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doi:10.2204/iodp.proc.317.106.2011

Lithostratigraphy

Sediment descriptions at Site U1354, located between Site U1351 on the outer shelf and Site U1353 on the inner shelf, are based on three holes at this site. Holes U1354A and U1354B were drilled with the APC to 85.4 and 77.2 m core depth below seafloor (CSF-A), respectively (unless otherwise noted, all depths in this section are reported in m CSF-A). Hole U1354C was drilled without coring to 65 m. The APC system was then used to recover Cores 317-U1354C-2H and 3H before the hole was drilled with the XCB to a total depth of 384.2 m. Holes U1354A and U1354B had a sediment recovery rate of 100%, and Hole U1354C had a recovery rate of 41.8% (Fig. F3). These rates are slightly exaggerated given that some cores contain downhole material from cave-in or flow-in (at the base of some cores).

Lithologic descriptions at this site lack the X-ray diffraction (XRD) data used in other site descriptions, and similarly coulometry data are limited because of the short time frame available for drilling and the short transit at the end of the expedition.

The succession was divided into two lithologic units, with Unit I being further divided into two subunits (Table T2). Unit I (0–250 m) covers the Holocene to early Pliocene and is characterized by a highly heterogeneous assemblage of facies dominated by mud, with more heterogeneity evident in Subunit IA and a more repetitive succession of alternating facies occurring in Subunit IB. Unit II is early Pliocene in age and is primarily composed of mud, sandy mud, muddy sand, and rare marl layers, distinguished from Unit I by the generally low calcareous content of the layers.

The dominant mud lithology is interpreted as a shelf facies and likely represents the deepest water depth during deposition, so that the increasing dominance of mud facies downhole represents progressively deeper depositional environments. The other lithofacies present in Unit I are interpreted as shoreface or estuarine to inner shelf, possibly representing either transgressive shoreface deposits or sediment deposited immediately after the transgression (see Browne and Naish, 2003).

Description of lithologic units

Unit I

• Intervals: Cores 317-U1354A-1H through 19H; 317-U1354B-1H through 15H; and 317-U1354C-2H to Section 23X-CC, 20 cm

• Depths: Hole U1354A: 0–85.43 m (total depth); Hole U1354B: 0–77.52 m (total depth); and Hole U1354C: 65–250.86 m

• Age: Holocene to early Pliocene

Unit I is divided into Subunits IA and IB, which are described below. Unit I is heterolithic but characterized by its overall muddy composition (Figs. F4, F5). The uppermost part of the unit is well defined in Holes U1354A and U1354B (Fig. F6). The dominant lithology is dark greenish gray homogeneous mud (Fig. F7) with <10% very fine sand beds/laminae. Shells are either rare and scattered or abundant and locally concentrated in layers as thick as 15 cm and beds ~1 m thick. Shells are dominated by the gastropod Stirocolpus and lesser amounts of Tawera or other bivalves, including oysters. Some shell layers and beds contain shell fragments that range in size from a few millimeters to several centimeters. Contacts between shell layers are, for the most part, gradational with the overlying and underlying mud. Bioturbation is common and has an ichnofabric index of 1 (no bioturbation) to 5 (complete bioturbation). Locally, the mud fraction can be clay rich or clay (see "Lithostratigraphy" in the "Methods" chapter for definitions), the latter either forming centimeter-thick, sharp-based beds or occurring as mottles within the mud (Fig. F8C).

Subordinate lithologies include

• Centimeter- to decimeter-thick beds of greenish gray and dark greenish gray very fine sandy shelly mud to shell hash (Fig. F8A) mixed with siliciclastic grains (very fine to medium sand) and abundant Stirocolpus, Tawera, echinoid spines, barnacle plates, bivalves, gastropods, and rare pebbles of graywacke sandstone as large as 1.5 cm in diameter and limestone as long as 7 cm.

• Very dark greenish gray, well-sorted, very fine to fine (mostly fine), highly micaceous lithic sand with broken shells (Fig. F9). Typically, the sand has a soft and soupy appearance in the core and may have been partially fluidized during drilling.

• Sandy marl (Fig. F8A–F8B), which occurs as a 4 m thick deposit in the uppermost part of the site (Cores 317-U1354A-1H and 317-U1354B-1H), presumably of Holocene age. The sandy marl is homogeneous, dark greenish gray, and micaceous, with very fine to fine sand.

• Dark greenish gray clay-rich mud (Fig. F8D) with intercalated, normally graded sand laminations. Each lamination has a sharp base and is composed of very fine to fine, well-sorted, dark gray sand that is slightly micaceous and of possible Torlesse provenance. Rare, angular graywacke clasts (5–7 mm in diameter) are present in the mud.

