized. Correlations of other surfaces and associated sediment packages at Site U1353 with seismic sequence boundaries remain tentative because of poor recovery (Fig. F17). Recovery from below 80 m at Site U1353, the most landward of the shelf sites, was either low or involved significant cave-in of overlying shell hash. The dominant recovered facies is mud, interpreted mostly to represent a highstand facies. Sandy intervals represent transgressive or regressive periods, whereas the shell hash, even when muddy, probably formed as a transgressive shoreface deposit. Although sediment recovered from Unit II was predominantly muddy, recovery was low and other lithofacies may have been present. The mud suggests deposition on a shelf below the fair-weather wave base, and the lack of abundant shell debris suggests that the setting was more distal than it was during deposition of Unit I. The sediment source for terrigenous sediments of both Units I and II at Site U1353 appears to have been the Torlesse graywacke to the west. This contrasts with sediments in Unit II at outer shelf Site U1351 and Subunit IB and Unit II at slope Site U1352, which include a major contribution from the Otago Schist to the south.

Site U1354 facies associations are similar to those at the other shelf Sites U1351 and U1353. Several potential unconformities were recognized within Unit I. U19–U15 were intersected in both Holes U1354A and U1354B at depths comparable to those predicted (Fig. F20). The strongest correlation of lithologic surfaces to seismic sequence boundaries was found at Site U1354: a burrowed probable sequence boundary in the same interval containing the Brunhes/Matuyama boundary that corresponds to U16. A ~0.3 m.y. gap that encompasses the Jaramillo normal chron, as demonstrated on nannofossil evidence, corresponds to U15. Furthermore, other older seismic sequence boundaries correspond to erosional gaps in the succession. In many cases, more than one candidate surface in the core was correlative with seismically mapped surfaces. These stratal relations suggest greater preservation of sediment at Site U1354, which could potentially contribute to an understanding of how sequence boundaries form and evolve. Correlations with seismic sequence boundaries therefore remain tentative, but good recovery in the uppermost 250 m at Site U1354 should allow for improved correlation between lithologic features and U19–U10. As at other Expedition 317 sites, poor recovery in the lower part of the hole makes core-seismic correlation more difficult.

Lithostratigraphic correlation of shelf sites

Packages of lithofacies can be tracked across the shelf between the three shelf sites, from Site U1353 in water depths of ~85 m, through Site U1354, to Site U1351 in water depths of ~122 m (Fig. F21). Generally, the landward Holes U1353B and U1354B show coarsening-upward trends. Muddy sand, presumably of Holocene age, drapes the shelf but thins offshore from 9 to 4 m thick. A similar trend is evident in a layer of very fine, well-sorted quartz sand, which thins offshore from nearly 10 m thick in Hole U1353B to 2 m thick in Hole U1351B. These sedimentation patterns are consistent with both the Holocene eustatic rise and a decrease in sedimentation rates at middle and outer shelf settings. Additional shore-based studies will provide insight into the numerous depositional environments in which observed sedimentary cyclicity and sediments were deposited, as well as their correlation between sites.

Slope site (U1352)

A number of distinct lithologic surfaces and their associated sediments present at slope Site U1352 have the potential to be resolvable by seismic profiles. The uppermost 500 m of Site U1352 contains coarse-grained deposits that have been tentatively correlated to U19–U13. These remain the best-constrained boundaries. However, because of improved recovery at Site U1352 (Fig. F17), candidate slope surfaces were also identified in all lithologic units down to and including the Marshall Paraconformity (Fig. F19).

Sedimentary cyclicity

Seismic sequences and Milankovitch cyclicity

The relationship between high-frequency Milankovitch sequences and longer, seismically resolvable sequences remains an active and unresolved area of research. Saul et al. (1999) showed that for the Wanganui Basin, classic fourth- and fifth-order cyclothems are grouped together into third-order cycles controlled by basinwide tectonic rhythms. Alternatively, Miller et al. (2005) pointed out that some third-order cyclicity may be the manifestation of longer wavelength Milankovitch periodicities of 1.2 and 2.4 m.y. or of interference between these cycles.

Sequences drilled during Expedition 317 span the boundary between Milankovitch-scale cyclicity and longer third-order seismically resolvable cycles: periods of most seismic sequences range from ~0.1 (seafloor to ~U17) to ~0.4 m.y. (U16–U7) (Table T2; Fig. F22). The two upper seismically delimited sequences on the Canterbury shelf (corresponding to the sequences overlying U19 and U18) almost certainly correspond to marine isotope Stages (MIS) 1–5 and MIS 6–7 sedimentary cycles, based on the magnetic susceptibility and natural gamma radiation (NGR) time series (Fig. F23). Surfaces U1351-S1 to U1351-S4 correlate with U18–U16, respectively. Between successive low spikes in this interval, gamma radiation shows four cycles of a gradual uphole increase followed by a decrease. These cycles correlate with fining- to coarsening-upward facies associations, which are formed by one cycle of transgression and regression (e.g., U1351-S2 to U1351-S4; Fig. F19). Based on biostratigraphic datums and the inferred position of the Brunhes/Matuyama boundary (~70 m at Site U1351), we can speculate that these sequence boundaries formed during periods of low global sea level and perhaps correlate with three of the four MIS 8, 10, 12, and 16 (Fig. F24). Three distinct intervals of low gamma radiation and high resistivity in wireline logging data across the three shelf sites correspond well to the predicted depths of U12, U11, and U10 (Fig. F25), which are also well resolved in preliminary synthetic seismograms generated from the logs. These features, which most frequently coincide with intervals of poor core recovery, appear to be associated with sand, shell, and gravel beds several meters thick that may be laterally continuous across the shelf.

