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Lithologic summaries of the six holes drilled at Site U1418 are shown in Figure F8. The sediment recovered at Site U1418 contains 15 facies. Detailed facies descriptions, information about common marine microfossils, facies occurrence in lithostratigraphic units, and tentative interpretations of depositional environments are summarized in Table T2. The dominant facies (F1a, F1b, and F2b) are predominantly gray (N 4) to dark greenish gray (10Y 4/1) mud and interbedded mud and silt and account for >90% of the core recovered. The remaining minor facies, although much less volumetrically significant, are distinctive and allow us to organize the cores into lithostratigraphic units. Photographs of the more common facies are shown in Figure F9. Based on characteristic facies associations, four major lithostratigraphic units and four subunits are defined (Table T3).

Facies description

Fifteen facies were identified and are outlined in Table T2. The numbering of the facies is based on facies documented for all Expedition 341 sites. Lithofacies include massive mud with lonestones (F1a), massive mud without lonestones (F1b), laminated mud (F1c), silt (F2a), interbedded silt and mud (F2b), very fine to coarse sand (F3a), muddy diamict (F4a), interbedded mud and diamict (F4d), diatom ooze (F5a), biosiliceous ooze (F5b), calcareous/carbonate-bearing mud (F5c), volcanic ash (F6), volcaniclastic mud and sand (F7), rock (F8), and intrastratal contorted mud and diamict (F9). These facies reflect deposition from suspension fall out, sediment gravity flows/large-scale mass wasting, ice rafting, variation in organic productivity, and volcanic eruptions.

The massive and bioturbated mud of Facies F1a is dark gray (N 4) to dark greenish gray (10Y 4/1) and has bed thicknesses that range from 2.5 to 964 cm (Table T2). Color banding ranges from gray (N 4) to dark greenish gray (10Y 4/1); individual bands are 0.1 to 5 cm thick. Bioturbation is mostly absent or slight but occasionally moderate to heavy (Fig. F10J–F10K). A diagnostic characteristic of Facies F1a is the absence or low abundance of microfossils. Based on smear slides, the composition of the mud is, on average, 75% clay-size particles and 25% silt particles, with no apparent compositional differences between different color bands. Lonestones consist mainly of argillite, siltstone, and metasiltstone, with subordinate amounts of granitoid and sandstone and minor amounts of basalt (greenstone), gabbro, limestone, quartzite, red chert, and rhyolite (Fig. F10A–F10H). Facies F1b is identical to Facies F1a, except for the absence of lonestones (Table T2; Fig. F9A). Facies F1c consists of laminated or bedded greenish gray (10Y 5/1) to dark gray (N 4) mud (Fig. F9B; Table T2). Facies thickness ranges from 0.5 to 485 cm. Bioturbation is generally absent and contacts with other units are gradational. Petrographically, the composition of Facies F1c is similar to that of Facies F1a and F1b. Laminations of Facies F1c are presumably related to slight changes in grain size and/or geochemical composition that are not easily recognizable from smear slide analysis.

Facies F2a consists of thin silt beds (Table T2) that range in color from dark gray (N 4) to greenish gray (5GY 5/1) and are interbedded with the mud of Facies F1a, F1b, and F1c, respectively. The silt beds have sharp lower contacts and more gradational upper contacts. Individual laminae and bed thicknesses are 0.2 to 7 cm and often exhibit normal grading. Facies 2a is often well sorted. The framework grain composition of this facies is almost entirely quartz and feldspar with minor amounts of heavy minerals based on smear slide analysis. Accessory framework grains include biotite and hornblende. Quartz in Facies F2a is monocrystalline, and few lithic grains were documented. Facies F2b consists of dark gray (N 4) to very dark gray (N 3) interbedded/interlaminated silt and mud (Fig. F9D, F9G). Facies thickness ranges from 3 to 1067 cm, and up to 50 laminae per core section were documented. Lower contacts of silt laminae are most often sharp. Upper contacts are sharp or gradational. Bioturbation is none to slight, and lonestones are present in low abundance.