• Bluish gray, poorly sorted, very fine to coarse sandy mud with abundant shells and shell fragments that range in size from 0–1 mm to 2 cm.

• Calcareous muddy sand to sandy mud and concretions (Fig. F9D).

A considerable amount of potential cave-in material was recovered in Holes U1354A and U1354B below Sections 317-U1354A-5H-CC (14.3 m) and 317-U1354B-4H-1 (12.1 m). The cores are typically capped by a shell-hash lithology in the top few decimeters that transitions, often through a soupy, mud-rich interval, into what is considered to be in situ sediment. Despite the issue of cave-in and flow-in, a discernible stratigraphy of mud with subordinate clay, sand, sandy marl, and sandy mud is evident. Muddy shell hash and shelly mud cave-in first occur at the top of Core 317-U1354B-4H. Nannoplankton biostratigraphy shows that shell hash is definitely cave-in in Core 317-U1354A-11H (55 m). It is possible that the shell hash from ~15 m to the occurrence listed above may represent in situ sediment.

Figure F6 illustrates the subtle variations in lithology, grain size, and shell content between Holes U1354A and U1354B, even though these holes are located 20 m from each other. Holes U1354A and U1354B can be correlated based on prominent sand and shelly mud beds. The sand intervals are thinner in Hole U1354B than in Hole U1354A. However, this could be an artifact of recovery because sand can be easily washed out from the bottom of APC cores. Both holes contain numerous shell beds, and a particularly thick example is present from 40 to 50 m in Hole U1354A and from 36 to 46 m in Hole U1354B (Fig. F6). In general, both holes show a coarsening-upward trend dominated by sand in the uppermost 30 m.

Subunit IA

• Intervals: Cores 317-U1354A-1H through 19H; 317-U1354B-1H through 15H; and Sections 317-U1354C-2H-1, 0 cm, to 12X-1, 110 cm

• Depths: Hole U1354A: 0–85.43 m (total depth); Hole U1354B: 0–77.52 m; and Hole U1354C: 65–145.8 m

• Age: Holocene to mid-Pliocene

Subunit IA is more heterolithic and contains dark greenish gray to olive-gray calcareous muddy sand, sandy marl, and homogeneous marl (e.g., Cores 317-U1354A-1H and 317-U1354B-10H; Fig. F9A–F9B) and very dark gray, massive, quartz-rich, very well sorted very fine to fine sand (e.g., Cores 317-U1354A-7H and 317-U1354B-10H; Figs. F6, F8B). This subunit also contains examples of sharp, bioturbated contacts between very fine muddy sand (sometimes calcareous) above and silty mud below (e.g., Sections 317-U1354B-13H-2 and 317-U1354C-12X-1; Fig. F9D).

Subunit IA at Site U1354, in common with the corresponding unit at all three other sites, is characterized by lithologic heterogeneity. The common lithologies are calcareous mud and calcareous sandy mud, which alternate with greenish gray shelly marl and sandy marl. Minor lithologies include very fine to fine sand, calcareous silty very fine to fine sand, and clay-rich mud. Most lithologies in Unit I contain >10% carbonate components based on smear slide estimates (Fig. F10) and the identification of sand- to gravel-sized bioclasts in the cores.

The terrigenous components identified in smear slides are dominated by quartz and feldspar, micas (including chlorite, muscovite, and occasionally biotite), rock fragments, ferromagnesian minerals (probably mainly hornblende), and dense minerals (zircon and epidote, among others). Rock fragments identified at this site, in common with previous sites, include fine-grained metasedimentary and metamorphic rock fragments.

Authigenic components include varying amounts of opaque minerals (infilling foraminifers, framboids, nodules, and alteration of mica) and carbonate (incipient micritic/microcrystalline cement and alteration). Glauconite is common in a few horizons, notably around Section 317-U1354C-8X-CC, where unusual turquoise patches, as well as corroded shells and pale gray muddy sediment, were observed. The glauconite observed here is different from the majority of that observed at previous sites in that it appears to be composed primarily of the alteration of shell fragments (and possibly opaque minerals or mica grains) rather than transported grains. Layers with unusually high alteration or carbonate cements occur in association with sharp lithologic contacts, either in the shelly sandy layers above the contact (e.g., Sample 317-U1354C-10X-1, 70 cm) or in the mud layers immediately below the contact (e.g., Sample 317-U1354A-11H-3, 37 cm).

