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doi:10.2204/iodp.proc.323.104.2011 LithostratigraphyFour holes were drilled at Site U1340, with the deepest (Hole U1340A) reaching 604.5 mbsf (608.7 m core composite depth below seafloor [CCSF-A]). The sediments recovered at Site U1340 are a mix of biogenic (mainly diatom frustules with minor amounts of calcareous nannofossils, foraminifers, silicoflagellates, sponge spicules, and radiolarians), volcaniclastic (fine to coarse ash), and siliciclastic (mainly silt-sized and minor pebble-sized clasts) sediments. Authigenic carbonates occur frequently below 230 mbsf. In general, the color of the sediment reflects its lithologic characteristics: sediment composed of siliciclastic sediment or mixed lithologies (clay, silt, or diatom silt) tends to be very dark greenish gray to dark gray, whereas diatom ooze tends to be olive-gray to olive or dark gray. Volcaniclastic ash layers are dark gray to black or shades of light gray to white and, rarely, weak red. Many are mixed with biogenic and siliciclastic sediment through bioturbation. Intervals showing soft-sediment deformation were observed intermittently above 1.40 mbsf. Three lithologic units were defined at Site U1340 (Figs. F6, F7, F8, F9). The Unit I/II boundary was defined as a major lithologic change from sediments dominated by diatom ooze (Unit II) and sediments that alternate between diatom ooze and diatom silt (Unit I). This boundary is early Pleistocene in age. The Unit II/III boundary was defined as the contact between a sponge spicule–bearing sand (Subunit IIIA) and Unit II. Subunit IIIB is a sponge spicule–bearing diatom ooze. Description of unitsSedimentary features common to all units at Site U1340 include thin to thick laminations and bedding that are often distinguished by variations in sediment color. Transitions between alternating beds can be sharp or gradational (and sometimes obviously bioturbated). Alternating laminae and beds vary in thickness from submillimeter to decimeter scale and occur throughout the hole, although the interval from 0 to 200 mbsf contains the greatest number of laminated intervals. Smear slide observations indicate that adjacent laminae are commonly characterized by the same lithology (mainly diatom ooze). However, individual laminae composed almost exclusively of fine or coarse ash, silicoflagellates, sponge spicules, or a single diatom species (Lioloma pacificum) (Fig. F10) were also observed (see "Site U1340 smear slides" in "Core descriptions"). Distinct volcaniclastic layers ranging in thickness from a few millimeters to 3 cm occur throughout (Figs. F6, F7, F8, F9). Most have sharp parallel or undulating contacts with the underlying sediment, whereas the top boundary is often gradational, with the volcaniclastic material mixed with the overlying diatom ooze, most likely from bioturbation. Most volcaniclastic layers are graded (fining upward; Fig. F11). Volcaniclastic ash layers are black to light gray, and distinctive weak-red beds as thick as 2 cm were found in all holes. Volcaniclastic sediment was also found in black patches and pods that likely resulted from the bioturbation of thin ash beds and laminae. Volcanic ash is a common secondary or trace lithologic component in most of the recovered sediment. X-ray diffraction (XRD) (see XRD in "Supplementary material") shows that opal-A, as well as the more mature silica phases cristobalite and tridymite, occurs together at all depths at Site U1340, regardless of lithology (Fig. F12). Thin tabular to acicular crystals found in some sediment were interpreted as zeolite minerals, which usually form during transformation phases of amorphous silica (both volcanic glass and biogenic opal). Bioturbation varies from slight to strong in all holes and is typically characterized by mottling defined by color changes. 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
