| IODP Proceedings Volume contents Search | |||
|
|||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.318.107.2011 LithostratigraphyFour holes were drilled at Site U1359. Holes U1359A–U1359D were drilled to total depths of 193.50, 252.00, 168.70, and 602.2 mbsf, respectively. In Holes U1359A and U1359B, the APC system was used to refusal, followed by XCB drilling. Only the APC was used in Hole U1359C. Hole U1359D was drilled using the RCB system, and core was only recovered below 152.2 mbsf. Silty clay with dispersed clasts is the dominant lithology observed throughout all holes at Site U1359 (Figs. F3, F4). Noticeable variations in the amounts of biogenic components and bioturbation and sedimentary structures are apparent, in particular the presence or absence of packages of silt/fine sand laminations and large variations in diatom abundance. Although texturally almost all of the sediments cored at Site U1359 are silty clays with dispersed clasts, five distinct lithofacies are identified based on variations in the style of laminations, bioturbation, or the relative abundance of the biogenic component. Three lithostratigraphic units are defined on the basis of observed changes in facies associations (Figs. F3, F4). Because four overlapping holes were cored at this site, unit boundary depths in meters below seafloor are not consistent between holes. The unit boundaries are defined in the hole where their depths (in meters below seafloor) are the greatest. Table T2 shows the unit boundary depths in all holes and conversion to meters composite depth as defined in “Stratigraphic correlation and composite section.” Lithostratigraphic Unit I (0–42.07 mcd) consists of decimeter-scale alternations of yellow-brown and olive-gray diatom-rich silty clays with dispersed clasts and occasional foraminifer-bearing clayey silt and sandy silt. Unit II (42.07–264.24 mcd) consists of bioturbated diatom-bearing silty clays interbedded with olive-gray diatom-bearing silty clays that are mostly massive but contain decimeter-scale packages of olive-brown silty clay with silt laminations. Unit III extends from 264.24 mcd to the bottom of the cored section at 613.46 mcd and consists of bioturbated diatom-bearing silty clays interbedded with laminated silty clays. The laminated silty clays contain more subtle, but persistent, submillimeter- to millimeter-scale laminations compared to Unit II. Clasts >2 mm in size occur throughout all lithostratigraphic units and are mostly dispersed in nature (i.e., trace to 1% in abundance). However, there are some variations in clast abundance (Fig. F3). Silty clays with common clasts (i.e., 1%–5%) are present between 200.1 and 247.1 mbsf (Cores 318-U1359D-7R through 12R), whereas below 372.4 mbsf (interval 318-U1359D-25R-1, 0 cm) clasts occur in trace amounts only (Fig. F4). Sediments from lithostratigraphic Units I and II are consistent with levee deposition by low-density turbidity currents, whereas the facies associations in Unit III probably represent deposition in an environment influenced by periodic variations in contour current strength or saline density flows related to bottom water production, with turbidity currents having less influence than the overlying units. The regular nature of the interbedding (i.e., beds 2–5 m thick) of the laminated and bioturbated facies within all three lithostratigraphic units suggests that the sedimentary record recovered at Site U1359 is cyclic in nature. Facies descriptionsFive lithofacies were identified at Site U1359:
Smear slide analysis indicates (Figs. F18, F19, F20, F21, F22; see also Site U1359 smear slides in “Core descriptions”) that carbonate-cemented clay occasionally occurs as a minor component throughout the core. Pyrite occurs as a minor component (2%–10%) in most of the major lithofacies. Pyrite is found in higher abundance within some dark-layered laminations (15%–40%), particularly within the diatom-rich silty clay lithofacies (see below). InterpretationThe diatom-bearing and diatom-rich silty clays (Facies 1 and 2) were probably deposited by hemipelagic sedimentation in a higher productivity environment relative to the other facies. The clays and silty clays (Facies 3–5) indicate high terrigenous sedimentation rates and/or lower biogenic productivity, perhaps related to the duration of seasonal sea ice cover regulating light availability in surface water or wind-regulated control of the mixed-layer depth, which in turn controls productivity. The opposite scenario may apply for the diatom-bearing to diatom-rich silty clay facies (Facies 1 and 2). An increase in terrigenous input may result from ice advance across the shelf or increase in sedimentation from bottom currents. The laminated silty clays (Facies 4 and 5) may originate from three potential depositional mechanisms: low-density turbidity currents, contour currents, and saline density flows. The rhythmic couplets of silt and clay laminations with sharp bases (Facies 4) are commonly overlain by massive silty clays (Facies 3) passing upward into bioturbated diatom-bearing silty clays (Facies 2). These facies associations have similarities to the distal muddy turbidite model of Stow and Piper (1984) (Bouma divisions TD–E). The absence of coarser grained deposits and ripple laminations in the Site U1359 section hints at levee deposition by low-density turbidity currents, which is consistent with the channel-levee complex interpretation of seismic profiles that cross the drill site (Escutia et al., 2008). Sediment redistribution may have occurred because of contour current influence, with variations in lamination thickness and particle size in Facies 4 or 5 indicating fluctuations in bottom current strength. The passage of cold saline density flows related to bottom water production at the Wilkes Land margin (e.g., High-Salinity Shelf Water flowing from the shelf into the deep ocean to form AABW) should also be considered as a potentially important sediment transport mechanism. The depositional model for recovered sediments at Site U1359 may represent a continuum of all three processes, in addition to pelagic and ice-rafted components as indicated by the presence of diatom remains and dispersed clasts throughout. Unit descriptionsBased on visual core descriptions and smear slide analyses, Site U1359 is divided into three lithostratigraphic units (Fig. F3). Unit I
Unit I consists of yellowish brown diatom-bearing to diatom-rich silty clays (Facies 1) interbedded at decimeter scale with olive-gray silty clay (Facies 3), both with dispersed clasts. Foraminifers are present throughout Unit I but are most abundant (>10%) in Cores 318-U1359A-4H, 318-U1359B-4H, and 318-U1359C-3H and 4H, where they are concentrated in decimeter-scale beds of clayey silt/silty sand (Fig. F11). The base of Unit I is defined as the lowermost foraminifer-bearing horizon identified by smear slide and macroscopic analysis. Millimeter-scale pockets of silt and fine sand occur throughout the unit. Dispersed clasts also occur throughout Unit I and are granule to pebble in size. Clast lithologies include basalt and quartzite. Millimeter- to centimeter-scale silt laminations are present in this interval below 5.6 mbsf (Section 318-U1359A-2H-4) and occur in ~25 cm thick packages (Facies 4) (Fig. F14). InterpretationDeposition of these facies is interpreted to result from hemipelagic sedimentation, with possible transport and resorting by currents related to low-density turbidity currents, saline density flows, or contour currents. Coarse-grained gravel clasts interpreted as ice-rafted detritus occur throughout this unit. The mottled appearance of the yellow-brown intervals (Facies 1) is attributed to moderate levels of bioturbation. Unit IISubunit IIa
Subunit IIb
Subunit IIc
Unit II consists of massive to laminated (olive-gray to liver-brown) silty clays interbedded with mottled greenish gray diatom-bearing/diatom-rich silty clays. Unit II is distinguished from Unit I on the basis of a lack of foraminifers (e.g., Facies 1) and a distinct change in color. The olive-gray silty clays (Facies 3) commonly pass sharply into and out of dark olive-brown silty clays that contain discrete intervals of silt laminations (Facies 4), many of which are defined as rhythmic couplets of silt and clay, as discussed in the facies section above. However, wispy and indistinct silt laminations, stringers, and silt/fine sand pockets (<4 mm in size) are also present. The olive-gray intervals commonly grade upward into greenish gray massive diatom-bearing (to diatom-rich) silty clays with a mottled appearance (Facies 2). The mottling is attributed to moderate levels of bioturbation, as there are common centimeter-scale oblate burrows, particularly near contacts with olive-gray and olive-brown silty clays. Dispersed clasts occur throughout Unit II. This unit is further divided into three subunits (IIa, IIb, and IIc). Subunit IIb is distinguished from Subunit IIa on the basis of a notable increase in diatom abundance with diatom-bearing and diatom-rich sediments (Facies 2, 3, and 4) becoming more common (Figs. F3, F4). Subunit IIc is defined on the basis of a significant increase in clast abundance with silty clays with common clasts (i.e., 1%–5%) occurring throughout. InterpretationThese facies associations are interpreted to result from hemipelagic sedimentation and deposition by contour currents, possibly interrupted by episodic low-density turbidity currents. The presence of well-defined packages of silt laminations (Facies 4) suggests levee deposition of low-density muddy turbidity currents, as discussed in the facies section. However, transport and resorting by currents related to saline density flows or contour currents is also a possibility. There is clear evidence of cyclical variability (meter scale) in the lithologic and physical properties expressed as alternating intervals of bioturbated silty clays (Facies 2) and massive (Facies 3) and laminated (Facies 4) silty clay facies (Fig. F5), as well as at decimeter scale between the massive and laminated silty clay lithofacies (Fig. F6). The downhole increase in diatom content associated with Subunit IIb and below is suggestive of increased surface water productivity or reduction in terrigenous input. Ice rafting of coarse-grained gravel clasts is interpreted to have occurred throughout the deposition of Unit II but was a more significant sediment contributor during the deposition of Subunit IIc. Unit III
