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doi:10.2204/iodp.proc.318.104.2011 LithostratigraphyThe sediments in Hole U1356A are poorly consolidated from the seabed to ~373 mbsf (Core 318-U1356A-40R) and moderately to highly disturbed by drilling. Preservation of core structure improves below ~373 mbsf (Fig. F3). The dominant lithofacies throughout the hole is moderately to strongly bioturbated claystone and calcareous claystone with Zoophycos or Nereites ichnofacies. Subordinate lithofacies include laminated silty claystones, diamictites, mudstones and sandstones with dispersed to common clasts, and graded or cross-laminated siltstones and sandstones. These facies associations are interpreted to result from hemipelagic sedimentation with variable bottom current and gravity flow influence. Smear slide data (Fig. F5; see also Site U1356 smear slides in “Core descriptions”) record a decrease in the relative abundance of diatoms and other siliceous microfossils from ~373 mbsf (Core 318-U1356A-40R) downward, coinciding with the appearance of carbonate and nannofossils and an increase in clay. Wavy submillimeter-thick black concretions below this depth are interpreted as a form of silica diagenesis. Carbonate-cemented sandstones and conglomerates are also present below this depth. Based on visual core descriptions and smear slide analyses, Hole U1356A is divided into 11 lithostratigraphic units (Fig. F3). Units I and II (0–278.4 mbsf) are composed of diatom oozes and diatom-rich silty clays with dispersed gravel, indicating hemipelagic sedimentation with ice rafting. Unit II comprises more indurated sediment than Unit I. Units III (278.40–459.4 mbsf), V (593.8–694.4 mbsf), and VII (723.5–782.7 mbsf) are characterized as repetitively interbedded light greenish gray bioturbated claystones and brown laminated claystones, indicating cyclical changes in bottom oxygenation, current strength, and fine-grained terrigenous sediment supply. Gravel-sized clasts are rare in these units, and there is only minimal evidence for ice rafting from rare dispersed sand grains. Extensive interbeds of contorted diamictites and other gravel-bearing lithologies in Units IV (459.4–593.8 mbsf) and VI (694.4–723.5 mbsf) indicate a strong gravity flow influence. Units VIII (782.7–879.7 mbsf) and IX (879.7–895.5 mbsf) are composed of mudstones with extensive contorted and convoluted bedding. Unit X (895.5–948.8 mbsf) is composed of crudely stratified and graded sandstones. These units are affected by several types of mass transport, including submarine slides and slumps. Repetition of the stratigraphy is suspected for portions of these units, based on the presence of vertically oriented stratification in extensively contorted beds. The lowermost unit in the hole, Unit XI (948.8–1006.4 mbsf), is characterized by bioturbated claystones with subordinate stratified siltstone and sandstone, indicating hemipelagic sedimentation with minor bottom current and gravity flow influence. Units IX, X, and XI (below ~880 mbsf) (Fig. F6) have a clay mineral assemblage dominated by smectite and kaolinite, indicating chemical weathering under relatively warm and humid conditions. This clay mineral assemblage is distinctly different from that of the overlying units (above ~880 mbsf), where the dominant clay minerals are illite and chlorite, indicative of physical weathering in a glacially influenced environment. Unit descriptionsUnit I
Unit I consists of unconsolidated clay-rich diatom ooze (Fig. F7) and diatom-rich silty clays. Rare pebble-sized clasts in the silty clays at the base of the unit include granite, diorite, and metasediments. Core recovery within Unit I was low and restricted to core catchers between 19.8 and 95.4 mbsf (Cores 318-U1356A-4R through 10R). Drilling disturbance precludes recognition of most sedimentary structures, although occasional strongly bowed mud laminae and sand stringers were observed. InterpretationThe high biogenic and low terrigenous content of these sediments indicates hemipelagic sedimentation in a high-productivity environment and/or a low-terrigenous sediment supply. Stratification is likely caused by current sorting, although drilling disturbance precludes a definitive diagnosis. The presence of dispersed gravel-sized clasts likely indicates ice rafting. Unit II