Biogenic components identified in smear slides are dominated by undifferentiated bioclasts (fragments of shells) in the sandy facies and by nannofossils and didemnid ascidian spicules in the muddy facies. Most shells, especially foraminifers, are altered, with calcite crystals growing on both the outside and inside of the shells.

All of the thin sections examined are from Subunit IA and belong to two different lithologies. The first lithology is sandy marlstone, in which cementation of the in situ sediment resulted in nodules or concretions. The cement comprises carbonate, either as matrix replacement or pore-filling cements (rhombic shapes are common). The other sediment components are generally the same as those in the surrounding sediments, with a slightly higher proportion of bioclasts and a lack of nannofossils (presumably from recrystallization). In one case (Section 317-U1354A-15H-CC), the cemented material comprises the interior of a shell, where cement is concentrated inside and around the shell, creating a nodule.

The second lithology identified in thin section is unusual and occurs in two isolated, bored cobbles, one of which was found in a core catcher sample (Section 317-U1354A-15H-CC) and the other in the shelly, sandy layer immediately above the Brunhes/Matuyama boundary (probably an unconformity; Sample 317-U1354B-13H-2, 77–78 cm; see "Paleomagnetism"). The cobbles are rounded and dark gray and have calcareous encrustations on the outside and extensive borings on their surfaces. The lithology of these samples is pyritized sandy or silty limestone. The matrix of each has been almost completely replaced by micrite, and opaque minerals (probably pyrite) occur throughout the matrix, concentrated around the margins of the borings. Sediment inside the borings is consistent with the surrounding sediment, but the lithified sediment inside the cobble itself is not consistent with the surrounding unlithified sediment. One of the differences noted is that although bioclasts (shell fragments and common foraminifers) occur within the limestones, specific bioclasts (mostly didemnid ascidian spicules) common to the surrounding and overlying sediments are not present. Burrowing in the sediment prior to cementation can be inferred by the occurrence of patches with no detrital grains, different cement texture, or randomly oriented detrital grains.

Subunit IB

• Interval: Sections 317-U1354C-12X-1, 110 cm, through 23X-CC, 20 cm

• Depth: 145.8–250.86 m

• Age: mid-Pliocene to early Pliocene

The Subunit IA/IB boundary is marked by a decrease in the percentage of ferromagnesian minerals and glauconite, as estimated in smear slides (Fig. F10). Subunit IB has a less varied lithology than that in Subunit IA; however, except for the above-mentioned differences, the overall composition observed in the smear slides is very similar. In this unit, the gray mud layers do not, in general, have very high carbonate component percentages and are only rarely calcareous.

Subunit IB lacks the aforementioned olive-gray marl and massive sand beds found in Subunit IA and is characterized by more repetitive assemblages of facies (e.g., Cores 317-U1354C-19X and 21X) that consist of (1) homogeneous greenish gray mud, which appears to be more clay rich (e.g., Sections 317-U1354C-18X-1 and 14X-1; Fig. F8D) than that in Subunit IA and which also contains a minor calcareous component, and (2) greenish gray to gray calcareous sandy mud to sandy marl that often contains calcareous concretions (e.g., Section 21X-CC).

Unit I/II boundary

The main difference between Units I and II is the calcareous nature of each (Fig. F10): Unit II contains far fewer calcareous beds than Unit I and is dominated by a monotonous succession of gray sandy mud. The boundary is placed below the last highly calcareous layer and at the base of a sandy interval in the core.

Unit II

• Interval: Sections 317-U1354C-23X-1 through 36X-CC

• Depth: 250.10–375.38 m (total depth)

• Age: early Pliocene

The sediments of Unit II are primarily composed of mud, sandy mud, muddy sand, and rare marl. In Unit II, the gray mud layers are not very calcareous, but they are also infrequent, and the unit is dominated by sandy mud (sometimes calcareous sandy mud). Greener layers with a sandier texture are still present. The percentage of carbonate in sandier greener layers, as estimated by smear slide observation, is much lower in Unit II than in Unit I and rarely exceeds 15%. The dominant lithology is gray, homogeneous silty mud with rare scattered shells. Bioturbation has an ichnofabric index of 1 (no bioturbation) to 4 (heavy bioturbation).

Correlation with wireline logs

Wireline logging data and physical property measurements were not processed sufficiently by the end of the expedition to enable detailed correlation of the logs to the described lithostratigraphy.

