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doi:10.2204/iodp.proc.339.106.2013 LithostratigraphyDrilling at Site U1388 penetrated a ~225 m thick sedimentary section with significant variations in recovery among the three holes drilled (Figs. F6, F7). The shipboard lithostratigraphic program involved detailed visual logging of all archive sections, visual assessment of sediment color, petrographic analysis of smear slides (no shipboard thin sections were created), and X-ray diffraction (XRD) analysis of 22 powdered bulk samples and 6 clay fraction samples. Bulk XRD samples generally were taken once per core, and the clay fractions of those samples were analyzed once per four cores. Sediment from Holes U1388A–U1388C was sampled regularly for smear slides during visual core descriptions. Hand-drawn logs showing the recovered sediment sequence, including the distribution and structure of bedding, are included in the DRAWLOG folder in “Supplementary material.” Total carbonate content of these cores, based on shipboard analyses, ranges from 18 to 30.1 wt%, with an average of 25.1 wt% assuming that all inorganic carbon was calcium carbonate (Fig. F8). These results are consistent with abundances of biogenic and detrital carbonate estimated from smear slides, so lithologic names determined from smear slide analysis have been used without modification through this text, the accompanying summary diagrams, and the visual core description sheets. The sediments recovered in all three holes at Site U1388 are considered to represent a single lithologic unit. Because of hole stability problems and the presence of sandy sediment, however, not all of Unit I was fully sampled. Following core recovery problems after Core 339-U1388A-1A (see “Operations”), Hole U1388B was spudded with the XCB and recovery was generally poor until Core 339-U1388B-12X. Drilling in Hole U1388C used rotary drilling to recover sediments that overlapped with the base of Hole U1388B. The sediment at Site U1388 is classified as one lithologic unit (Fig. F7). Unit I is a Holocene to Pleistocene sequence (see “Biostratigraphy”) with interbedded nannofossil mud, mud with biogenic carbonate, silty mud with biogenic carbonate, silty sand with biogenic carbonate, sandy silt with biogenic carbonate, and sand. The thickest sand bed (3.64 m) is at the sediment surface (all of Core 339-U1388A-1H) and is primarily fine sand with shell fragments; however, grain size approaches coarse sand to granule-grade in horizons where shell fragments are common. Most of the coarser beds recovered deeper in the section are silty muds, muddy sands, and silty sands. These are predominantly medium to thick bedded with occasional very thick beds (the thickest observed is ~7.5 m thick). In contrast, the finer sediments are muds with biogenic carbonate. The character of sediment physical properties, including natural gamma radiation (NGR), magnetic susceptibility, color reflectance parameters, and density, records the distribution of these various lithologies and sediment components (see “Physical properties”). Characteristics of the sedimentary sequence cored at Site U1388, together with some of these additional properties, are summarized in Figure F9. Unit I description
Lithologies and beddingThe major lithologies in Unit I are silty mud with biogenic carbonate, silty sand with biogenic carbonate, and mud with biogenic carbonate. Secondary lithologies are calcareous sand and sand with biogenic carbonate; sandy silt and sandy mud, both with biogenic carbonate; and nannofossil mud and calcareous silty mud. Most of Unit I contains 10–25 wt% carbonate, although some coarser sandy layers contain >25 wt% carbonate because of abundant shell fragments. Uncertainty about the lithologies of the unrecovered intervals has implications for the ability to define lithologic units and subunits at Site U1388. Sandy beds (including calcareous sand, sand with biogenic carbonate, silty sand with biogenic carbonate, sandy silt with biogenic carbonate, and sandy mud with biogenic carbonate) and silty beds (including silty mud with biogenic carbonate) form ~75% of the sediment recovered above ~100 mbsf and ~25% of the sediment recovered below that level (Figs. F7, F10). However, the overall rate of sediment recovery was much lower above ~100 mbsf, so the sandy and silty beds that were recovered average 15%–20% of the total depth intervals cored below ~5 mbsf. We attribute the poor recovery at this site to the use of XCB coring in unconsolidated sands, but note that our interpretation of the lithologic sequence would change if the unrecovered sediments above 100 mbsf are different from the sediments recovered by coring in that interval. Sandy units are quite difficult to recover using the XCB because there is no closure on the bottom of the pipe, so this part of the section would be quite coarse if all unrecovered sediments above 100 mbsf are sandy. However, if all the unrecovered sediment in this hole was mud (weakly consolidated muds are also difficult to recover using XCB coring), then the percentage of sandy and silty beds relative to total thickness cored would be nearly identical at 17.4% above 100 mbsf and 16.7% below 100 mbsf (Fig. F10). Although the latter may not be a likely scenario, we are forced to conclude that it is not possible to divide Unit I based on visual observations of the recovered sediment alone. Additional studies will be needed to determine whether Unit I can be further divided. The coarsest bed in Unit I is the ~3.64 m thick bed of fine sand recovered in Core 339-U1388A-1H. This