Facies F3a (Table T2) consists of dark gray (N 4) to gray (N 5) fine to coarse sand (Fig. F9E). This facies often has a sharp lower contact and a gradational upper contact. Bed thickness commonly ranges from 1 to 11 cm, but maximum bed thickness is 38 cm. The sand has a muddy matrix, is poorly sorted, and sometimes contains minor amounts of diatom fossils and volcanic ash. The composition of framework grains of sand is mainly quartz and feldspar with few lithic grains identified in smear slides. Quartz in Facies F3a is mainly monocrystalline. Heavy minerals are minor but consistent constituents.

Facies F4a is a dark gray (N 4) diamict that has variable clast concentration (Table T2; Fig. F9I–F9J). Common clast sizes are granule and pebble, with clasts being subangular to subrounded. Common clast types are argillite, siltstone, metasiltstone, granitoid, sandstone, basalt (greenstone), gabbroic, chert, and rhyolite (Fig. F10A–F10H). Based on smear slides, the composition of the sand fraction is primarily quartzofeldspathic and the mud fraction is typically 35% silt and 65% clay-size particles. Beds of this facies thickness are 2 to 1435 cm. Interbedded mud and diamict define Facies F4d (Table T2; Fig. F9H). This facies is characterized by laminated mud, with diamict beds up to 4 cm thick. Maximum clast size in the diamict is 2 cm. Clast types in the diamict are similar to those described for Facies F4a. The contacts between the mud and diamict interbeds in Facies F4d are gradational in most cases. Intervals of this facies are 12 to 441 cm thick. It is often color banded, with the diamict forming the darker parts of the color banding and the laminated mud the lighter parts.

Facies F5a is composed of diatom ooze (Table T2; Fig. F9C). By definition, lithostratigraphic units within this facies contain >50% diatoms (see “Lithostratigraphy” in the “Methods” chapter [Jaeger et al., 2014a]). The remaining constituents in this facies are typically mud, and in some cases, calcareous mud was documented. Contacts vary between gradational and sharp. The distinction between Facies F5a and F1a, F1b, and F1c can be very subtle and only recognizable by the documentation of diatoms in smear slides. Bed thickness for Facies F5a varies from 3 to 291 cm. Bioturbation and black mottling is common. The biosiliceous ooze of Facies F5b is similar to the characteristics described for Facies F5a, except that the biogenic material consists of some combinations of diatoms, sponge spicules, radiolarians, and/or nannofossils. Facies F5c is dark gray (N 4) calcareous/carbonate-bearing mud (Table T2). This facies includes calcareous-rich laminae as thick as 0.2 cm, as well as larger intervals with calcareous-bearing mud. It can be as thick as ~195 cm.

Facies F6 is defined by gray (5Y 4/1) to brown (5YR 5/3) volcanic ash (Table T2; Fig. F9F). Bed contacts range from sharp to gradational. Compositionally, this facies consists of 90% glass shards (vitric fragments). The remaining framework grains are feldspar, quartz, and opaque minerals. In intervals of heavy bioturbation, the ash becomes mixed with the underlying and overlying mud and is sometimes concentrated in burrows. Volcaniclastic mud and sand define Facies F7 (Table T2). Bed thickness commonly ranges from 0.5 to 558 cm. These beds consist of a mixture of volcanic glass (typically 10%–20%), quartz, feldspar, and often diatom microfossils. Facies F7 is often bioturbated. Facies F8 consists of 1–6 cm thick intervals of lithified siltstone with calcite cement (Fig. F10I) that most probably reflects in situ precipitation of calcite related to diagenesis.

Facies F9 consists of intrastratal contorted mud and diamict (Table T2; Fig. F11). Very dark greenish gray (10Y 3/1) mud mixed with very dark gray (N 3) muddy diamict with variable clast content are the two major lithologies (Fig. F11A–F11B). We merged these two lithologies into one facies category because of the common occurrence together and limited extent (Cores 341-U1418F-70R through 72R). Facies F9 is characterized by soft-sediment deformation and contorted bedding (Fig. F11A). Contorted mud occurs in beds with thicknesses of 50–100 cm, whereas muddy diamict occurs in beds that average 20–40 cm. Contacts between the two lithologies appear to be gradational. There is a range of soft-sediment deformation fabrics, including

  • A flow slurry where the long axis of centimeter-scale intraformational clasts are oriented parallel to the fabric in the lighter colored, finer grained matrix (Fig. F11C). This fabric has well-defined flow lineation. In some cases, clasts form a greater proportion than the matrix, and in other cases, clasts and matrix are in roughly equal proportions.
  • A mud breccia where angular clasts of mud are separated by smaller amounts of lighter colored matrix (Fig. F11D). This fabric does not display well-developed flow lineation, and clasts are much more common than matrix.
  • A flow-parallel fabric defined by laminated millimeter-scale bands of mud with internal evidence of soft-sediment deformation that contains no intraformational clasts (Fig. F11E).