Postcruise analysis, including improved age control for deeper sequence boundaries, will document and clarify the probable relationship between seismic sequences and Milankovitch cycles. Careful study will be needed to distinguish lithologic changes corresponding to genuine Milankovitch cyclicity from those resulting from autocyclic variation within cycles controlled by sedimentary changes of local significance. It is already clear that Expedition 317 data will provide important insights into the hierarchy of sedimentary cyclicity and into how and why certain cycles and/or periodicities are preferentially imaged using seismic data.

Pleistocene to Holocene glacial–interglacial cycles

Expedition 317 shelf sites (landward to basinward: U1353, U1354, and U1351) provide highly significant high-recovery sections through the Holocene–Pliestocene for high-resolution study of recent glacial cycles in a continental shelf setting. The smaller scale cycles penetrated during Expedition 317 drilling are similar in part to the Milankovitch-scale rhythms documented at nearby Site 1119 (Shipboard Scientific Party, 1999a; Carter and Gammon, 2004) and the New Jersey shelf (McHugh and Olson, 2002) and correspond to cycles with inferred lengths of 100,000 and 40,000 years. The seismic stratigraphy of the sequences deposited during the last (MIS 1–5) and penultimate (MIS 6–7) glacial–interglacial cycles on the Canterbury and Otago shelves has been described by Browne and Naish (2003) and Osterberg (2006).

Cycles drilled during Expedition 317 have differing sedimentary architectures depending on whether they were deposited seaward (Site U1352) or landward (Sites U1353 and U1354) of the inferred position of the last glacial lowstand shoreline (~125 m). Site U1351 at 122 m water depth is located near the lowstand shoreline. Cycles at slope Site U1352 are always seaward of the lowstand shoreline. The main sedimentary facies association on the slope is composed of a sharp basal contact with sand-filled Thalassinoides burrows overlain by a few decimeters of shelly sand that grades upward into sandy mud. Similar cycles at Site 1119 were shown to contain temperate-water planktonic foraminifers in the basal sand (Shipboard Scientific Party, 1999a), with a colder water microfauna in the intervening muds; these facts are consistent with each cycle representing a full interglacial–glacial rhythm (though perhaps with a diastem of a few thousand years, represented by the sharp, burrowed bases of the sand units). These rhythms are interpreted as resulting from oscillations in flow intensity of cold-water SAMW, perhaps linked also to migrations of the Southland (Subtropical) Front across the site (Carter et al., 2004b). The rhythms also have to be influenced by terrestrial sediment supply, which is likely highest at lowstands and minimal to absent during highstands.

Glacial–interglacial deposits landward of the position of the lowstand shoreline are well developed in the upper parts of Sites U1353 and U1354. These are composed of a sharp and sometimes burrow-penetrated base, inferred to sometimes represent a shoreface ravinement surface, overlain by well-sorted sands as thick as several meters that grade rapidly up into sandy siltstone and siltstone. A pedogenic profile may be preserved below one or more of the ravinement surfaces that coincide with seismic sequence boundaries; if confirmed, this would be an unusual circumstance because virtually all of the Pleistocene sequence boundaries in the onland Wanganui Basin are characterized by sharp (albeit often burrowed) marine ravinement surfaces cut during transgression, and only one soil profile has been recorded (Abbott, 1992).

Some basal sandy units have a lower shelly portion or possess a shell-hash facies similar to the Type A transgressive shell beds described from middle Pleistocene cyclothems in the onland Wanganui Basin (Abbott and Carter, 1994). The associated mollusk faunas are firmly indicative of a proximal shoreface–beach environment for the sandy and shelly units (Austrodosinia, Phacosoma, Anchomasa, Maoricolpus, and Alcithoe) and a distal shoreface–inner shelf environment (Atrina, Dosinia lambata, Divaricella, Maorimactra, Scalpomactra, Antisolarium, and Stiracolpus) for the slightly deeper water siltstones.

Overall, the sedimentary cycles penetrated in the upper parts of Sites U1353 and U1534 show striking similarities to Pleistocene cyclothems described from the Wanganui Basin. Although in detail individual cycles display differing sedimentary architectures that await full description and interpretation, elements of the Castlecliff motif (Abbott and Carter, 1999; Saul et al., 1999) and the Birdgrove and Maxwell motifs (Abbott et al., 2005) may all be represented.

Located close to the lowstand shoreline, Site U1351 displays cyclicity characteristic of the more shoreward facies associations in its upper part and of the upper slope facies associations deeper in the hole. However, the uppermost cycles at Site U1351 are thicker and represent deeper water mud facies settings than those of their shelf counterparts at Sites U1353 and U1354, as would be expected. Some Site U1351 cycles coarsen upward into sands before being truncated by the superjacent sequence boundary, in similar fashion to Pliocene cycles described from the Wanganui Basin (Naish et al., 2005).

Sedimentary processes

One goal of Expedition 317 was to provide insights into the role of contour-current deposition in a location where prominent sediment drift geometries are absent. Slope Site U1352 is most relevant to this question, but the deeper parts of Site U1351 also sampled slope facies between U6 and U8.

Northeasterly flowing currents (Southland Current, inboard of the Subtropical Front, and a gyre of the Subantarctic Front) are active today. At present, the core of the Southland Current is over the ~300 m isobath (Chiswell, 1996). The local gyre of the Antarctic Circumpolar Current circulates clockwise within the head of Bounty Trough parallel to the Southland Current to at least 900 m (Fig. F2) (Morris et al., 2001). The sediment drifts in the northern part of the Canterbury Bight, as well as outcrop evidence of current-related deposition, suggest that similar currents have been active for the last 30 m.y.

Early Pliocene and late Miocene nannofossil assemblages at Site U1351 contain reworked older material, some as old as Oligocene, but mainly early to middle Miocene. The downhole increase in reworking began while paleowater depths were still relatively shallow (inner to middle shelf) and before the downhole transition from shelf to upper slope facies, between U8 and U7 (~725 m), where paleowater depths became more equivalent to those influenced by modern ocean currents. Nannofossils are small and easily transported. Reworked nannofossils at Site U1351 may have been reworked from the Campbell Plateau, where Miocene sediment is shallowly buried and may have been exposed. This suggests an along-strike component of deposition.

