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doi:10.2204/iodp.proc.323.105.2011 LithostratigraphyThree holes were drilled at Site U1341; the deepest, Hole U1341B, reached 601.87 mbsf (621.82 m core composite depth below seafloor [CCSF-A]). The sediments recovered at Site U1341 are a mixture of diatoms and siliciclastic sediments with minor amounts of ash, foraminifers, calcareous nannofossils, and sponge spicules. The sediments are predominantly very dark greenish gray to dark gray or dark gray to olive-gray to light olive in color. Authigenic carbonates occur frequently at this site below 130 mbsf. Three lithologic units were defined at Site U1341, and Unit III was further divided into two subunits (Fig. F6). The Unit I/II boundary is defined by a major lithologic change from sediments dominated by laminated diatom ooze to sediments that alternate between diatom silt/clay and diatom ooze. The boundary between the two units occurs with a significant increase in magnetic susceptibility at ~205 mbsf in Hole U1341B and is early Pleistocene in age. The boundary between Unit II and Subunit IIIA is defined by a change in lithology from clays rich in authigenic carbonates to mostly diatom ooze. Subunit IIIB consists of alternations between diatom clay and diatom ooze. The bottom of Hole U1341B reaches the middle Pliocene. Description of unitsThe most abundant terrigenous grain types are silt-sized feldspar, quartz, clay, mica, and rock fragments (mainly polycrystalline quartz; see also "Site U1341 smear slides" in "Core descriptions"). Pebble-sized clasts occur frequently throughout this site and are often composed of basalt, pumice, or scoria. Ash layers occur less frequently than at Site U1340 on the eastern side of Bowers Ridge. These ash layers are typically black, light gray, dark reddish gray, or light reddish gray. Volcaniclastic grains are often mixed with biogenic and siliciclastic sediments through bioturbation. Dolostones and micrometer-scale crystals of authigenic carbonate occur below 130 mbsf in all holes at Site U1341. Authigenic carbonate patches, nodules, and layers are light olive-gray and olive-gray, are often characterized by a granular texture, and are more indurated than surrounding sediments. Intervals with soft-sediment deformation are less common than at Site U1340 and are largely confined to Cores 2H and 3H in all holes. Deformation appears as folded and tilted bed boundaries, suggesting synsedimentary slumping as a potential mechanism. Some deformation occurs deeper in holes without significant disturbance to the overall stratigraphic relationships of the sediments. Clasts occur from 0 to 400 mbsf with no apparent patterns in distribution. The clasts are mostly pebble-sized, vary from angular to rounded, and are volcanic or volcaniclastic in origin. However, in Cores 323-U1341B-57X and 58X, many dolostone concretions occur in an interval of clay lithology. This interval is described more fully below in the description of Unit III. In the database and summary figures, these concretions were sometimes classified as clasts, although an authigenic origin is likely, because authigenic carbonates were observed in smear slides from these cores. The core tops from Core 323-U1341B-25H and deeper often contain a mixture of black basalt clasts and whitish carbonate clasts. These clasts are not in place and fell in from the hole walls during coring. Clasts were also sometimes observed along the core liner and were probably pushed there during the coring process. These clasts were recorded in the database not as clasts but as the drilling disturbance type "fall-in" because they are apparently not in place. Boundaries between different sediment colors and/or lithologies are dominantly gradational and bioturbated, but occasional sharp contacts occur as well. Variations in color and lithology can generally be described as irregular bedding on a decimeter to meter scale in all holes and at all depths. Intervals with thin, distinct parallel laminations are confined to Unit I (Fig. F7B). In all units there are thickly laminated to thinly bedded intervals that have undulating or wavy boundaries that are slightly to moderately bioturbated (Fig. F7A). Sequences with no visible bioturbation are massive and may be intensely bioturbated. Individual burrows are not often well preserved, and hence bioturbation cannot be related to specific ichnofacies types in most cases. Where visible, bioturbation was mostly assigned to the Chondrites ichnofacies with minor Skolithos and Planolites. Unit I
This unit is characterized by an alternation between diatom ooze, mixed lithologies of diatoms with clay/silt, and silt/clayey silt (see "Site U1341 smear slides" in "Core descriptions"). The sediment color ranges from dark greenish gray (10Y 4/1) to dark olive (5Y 4/1) and from dark grayish brown (5Y 4/2) to olive (5Y 5/4). The alternation occurs on a decimeter to meter scale. In addition, this unit has intervals with abundant calcareous nannofossils, foraminifers, or sponge spicules. These microfossils tend to be most abundant in light-colored laminae within laminated intervals and also within some patches or burrows. There is an interval in Cores 9H and 10H in all holes where nannofossils are notably abundant (5%–40%) and the sediment is slightly lighter colored (light gray, 10YR 7/1), with more intensely bioturbated boundaries with surrounding units (Fig. F7). Intervals with thin parallel laminations are usually 5–30 cm thick and can sometimes be correlated between holes. In this unit, thick laminations and thin bedding are common and frequently very faint, and there are instances, such as in Core 12H in all holes, where different observers judged sediment that otherwise appears identical as having laminations, bedding, or neither. This can account for the differences in the recorded occurrence of laminations and bedding between holes. Unit II