Common clast types in the diamict of Facies 9 are rhyolite, basalt (greenstone), siltstone, sandstone, argillite, quartz, gneiss, and granite (Fig. F11B). Clasts are typically subrounded, but some smaller clasts are subangular; the largest clast diameters average 2 cm. The diamict contains a wide range of grain sizes with a mud-rich matrix. Smear slide analysis indicates that the mud is poorly sorted, a mixture of silt and clay grain sizes. Normal faults are common in this unit (Fig. F11F–F11G); these faults commonly have <2 cm of displacement. Some of these faults are at a high angle to surrounding beds, and others are at a low angle. A few rare lonestones were identified in this unit.

Lithostratigraphic units

Based on facies associations, four major lithostratigraphic units (I–IV) are defined (Table T3). Unit II is further divided into Subunits IIA–IID based on the characteristics of diamict intervals. The contacts between lithostratigraphic units at Site U1418 are usually gradational, and the criteria used to define units are discussed below.

Unit I

  • Intervals: 341-U1418A-1H-1, 0 cm, to 33H-CC, 25 cm; 341-U1418B-1H-1, 0 cm, to 2H-CC, 16 cm; 341-U1418C-1H-1, 0 cm, to 32H-CC, 18 cm; 341-U1418D-2H-1, 0 cm, to 32H-CC, 24 cm; 341-U1418E-2H-1, 0 cm, to 13H-CC, 14 cm
  • Depths: Hole U1418A = 0–209.9 m core depth below seafloor (CSF-A); Hole U1418B = 0–17.0 m CSF-A; Hole U1418C = 0–230.76 m CSF-A; Hole U1418D = 3.0–257.3 m CSF-A; Hole U1418E = 78.0–181.6 m CSF-A
  • Age: Middle Pleistocene to Holocene

Dark gray (N 4) to dark greenish gray (10Y 4/1) mud with interbedded silt alternates with intervals as thick as 15.5 m of color-banded dark gray (N 4) mud (Table T3; Fig. F9A–F9B). Silt laminae are commonly <5 mm thick and occur in groups of between 5 and 34 laminae at variable spacing within the mud (Fig. F9D). Bioturbation is mostly absent or slight. Diatom ooze occurs in two broad intervals, at the top of the hole and sporadically from 100 to 200 m core composite depth below seafloor (CCSF-B) (Fig. F12). Diatom-bearing to diatom-rich mud occurs within numerous cores (see Fig. F8 unit descriptions for specific cores). Each interval with diatoms has gradational contacts and greater bioturbation intensity, and black mottling is often present. Graded sand beds with sharp lower contacts (Fig. F9E) occur infrequently. Five ash beds are present, and volcaniclastic-bearing to volcaniclastic-rich mud occurs in irregular patches. Lonestones appear deeper than 3 m CSF-A and are present throughout. Common lithologies of the lonestones are siltstone, metasediment, and igneous rock (Fig. F10A–F10H).

Unit II

This unit is dominated by mud interbedded with intervals of clast-poor diamict that range in thickness from a few centimeters to >1 m. Mud with clast concentrations ranging from dispersed to abundant is also present. Unit II extends over 546 m at Site U1418 and is divided into four subunits based on the relative thickness and occurrence of the diamict facies within the mud facies.

Subunit IIA
  • Intervals: 341-U1418D-32H-CC, 24 cm, to 37X-CC, 43 cm; 341-U1418F-4R-1, 0 cm, to 9R-5, 70 cm
  • Depths: Hole U1418D = 257.3–297.93 m CSF-A; Hole U1418F = 279.4–334.6 m CSF-A
  • Age: Middle Pleistocene

Subunit IIA contains dark gray (N 4) to dark greenish gray (10Y 4/1) muddy diamict with generally lower clast abundance (Fig. F9I) interbedded with dark gray (N 4) mud, as well as mud with dispersed clasts. Contacts between beds appear gradational. The mud is weakly laminated. Coarse sand, granules, and lonestones of variable composition occur within the diamict and mud.