Downhole trends in sediment composition and mineralogy

As at the other shelf sites, the compositional trends are gradational from Unit I to Unit II, but the lack of shipboard XRD data limits the description of downhole mineralogical trends. Unit I composition is highly variable depending on the lithology, which is consistent with the heterolithic nature of the unit. Quartz/feldspar, carbonate, clays, micas, ferromagnesian minerals, glauconite, rare dense minerals, and siliceous bioclasts occur throughout Unit I, but their concentrations are quite variable (Figs. F10). The carbonate, siliceous bioclast, and ferromagnesian mineral contents are notably higher in Subunit IA relative to deeper in the hole. However, no discernible downhole trends in composition are apparent in Subunit IB, and there are no recognizable changes in composition between Subunit IB and Unit II.

Description of lithologic surfaces and associated sediment facies

Because of time restrictions on board ship, surfaces were only examined close to the predicted depths of seismic sequence boundaries; therefore, the lithologic surfaces identified here are implicitly linked to the predicted occurrences of sequence boundaries identified on the seismic (Lu and Fulthorpe, 2004). A similar approach was used on board Ocean Drilling Program (ODP) Legs 150 and 174A, the objectives of which were also the study of sea level changes. Postcruise study will attempt to clarify the exact relationship of all lithologic surfaces and facies associations to sea level changes and seismic stratigraphy. The numbering system used in the site chapters, tables, and summary diagrams comprises a hole-specific prefix and a surface designation (e.g., U1354A-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.

Definitions of Type A–C surfaces are detailed in "Lithostratigraphy" in the "Site U1352" chapter. Type A contacts and facies associations (sharp, commonly bioturbated basal contacts with overlying muddy shelly sand or sand beds ~1 m thick) were identified in Holes U1354A–U1354C. This type of contact and facies association dominates the uppermost 250 m of sediments at this site. These contacts were classified as lithologic surfaces U1354A-S1 to U1354A-S5, U1354B-S1 to U1354B-S5, and U1354C-S5 to U1354C-S8. One major characteristic of Site U1354 sediments is that in several intervals two or three Type A contacts and facies associations that could be used for correlation occur in one core. In such cases, the thickest bed was chosen.

Surfaces U1354A-S1 to U1354A-S4 and U1354B-S1 to U1354B-S4

Type A lithologic surfaces are present in the uppermost 64 m in Holes U1354A and U1354B (see Table T3 for associated depth intervals), revealing a very good correlation between holes. Surface depths differ across holes by as much as 4.8 m for U1354A-S4 and U1354B-S4 (Table T3). As at shelf Sites U1353 and U1351, very fine, well-sorted sand forms beds as thick as 9 m, and these beds are associated with U1354A-S1 and U1354B-S1 (base of shelly sand layer) and U1354A-S2 and U1354B-S2 (debatable top of liquefied sand interval) (Fig. F11). The strongest correlation between holes is for surfaces U1354A-S4 and U1354B-S4, which were dated and correlated across holes by the Brunhes/Matuyama boundary.

Surfaces U1354A-S5, U1354B-S5, and U1354C-S5

Site U1354 is the only location at which three adjacent holes recovered surface S5 at comparable depths ranging from 74 to 80 m (Table T3). Muddy shelly Type A sand was recovered in each hole. However, a sharp burrowed contact was only recovered in Hole U1354C.

Surfaces U1354C-S5.1 and U1354C-S6

These two Type A surfaces are characterized by muddy shelly sand above a sharp contact that is bioturbated beneath. Surface U1354C-S5.1 is at 94 m and U1354C-S6 is at 118 m. (Table T3). The Pliocene/Pleistocene boundary was picked by biostratigraphy between 122 and 133 m, below U1354C-S6. Above this, the sediment age is between 1.73 and 1.81 Ma (NN) and below this the age of the sediment is >2.78 Ma (NN).

Surfaces U1354C-S7, U1354C-S7.1 and U1354C-S8

A particular characteristic of outer shelf Site U1354 is the occurrence of several Type A contacts and facies associations within a few meters of each other. For example, sharp, burrowed contacts between shelly sand above and mud below that could be assigned to U1354C-S7 were found at 132, 146, and 163 m.

U1354C-S7.1 is characterized by shelly sandy mud with a gradational lower contact containing concretions. This contact was placed at 181 m.

U1354C-S8 is another example of multiple lithologic contacts occurring in a short interval of stratigraphy. In all cases, the associated sediments are composed of shelly sandy marl. The sediment is more calcareous, and increased diagenesis and concretions are associated to this surface. Several lithologic contacts and associated sediments linked to U1354C-S8 are present at 218, 222, and 244 m.

Discussion and interpretation

Interpretation of Unit I

Unit I represents a heterolithic assemblage whose deposition throughout was likely influenced by fluctuating sea level. The Type 1 and 2 cycles, as described in "Lithostratigraphy" in the "Site U1351" chapter, also occur at Site U1354.