A nearly complete Neogene continental slope sedimentary record dominated by pelagic to hemipelagic sedimentation with minor traction and gravity flow sediments was cored at slope Site U1352, as was a condensed late Eocene to early Oligocene section below the Marshall Paraconformity. This site represents a downhole record of unlithified sediments transitioning to lithified carbonates at depth. The gradual downward transition in lithofacies from more siliciclastic-rich Holocene–Pleistocene muddy facies into pelagic limestones and glauconitic marlstones and marls appears to reflect the downhole transition, seen on seismic profiles, from an upper slope location on a clinoformal margin with sharp shelf-slope breaks in the Holocene–Pleistocene toward a toe-of-slope location on a margin with more rounded shelf-slope breaks and a more gently inclined slope in the Miocene (Fig. F14). The lower carbonate content in the upper part of this interval may be linked to higher terrigenous input related to the uplift of the Southern Alps and/or Neogene climate change.

Subunit IA at Site U1352 may be composed of sediment gravity flows that fed a series of lowstand deltas. Evidence for this is found in the composition of the sediments. The gray sand beds of Subunit IA have a Torlesse Terrane mineralogy, indicating that they are derived from the Canterbury region. This contrasts with the rest of Site U1352 (Subunit IB and below) and with Site U1351, which appear to be predominantly sourced from the Otago Schist to the south. Subunit IB also seems likely to contain evidence of mass transport deposition, although the equivalent interval at Site 1119 was interpreted as being contourites. Subunit IB may be a transitional (downhole) downslope-to-drift interval culminating in Subunit IC, which shows evidence (well-sorted, very fine–grained sand fraction) consistent with considerable transport distance and seismic evidence of sediment drift deposition.

Unit II is dominantly calcareous (sandy marl and marlstone). Hemipelagic to pelagic sediments of Subunit IIA (709–1189 m) may have been deposited as sediment drifts, based primarily on seismic evidence downslope and along strike (Fig. F14) (Lu and Fulthorpe, 2004): diagnostic sedimentary structures are rare, in part because of extensive bioturbation. More calcareous lithologies may represent condensed nondepositional intervals within the drifts. The increasing frequency of calcareous intervals toward the base of Subunit IIA may reflect reduced rates of terrigenous deposition near clinoform toes.

Subunit IIB (1189–1694 m) contains intermittent current-generated structures (wavy laminations and ripple cross laminations) interpreted as contourites. An alternation of light-colored marlstone with darker mudstone suggests switching between more hemipelagic carbonate (highstand?) sources and terrigenous (lowstand?) sources. Subunit IIB also contains mass transport complexes (slumping) near its base. Subunit IIB lithologies therefore suggest a complex interplay between along-strike and downslope sedimentary processes.

Subunit IIC marlstones and limestones lack traction features, suggesting pelagic deposition from suspension in deep water. Micropaleontology indicates a lower bathyal setting. The intercalated glauconitic sandstone laminae characteristic of this unit are inferred to be injected sands derived from the underlying Oligocene greensand (Kekenodon Group), which was not recovered at this site. The Marshall Paraconformity (see below) forms the boundary between Units II and III. Unit III is Amuri Limestone–equivalent pelagic nannofossil limestone deposited in lower bathyal depths.

Marshall Paraconformity

The Marshall Paraconformity is a regional unconformity terminating the postrift transgressive phase and capping the widespread Amuri Limestone. The paraconformity has been dated at its onshore type section using strontium isotopes as representing a hiatus between 32.4 and 29 Ma (Fulthorpe et al., 1996), where it occurs at the base of middle–upper Oligocene cross-bedded glauconitic sand (Concord Formation) and calcarenite limestone (Weka Pass Formation). The paraconformity was the deepest target of Expedition 317 drilling and was hypothesized to represent intensified current erosion or nondeposition associated with the initiation of thermohaline circulation (Deep Western Boundary Current) and associated shallower circulation upon the opening of the seaway between Antarctica and Australia (~33.7 Ma) prior to the opening of the Drake Passage (Carter, 1985; Fulthorpe et al., 1996; Carter et al., 2004c).

The Marshall Paraconformity is represented by a lithologic change at 1853 m in Hole U1352C, a record penetration for a Deep Sea Drilling Project (DSDP)/ODP/IODP hole drilled during a single expedition (total depth was 1927 m). The paraconformity marks the boundary between overlying lower Miocene glauconitic limestone and underlying lower Oligocene recrystallized pelagic nannofossil limestone containing trace fossils and stylolites, equivalent to the onshore Amuri Limestone. Recovery was low across the paraconformity, which is represented by a decimeter-scale rubble zone in the recovered core. No equivalent to Concord Formation glauconitic sand was recovered, although logs from the Clipper well suggest the presence of a boundary sand layer. The hiatus at the paraconformity is estimated to be 11–12 m.y. at Site U1352, conforming with previous observations that the hiatus is longer offshore than at the type section, perhaps owing to greater current-induced erosion at deeper water locations (Shipboard Scientific Party, 1999a; Carter et al., 2004c). Hole U1352C terminated in upper Eocene (35.2–36.6 Ma) limestone with clayey interbeds.

Drilling results indicate that the Marshall Paraconformity correlates with the "Green" reflection observed by industry interpreters (e.g., Hawkes and Mound, 1984). Our original interpretations assumed a correlation with a reflection ~160 m shallower than "Green." This, in turn, means that "Green" does not represent the base of the Amuri Limestone, as was previously inferred.

