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doi:10.2204/iodp.proc.342.112.2014 LithostratigraphyAt Site U1411, we recovered a 254.5 m thick sedimentary succession of deep-sea sediment of Pleistocene to late Eocene age. The full sequence of sediment was only recovered from Hole U1411B. Hole U1411A was terminated after one core because of failed mudline recovery. Hole U1411C was washed down from 9.2 to 100.0 mbsf to ensure sufficient time for the recovery of the Eocene–Oligocene transition between 130 and 170 mbsf in the time available. The sedimentary sequence at Site U1411 is composed of three lithostratigraphic units (Figs. F4, F5, F6, F7, F8, F9; Table T2). Unit I is a 14.45 m thick succession of Pleistocene sediment with alternating gray and reddish brown clayey foraminiferal ooze, gray silty sand with foraminifers, and brown to grayish brown silty clay with foraminifers. Minor lithologies include reddish brown clay and gray silty sand with foraminifers. Occasional sand-sized lithics and pebble-sized dropstones are also present (Fig. F5). The Pleistocene succession at Site U1411 is very similar to that drilled at the other Southeast Newfoundland Ridge sites but is, in general, more clay rich. The Unit I/II boundary is a sharp contact between the banded brown–gray Pleistocene sediment and the underlying greenish gray silty clay. Unit II is a 198.23 m thick succession of silty clay, clay with nannofossils, silty nannofossil clay, and clayey nannofossil ooze with silt of early Miocene to late Eocene age (Figs. F6, F8). Minor lithologic accessories include prominent green glauconite- and chlorite-rich horizons. The base of Unit II is defined by the occurrence of foraminifers as a common lithologic component (common to very abundant) and diminishing silt content. Unit III is 41.92 m thick and composed of greenish gray and dark greenish gray nannofossil clay with foraminifers, clayey nannofossil chalk with foraminifers, and clayey foraminiferal chalk. Laminated intervals, some of which are highly concentrated in foraminifers, are common in Unit III. Three lines of evidence suggest that these intervals have undergone significant reworking and winnowing by currents: the presence of (1) plane-parallel laminae, (2) foraminiferal sands, and (3) intraformational conglomerate with pebble-sized clasts of clayey nannofossil chalk. Lithostratigraphic units and boundaries are defined by changes in lithology (as identified by visual core description and smear slide observations), physical properties, color reflectance (L*, a*, and b*), and biogenic content (calcium carbonate and silica) (Fig. F4). The lithologic differences observed between units are primarily attributable to varying abundances of nannofossils, diatoms, radiolarians, and foraminifers (Figs. F8, F9). Lithologic descriptions are based on sediment recovered from Hole U1411B and refined with observations from Holes U1411A and U1411C. Unit I
Unit I is an ~14.35 m thick succession of the Pleistocene sediment typical of the Southeast Newfoundland Ridge sites. Major lithologies are interbedded on the decimeter scale and include gray and reddish brown (10YR 6/1 and 5Y 5/1) clayey foraminiferal ooze, gray (5Y 5/1) silty sand with foraminifers, and brown to pale brown (7.5YR 5/3 to 10YR 6/3) and gray to grayish brown (7.5YR 6/1 to 10YR 5/2) silty clay with foraminifers (Figs. F4, F5, F6, F7, F8, F9). Minor lithologies include strong brown (10YR 5/4) clay and light gray (2.5Y 5/1) silty sands with foraminifers (Figs. F4, F5, F8). Bioturbation is extensive to complete; discrete burrows are typically not apparent. Sand-sized lithics and pebble-sized dropstones (limestone, arkosic sandstone, and granite with red-till matrix cemented to surface) are common (Fig. F5). The Unit I/II boundary is a sharp contact with underlying lower Miocene greenish gray (5GY 6/1) silty clay in Section 342-U1411B-3H-3, 95 cm (Fig. F6). Unit II