Subunit IIB
  • Interval: 341-U1418F-9R-5, 70 cm, to 44R-1, 111 cm
  • Depth: Hole U1418F = 334.6–668.5 m CSF-A
  • Age: Middle Pleistocene

Dark gray (N 4) to dark greenish gray (10Y 4/1) laminated mud with layering ranging from submillimeter (a few coarse silt grains) to 4 mm is interbedded with mud with dispersed clasts and diamict with generally low clast content (Fig. F9H). Commonly, laminae occur in packages of variable thickness bounded with gradational contacts by thin beds of diamict with generally low clast content forming interbedded mud and diamict facies. Some intervals are calcareous bearing, as seen in smear slides, and may include authigenic carbonate and in some cases foraminifers. Volcanic ash occurs as burrow fills and in dispersed intervals of moderate to heavy bioturbation. Lonestones occur throughout. A few normal faults with <1 cm of displacement were documented in this unit (Fig. F10N).

Subunit IIC
  • Interval: 341-U1418F-44R-1, 111 cm, to 47R-CC, 19 cm
  • Depth: Hole U1418F = 668.5–706.2 m CSF-A
  • Age: early Pleistocene

Subunit IIC contains very dark greenish gray (10Y 4/1) massive clast-poor diamict interbedded with mud. Thirteen diamict intervals with gradational contacts occur, ranging from 30 to 150 cm in thickness. Mud is massive, thinly color banded, or laminated. Diatom ooze occurs in Section 341-U1418F-46R-2A (Fig. F9C). Moderate bioturbation occurs in the diamict. Volcanic ash is present in bioturbated mud and ooze. Lonestones occur within the mud.

Subunit IID
  • Interval: 341-U1418F-48R-1, 0 cm, to 58R-1, 10 cm
  • Depth: Hole U1418F = 706.2–803.3 m CSF-A
  • Age: early Pleistocene

Subunit IID contains very dark greenish gray (10Y 4/1) to dark gray (N 4) laminated mud with some color banding interbedded with thin-bedded diamict with generally lower clast content (Fig. F9H). Gravel and coarse sand occur in beds a few centimeters thick with gradational contacts. Stratification varies from millimeter-scale lamination to thin bedded. Lonestones are scattered throughout. Section 341-U1418F-50R-1A contains small-scale ripple stratification (Fig. F10L). Sections 50R-2A and 3A contain two slump features that are ~50 cm in individual thickness and result in discordant bedding orientations relative to the horizontal bedding in the rest of the core (Fig. F10O–F10P). These cores also contain minor unconformities recognized by erosional surfaces and/or slightly discordant bedding (Fig. F10M).

Unit III

  • Interval: 341-U1418F-58R-1, 10 cm, to 70R-1, 0 cm
  • Depth: Hole U1418F = 803.3–919.6 m CSF-A
  • Age: early Pleistocene

Laminated and bioturbated dark gray (N 4) mud is the dominant lithology. Thinly bedded very dark gray (N 3) sandstone and gray (N 6) siltstone are minor but distinctive components. Two 10 cm thick beds of diamict with generally higher clast content containing rip-up clasts occur near the base of the unit. Minor units often have calcareous cement. Thin intervals of well-cemented carbonate mud, as well as small numbers of lonestones, occur throughout this unit (Fig. F10I). Microfossils are rare in this unit based on smear slide analysis. Ash is also rare, but a well-preserved example was documented in interval 341-U1418F-58R-5A, 83–89 cm (Fig. F9F). Although volcaniclastic-bearing to volcaniclastic-rich mud was documented in Units II and III, Core 58R contains the only occurrence of a bed of >90% ash. Other ash falls appear to have been bioturbated into the surrounding mud, leaving discontinuous mixed pods.

