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doi:10.2204/iodp.proc.323.106.2011 LithostratigraphyFour holes were drilled at Site U1342; the deepest, Hole U1342D, reached 127.7 meters below seafloor (mbsf) and recovered basement rocks composed of dominantly volcaniclastic sedimentary rock below 62.6 mbsf. The sediments recovered at Site U1342 are a mixture of biogenic (mainly diatom frustules and foraminifers, with minor amounts of nannofossils, silicoflagellates, sponge spicules, and radiolarians), volcaniclastic (fine to coarse ash), and siliciclastic (clay- to pebble-sized clasts) sediments. In general, the color of the sediment reflects its lithologic characteristics: sediment composed of siliciclastic sediment or mixed lithologies tends to be very dark greenish gray to dark gray, whereas biogenic sediment is olive-gray to olive. Volcaniclastic ash layers are dark gray to black or shades of light gray to white. The recovered sediments and volcanic and volcaniclastic sedimentary rock are classified into three units based on major lithologic changes. The bottom boundary of Unit I (sediment composed mainly of diatoms, silt, and clay) is defined by a sharp change in grain size and diatom abundance. Unit II is predominantly sandy silt and silty sand. The bottom of Unit II is the contact between unconsolidated sediments and basement rock (Unit III), where drilling commenced. Description of unitsUnit I
Unit I is composed of two alternating, mainly siliciclastic and biogenic lithologies: silty clay to clayey silt and diatom ooze (Figs. F6, F7, F8) and is similar to Unit I described at other sites. The main siliciclastic lithologies are diatom-bearing to diatom- and foraminifer-rich silty clay or clayey silt. These lithologies tend to be very dark gray to olive-gray (5Y 4/1, 10Y 4/1, 5Y 3/2, 5Y 3/1, and 10Y 3/1). The main biogenic lithology is olive foraminifer-rich diatom ooze with variable amounts of dispersed vitric ash. The biogenic ooze is frequently laminated and ranges from olive to olive-gray (5Y 4/3, 5Y 4/4, and 5Y 4/2). Foraminifers occur scattered throughout all lithologies. The preservation of siliceous microfossils is poor in the siliciclastic lithologies (see "Biostratigraphy"). The most dominant sedimentary features are thin to thick laminations that occur frequently in foraminifer-rich diatom ooze and occasionally contain nannofossils or silicoflagellates. Laminations are generally parallel, but occasionally cross-laminations occur. Laminated diatom ooze generally has a gradational top boundary characterized by burrows or mottling, whereas the lower boundary is either gradational or sharp. In some cases, a distinct, nearly monospecific layer of pennate diatoms (Lioloma pacifium) occurs at the bottom of the layer. Pennate diatom laminae are subhorizontal and are visible at the lower boundaries of laminated diatom ooze. These laminae are similar to those observed in Hole U1340D (Fig. F10 in the "Site U1340" chapter). Fifty laminated intervals were described in Holes U1342A, U1342C, and U1342D. Nineteen of these were correlated between all holes based on their core composite depth below seafloor (CCSF-A), lamina thickness, and other visual characteristics (Fig. F9). The remaining 31 laminated layers were recognized in only one or two holes. In these intervals, slightly to moderately bioturbated diatom ooze is likely to occur in the other hole without apparent lamination (Figs. F9, F10). Bioturbation varies from slight to moderate throughout the unit and is typically characterized by a mottling defined by color or texture changes. Typically, individual burrows are well preserved at the upper boundary of the laminated intervals and even within the laminations. Where recognizable, trace fossils were mostly assigned to the Chondrites and Skolithos ichnofacies. Clasts occur throughout the unit and do not appear to be associated with any particular lithology. Distinct ash layers occur mainly between 20 and 40 mbsf in all holes. Volcanic ash layers are gray (5YR 6/1 and 5Y 6/1), grayish brown (2.5Y 5/2), dark grayish brown (2.5Y 4/2), and black (5Y 2.5/2). Unit II