Unit II is a 198.13 m thick succession of silty clay, silty clay with nannofossils, nannofossil clay with silt, clayey nannofossil ooze, clayey nannofossil chalk, and clayey nannofossil chalk with foraminifers (Figs. F4, F6, F8, F9). Colors vary among gray (2.5Y 5/1, 5Y 5/1, 2.5Y 6/1, and 5Y 6/1), dark gray (5Y 4/1 and 2.5Y 4/1), greenish gray (5GY 5/1 and 10GY 5/1), and dark greenish gray (10Y 4/1). Lithologic accessories include dark green glauconite-rich horizons and dark gray to black concentrations of sulfides. Small blebs of angular, very fine quartz sand and silt on core surfaces are very common in the Miocene and Oligocene sediment of Unit II (Fig. F10). We interpret these blebs as formerly carbonate-cemented coarse silt–sized clasts of silty quartzstone transported as ice-rafted debris. Moderate bioturbation and secondary sulfide mineralization in burrows produce prominent mottling that characterizes the Unit II appearance (Fig. F6). Discrete burrows include Planolites, Zoophycos, and Chondrites, which are particularly well defined in XCB cores (342-U1411B-20X through 23X). Unit II is differentiated from Unit I by a sharp contrast in color, the presence of glauconitic nodules and hardground (Fig. F6), reduced color variability at the core-length scale, a significant decrease in the abundance of foraminifers, and the absence of very common siliciclastic grains larger than very fine sand sized (Figs. F4, F6, F8, F9). The Unit II–III transition is defined by the presence (Unit III) or absence (Unit II) of foraminifers as a major lithologic component. It should be noted that smear slide data do not accurately reflect this transition. This is a result of the large size of Unit III foraminifers, which are not easily incorporated into smear slides. The high abundance of foraminifers in Unit III is clearly evident from examination of section-half surfaces. Unit II contains an expanded record of the Eocene–Oligocene transition in Cores 342-U1411A-13H through 18H (discussed in detail below). The Eocene–Oligocene sequence is indicated lithologically by the presence of decimeter-scale beds of clayey nannofossil ooze and an increase in carbonate content on the basis of smear slide analyses, also seen in carbonate content data (Figs. F4, F11). Unit III
Unit III is a 39.70 m thick sequence of greenish gray (10Y 5/1 and 10GY 5/1) to dark greenish gray (10Y 4/1) clayey nannofossil chalk and clayey nannofossil chalk with foraminifers and greenish gray (5GY 5/1) nannofossil foraminiferal chalk and clayey foraminiferal chalk of Eocene age (Figs. F4, F6, F7, F8, F9). Minor lithologic components include common, thin, green glauconite- and chlorite-rich bands and sulfide blebs. Bioturbation is very well expressed on the XCB core surfaces. Finely detailed Zoophycos and Planolites burrows with inset Chondrites are common and produce a finely mottled core surface. In general, the degree of bioturbation is moderate to low and decreases downhole. We observed an increasing occurrence of millimeter-scale plane-parallel laminations with occasionally wavy and crenulated surfaces and increasing concentrations of foraminifers downhole from Section 342-U1411B-26X-3, 87 cm, to the base of Unit III. We interpret these features to reflect the influence of bottom currents (Fig. F7). An intraformational chalk-pebble conglomerate composed of locally reworked clasts in a foraminiferal ooze matrix is further evidence for strong bottom currents and winnowing as the source of the foraminiferal chalk in Unit III. Disruptions in bedding, fine-scale faulting, and folded beds, common in Core 342-U1411B-28X, are the result of slumping. The degree to which bottom currents and slumping are associated is speculative, but at Site U1410 we also observed slump features in association with probable winnowed horizons. Lithostratigraphic unit summaryAt Site U1411, we recovered a 254.5 m thick sedimentary succession of deep-sea, pelagic sediment of Pleistocene to late Eocene age and an expanded record of the EOT. The sedimentary sequence at Site U1411 is composed of three lithostratigraphic units. Unit I is a 14.35 m thick succession of Pleistocene sediment typical of other Southeast Newfoundland