Unit IV

  • Interval: 341-U1418F-70R-1, 0 cm, to 72R-CC, 21 cm
  • Depth: Hole U1418F = 919.6–941.4 m CSF-A
  • Age: early Pleistocene

Very dark greenish gray (10Y 3/1) mud mixed with a very dark gray (N 3) muddy diamict with generally higher clast content are the two major lithologies (Fig. F11A, F11C). Both lithologies are characterized by soft-sediment deformation and intrastratal contortions (Fig. F11A–F11E). Convoluted gray mud occurs in beds with thicknesses of 50–100 cm, whereas muddy diamict occurs in beds that are, on average, 20–40 cm thick. Contacts between the two lithologies appear to be gradational. A range of soft-sediment deformation fabrics is discussed in the description of Facies F9. Normal faults are common in this unit (Fig. F11F–F11G). A few rare lonestones were identified.


Clast lithologies

The main lithologies of the diamict clasts and lonestones contained in Site U1418 sediment are, in order of decreasing abundance, argillite, siltstone, basalt, sandstone, granitoid, and limestone (Fig. F10A–F10H). The granitoid group includes intermediate and felsic intrusive rocks. Gabbros represent a minor lithology, as do quartzite, metasiltstone, chert, rhyolite, gneiss, and diorite. These lithologies are unevenly distributed in Holes U1418A–U1418F. Composition for each hole and the site is determined according to the main lithology types, metamorphic (M), sedimentary (S), and igneous (I) (Fig. F13), and reveal variation between holes and also the predominance of metamorphic and sedimentary lithologies over igneous. The average clast ratio for Site U1418 is M39S23I38.

Bulk mineralogy

X-ray diffraction (XRD) analyses were performed on 108 powdered bulk samples from Holes U1418A, U1418D, and U1418F to delineate bulk mineralogy and identify compositional trends with age or depth in the cores. Representative diffraction patterns are shown in Figure F14, and the relative mineral diffraction peak intensities, as defined in “Lithostratigraphy” in the “Methods” chapter (Jaeger et al., 2014a), are listed in Table T4. In general, the mineralogy is uniform downhole, although there are some variations in relative peak intensities that may indicate slight variations in mineral content. The primary minerals identified include quartz, plagioclase (feldspar), mica (muscovite/illite and biotite), and the minerals chlorite and/or kaolinite (Fig. F14A). Quartz and plagioclase are the dominant peaks, with quartz generally the larger. Figure F14B shows the comparative XRD patterns from 4° to 24°2θ, where the scans were run before and after the samples had undergone a glycolation treatment (see “Lithostratigraphy” in the “Methods” chapter [Jaeger et al., 2014a]). This treatment was used to determine the presence of expandable clay minerals (e.g., smectite). The scans show no indication of expandable clay minerals in the cores until ~800 m CCSF-B. On the last sample, we observed a shoulder at ~7°2θ in the whole samples. After the treatment, a small peak was observed at ~5°2θ, suggesting the presence of expandable clay minerals. Additional minor mineralogical phases documented in the bulk mineralogy of the cores include calcite associated with well-indurated siltstone intervals and pyrite associated with localized structures such as burrows. Our preliminary findings are similar to the results of Molnia and Hein (1982) from samples collected on the continental shelf of the Gulf of Alaska.