Unit II consists of olive-brown interbedded diatom ooze and laminated diatom-rich silty clay. This unit is distinguished from Unit I on the basis of increased consolidation of the sediments, resulting in less disturbance by RCB drilling and preservation of original bedding structures. Core recovery and preservation in the upper ~125 mbsf (Unit I) is extremely low, and the top of Section 318-U1356A-14R-2 is the first occurrence of sedimentary structures preserved in drilling biscuits. Two styles of laminae occur within Unit II: (1) intervals of millimeter- to centimeter-scale light and dark color alternations (Fig. F8) and (2) intervals that contain submillimeter-scale (pinstripe) laminae of silt-sized grains. Locally, the sediments contain rhythmic couplets of diatom-rich and diatom-poor claystone laminae. Dispersed gravel clasts of various lithologies (granites, diorite, and metasediments) occur regularly throughout this unit, with a higher concentration below 162.5 mbsf. Occasional clusters or beds (<10 cm) of coarse sand grains and granules are also observed. Bioturbation is sparse to absent throughout. An isolated carbonate-cemented claystone in a drilling biscuit contains subhorizontally oriented burrows with backfill. InterpretationThe high-biogenic and low-terrigenous content of these sediments indicates hemipelagic sedimentation in a relatively high productivity environment. Submillimeter-scale laminations likely result from current sorting. The pebble-sized clasts and coarse sand clusters or interbeds likely indicate ice rafting. Unit III
Unit III is differentiated from Unit II on the basis of increased bioturbation and a lack of gravel-sized clasts. The boundary between Units II and III is gradational. This unit exhibits repetitive interbedding of light greenish gray biogenic or bioturbated (silty) claystones and dark olive-brown laminated claystones, with average bedding thicknesses between ~0.5 and 3.0 m. Diatom-bearing to diatom-rich (silty) claystones are dominant above 373.2 mbsf, and diatoms decrease in abundance below this depth. Rare ripple cross-laminated sandstone beds and laminae are present below 373.2 mbsf. The proportion of nannofossils and carbonate-cemented beds increases significantly at the base of the unit (below 442.46 mbsf). The increased consolidation of this unit allowed identification of trace fossil assemblages. Subhorizontally aligned backfilled burrows are interpreted as Zoophycos, whereas Chondrites-style burrows are also identified, suggesting alternations between Zoophycos and Nereites ichnofacies. The first downhole amorphous silica cement is recorded within Core 318-U1356A-35R (~327 mbsf). From Core 318-U1356A-40R (~373 mbsf) downward, the well-preserved biogenic silica component in smear slides decreases, whereas amorphous silica fragments increase in abundance. Toward the base of Unit III, biogenic silica is present only as trace diatom fragments and, when present, is commonly replaced by pyrite. Coincident with the absence of diatoms are distinctive wavy submillimeter-scale black concretions that crosscut in situ sedimentary structures. Interval 318-U1356A-40R-1, 13–17 cm, also marks the first downhole occurrence of in situ carbonate-cemented sandstone. InterpretationThe cyclic alternations between laminated and bioturbated claystone facies are indicative of hemipelagic sedimentation with variability in bottom current strength and fine-grained terrigenous supply. The lack of gravel-sized clasts suggests minimal iceberg rafting. The poor preservation of diatoms below ~373 mbsf is interpreted to be caused by diagenetic alteration of biogenic opal. The wavy submillimeter-thick black concretions observed below this depth persist until the bottom of the hole and are interpreted as a form of silica diagenesis. Unit IV
Unit IV is characterized by lithologies that contain gravel-sized clasts interbedded with intervals of alternating bioturbated claystones and cross-laminated and decimeter-scale graded silty claystones (Fig. F9). The top of the unit is placed at the top of a bed of diamictite. Occasional <1 m thick calcareous and rare nannofossil-bearing interbeds are present within the bioturbated claystones. Coarse-grained lithologies include diamictites, conglomerates, and silty claystones with dispersed to common clasts. Where diamictites overlie claystones, laminae of the underlying units are commonly contorted and sheared (Fig. F9). Internal bedding in the diamictites is often strongly contorted, and rotational shear features are common. Occasional sand-filled clastic dikes are present. Clast composition in the clast-bearing lithologies is diverse and includes granite, diorite, and metasediments, as well as common intraformational clasts of variable claystone facies. Shell fragments are locally present within the coarse-grained facies. InterpretationThe clast-bearing lithologies in this unit are interpreted as a combination of debris flow deposition and ice rafting. Deformational features and intraclasts in the diamictites result from gravity flows. Thin, decimeter-scale graded silty claystones could have been produced by turbidity currents. Intermittent pelagic and hemipelagic sedimentation with bottom current influence is indicated by the interbeds of calcareous, bioturbated, and laminated claystones. Unit V