Unit II is defined by a shift in texture from silt- to sand-sized grains and a corresponding decrease in biogenic components composed of sandy silt to silty sand (Fig. F6). The main lithologies are very dark greenish gray to gray diatom-bearing clay to sand and very dark gray to black (5Y 3/1 to 2.5N) diatom-rich sponge spicule–bearing fine-ashy sand. One dark gray ash layer is present in all three holes, and dispersed vitric ash mottles are present throughout. Isolated clasts were also observed. In general, the sediment is highly disturbed. The sand is composed of moderately to well-rounded igneous and metamorphic rock fragments, quartz, feldspar, pyroxene, sponge spicules, diatom frustules, and well-rounded glauconite grains (Fig. F11; see also "Site U1342 thin sections" and "Site U1342 smear slides" in "Core descriptions"). Bioturbation varies from moderate to strong throughout the unit and is characterized by large Skolithos burrows. Because of the highly chaotic nature of the sediments, it is unclear whether the visible structures are a result of coring disturbance, slumping, or primary deposition. Bedding and parallel laminations were described in Hole U1342C; these features are also found in Holes U1342A and U1342D but were not described as such. In Holes U1342A and U1342C (Sections 323-U1342A-5H-4, 88 cm, to 7H-CC and Sections 323-U1342C-5H-CC, 0 cm, to 5H-CC, 36 cm), black sand sediments are soupy and include no significant biogenic components as a result of sediment sorting by flow-in. Because of poor recovery and flow-in of sand, it is not possible to define the exact thickness of these sandy lithologic units. In Core 323-U1342A-7H, the APC was advanced 1 m but recovered 7.4 m of sediment. In Figure F6, only 1 m of sediment from Core 323-U1342A-7H is illustrated to show the depth of the drilled interval (323-U1342A-8D) and the rocks recovered in Core 323-U1342A-9X. The base of Unit II is defined by the termination of APC coring at Core 323-U1342A-7H (49.3 mbsf), where the lithified basement was reached. Unit III
Unit III is composed of mafic volcaniclastic rocks that range from sandstone to breccia and possibly minor basalt (Fig. F12). Basalt is present only in Core 323-U1342A-7X, extending from 52.30 to 52.76 mbsf. The basalt is porphyritic and vesicular, with a very dark gray interstitial groundmass. Phenocrysts are mostly plagioclase laths as large as 6 mm (~20%) and pyroxene as large as 2 mm (~2%). Many phenocrysts are in glomeroporphyritic aggregates. Vesicles are ovoid to irregular in shape and have a diameter as wide as 15 mm, with an average of 2–3 mm. They have a pale blue-gray or white mineral coating. The groundmass consists of plagioclase laths with brown glass in the interstices. Below the basalt are interbedded gray, green, and red volcanogenic sandstone and breccia. The contact between the basalt and sedimentary rocks was not recovered. Most of the bedding contacts recovered in the volcaniclastic interval are parallel and are more or less sharp. Thin to thick laminations are common. The sandstone and some of the breccia beds are polymictic, consisting of a range of scoriaceous (black and red), vesicular, and nonvesicular glassy and porphyritic basalt fragments and plagioclase and pyroxene crystals. Monomict breccias with irregularly shaped porphyritic and vesicular mafic clasts are also present. Breccia clasts range from very angular to subangular and, less frequently, subrounded. Some mafic clasts have alteration rims, and rare clasts in the polymict breccia have cored lapilli consisting of a central mafic rock fragment and a coating of basalt. Most mafic clasts have flow-aligned plagioclase laths. The matrix of both breccia types is made up of glassy fragments. Most beds are moderately to well sorted, and some sandstone beds have dewatering structures. Some breccia beds are more poorly sorted. Beds are both massive and normally graded. One of the most common sedimentary structures in the laminated volcaniclastic layers is soft-sediment deformation. Tightly folded and tilted volcaniclastic beds were observed in Sections 323-U1342D-10X-2, 9X-2, 10X-3, and 12X-2. Faulted volcaniclastic layers showing ~3 cm of apparent vertical displacement (throw) were also observed; the largest was found in Section 323-U1342D-12X-2 above a slightly slumped interval (Fig. F13). The relative offset of the hanging wall and footwall indicates that this is probably normal faulting produced in an extensional regime. Based on regional information from the Aleutian Ridge, the volcanic basement rock of Bowers Ridge probably first formed in the Eocene and ceased to accumulate in the Miocene. DiscussionUnit I is characterized by repeated alternations between laminated foraminifer-rich diatom ooze layers and siliciclastic layers. The occurrence of well-preserved laminations indicates the absence of bioturbating fauna and thus suggests low-oxygen conditions in bottom waters and within the surface sediments. Burrows or mottles at the gradational tops of laminated sediment intervals indicate an