The dominant lithology in this unit is diatom ooze, with one interval in Hole U1341B (Cores 37H through 46H) dominated by diatom clay. The diatom ooze is dark grayish brown (2.5Y 4/2), dark greenish gray (10Y 4/1), dark olive-green (5Y 4/1), and olive-gray (5Y 4/2). The diatom clay is mostly dark greenish gray (10Y 4/1). In contrast to Unit I, calcareous microfossils never reach concentrations >5%, but sponge spicules and sponge spicule aggregates are abundant (>5%) in a few intervals (see also "Site U1341 smear slides" in "Core descriptions"). Laminations and thin bedding are pervasive throughout Unit II and occur more frequently than in Units I or III. Unit IIISubunit IIIA
This subunit consists of olive (5Y 5/3) silty clay and is distinguished from Unit II by a sharp change in lithology. The silty clay contains finely disseminated authigenic carbonates, numerous angular pebble-sized authigenic carbonate concretions, and two dolostone layers (Fig. F8B). Significant concentrations of sponge spicules are not observed in this interval, although they make up 5%–10% of the sediments in the lithologies immediately above and below (see "Site U1341 smear slides" in "Core descriptions"). Subunit IIIB
This subunit consists of alternations between light olive-gray (5Y 6/2) and olive-gray (5Y 4/2) diatom ooze and diatom silty clay. Sponge spicule aggregates were frequently observed, and sponge spicules are abundant (>5%) in a few intervals (see "Site U1341 smear slides" in "Core descriptions"). There are occasional narrow intervals with faint reddish or brownish laminations in this unit. Authigenic carbonates occur in one interval within this subunit in Cores 323-U1341B-62X through 64X. DiscussionUnit I at this site spans the same time period as Unit I at other sites. Units II and III at Site U1341 appear to span the same time periods as Units II and III at Site U1340, and there are also similarities in the lithologies of Sites U1341 and U1340. Unit I at both sites consists of alternating diatom ooze and mixed diatom-siliciclastic lithologies. Unit II is mostly diatom ooze, with an interval centered at ~2.5 Ma dominated by mixed diatom-siliciclastic lithologies (silt at Site U1340 and clay at Site U1341), and Subunit IIIA contains an interval just below the 3.8 Ma datum (see also "Paleomagnetism" and "Biostratigraphy" both in the "Site U1340" chapter) that is dominated by siliciclastic lithologies (sand in Site U1340 and clay in Site U1341). The similarities in lithology and age between Sites U1341 and U1340 suggest broadly similar changes in sedimentation history. However, there are also some interesting differences. The intervals where clasts are observed are similar at the two sites, but clasts are notably more abundant in Unit I at Site U1340. There is no similar increase in clast abundance in Unit I of Site U1341. The laminations and bedding characteristics of Unit II at Site U1341 are similar in appearance to those in Unit II at Site U1340. At both sites, the laminations and bedding are particularly pervasive during the siliciclastic-rich interval in Unit II. The frequency of thin, parallel laminations in Unit I at Site U1341 appears to be lower than in Unit I at Site U1340, but as discussed above, it is possible that this is an artifact of the faintness of some laminations and of inconsistency in how these features were recorded. The occurrence of soft-sediment deformation related to slumping was less extensive at Site U1341 than at Site U1340, but the deformation may have the same trigger mechanism (see "Lithostratigraphy" in the "Site U1340" chapter). Ash layers at Site U1341 were a less prominent feature than at Site U1340, perhaps reflecting the more distal location of Site U1341 to the Aleutian arc. Ash-filled mottles and dispersed ash may have originally been thin ash layers that were mixed with the surrounding sediment through bioturbation. Low gamma ray attenuation (GRA) bulk density broadly correlates with high diatom abundance (Figs. F6, F9, F10). Diatom ooze may reflect interglacial conditions, whereas mixed diatom-siliciclastic lithologies may reflect glacial conditions. This is consistent with the pattern of biogenic opal mass accumulation rates (MARs) observed in piston cores from the Bering Sea (Katsuki and Takahashi, 2005). The diatom clay lithology in Unit II represents a period of relatively high siliciclastic grain deposition at Site U1341, perhaps reflecting the onset of glaciation in the Bering Sea and surrounding continental areas. The increase in clay in this interval is also visible in the logging data as higher natural gamma radiation (NGR) and potassium and was defined as logging Unit 3 (see "Downhole measurements"). The lithologies in Unit I are richer in diatoms, and the siliciclastic grains are dominated by silt rather than clay, leading to lower NGR and potassium signals. The increase in magnetic susceptibility at the base of Unit I may reflect a change in the nature or preservation of magnetic grains deposited at Site U1341 after 1.6 Ma (see "Paleomagnetism"). Coarser grains may be better preserved in the sediment, thereby delivering a stronger magnetic signal after 1.6 Ma. If coarser grains were transported by ice rafting, this would reflect the intensification of glaciations after 1.6 Ma. An interesting feature of Site U1341 is the occurrence of sediment intervals rich in nannofossils (Fig. F11). Calcareous nannofossils are rare in the sediments drilled at IODP Expedition 323 sites, with the exception of Site U1341, where a small interval of nannofossil-diatom ooze was recovered in all holes. This sudden increase in nannofossil abundance is associated with increased foraminifer abundance and may be explained by increased productivity, increased carbonate preservation, or both. The calcareous nannofossil assemblage is dominated by small gephyrocapsids and includes specimens of the heavily calcified Gephyrocapsa caribbeanica and Coccolithus pelagicus. Increased calcareous nannofossil preservation at the seafloor may be due partially to the heavy calcification of Gephyrocapsa coccoliths (see "Biostratigraphy"). Below ~200 mbsf, the presence of authigenic carbonate crystals and dolostones suggests diagenetic carbonate precipitation as a result or by-product of microbial activity. The coincidence of authigenic carbonates, nodules, and dolostones with the silty clay lithology in Subunit IIIA is intriguing. It is not clear what mechanisms could have caused this relationship. |
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