Biostratigraphy

Subtidal to lower bathyal sediments of Holocene to late Eocene age (0–36 Ma) were cored in a transect across the Canterbury Basin continental shelf and slope. Sequence stratigraphic processes influenced calcareous nannofossil and planktonic foraminiferal assemblages, which is strongly reflected in the overall abundance, preservation, and assemblage variations in relation to lithology. These variations may have been influenced by a number of factors, including global climate, local paleoceanography, changes in sea level, and tectonic uplift along the Alpine Fault. In general, Expedition 317 age assignments relied on nannofossils from the Holocene to middle Pliocene and planktonic foraminifers from the early Pliocene to late Miocene. Both fossil groups were integral for biostratigraphic control from the middle Miocene to Eocene. The following biostratigraphy and resulting interpretations are described in the order of sequences drilled, from youngest to oldest.

Holocene

The Holocene was not identified biostratigraphically, but, based on lithologic evidence, the thickness of Holocene sediments decreased oceanward, from a maximum of 9.9 m at the innermost shelf Site U1353 to 1.2 m at slope Site U1352. Mudline samples from Sites U1352, U1353, and U1354 contained temperate planktonic foraminiferal assemblages consistent with the Holocene highstand and the present-day landward position of these sites relative to the Southland Front.

Pleistocene

Calcareous nannofossil dating provided robust age control through the Pleistocene and contributed to the identification of several intra-Pleistocene hiatuses at all three shelf sites. The hiatuses potentially correspond to seismic surfaces inferred to represent sequence boundaries (Fig. F22). Calcareous nannofossil abundances appear to be cyclical. Maximum abundances are thought to be correlated with highstands and lowest abundances with lowstands. This is consistent with observed lithologic variations. In general, greenish gray sandy marls were associated with high numbers of calcareous nannofossils and with temperate populations, whereas gray sandy muds were associated with low numbers of nannofossils and cool-water populations. The Pleistocene section at slope Site U1352 was expanded relative to the shelf sites, and the hiatuses seen at the shelf sites were either absent or less pronounced at Site U1352.

Planktonic foraminiferal assemblages through the Pleistocene sections of shelf Sites U1351, U1353, and U1354 were generally characterized by low abundances and small, thin-walled neritic forms typical of deposition under inner neritic conditions (Fig. F26). These low abundances are consistent with an offshore westerly wind pattern, similar to that of the present day. However, higher abundances indicative of deposition under outer neritic or extraneritic conditions were noted in some samples, especially greenish gray sandy marls. These assemblages included larger temperate forms, whereas interbedded gray calcareous muds were dominated by smaller cold-water forms, including the subantarctic species Neogloboquadrina pachyderma. Similar alternations between temperate and cold-water assemblages were noted throughout the Pleistocene succession. Age-diagnostic planktonic foraminifers that need deep water to complete their reproductive cycles were generally rare, which contributed to poor planktonic foraminiferal dating through the Pleistocene. Planktonic foraminifers were better represented at slope Site U1352 (Fig. F26), but temperate species that facilitate a robust correlation with the New Zealand timescale were relatively rare and were generally only found in greenish gray sandy marls. There were few age-diagnostic species in intervals of gray calcareous mud.

Pliocene

The Pliocene/Pleistocene boundary is unconformable at all four sites and is a major correlation surface close to the predicted depth of U13 (Table T2). Biostratigraphic evidence indicated that most, if not all, of the upper Pliocene was missing. The thickness of the Pliocene succession was poorly constrained at the innermost shelf Site U1353, but at middle shelf Site U1354 the middle Pliocene was 50 m thick, and the lower Pliocene was at least 185 m thick. At the outermost shelf Site U1351, where the Pliocene was represented by a progradational middle and lower Pliocene succession, the middle and lower Pliocene sections were 100 and 580 m thick, respectively. The same progradational succession at slope Site U1352 was 390 and 380 m thick, respectively.

Standard calcareous nannofossil zonation schemes (typically derived from mid- to low latitudes) could not be applied to the Pliocene section at any site in spite of their published presence at even higher southern latitudes. This is attributed to cold subantarctic waters that strongly influence the oceanographic setting of the Canterbury Basin. Age-diagnostic planktonic foraminifers associated with the Globoconella sphericomiozea-puncticulata-inflata group are common throughout the Pliocene section of slope Site U1352 and the lowermost Pliocene section of outer shelf Site U1351. This, in combination with calcareous nannofossils and seismic stratigraphy, provided age control through the Pliocene sections of the shallower shelf Sites U1353 and U1354.

At Site U1351, planktonic abundances increased downhole throughout the lower Pliocene section below 590 m, marking a downhole change from shelfal to slope sedimentation and reflecting oceanward migration of the shelf break over the site. Significantly, a major influx of reworked late–middle Miocene calcareous nannofossils occurred in this same interval. The absence of low-latitude zonal markers in the reworked calcareous nannofossil assemblage indicated that it must have come from a cold southerly source. This has implications concerning the role of along-strike sediment transport, the focus of which appears to be at the shelf break.

Similar increases in planktonic foraminiferal abundance related to slope deposition and reworking of Miocene calcareous nannofossils were not seen in the Pliocene sections of the shallower shelf sites. In fact, the Pliocene–?upper Miocene section of the most landward Site U1353 is largely devoid of microfossils, except for occasional pulses of calcareous nannofossils and planktonic foraminifers.

Miocene

Upper Miocene sediments were recovered at Sites U1351, U1352, and U1353, although the dating and subdivision of the Miocene was very poorly constrained at the latter site. The Miocene/Pliocene boundary was identified between U7 and U6 and is presumably conformable.

Reworked calcareous nannofossil material continued through the 204 m thick progradational upper Miocene slope succession, except reworked taxa were mid-Miocene to Oligocene in age. The continuity of the upper Miocene section at Site U1351 was interrupted by a major hiatus between U6 and U5, where a gap of ~5 m.y. exists between 5.6 and 10.5 Ma (Table T2).