Unit I is composed of alternating beds of olive (5Y 4/2) to dark gray (5Y 4/1) diatom ooze and dark gray (5Y 4/1) to dark greenish gray (10Y 3/1) diatom silt (Fig. F13). The diatom ooze is predominantly composed of both centric and pennate diatoms. The preservation of diatom frustules is generally good. Diatom frustules hosting pyrite framboids were observed. Additionally, minor (<20%) amounts of other biogenic components, including nannofossils, foraminifers, and trace (<5%) amounts of silicoflagellates and sponge spicules, occur mainly in thin laminae mixed with variable amounts (<5%–40%) of silt-sized volcaniclastic and siliciclastic material, including quartz, feldspar, and clay minerals and trace amounts of mica, ferromagnesian, and zeolite minerals. The diatom silt contains subequal proportions of siliciclastic (silt-sized quartz, feldspar, and rock fragments and/or clay) and biogenic (mainly diatoms and secondarily nannofossils, foraminifers, silicoflagellates, and sponge spicules) components with a minor volcaniclastic component (see "Site U1340 smear slides" in "Core descriptions"). Variable amounts of dispersed vitric ash and isolated pebbles occur throughout, as do distinct ash layers, bioturbated ash/diatom ooze layers, and laminated intervals. Light-colored sediments (olive) tend to contain predominantly biogenic components, whereas dark-colored sediments (gray) tend to contain subequal proportions of siliciclastic and biogenic components. The volcanic ash layers are typically black (5Y 2.5/1), light gray (10Y 7/1 and 10YR 7/2), and, rarely, weak red (2.5YR 5/2). Solitary pebbles occur mainly in two intervals in Unit I: from Cores 323-U1340A-1H through 8H (0–67.3 mbsf) and from Cores 323-U1340A-17H through 30H (153.2–258.9 mbsf) (Figs. F6, F7, F8, F9; black diamonds). The petrology of most pebbles was not described; however, some pebbles are composed of basalt and pumice or scoria, indicating a volcanic source. The most prominent sedimentary structure in Unit I at Site U1340 is soft-sediment deformation of laminated and bedded diatom ooze and diatom silt (Fig. F14). Intervals with soft-sediment deformation extend from Core 323-U1340A-3H through Section 323-U1340A-8H-2 and throughout Cores 323-U1340A-11H, 12H, 14H, and 15H. In Hole U1340B, soft-sediment deformation is present in Cores 323-U1340B-3H through 6H. In Hole U1340C, soft-sediment deformation is present in Cores 323-U1340C-2H and 3H, and, in Hole U1340D, deformation is present in all cores (Figs. F6, F7, F8, F9). Folded and tilted bed boundaries are clearly visible in these cores, suggesting the occurrence of synsedimentary slumps as a potential mechanism for deformation. Unit II
Unit II is composed of diatom ooze with minor amounts of diatom silt and volcaniclastic sediment. This unit differs from Unit I in having significantly more diatom ooze, less interbedded diatom silt, and fewer laminated intervals. The main lithology is therefore an olive-gray (5/Y 4/2 and 5Y 5/2) to olive (5Y 4/4, 5Y 5/4, and 5/Y 5/3) to dark olive-gray (5Y 3/2) diatom ooze with variable amounts of dispersed vitric ash, isolated pebbles, and distinct ash layers, as well as bioturbated ash/diatom ooze layers and laminated intervals. Bioturbation varies from slight to strong. A layer of gravel was recovered at 380 mbsf in Hole U1340A. This layer is in the lower portion of an interval between 326.8 and 384.1 mbsf that contains numerous pebbles isolated within diatom ooze and diatom silt (Figs. F6, F7, F8, F9; black triangles). Rare semi-indurated to indurated layers of authigenic carbonates were found between 275 and 435 mbsf in Hole U1340A. Semilithified or lithified yellowish layers are 5–10 cm thick. Additionally, authigenic carbonate microcrystals were found scattered throughout the sediment. Several of these intervals were examined using XRD and were determined to be dolomite (Fig. F15). Unit III
Unit III is distinguished from the other units by the predominance of ash and siliciclastic sediments and the abundance of sponge spicules (>5%). Unit III is composed of diatom-bearing, sponge spicule–rich ashy sand; sponge spicule–rich diatom ooze; diatom ooze; and minor amounts of volcaniclastic and siliciclastic sediment. Subunit IIIA