Lithostratigraphy and depositional interpretations

Figure F12 shows the distribution of observed primary sedimentary lithologies and characteristics and how they relate to unit boundaries and core recovery at Site U1418. Unit I is characterized by the high abundance of interbedded mud and silt and sand beds. Although a minor constituent, the highest abundance of diatom ooze and diatom-rich mud lithofacies observed at Site U1418 was between 100 and 175 m CCSF-B. The number of clasts per meter, which was only recorded within this unit because they become too abundant to individually count in deeper units, increases toward the top of Unit I, culminates at 125 m CCSF-B, and then declines toward the top of the sequence. Relative to the underlying units, we also observe a higher number of volcanic ash layers and increased occurrences of volcaniclastic-bearing sand and mud. Subunit IIA is defined by the abrupt appearance of diamict and mud with variable clast abundance and the disappearance of the interbedded mud and silt and sand layers observed in Unit I (Fig. F12). Subunit IIB is characterized by the alternation of mud with clasts, laminations, and mud. The mud between 525 and 625 m CCSF-B exhibits bioturbation intensity index values of 3, and at a finer (meter level) observational scale, these bioturbated intervals are found between laminated mud and/or mud with clasts. The abrupt appearance of diamict interbedded with units of laminated mud typifies Subunit IIC. The Matuyama/Brunhes boundary, which marks the transition between the early and Middle Pleistocene (0.781 Ma), was identified within the upper portion of Subunit IIC (see “Paleomagnetism”). Subunit IID consists of alternation of mud with clasts, mud with laminations, and mud with extensive bioturbation and overall has a similar appearance to Subunit IIB (Fig. F12). The abrupt downcore appearance of interbedded mud and silt at ~800 m CCSF-B, in conjunction with mud with laminations and mud with clasts, characterizes Unit III. Volcanic ash is present near the top of the unit, and clast-rich diamict beds are found near the base at ~900 m CCSF-B. The top of Unit IV is at 920 m CCSF-B and is typified by a mixture of gray to dark gray (N 3 to N 4) mud and clast-rich muddy diamict with soft-sediment deformation and intrastratal contortions. Bioturbated and laminated mud are notably absent from this unit. We compared the lithologic characteristics described above with physical properties data collected by the onboard core logging systems but did not encounter consistent associations between natural gamma radiation (NGR) and abs (a*/b*) and the sedimentary components as we did at Site U1417 (see “Lithostratigraphy” in the “Site U1417” chapter [Jaeger et al., 2014b]). We do, however, see some association between the distribution of lithostratigraphic units and changes in magnetic susceptibility where peaks in clast abundance in Unit I are coincident with peaks in magnetic susceptibility, and these associations are described in more detail in “Core-log-seismic integration.”

Unit I

Lithostratigraphic Unit I consists mainly of dark gray (N 4) mud with thin silt beds (Fig. F9D). We interpret most of the mud in Unit I as originating from suspension settling through the water column and mud-rich sediment gravity flows. The thin silt beds are interpreted to represent frequent and regular deposition from silt-rich sediment gravity flows in the overbank environments of the proximal Surveyor Fan system. The documentation of thin sand beds with normal grading and erosive bases (Facies F3b on Table T2), although uncommon, suggests that periodically more tractive, higher velocity currents reached the overbank environments. Subaqueous sediment gravity flows could have been generated from the continental shelf/slope by many episodic processes that likely include seasonal fluctuations at the glacier terminus, earthquakes, high sedimentation rates that created slope oversteepening, wave-induced liquefaction, and iceberg calving (Powell and Molnia, 1989). The lonestones in Unit I are interpreted to be ice-rafted debris mostly deposited by icebergs calved from tidewater glaciers and/or from sea ice. The diatom intervals in Unit I might be related to one or several processes:

  • Increased biological productivity in the water column in the vicinity of sea ice margins (Sakshaug, 2004; Smith et al., 1987);
  • Increased biological productivity due to optimized oceanographic conditions (e.g., reduced sea ice cover, surface layer overturning, and/or mixing by gyres);
  • Enhanced macro (N, P) and/or micro (Fe) nutrient supply from land (volcanic ash, dust, etc.) leading to increased biological productivity (Hamme et al., 2010);
  • Seawater silica saturation, leading to a higher diatom productivity and preservation (e.g., Brzezinski et al., 1998; Dugdale et al., 1995); and
  • Decreased input of terrigenous sediments (i.e., less dilution).

The sources for lonestones documented in Unit I are interpreted to be the onshore St. Elias Mountains, Chugach Mountains, and coastal ranges located along the southern coast of Alaska and northwestern British Columbia, Canada. Metasedimentary rocks, common lithologies in the lonestones, occur in all these ranges (Plafker, 1987; Plafker et al., 1994; Gasser et al., 2011), but the combination of metagabbro, quartzite, rhyolite, and limestone suggest that the rocks of the Alexander Terrane (Gehrels and Saleeby, 1987; Gehrels and Berg, 1994) may be considered as a possible source for some of the ice-rafted debris. Felsic igneous clasts may be derived from the Sanak-Baranoff plutons found dispersed along the southern Alaska margin (Sisson et al., 2003). A previous study of lonestones collected from fjords along the coast of Alaska found these lithologies and interpreted the Alexander Terrane as a probable source (Cowan et al., 2006). The rare volcanic ash and volcaniclastic-bearing sand at Site U1418 indicates that the location was proximal enough to either the Aleutian or Wrangell volcanic belts to have periodic influxes of pyroclastic detritus.