Ridge sites composed of alternating gray and reddish brown clayey foraminiferal ooze, gray silty sand with foraminifers, and brown to grayish brown silty clay with foraminifers. Dropstones and sand-sized lithics are prominent in Unit I. Unit II is a 198.23 m thick succession of silty clay, clay with nannofossils, and silty nannofossil clay of early Miocene to late Eocene age. Nannofossil oozes are also present in Unit II but are only present in the interval immediately above the Eocene/Oligocene boundary. Small blebs of angular, very fine quartz sand and silt on core surfaces are very common in Unit II and are interpreted as formerly carbonate-cemented coarse silt–sized clasts of silty quartzstone transported as ice-rafted debris. Unit III is 41.92 m thick and composed of greenish gray and dark greenish gray nannofossil clay with foraminifers, clayey nannofossil chalk with foraminifers, and clayey foraminiferal chalk. Laminated intervals, some of which are extremely rich in foraminifers, are common in Unit III. The foraminifer sand is interpreted as evidence for significant reworking, winnowing, and downslope transport of foraminifers from the seamount crest. This interpretation is supported by seismic evidence that the middle Eocene crops out near the crest of the seamount. On the origin of silty sand blebs on core surfacesDuring Expedition 342, we noted the presence of small, 1–2 mm sized white blebs composed of highly angular, well-sorted, uncemented quartz silt and very fine grained sand within muddy lithologies. These blebs are very common on section-half surfaces (Fig. F10). We also found the same very fine, angular quartz sand present in the 63 μm fraction of sieved sediment. A coarse sand–sized, presumably ice-rafted clast of very fine angular quartz silty sandstone was recovered from Section 342-U1411B-19H-1, 140 cm, and is similar in composition to the white blebs. The clast was thoroughly washed of its nannofossil claystone matrix and treated with dilute hydrochloric acid. The sandstone clast reacted vigorously, with bubbles emanating from within the clast, confirming that it was cemented by calcite. This suggests that the very common blebs were transported to J-Anomaly Ridge and Southeast Newfoundland Ridge as coarse sand–sized clasts but have become disaggregated because of dissolution of calcite cement or they could not withstand core splitting and washing intact. The compositional uniformity of the blebs and their persistence in the Oligocene–Miocene sediment of J-Anomaly Ridge and Southeast Newfoundland Ridge is enigmatic. The Eocene–Oligocene transitionAn expanded EOT section with excellent calcareous microfossil preservation was recovered at Site U1411 (see “Biostratigraphy”) (Fig. F11). Sedimentation rates across the transition (including nannofossil Zones NP22, NP21, and NP19/20) were as high as 5 cm/k.y., much higher than comparable sedimentation rates of <0.25 to 0.6 cm/k.y. at Sites U1404, U1406, and U1409 for the same interval. High mass accumulation rates (see “Age-depth models and mass accumulation rates”) across the EOT may explain the subtle lithologic expression of the transition in comparison to the other J-Anomaly Ridge and Southeast Newfoundland Ridge sites. The Chron C13r/C13n boundary can be used to approximate the EOT and is well resolved in Hole U1411B (Fig. F11). Just above this tentative chron boundary, color reflectance (L*) decreases, but this largely coincides with core disturbance (flow-in). Sediment from Cores 342-U1411B-15H to 17H are a slightly lighter greenish gray (5Y 6/1 to 5Y 5/1) than surrounding intervals (5Y 4/1 to 10Y 4/10) and are composed of nannofossil clay with silt to clayey nannofossil ooze with silt. This relatively carbonate rich lower Oligocene interval at Site U1411 may represent the carbonate overshoot following the early Oligocene glaciation (Oi-1). The carbonate mass accumulation rate peak present at Site U1411 is temporally correlated with peaks observed at Sites U1404 and U1406. |
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