Unit V consists of light greenish gray strongly bioturbated claystones and micritic limestones interbedded with dark brown, sparsely bioturbated, parallel-laminated claystones. Minor cross-laminated sandstone interbeds are present as well. The top of Unit V is at the base of a diamictite at 593.8 mbsf. Gravel is absent below this depth to the base of Unit V, and sand-sized grains are rare. Lithologic successions are locally strongly cyclic with repetitions of sharp-based dark brown laminated claystones that transition upward into the light greenish gray bioturbated claystones with lighter colored calcareous claystones or limestones at the top (Fig. F10). The laminated claystone contains packages of <6 mm thick silt laminae (with pinstripe lamination and ripple cross-lamination) and <3 cm thick cross-laminated sandstone interbeds. Intervals containing rhythmic couplets of dark and light laminae (millimeter to centimeter scale) are present locally. Smear slides indicate that most silt laminae are composed of well-sorted quartz. This is confirmed by a bulk X-ray diffraction (XRD) analysis of two laminae from Core 318-U1356A-68R (XRD samples at interval 318-U1356A-68R-3, 27–28 cm). Ichnofossils within this unit include Chondrites and subhorizontal Zoophycos. InterpretationThe bioturbated and calcareous claystones and limestones represent pelagic and hemipelagic sedimentation. The absence of gravel and presence of rare dispersed sand grains indicate that ice rafting was not an important sedimentary process. The laminated claystones and ripple cross-laminated sandstones probably indicate variations in bottom current strength and fine-grained terrigenous supply. The sharp-based facies successions indicate that sedimentation was strongly cyclic. Unit VI
Unit VI is dominated by contorted beds of mudstones with dispersed (<1%) to common (1%–5%) clasts of various lithologies, including granite and diorite (Fig. F11). These intervals are interbedded with thinner (~1–2 m thick) intervals of bioturbated claystones and carbonate-bearing claystones. The top of the unit is marked by the first downhole occurrence of a contorted claystone with dispersed clasts. The beds of clast-bearing mudstone are characterized by soft-sediment mixing and rotational shear features, as well as abundant (5%–30%) intraformational clasts of mudstone. The basal contacts of the deformed mudstones with clasts are sharp, and in situ bioturbation is absent in these beds. InterpretationBeds of mudstones with clasts may represent the distal ends of debris flows affecting an environment characterized by hemipelagic sedimentation, represented by the bioturbated claystones and carbonate-bearing claystones. The abundant mudstone intraclasts are interpreted as rip-up clasts derived from the underlying bioturbated claystone lithologies. Unit VII
Unit VII consists of bioturbated claystones and calcareous claystones interbedded with laminated to massive claystones. The lithologies are similar to those observed in Unit V. The upper boundary of Unit VII is placed at the top of a laminated claystone. Bed thicknesses are generally ~0.5–3.0 m. Silt laminae are present as either pinstripe (submillimeter scale) or parallel laminations and ripple cross-laminations up to 10 mm thick. Gravel-sized clasts are absent. Ichnofossils within this unit include Chondrites and subhorizontal Zoophycos. InterpretationThe bioturbated and calcareous claystones and limestones represent pelagic and hemipelagic sedimentation. The pinstripe laminated and ripple cross-laminated claystones probably indicate variations in bottom current strength and fine-grained terrigenous supply. Unit VIII
Unit VIII consists of interbedded bioturbated claystones, calcareous claystones, laminated claystones, and massive structureless claystones. The bioturbated claystones are characterized by Zoophycos ichnofacies. Ripple cross-lamination in laminated silty claystones is common. Locally, decimeter-thick sand and granule-rich interbeds are present. Contorted or inclined bedding, intraformational clasts (Fig. F12), and synsedimentary microfaults are common throughout this unit. Convolute bedding with vertical orientation of stratification signals possible repetition of stratigraphy on a meter scale at ~807–810 and ~824–832 mbsf (Cores 318-U1356A-85R and 87R, respectively). InterpretationThe bioturbated claystones and laminated claystones indicate hemipelagic deposition. Ice rafting could be responsible for the gravel component in the sediments. Contorted bedding results from mass movement. The convolute bedding and possible repetition of stratigraphy with preservation of undeformed beds between deformed intervals suggests that submarine slides and slumps occurred periodically during deposition of the unit. Unit IX