Unit II

The strong influence of icebergs (and/or sea ice) on the deposition of facies within Unit II is indicated by the presence of ice-rafted debris in the form of lonestones deposited in massive or laminated mud, as well as the accumulation of diamict. Subunits IIA and IIC are both identified by the presence of massive clast-poor muddy diamict with pebbles, granules, and coarse sand scattered throughout. Laminae are present but rare within the interbedded mud and diamict. Subunit IIA is similar to Subunit IIC: a regular pattern of massive diamict grades into laminated to very thin bedded mud. The 13 diamict beds in Subunit IIC range in thickness from 30 to 150 cm. These thick diamict beds appear to indicate periods of more intense iceberg rafting. Most are associated with moderate to occasionally heavy bioturbation, which may suggest a slower sediment accumulation rate than in the laminated mud intervals. Further, the addition of micronutrients (associated with the terrestrial sediments) may have stimulated marine productivity. Periods with higher marine productivity are indicated by diatom ooze that occurs in Core 341-U1418F-46R within Subunit IIC.

The diamict beds within Subunits IIB and IID are more variable in thickness and composition. Most commonly they are thin and interbedded with laminated mud or mud with dispersed clasts. Interbedded mud with diamict facies occurs commonly within these subunits and is characterized by several-centimeter-thick diamict that is composed of granules and coarse sand but notably lacking in lonestones (Fig. F9H). These thin, bedded diamict intervals are bounded by thinly laminated packages of mud that appear to be aggradational. This lithology is similar to gravelly mud beds and laminated mud described from Southeast Alaskan glacial fjords (Cowan et al., 1999) that was deposited by sea ice rafting and turbid meltwater plumes. In the modern glacimarine setting of Alaska (restricted to bays and fjords), sea ice forms in winter from freezing of freshwater at the sea surface and debris is entrained along beaches and fan deltas as ice is stranded along the shore by tidal variations. During the summer, meltwater plumes deposit laminated mud (Cowan et al., 1999). The silt-mud couplets, termed cyclopels, form by suspension settling that is influenced by variations in meltwater stream discharges, wind-generated waves and currents, and tidal fluctuations (Powell and Molnia, 1989). The thin diamict beds at Site U1418 may record the presence of sea ice during glacial periods; however, further multiproxy studies are needed to verify the presence of sea ice. Although sea ice is not present in this region today, studies of diatoms and dinocysts indicate that it was widespread during colder periods such as the Younger Dryas and Last Glacial Maximum (Barron et al., 2009; de Vernal and Pedersen, 1997).

Unit III

Unit III is similar to Unit II, with massive/bioturbated mud occasionally interbedded with laminated mud. The diagnostic characteristics of Unit III relative to Unit II are the introduction of siltstone and sandstone beds, two thin clast-abundant muddy diamicts beds with rip-up clasts, a decrease in lonestone occurrence, and an overall decrease in diamict beds. The well-sorted, thin silt/sand beds at the top of this unit have sharp bases and are interpreted as distal sediment gravity flow. Clast-rich diamicts at the base of the unit are interpreted as sediment gravity flows. Despite the decrease in the number of lonestones and diamict beds relative to Unit II, their presence in this unit indicates that glacially derived and transported sediment was an important contributor during deposition. The cycles of interbedded massive and bioturbated mud (with dispersed clasts and few diamict beds) and laminated mud may point to periods characterized by a weaker glacimarine signal that alternate with periods of more intense ice rafting.

Unit IV

The mixture of gray to dark gray (N 3 to N 4) mud and clast-rich muddy diamict characterized by soft-sediment deformation and intrastratal contortions of Unit IV are interpreted as the products of one or more mass transport events, which were interpreted from seismic reflection data (see “Background and objectives;” Reece et al., submitted). The range of soft-sediment deformational fabrics suggests that some of the beds were partially lithified before being transported and redeposited at Site U1418. The large amount of fine-grained matrix with flow textures surrounding angular blocks in this deposit suggests that large amounts of fluid or slurry were available during transport and redeposition of the failed sediments. The timing of the mass transport event appears to be at ~1.2 Ma based on our current understanding of the age model for this site (see “Stratigraphic correlation” and “Paleomagnetism”).