Subunit IIIA is composed of black (5Y 2.5/1), massive diatom-bearing sponge spicule–rich coarse-ashy sand. Although the coring device was advanced only 4 m, 8 m of sediments was recovered in this subunit from flow-in; therefore, the true subunit thickness could not be determined. Significant coring disturbances occurred during the recovery of Core 323-U1340A-64H. Because Cores 323-U1340A-60X through 62X retrieved no sediments using XCB coring, APC coring was resumed for Core 323-U1340A-63H. It is possible that Cores 323-U1340A-60X through 62X were empty because coring occurred in an unconsolidated sandy layer with low cohesion. The rotation of the XCB core barrel and washing decreased cohesion even more by adding water to the sandy layer and possibly washing away silt- and clay-sized particles. The sandy sediments could not be retrieved without suction. Once APC coring resumed, the first core (323-U1340A-63H) penetrated through the sandy fill and retrieved diatom ooze, and the force of the piston compacted the sandy layer. The next core (323-U1340A-64H) retrieved the recompacted sandy layer. Subunit IIIA has sharp upper contact with a thin (2 cm), light-colored ash bed of Unit II (Fig. F16). The sand extends to the bottom of the core catcher of Core 323-U1340A-64H and is absent from Core 323-U1340A-65H, where Subunit IIIB begins. The coarse-ashy sand consists of 40%–50% siliciclastic grains composed mainly of plagioclase and amphibole with minor biotite, 25%–30% plagioclase-phyric vitric clasts, and 25%–30% biogenic grains, including 17%–20% sponge spicules and 8%–10% diatoms (Fig. F17; see also "Site U1340 thin sections" in "Core descriptions"). Sand grains are angular and moderately well sorted. The finer components of this bed may have been washed away during XCB coring; however, it is likely that low sand cohesion led to the drilling problems, so sand may have been present in high amounts in the layer. The upper sand bed contains a 6 cm long, light-colored clast of diatom-rich fine ash. The ash has sharp contacts with the surrounding sand. Because of drilling disturbance, it is unclear whether this clast is isolated or is a disturbed ash bed within Unit III. There were no gravel-sized clasts in this subunit. Core disturbance was observed in all sections collected in Subunit IIIA. The sediment was described as a slurry with water-rich areas at the core surface and along the liners. Subunit IIIB
The sponge spicule–rich diatom ooze is olive (5Y 4/3) or olive-gray (5Y 4/2) to dark olive-gray (5Y 3/2) and contains 60%–70% diatom frustules, 10%–30% sponge spicules, minor siliciclastic grains, and glass shards (Fig. F18; see also "Site U1340 smear slides" in "Core descriptions"). The diatom ooze is similar to that found in Units I and II. There are several intervals of lithified diatom ooze with authigenic carbonate. Bioturbation varies from absent to moderate. DiscussionThe estimated ages of the three units at Site U1340 are similar to the three units identified at Site U1341. In addition, the depositional history of the terrigenous sediments at this site has some similarities to that at Site U1341. See "Lithostratigraphy" in the "Site U1341" chapter. Five intervals of high terrigenous content were noted at Site U1340:
Potential mechanisms of siliciclastic sediment transport include density or gravity flows, bottom currents, and sea ice or glacial ice rafting. Potential sources of siliciclastic sediments include the Alaskan continent, the Aleutian Islands, or Bowers Ridge, if it was exposed at this time. The occurrence of isolated clasts overlaps the onset of increased siliciclastic components and is coeval with a subtle increase in gamma ray attenuation (GRA) bulk density (Figs. F6, F7, F8, F9). Many of these isolated clasts are well rounded, suggesting that their source was a coastal environment. They are matrix supported, which favors a hypothesis that they were picked up, transported, and deposited as a consequence of either sea ice or glacial ice rafting. Additionally, the extensively laminated mixed sediments deposited at ~2.5 Ma correlate with an interval of laminated mixed sediments at Site U1341 (Cores 323-U1341B-37X through 46X) (see "Lithostratigraphy" in the "Site U1341" chapter). If the interpretation of isolated matrix-supported pebbles as dropstones is correct, their deposition, along with the deposition of laminated mixed sediments, may reflect increased mechanical weathering of the continents by terrestrial ice and greater exposure of the continental shelf