The dominant homogeneous mud lithology is interpreted as a shelf facies and likely represents the deepest water depth during deposition. Benthic foraminifers in Subunit IA indicate fluctuations between estuarine or subtidal and middle shelf environments between 0 and 30 m (Cores 317-U1354A-7H and 317-U1354B-8H) and generally inner shelf to outer shelf environments (see "Biostratigraphy"). Intercalated, gray clay beds may represent episodic fluxes of fine-grained sediment derived from fluvial or glacial discharges.

We interpret the other lithologies within Unit I as shoreface or estuarine to inner shelf, particularly in the basal portions of the Type I and II cycles deposited during transgressive phases. We suggest that these lithologies represent either transgressive shoreface deposits or sediment deposited immediately after the transgression (Browne and Naish, 2003; see "Lithostratigraphy" in the "Site U1351" chapter). Shell-hash intervals likely formed in a shoreface and/or beach setting, and were probably also a transgressive shoreface deposit. The green sandy shelly mud may represent periods of late highstand and regression, when more calcareous sediment was deposited and perhaps when terrigenous sediment diminished. These conclusions are similar to our interpretation of lithologies within Unit I at Sites U1351 and U1353 (see "Lithostratigraphy" in the "Site U1351" chapter and "Lithostratigraphy" in the "Site U1353" chapter).

Interpretation of Unit II

Poor core recovery in Unit II may have resulted in a bias toward the recovery of muddy lithologies at the expense of other lithologies. The generally muddy character of the sediments means that they are difficult to interpret from a sedimentologic perspective. Benthic foraminifers indicate depositional water depths that range from inner to outer shelf (see "Biostratigraphy").

The generally mud-rich nature of the unit suggests a relatively quiet depositional setting on a shelf below a fair-weather wave base. Muddy sand may represent periods when more terrigenous sediment was supplied from the margin. Biostratigraphy reveals significant hiatuses (e.g., during the late Pliocene; see "Biostratigraphy") and suggests considerable periods of erosion that were likely caused by sea level and/or tectonic fluctuations that affected the shelf environment.

Interpretation of lithologic surfaces and associated sediment facies

Correlation at Site U1354 was found between Type A contacts and facies associations and the predicted depths of seismic sequence boundaries (Table T3). These lithologic contacts were found within 2–30 m of the predicted depths of sequence boundaries (Fig. F12). In some cases, several contacts and facies potentially correlate to each respective sequence boundary.

Surfaces U1354A-S1 to U1354A-S4 and U1354B-S1 to U1354B-S4

These surfaces are correlated to seismic sequence boundaries U19, U18, U17, and U16. The depths at which these surfaces occur and the predicted depths of U19–U16 exhibit good correlation, with minor offsets of 5 m for U1354A-S3 and U1354B-S3 and U1354A-S4 and U1354B-S4. The Brunhes/Matuyama boundary was located and correlated across Holes U1354A and U1354B between 65 and 69 m, and the paleomagnetic age further contributed to the correlation of surfaces U1354A-S4 and U1354B-S4.

Surfaces U1354C-S5, U1354A-S5, U1354B-S5, U1354C-S5.1, and U1354C-S6

Type A surfaces and facies associations associated with U1354C-S5 were also identified in Holes U1354A and U1354B. These surfaces occur in the general depth range of U15, which has a predicted depth of 79 m. Surface U1354C-S5 was picked at 75 m, U1354B-S5 was picked at 74 m, and U1354A-S5 was picked at 80 m. U1354C-S5.1 is correlated to U14, which has a predicted depth of 93 m. This surface is also present at Site U1352, where it is also correlated to U14. U1354C-S6, located at 118 m, is correlated to U13, which has a predicted depth of 121 m.

Surfaces U1354C-S7, U1354C-S7.1, and U1354C-S8

Type A contacts and facies associations related to U1354C-S7 occur at 132, 146, and 163 m. The lithologic surface at 146 m is correlated to U12 at 156 m. U1354C-S7.1 at 181 m is correlated to U11 at 177 m. Hole U1354C is the only location at which a lithologic surface was correlated to U11. Four Type A deposits (at 218, 222, and 244 m) are related to surface U1354C-S8 and are potential lithologic surfaces of correlation for U10 at 211 m. The surface at 218 m has been tentatively selected. Future studies will investigate in more detail the role of contrasting lithologies, overlying strata thicknesses, the number of events, and other potential sources that may generate seismic impedance contrast and acoustic signal.

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