At slope Site U1352, the progradational upper Miocene section was 220 m thick, overlying a 190 m thick middle Miocene section and a 170 m thick lower Miocene section. Deposition at this site occurred in a lower to basal slope environment under oceanic conditions (Fig. F26). The high abundance of planktonic foraminifers and the presence of bolboformids contributed to reliable dating at this site, although preservation was generally poor, especially in intervals of cemented sandy marl. Calcareous nannofossil assemblages in the Miocene section of Site U1352 included pulses of reworking, but the total amount of reworked material was less than that at Site U1351. The major hiatus recognized between U6 and U5 in the lower upper Miocene section at Site U1351 was also recognized at Site U1352, although it occurs above, rather than below, U6. This suggests that the seismic mapping of U6 needs review. A significant hiatus was also noted between the middle and upper Miocene at Site U1352, where at least 1.3 m.y. was missing. The same hiatus may have also occurred in the lowermost part of Site U1353, although ages were poorly constrained at that site, which reduces the reliability of the correlation. A marked change to warmer planktonic foraminiferal and calcareous nannofossil assemblages occurred in the lower Miocene below the level of the hiatus. This coincided with the return of standard nannofossil zonal markers in the lower Miocene. Another major hiatus associated with the Marshall Paraconformity was recognized at the base of the Miocene succession, where ~12 m.y. was missing. At Site 1123, located on the deep northeast slope of Chatham Rise ~1000 km north-northeast of Site U1352, a similar amount of time was missing at the Marshall Paraconformity (Shipboard Scientific Party, 1999a).

Oligocene to Eocene

Slope Site U1352 was the only site where Oligocene and Eocene sediments were recovered. Microfossil preservation was generally poor, but nannofossil and planktonic foraminifer markers were identifiable, providing solid age control. The extremely high abundances of planktonic foraminifers throughout this succession are consistent with basin floor deposition under an open oceanic watermass (Fig. F25). The Oligocene/Miocene boundary is unconformable (Marshall Paraconformity). This hiatus extended from lower Miocene (18–19 Ma) to lower Oligocene (30.1–32.0 Ma), a gap of ~12 m.y. The Eocene/Oligocene boundary is also unconformable, falling within a hiatus of ~2.3 m.y. from the lower Oligocene (32.5–32.9 Ma) to the upper Eocene (35.2–36.0 Ma) (Fig. F22). The age at the bottom of the hole is 35.2–36.0 Ma.

Oceanicity

Planktonic foraminiferal abundance provides a measure of overlying oceanicity and serves as a measure of the distance from land in paleogeographic reconstructions. Test size, diversity, and composition of the planktonic foraminiferal assemblages also serve as proxies for oceanicity.

Oceanicity records from Expedition 317 sites were markedly different, except in the Pleistocene. The three shelf sites all exhibited high-amplitude Pleistocene fluctuations in planktonic abundance (Fig. F26). These high-amplitude peaks may correspond to highstands and could potentially be correlated across the shelf.

The Pliocene–Miocene succession of landward shelf Site U1353 was characterized by the absence of planktonic foraminifers, except for a single specimen found in the last sample (Fig. F26). This is unusual even in such a shallow marine environment and is in marked contrast with the continental shelf in the Taranaki Basin, where planktonic foraminifers were relatively common. This difference is attributed to the prevailing westerly wind pattern, which in Taranaki drives planktonics inshore and in Canterbury drives them offshore. Turbid water over the Canterbury shelf, related to high-volume river discharge, may also have contributed to the paucity of planktonic foraminifers.

The Pliocene–Pleistocene record of oceanicity at shelf Site U1354 was similar to the correlative succession at Site U1353, except planktonic abundances were slightly higher (Fig. F26). This is consistent with the site location closer to the shelf break.

Planktonic abundances throughout the lower Pliocene–Miocene succession of outermost shelf Site U1351 increased from inner neritic to suboceanic values (Fig. F26). The onset of this downhole increase in the lower Pliocene occurred when the shelf break was located near the site. The subsequent increase in oceanicity below this level suggests that the site was located progressively more oceanward of the shelf break. This trend was also reflected in the test size and diversity of planktonic foraminiferal assemblages. Second-order high-amplitude perturbations in the oceanicity record of the slope succession may be related to changes in sea level, although more detailed sampling is needed.

The abundance of planktonic foraminifers at slope Site U1352 generally increased downhole from suboceanic values in the Pleistocene to fully oceanic values in the Oligocene (Fig. F26). This is consistent with deposition on the mid- to lower slope and possibly the basin floor in the lowermost part of the succession. It also indicates that the site was always located oceanward of the shelf break.

Benthic foraminifers and paleowater depths

Although their abundance varied, benthic foraminifers constituted the major microfossil component within the 150–1000 µm size fraction at all Expedition 317 sites. The preservation of benthic foraminifers was generally good in the Pleistocene, became poor to moderate in the Pliocene and Miocene at shelf Sites U1353, U1354, and U1351, and was poor in the Oligocene and Eocene at slope Site U1352.

The Pleistocene assemblage was composed of mainly inner to outer shelfal taxa at shelf Sites U1351, U1353, and U1354 and outer shelf to upper bathyal taxa at slope Site U1352. Subtidal to inner shelfal assemblages alternated with mid- to outer shelfal assemblages at Sites U1351, U1353, and U1354, implying cyclic changes in water depth. General trends of deepening or shallowing can be provisionally correlated with some seismic sequence boundaries (Fig. F27). Pliocene assemblages at the shelf sites are similar to those of the Pleistocene, with marker species suggesting slightly deeper shelfal environments oceanward across the transect.

In spite of the poor preservation and rarity of earliest Pliocene to latest Miocene benthic foraminifers at slope Site U1352, assemblages suggested a downhole decrease in shelfal species and an increase in upper bathyal marker species. Deep middle to deep lower bathyal species were the main components in the middle Miocene. Water depths at slope Site U1352 therefore shallowed from lower bathyal to uppermost bathyal since the middle Miocene as the shelf-slope sediment prism prograded out into the basin (Fig. F27). Early Miocene and Oligocene benthic foraminifers were too rare for reliable estimation of paleowater depths.