Unit III is characterized by dark greenish gray planar-laminated clays and silty clays (Facies 5) that are interbedded at meter scale with light greenish gray diatom-bearing to diatom-rich silty clay with abundant bioturbation (Facies 2). Unit III is differentiated from Unit II on the basis of dark greenish gray clay and silty clay interbeds that are sparsely bioturbated and display planar submillimeter-scale (pinstripe) to millimeter-scale laminations throughout (Facies 5). The diatom-rich silty clays with abundant bioturbation (Facies 2) are similar to those documented in Unit II. The increased lithification of Unit III allows for improved identification of bioturbation style and intensity, and well-defined centimeter-scale horizontal and oblate burrows are common (Fig. F23). Above 452.6 mbsf (interval 318-U1359D-33R-3, 44 cm), laminations are faintly defined by color or very subtle grain-size variations and are submillimeter (pinstripe) to millimeter scale in thickness. Below this depth, laminations are characterized by more distinct grain-size variations and consist of pinstripe to millimeter-scale (generally <5 mm thick) layers of well-sorted quartz-rich silt. Dispersed granule- to pebble-sized clasts occur throughout, but below 372.4 mbsf (Section 318-U1359D-25R-1) they are rare relative to the overlying units (Fig. F3). Unit III contains a conspicuous diatom-bearing nannofossil-bearing ooze (Fig. F24) interval between Sections 318-U1359D-24R-6 and 25R-1 (see “Biostratigraphy”). This ooze also contains a minor foraminifer component. InterpretationThe laminated intervals most likely represent an environment that is influenced by saline density flows and contour currents and, relative to overlying Units I and II, probably less turbidity current influence was present. This may represent a depositional environment that is more distal on the levee than Units I and II. However, meter-scale cyclicity similar to that noted in Unit II continues into Unit III, with alternating beds of bioturbated silty clays (Facies 2) and laminated silty clays (Facies 5) occurring throughout. The intervals of bioturbated silty clays (Facies 2) may represent variations in terrigenous input or bottom water oxygenation that in turn may ultimately be tied to changes in ice sheet or sea ice extent. Evidence of ice rafting of gravel-sized clasts is present throughout this unit but is noticeably less below 372.4 mbsf (Section 318-U1359D-25R-1). Clay mineralogyForty-eight samples from Site U1359 were prepared for X-ray diffraction (XRD) analysis of the clay fraction. These samples were taken from all four holes drilled at this site (Holes U1359A–U1359D), but the majority are derived from Holes U1359A and U1359D. The combined sample set provides a low-resolution clay mineral stratigraphy for the entire section (0–596 mbsf), with an average sample spacing of one sample every 15 m (Fig. F25). The clay mineral assemblages at Site U1359 are dominated by the illite, smectite, and chlorite groups (Fig. F25). The relative proportion of illite remains relatively invariant through the sequence, whereas larger fluctuations are noted in the chlorite and smectite records. Specifically, a higher fraction of smectite (corresponding to a lower fraction of chlorite) is present between ~14 and 64 mbsf and between ~120 and 240 mbsf. Increased concentration of smectite in these intervals may represent periods of increased supply of clay material transported by currents from the Ross Sea because smectite is not presently produced in abundance locally on the Wilkes Land margin but instead is thought to be derived from weathering of volcanic rock in the Ross Sea (see Damiani et al., 2006). Kaolinite is generally present in trace amounts in the samples analyzed from Site U1359. The only interval in which kaolinite is present in greater abundance, but still representative of a minor clay component, is between ~14 and 25 mbsf within the Pleistocene section (Cores 318-U1359A-3H and 4H) (Fig. F25). Kaolinite has been previously observed in Quaternary cores from the Wilkes Land margin (Busetti et al., 2003; Damiani et al., 2006) and represents reworked material from Paleogene or older sediments. Overall, the clay mineral assemblages present in the upper Miocene–Pleistocene succession recovered at Site U1359 are similar to those previously reported from Neogene records in the Antarctic margin (e.g., Ehrmann et al., 1991; Hillenbrand and Ehrmann, 2005). Abundant illite and chlorite indicate active physical weathering of granitoid and metamorphic parent rocks in a glacial regime. Given the low-resolution sampling for clay XRD analyses, the degree of variability on short-length scales through the section is uncertain. Five samples, however, were taken from different lithofacies in Core 318-U1359D-4R (171.30–180.00 mbsf). Within this core, substantial variation in the relative abundance of smectite (Fig. F25) suggests that higher resolution analysis may reveal significant changes in the clay composition of the cyclically deposited diatom-rich and diatom-poor beds present in Units II and III. |
|||