increase in oxygenation of bottom waters after the deposition of laminated sediments. In contrast, the sharp bottom boundaries could be the result of a decrease in bottom water oxygen and cessation of bioturbation at the onset of laminated intervals or a hiatus between the laminated sediments and underlying siliciclastic sediments. Occasional cross laminations indicate either migration of sediment by bottom currents or soft-sediment deformation. The sedimentation rate at this site is on average very low (3 cm/k.y.) (see "Paleomagnetism"). The winnowing of sediment by bottom currents may have caused the apparent low average sedimentation rate, and the sedimentary record may have inherited strongly variable sedimentation rates or even phases of nondeposition or erosion. High values in color reflectance parameter b* correlate well with the laminated sediments in all holes (Fig. F14) because of the difference in color between siliciclastic sediments (dark gray to greenish gray) and diatom ooze (olive). Sudden changes in b* were also recognized at depths where prominent laminated intervals were found in other holes. The nonlaminated intervals are generally slightly to moderately bioturbated diatom oozes. This bioturbation indicates that these nonlaminated diatom oozes are likely the result of postdepositional bioturbation of the laminated structure (see example in Fig. F10) and might represent similar depositional conditions as their laminated equivalents. The total number of well-correlated laminated intervals (16) is similar to the total number of interglacial cycles (15) that occurred during the last 1.1 m.y. (Lisiecki and Raymo, 2005). This match suggests that the changes controlling laminations occur on the same timescale as glacial–interglacial cycles. The laminations may be related to high biological production in the surface waters and elevated organic matter export, which create anoxic bottom water conditions that prevent bioturbation of the sediments by macrofauna. Biological production tends to increase during interglacial periods in the Bering Sea (Okazaki et al., 2005). A preliminary interpretation is that the occurrence of laminated ooze reflects interglacial times. This interpretation is consistent with the preliminary paleomagnetic age model (see "Paleomagnetism"). However, this interpretation is tentative because these laminated intervals have not yet been well dated. Unit II is characterized by its sandy texture with low abundances of biogenic components (Figs. F6, F7, F8). Moderately to well-rounded and mixed volcaniclastic and metamorphic mineral or rock fragments in the sand suggest that the source is terrestrial. The contact between Units I and II is sharp. It is unclear whether the sand was redeposited from a shallower water depth by sediment gravity flows or the paleowater depth at this site was shallower than it is today. Seismic profiles that cross the crest of Bowers Ridge appear to document that its basement rock framework was truncated by wave-base erosion prior to subsidence to depths of 1 km and deeper. Furthermore, the age of the unit is also ambiguous. Several Miocene diatom species were observed in this unit (see "Biostratigraphy"), but the base of Unit I is dated to only 1.1 Ma. Thus, the occurrence of Miocene species can be explained by reworking by sediment gravity flows, an extremely low sedimentation rate, or a hiatus between Units I and II. The basement volcanic rock (basalt) was most likely deposited as lava, as indicated by the flow alignment of the plagioclase laths and the vesicular texture. The volcaniclastic rocks beneath have a mafic volcanic source and contain both fragments and tephra fragments (scoria). The complexly zoned plagioclase shows resorbed and sieve-textured cores typical of arc lavas. The massive to graded nature of beds and the unmodified (angular) grain shapes suggest that rapid deposition by mass flows was one of the primary depositional mechanisms. However, other sedimentary structures, including the sorting of particles, soft-sediment deformation, and microfaulting, may indicate alternate depositional modes that reflect resedimentation by mass wasting of sediments having different degrees of coherence and lithification, which, together with the presence of lava flows, are common in stratovolcanoes. It is unclear at this stage whether the mass flows were subaerial or subaqueous and whether the flows are gas or water supported. The red color of many of the fragments suggests a subaerial and oxidized source, at least in part. |
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