Age-depth plots

Age-depth plots (Figs. F28, F29, F30, F31) were created for each site using biostratigraphic datums, New Zealand stage determinations based on foraminiferal assemblages, and paleomagnetic reversal data at Site U1354. At all sites, age control data points were spaced too far apart to determine sedimentation rates with accuracy; however, average sedimentation rates were calculated over the gross intervals between major hiatuses, where the rate appeared to remain constant (dashed lines). These rates may be averaged over unresolved hiatuses, so the relatively low numbers may actually represent high rates of sedimentation separated by periods of nondeposition and/or erosion.

The pattern of sedimentation rates across the sites indicates relatively slow deposition during the early to middle Miocene (data mainly from slope Site U1352; Fig. F31), with one or more hiatuses in the late Miocene, dividing this succession from a latest Miocene to Pliocene period of rapid deposition across shelf sites (data from Sites U1354, U1351, and U1352; Figs. F29, F30, F31).

Note that the benthic foraminiferal datums included in the plots are in some cases likely to be strongly affected by environmental changes. An example of this effect is the highest occurrence (HO) of Hopkinsina mioindex at 1220 m in Hole U1352C (Fig. F31). This datum (3.62 Ma) appears displaced from the rest of the data points in this zone, perhaps because the species occurs only in water depths greater than ~1000 m. This HO is therefore actually an indication that water depths in the depositional environment reached 1000 m at this horizon.

Hiatuses were identified by the biostratigraphy group based on the occurrence of multiple datums at a single stratigraphic level, as well as on "softer" data such as sudden changes in abundance, preservation, and faunal assemblages. Sometimes these hiatuses corresponded with lithologic changes or surfaces identified in the cores, but, more often, low recovery meant that unconformity surfaces were not recovered. The exception is Site U1354, where several sharp lithologic changes were recovered in intervals where biostratigraphy identified hiatuses; the good recovery allowed the identification of two unconformity surfaces between 60 and 80 m (Fig. F29), whereas at other sites only one unconformity was definitely identified. Otherwise, the unconformities first identified at Site U1351 were also identified at other sites, particularly a mid-Pleistocene unconformity (two surfaces at Site U1354), a significant unconformable surface at the Pliocene/Pleistocene boundary (identified at all sites), and a late Miocene unconformity (or possibly two, identified at Site U1352).

Porosity and backstripping

One of the goals of Expedition 317 was to provide the data necessary to allow quantitative analysis of subsidence and, ultimately, sea level change in the Canterbury Basin using two-dimensional backstripping. In addition to paleoenvironment and age data, backstripping requires an estimate of the history of sediment compaction. Porosity measurements reveal variations in compaction of mixed, largely terrigenous sediments as a function of both burial depth and location across the shelf to the upper slope (Fig. F32).

All sites showed rapid compaction in the upper several tens of meters, suggesting a rapid change from loosely to closely packed grains. Beneath this is a zone in which little porosity loss occurs with depth. The thickness of this zone increases from the middle shelf to the upper slope (Fig. F32). This zone is all but absent at landward shelf Site U1353, where compaction as a function of depth is most rapid. At upper slope Site U1352, two overlapping compaction trends were observed (Fig. F32). The coarser grained facies display rapid cementation and porosity loss, whereas interbedded finer grained sediments remain unlithified to greater depths and lose their porosity more slowly. Improved understanding of sediment compaction and diagenesis that arises from these data will be incorporated into backstripping models.

Geochemistry

The transect of four sites drilled during Expedition 317 provided the opportunity to study pore fluid and sediment geochemistry from middle shelf to upper slope settings spanning present-day water depths of 85–344 m. This contrasts with the majority of scientific ocean drilling sites, which involve coring in water depths of 1500–5000 m. Shelf sediments display variability imposed by glacial–interglacial changes in sea level and changes in conditions associated with discontinuities in sedimentation, including subaerial emergence, uplift, and erosion. These conditions introduce complications such as missing section and major changes in variables such as temperature and pressure that affect the ephemeral properties of pore fluids.

Many of the geochemical processes studied in the shipboard geochemistry program are driven by the microbially mediated remineralization of organic matter buried with the sediments. In addition, other processes increase with burial depth as unstable mineral assemblages are reorganized by dissolution and precipitation. By sampling and analysis at successive core depths, we attempted to piece together the dominant geochemical processes that have taken place in the sediments. The analyses are grouped conveniently by gas, liquid, and solid.

Gas and shallow biogeochemical zones

During Expedition 317, cores at all four sites were routinely sampled for traces of residual dissolved gas, both as a hydrocarbon safety procedure and as a sensitive technique to determine the status of the microbial and thermal degradation of organic matter. Combined with high-resolution water samples (taken in every section of recovered cores), this approach led to detailed profiles covering the biogeochemical zones of sulfate reduction, anaerobic oxidation of methane (AOM), and methanogenesis at all sites (Fig. F33).