during low sea level stands due to the onset of large glacial–interglacial cycles as well as the onset of large glaciations, known as Northern Hemisphere glaciation (NHG), at 2.6 Ma (Lisiecki and Raymo, 2005). Additionally, at ~260 mbsf (~1.78 Ma) (see "Biostratigraphy"), magnetic susceptibility values increase (Figs. F6, F7, F8, F9), possibly reflecting the more magnetic nature of grains in the siliciclastic component, such as ferromagnesian minerals and rock fragments. The composition and combination of siliciclastic grains (quartz, feldspar, and polycrystalline quartz aggregates) suggest both volcanic and sedimentary/metamorphic provenances and therefore favors elevated areas of the Alaskan continent as the source of the detritus (VanLaningham et al., 2009). Unit I, on the other hand, consists of a relatively large amount of silt- to clay-sized siliciclastic grains. At this point, it is not possible to interpret the transport mechanism for these grains; however, note that sea ice may selectively entrain silt- to clay-sized particles in regions where ice formation occurs (Reimnitz et al., 1998). The occurrence of extensive intervals with soft-sediment deformation related to slumping is somewhat unexpected because the slope of Bowers Ridge is only slightly inclined at the drill sites. The trigger for movement of sediment downslope could have been seismogenic activity related to the magmatic arc of Bowers Ridge or the nearby Aleutian arc. Sediment instability could also be due to water loss during mineral phase changes in deeper sediment, resulting in less cohesive sediment packages prone to deformation. The combination of grading, sharp bases, and bioturbated tops is typical of air fall ash deposited in deep marine settings. Alternately, the ash may have been delivered to the site by sediment gravity-flow mechanisms. The source of the darker colored, more mafic ash is likely to be proximal, whereas lighter colored, more felsic ash (which tends to be more explosive and thus be spread farther from the source) could be from proximal or distal sources. The primarily biogenic component of Unit II and Subunit IIIB may reflect warmer, highly productive Pliocene conditions or a higher sea level and little exposure of the continental shelf. High productivity prior to 1.9 Ma (~300 mbsf) is also corroborated by a change in foraminifer occurrences and preservation at Site U1340. Benthic foraminiferal assemblages changed from species building an agglutinated test in Unit II to species producing a carbonate test in Unit I, whereas planktonic foraminifers were only observed in Unit I (see "Biostratigraphy"). There are at least two interpretations for Subunit IIIA, a coarse-ashy sand. As stated above, significant drilling disturbances, particularly flow-in and fill, prevent us from knowing the thickness of the layer or its original composition because finer grained components may have been removed. Nevertheless, a sand-rich layer is present near 550 mbsf. If the sharp upper contact, moderately well sorted sand-sized grains, and massive to subtly coarsening-upward structure are primary structures, then this subunit may have been deposited by a submarine grain flow. Grain sorting may have occurred on a beach adjacent to an active andesitic volcano. Alternatively, sorting and possibly grading may have occurred during drilling. The angular and compositionally immature nature of the particles suggests they were rapidly redistributed to water depths below wave base after initial sorting. Biostratigraphic marker species (see "Biostratigraphy") and the lack of dropstones in this subunit compared to Units I and II also indicate rapid sedimentation and favor deposition by mass movement. This sand unit is correlative to Subunit IIIA at Site U1341, which is composed of authigenic carbonate–rich clay, suggesting an increase in terrigenous deposition on both sides of Bowers Ridge. Unit III was defined as being particularly rich in sponge spicules when compared to other units. Although we cannot point to a reason for such high accumulation of sponge spicules, this may also reflect high productivity or ecological change during the early Pliocene. |
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