Unit IX is composed of light green laminated claystones, dark green massive (structureless) claystones, and contorted massive and stratified sandstones. It is distinguished from Unit VIII by a color change to distinct green lithologies, the appearance of massive (structureless) claystones, and the presence of sandstone interbeds. Locally, beds are composed of convoluted mudstones and sandstones with vertical stratification and abundant intraformational mudstone clasts. Repetition of a conspicuous green mudstone lithology on a meter scale is also observed. Therefore, repetition of stratigraphy is suspected. InterpretationThe claystones probably indicate hemipelagic sedimentation, whereas the sandstones indicate stronger winnowing by currents or gravity flow. Repetition of stratigraphy and convolute bedding suggests that submarine slides and slumps were a dominant type of mass movement. Alternatively, proglacial or subglacial deformation of strata may have occurred. Unit X
Unit X consists of interbedded stratified and massive sandstones, diamictites, silty claystones, and siltstones with graded bedding and parallel lamination. The upper boundary of Unit X marks the top of a section with fining-upward and coarsening-upward beds that are stratigraphically intact. Contorted bedding is occasionally present in the clast-bearing sandstones and diamictites. Abundant intraformational clasts are present in the fining-upward sandstones. The sandstones also contain mica and pyritized shell fragments. In the lowermost intervals of this unit, distinctive reddish brown silty claystones are repetitively interbedded at the decimeter to meter scale with greenish gray and brown sandy mudstones with parallel laminae. InterpretationThe presence of contorted bedding in clast-bearing sandstones and diamictites and graded sandstones and siltstones in Unit X could indicate an environment with debris flows and turbidity currents or, alternatively, a storm wave–influenced environment in a shallower setting, as indicated by the micaceous fining-upward sandstone beds. The distinctive interval of interbedded reddish brown claystones and greenish gray-brown sandy mudstones likely resulted from changes in bottom current intensity or episodic gravity flow. The anomalous colors of these beds may have resulted from differential postdepositional redox reactions in the different lithologies caused by changing oxygenation conditions upon burial. Unit XI
The dominant lithology in Unit XI consists of dark green bioturbated claystone that is faintly stratified (millimeter to meter scale) as defined by color variations (light to dark bands). The upper boundary of this unit is defined by an abrupt downward change from sandstone-dominated lithologies (Unit X) to claystone-dominated lithologies. Semihorizontal backfilled burrows are present in the claystone and, although these have been compressed by sediment compaction, they likely represent Zoophycos. This interval has rare interbeds of massive or cross- and parallel-laminated fine sandstone and siltstone as thick as 6 cm. The sandstones contain mica. One ~0.3 m thick interval of highly indurated, massive, poorly sorted, very angular fine to medium quartzose sandstone is present. The sandstone is also micaceous, like the other sandstone beds in this unit. One enigmatic mafic igneous pebble was also observed in a strongly fractured interval at 318-U1356A-104R-1, 46–47 cm (Fig. F13). InterpretationThe bioturbated claystones were likely deposited through hemipelagic sedimentation. The rare laminated siltstone and sandstone interbeds probably indicate sporadic gravity flows or bottom current activity, although these processes appear to be much less common than in overlying units. The 0.3 m thick sandstone interval may constitute the cored component of a boulder-sized clast, as there is no lithologic change in the underlying or overlying lithologies, and the basal contact of this bed is very sharp and “V-shaped,” although disturbed by drilling. Alternatively, this sandstone interval may constitute a cemented bed of immature sandstone. A sandy debris flow origin is envisioned, based on the angular nature of the grains and the presence of matrix, including fragile micas that would not