Sites U1351 and U1352 are located farther offshore and have similar geochemical profiles distinct from those at Sites U1353 and U1354. At both sites a classic sulfate–methane transition (SMT) zone is located at ~15–16 m. In sediments below the SMT, sulfate is virtually absent, and methane concentrations increase dramatically from background levels in the low ppmv range to >10,000 ppmv. Alkalinity at the SMT maximizes at 10 and 24 mM for Sites U1351 and U1352, respectively (Fig. F33). The apparent levels of carbon oxidized and the low levels of ammonium and phosphate generated at Site U1351 suggest that sulfate reduction is primarily fueled by AOM, whereas at Site U1352 both AOM and organic matter oxidation are coupled with the removal of sulfate. Residual methane concentrations below the SMT remain constant at 10,000–20,000 ppmv for both sites, approximately equivalent to 4–8 mM of methane per liter of pore volume, which probably represents only 5%–10% of actual subsurface dissolved gas contents. However, ethane levels (shown by the methane/ethane [C₁/C₂] ratio) vary significantly. C₁/C₂ at Site U1351 increases gradually with depth and temperature, as expected, but C₁/C₂ ratios at shallow depths are unusually low (400–600). This suggests some combination of higher temperatures associated with warmer bottom waters, some removal of sediment at unconformities, preferential loss of methane by anaerobic oxidation, or gas loss from sands during core recovery and sampling. At Site U1352, which is located farther offshore on the slope, gas just below the SMT has a C₁/C₂ ratio of 16,000, which is lower than most deepwater coring sites. The average C₁/C₂ ratio decreases by three orders of magnitude to ~60 at the bottom of Hole U1352C at 1920 m. At an apparent discontinuity near 1390 m, only very low gas content was found (~40 ppmv methane) and C₁/C₂ drops as low as 7, mainly from the near absence of methane. Below the discontinuity the gas resumes the normal trend of residual methane contents (>10,000 ppmv). At both Sites U1351 and U1352, higher hydrocarbons (C₃–C₅) were detected in increasing amounts with depth, apparent products of the low-temperature alteration of organic matter in the deeper cores.

Sites U1353 and U1354 are located on the shelf in water depths of 85 and 113 m, respectively. Both sites are characterized by the absence or low levels of methanogenic activity in the uppermost 150 m, leading to low methane concentrations (Fig. F33). At Site U1353 no hydrocarbons above background levels were detected, and sulfate remains close to seawater concentration, with the exception of slightly decreased sulfate in a lower salinity interval at 20–70 m. No SMT is apparent, suggesting that either (1) methanogenesis did not occur in the sediments, (2) previously generated methane was lost when the shelf was emergent, or (3) methane was oxidized when sulfate was replenished by diffusion after a subsequent sea level rise. Alkalinity never rises above 6.5 mM and declines gradually with depth.

Hydrocarbon monitoring at Site U1354 showed two peaks in methane, a small peak (23 ppmv) at 33–75 m, and a large peak below ~200 m, where methane increased to >20,000 ppmv. In both cases methane begins to increase when sulfate drops to zero, and the shallow methane peak disappears when sulfate reappears in the cores. These alternations of methanogenesis appear to be related to changes in sedimentation rate. The upper and lower methane zones correspond to periods of rapid sedimentation (200–400 m/m.y.), during which sulfate was depleted and methane was generated. The intermediate interval (60–178 m) corresponds to periods of slower sedimentation (<50 m/m.y.), when pore waters could be replenished with sulfate by diffusion from overlying seawater.

Interstitial water geochemistry

Interstitial water was squeezed from sediments in most of the sedimentary sequence for the three shelf sites but only in the uppermost 1400 m of slope Site U1352. At this site pore water yields decrease from ~2–5 mL/cm of whole round squeezed in the shallower sediments to <1 mL/cm below 1200 m, reflecting progressive compaction, porosity and water loss, and the general sediment to rock transformation. Pore waters in the Canterbury Basin have quite variable properties depending on site location on the transect.

Below the sulfate reduction zone calcium always increases with depth, but at different rates and to different values (Fig. F34). At Sites U1353 and U1354, the increase occurs mainly in the uppermost 200 m and reaches only moderate values (<20 mM), whereas at Sites U1351 and U1352 the calcium increase is noticeably sharp (at ~250 and ~500 m, respectively) and reaches values of 30 mM and higher in the deepest sediments at each site. These variations are likely related to carbonate diagenesis, and possibly contributions from deeper basinal brines, and are broadly consistent with dissolution of carbonates and the poor preservation of microfossils. More particularly, there is evidently some exchange of magnesium for calcium in the dissolution and reprecipitation of carbonates during diagenesis at Sites U1351 and U1352, as indicated by the greater declines in the Mg/Ca ratio and the higher carbonate content at these sites.

Alkalinity, mainly as bicarbonate ion produced by oxidation of organic matter, always decreases with depth below the sulfate reduction zone and generally reaches values <3 mM below 200 m (Fig. F33), although this transition, together with the increase in calcium, occurs at ~500 m at slope Site U1352.

Chloride and salinity generally co-vary, but there are other controls on sodium content. The salinity and chloride contents of pore water remain constant and near seawater composition throughout slope Site U1352, except for the deepest sample, which has a significantly lower chloride content. At Site U1351 the chloride becomes more concentrated than seawater between 50 and 200 m and again below 900 m. The highest salinity and chloride values occur at Site U1354, where they increase throughout the sequence and reach values of 3.8 and 694 mM, respectively. The deepest samples from Site U1353 are also the most saline, but this site is also characterized by a salinity minimum in the uppermost 150 m that reaches 2.4 mM or 70% of seawater at ~50 m. This less saline lens affects the pore water composition of several other ions, including sodium, potassium, and magnesium (Fig. F34). The presence of this less saline lens could be explained by either modern intrusion of meteoric water from land or by the historic remains of freshwater that was emplaced when the shelf was emergent and is now being slowly replaced by the downward diffusion of seawater. However, this less saline water lens is not present at the more seaward Sites U1354 and U1351. Water depths are 85 m at Site U1353, compared with 113 m at Site U1354 and 122 m at Site U1351. Global sea level was ~125 m below today's sea level at the last glacial maximum about 20 k.y. ago, so both Sites U1353 and U1354 should have experienced periods of complete emergence. Therefore, the lack of less saline water at Site U1354 and its presence at Site U1353 are more likely explained by the modern intrusion of meteoric water from land, rather than by the historic remains of freshwater emplaced when the shelf was emergent.

Potassium concentrations decrease with depth at all four sites, starting from slightly higher than seawater values of ~11 mM in the shallowest samples (Fig. F34). The decrease to <6 mM is rapid within the uppermost 150 m at the shelf sites but is much more complex at the slope site, with an initial decrease in the sulfate reduction zone, an increase back to seawater values, and finally a steady decrease to <4 mM below 800 m (Fig. F34). The decrease in potassium, and to some extent sodium, is likely related to diagenetic glauconite precipitation, which is a common mineral at all four sites and usually appears around the depth of the potassium drawdown.

Silica and silicon concentrations are always <1000 µM and generally rapidly fluctuate in abundance in the uppermost 100 m. At the slope site this variability persists throughout the section with interstitial water samples, but silica and silicon are more consistent and less abundant below 700 m. At Site U1351 silicon concentration levels are ~400–500 µM below 100 m, whereas they are ~200 µM at nearer shore sites. This suggests that silica mobilization and precipitation is more active nearer to shore, consistent with the lower carbonate and greater clastic component at these sites.

Lithium concentrations generally increase with depth at all four sites, with very smooth profiles to <100 µM at Sites U1353 and U1354 and more complex patterns and higher overall concentrations at the two more distal sites. The lithium increases may be caused by dehydration reactions that removed lithium from clay interlayer exchange sites and in some cases are related to lithologic changes.

Barium concentrations increase rapidly in the shallowest samples (<40 m) and then generally remain steady until >250 m, below which they increase. At Sites U1353 and U1354 this increase never exceeds 10 µM, but barium concentrations are much higher (>20 µM) in Site U1352 samples below 500 m. Site U1351 has intermediate barium concentrations in the deeper sediments (5–12 µM) but higher abundance (up to 23 µM) in the four shallowest samples, which so far remains unexplained.

Boron concentrations also increase with depth at all four sites, but to different values. Boron reaches ~0.6 mM in the deepest sample (586 m) at Site U1353, 1.26 mM at 318 m at Site U1354, and maximum values of 4–5 mM at Sites U1351 and U1352. Thus, there is a trend of higher boron farther offshore and deeper in the cores, which may be consistent with a diagenetic opal-A/opal-CT transition and microbial degradation of organic matter.

Sediment geochemistry

The sediments cored at Site U1351 show marked differences in geochemistry with depth, with higher carbonate, higher nitrogen, and lower sulfur above 200 m (Fig. F35). Organic carbon contents range from <0.1 to 1.5 wt%, with more frequent higher values in the uppermost 200 m. Pyrolysis characterization suggests the organic matter is dominated by degraded higher plant debris.

Analyses of sediment samples at Site U1352 also distinguished the clay-rich lithologic Unit I from the carbonate-dominated Unit II. Organic carbon content was generally low (<0.6 wt%), with only a few samples having >1% total organic carbon (TOC) (Fig. F35). The character of the organic matter changes from relatively labile volatile material in the shallower sediments to more stable proto-kerogen downhole, with evidence for increasing thermal maturity at total depth. The organic matter appears to be mainly terrestrial plant or degraded marine in origin.

At Site U1353 the average carbonate content is highly variable but generally is lower below 300 m (Fig. F35). The decrease in TOC in the uppermost 100 m can be correlated with intervals of increased alkalinity and decreased sulfate and might represent active biological oxidation. Pyrolysis characterization of organic matter suggests a major contribution from terrestrial plants, whereas ratios of TOC by difference (TOC_DIFF) to total nitrogen (TN) from elemental analysis are consistent with a significant marine influence.

Only 18 Site U1354 sediment samples were analyzed for carbonate content and by the elemental analyzer because of time constraints at the end of the expedition. Calcium carbonate contents range from 1.3 to 52 wt% in the sediments analyzed down to burial depths of 81 m. Organic carbon ranges from 0.02 to 1.1 wt%, with the highest value at 50 m (Fig. F35). TOC_DIFF/TN generally decreases with depth, with the exception of some of the high-carbonate samples in the 73–76 m depth interval.

Microbiology

Microbiological studies on sediment cores collected during DSDP, ODP, and IODP expeditions have demonstrated the presence of microbial communities in deeply buried marine sediments down to >1600 mbsf. The Canterbury Basin is a promising place to expand our knowledge of the deep biosphere in a complex setting, representing the history of life under a variety of environmental constraints. The basin is heavily influenced by the input of terrestrial organic matter and is therefore an excellent end-member environment to complement marine settings that have been studied during previous drilling expeditions.

The Expedition 317 shipboard microbiology program included collecting samples for onshore microbiological investigations, assessing the degree of seawater contamination of microbiological sediment samples, and testing a new cell-counting technique. Three sites were sampled for microbiology investigations (Sites U1351, U1352, and U1353). Eleven whole-round samples between 1630 m and the bottom of Hole U1352C at 1927 m were taken for microbiological and organic geochemical characterization of in situ microbial communities. These samples will be studied in more detail onshore and could potentially extend the maximum known depth of habitable sediments.

A cell-counting technique recently developed by Morono et al. (2009) was also tested. The procedure proved to be highly efficient, and discriminative detection and enumeration of microbial cells in sediments using diluted (1%) hydrofluoric acid was achieved at Site U1351. The greatest abundance of cells was found in the near-surface sample, which contained 1.08 × 10⁸ cells/cm³, and the sample containing the smallest number of cells (1.34 × 10⁵ cells/cm³) was found at 930.9 m. Generally, the total number of cells decreased rapidly with depth in the uppermost 4 m of sediment. This depth profile follows a trend observed at ODP sites, but the absolute number of prokaryotes is lower than the averages for all previously examined sites, particularly below 4 m.

Contamination tests were performed on board using particulate tracers, providing confirmation that uncontaminated samples can be recovered and there is no difference in contamination produced by the three types of coring (advanced piston corer [APC], XCB, and RCB). Nevertheless, the dispersion of these tracers was found to not be homogeneous along the core liner, and microsphere concentrations were found to be diluted during RCB coring.

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