survive extensive traction transport. Zoophycos and Nereites communities are commonly associated with the environmental conditions found in bathyal to abyssal depositional settings (Seilacher, 1967). However, they may be found in shallower depths when influenced by other factors, especially oxygen levels in the case of Zoophycos (Frey et al., 1990). Clay mineralogyApproximately 100 samples from Hole U1356A were prepared for XRD analysis of clay mineral assemblages. Strong reflection patterns were obtained from 92 of these samples. Results from preliminary assessment of the data are presented in Figure F6. Four clay intervals defined in the hole, CI, CII, CIII, and CIV, are characterized by similar clay mineralogy. IntervalsThe uppermost interval, CI, ranges from Sample 318-U1356A-1R-2, 49–51 cm, to 2R-6, 57 cm (2.01–17.58 mbsf). The clay mineral assemblage in this interval is dominated by smectite and illite, with a minor component of kaolinite (Fig. F6). Chlorite was not identified in the three samples analyzed from this interval. Clay interval CII spans the thick section between Samples 318-U1356A-3R-CC, 6 cm, and 93R-2, 38–39 cm (19.64–878.34 mbsf). The clays in this interval are dominated by a mixture of illite, chlorite, and smectite, with only trace amounts of kaolinite. An apparent downcore increase in the amount of smectite associated with a concomitant decrease in illite occurs in the lower part of this interval between ~600 and 878 mbsf (Fig. F6). A thin interval between Samples 318-U1356A-93R-3, 100–101 cm, and 95R-4, 47–48 cm (880.06–895.42 mbsf), is identified as clay mineral interval CIII. Samples from this interval are dominated by smectite, with a very minor component of illite, chlorite, and kaolinite. Two samples from this interval (Samples 318-U1356A-93R-3, 100–101 cm, and 95R-4, 47–48 cm) contain a near-monomineralic assemblage of highly crystalline smectite. The lowermost clay interval of the hole, CIV, is between Sample 318-U1356A-96R-CC, 16–17 cm, and 106R-1, 75–77 cm (900.77–997.55 mbsf). The clay fraction in samples from this interval is composed primarily of smectite and kaolinite, with very minor amounts of illite. One exception to this characterization is Sample 318-U1356A-104R-3, 133–134 cm (981.88 mbsf), which is dominated by an illite group mineral. Mica-bearing sandstones are present within lithostratigraphic Units IX–XI. Sample 318-U1356A-104R-3, 133–134 cm, was taken from a claystone interval, but it is possible that some of the finer grained lithologies also contain mica. The most prominent overall change in clay mineral assemblages in Hole U1356A occurs at the boundary between clay intervals CII and CIII (~880 mbsf), corresponding to the boundary between lithostratigraphic Units VIII and IX. At this level, there is sharp downcore decrease in illite and chlorite and an increase in kaolinite (Fig. F6). The succession above ~880 mbsf is assigned an age of early Oligocene to Pliocene (see “Biostratigraphy”). The presence of abundant chlorite and illite in this interval suggests a low degree of chemical alteration of clay minerals during transport and deposition in the hemipelagic environment at Site U1356. The illite and chlorite mineral groups represent fresh weathering products produced as a result of physical erosion of metamorphic and granitoid igneous parent rocks by glaciers. Similar clay mineral assemblages are documented in early Oligocene to early Miocene cores on the Antarctic margin and in the Southern Ocean (e.g., Ehrmann, 1991; Robert et al., 2002; Ehrmann et al., 2005) and are interpreted as the weathering products of widespread glacial activity in East Antarctica through this time interval. The distinct shift to smectite- and kaolinite-dominated clays below ~880 mbsf (clay intervals CIII and CIV; Fig. F6) corresponds to a major hiatus, with the lower succession dated as early Eocene to earliest early Oligocene (see “Biostratigraphy”). If the clays present in this interval are of detrital origin, the abundance of smectite and kaolinite indicates chemical weathering under warm and humid conditions. Similar clay assemblages previously have been observed in late Paleocene and early Eocene cores in the Southern Ocean (Robert and Chamley, 1991; Robert and Kennett, 1992, 1994). The presence of highly crystalline smectite in some samples from this lower section of Hole U1356A, however, may indicate that there is an authigenic component to the clay mineral assemblages in this interval. The relative proportion of detrital versus authigenic clay components will need to be examined more closely in postcruise analysis. |
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