bed has layers of coarse sand to granules, with shell debris, at intervals 339-U1388A-1H-1, 139–148 cm; 1H-2, 111–116 cm; 1H-3, 0–13 cm; and 1H-3, 20–30 cm. Sediment in Holes U1388B and U1388C is not as coarse as that in Hole U1388A, with the coarsest beds in Holes U1388B and U1388C described as silty sand. As sampled in Holes U1388B and U1388C, Unit 1 predominantly is medium to thick bedded, with some coarse-grained beds as thick as ~7.5 m. The finer grained beds are muds with biogenic carbonate and range in thickness from ~0.5 to >15 m. The contacts between lithologies are generally gradational and bioturbated, although some of the coarser grained beds have sharp to erosional lower contacts. Structures and textureOnly Core 339-U1388A-1H was recovered by APC. All cores in Hole U1388B were recovered by XCB and as such have been biscuited to some extent, thus obscuring sedimentary structures. Shell fragments are common in coarser grained layers, especially in many of the coarser beds (e.g., Sections 339-U1388B-7X-1A and 339-U1388A-1H-2A), perhaps related to the movement of large bedforms. Normally graded and inversely graded beds are present in Unit I. Many of the coarser beds have gradational upper and lower contacts, exhibit bi-gradational bedding, and are considered to be of contourite origin. Figure F11 shows a typical silty contourite, 24 cm thick, in Section 339-U1388B-13X-3. Another typical bi-gradational sequence is present in Core 339-U1388B-23X that is ~3.5 m thick and also interpreted as a contourite. Other beds exhibit normal grading and sharp to erosional lower contacts (Fig. F12) and are interpreted as turbidites. Beds with inverse grading are also present (Figs. F13, F14, F15) and generally are medium to thick to very thick bedded. These reverse-graded beds exhibit a variety of upper and lower contacts, ranging from erosional to gradational. Because of this variability in contact type, the deposition of these beds is most likely a result of current activity. In general, the contacts between other beds also range from sharp to gradational and bioturbated, but it is not generally clear whether biscuiting had occurred at those contacts. Parallel laminated sands are present in interval 339-U1388B-20X-5, 50–105 cm (Fig. F16), formed by alternating lighter and darker laminations each 2–6 mm thick. The principal difference between the light and dark sand laminations appears to be a lower mud content in the light-colored sands. We interpret these as primary parallel laminations formed by currents, although the role of drilling disturbance in their formation is not known. Deposits originating from sediment failure are also noted at this site. A possible debrite is identified at interval 339-U1388B-10X-2, 41–121 cm, and a possible slump is identified at interval 339-U1388B-24X-5, 33–54 cm (Fig. F17). These deformed, thinly bedded sediments have inclined contacts between muddy and sandy intervals, as well as possible muddy intraclasts in a more sandy matrix. An analysis of the number of contourite and turbidite beds observed (Fig. F7) suggests that turbidite and contourite beds are distributed throughout the recovered section. The number of silt-dominated and sand-dominated beds per core shows some variation, with more beds per meter observed at ~120–140 mbsf (Cores 339-U1388B-14X and 15X). Sandy contourite beds are also somewhat more common in this depth range. Burrows are common secondary sedimentary structures, although bioturbation is generally sparse to slight. In many cases, burrows have a sandy fill similar to the overlying or underlying bed. One possible subvertical burrow found near a lithologic contact is >1 cm wide and 28 cm long (interval 339-U1388-6X-2, 7–35 cm) and is filled with clean medium-grained sand that is coarser than either the overlying or underlying unit. CompositionThe results of smear slide analysis showing sediment composition and grain size are given in Table T2. These observations indicate that detrital carbonate is more common in lithologies with intermediate grain sizes (silty sand, sandy silt, and silty mud) than in the finer (mud with nannofossils) or coarser grained sediments (sand). Biogenic silica is rarely observed. A diverse suite of minerals from a variety of source rocks is present in many samples (Fig. F18). Several well-preserved gastropods are present in Unit I (Fig. F19). These species can be found living on the Iberian shelf today, but it is not certain if they can live in deeper water. However, the gastropods recovered at this site were found in a variety of lithologies—in a silty sand contourite, a mud, and the surface sand layer (Fig. F19, top, center and bottom, respectively)—suggesting that the shells were found in situ. ColorSediment colors are generally described as light to dark gray near the surface, greenish gray or grayish brown in Cores 339-U1388C-3R through 4R, and varying downhole between greenish gray, dark greenish gray, and very dark greenish gray. The most common color is dark greenish gray. Bulk mineralogyXRD analysis was conducted on 22 powdered bulk samples, which generally were taken once per core the first time a particular stratigraphic level was sampled. Six samples were analyzed for clay fraction mineralogy, chosen as a subset of the suite of bulk mineralogy samples (Table T3; Figs. F20, F21). The bulk XRD results indicate that the mineral composition varies considerably above ~50 mbsf and is somewhat more uniform through the rest of the cored interval. The samples at 2.6 mbsf in Hole U1388A (Sample 339-U1388A-1H-2, 115–116 cm) and 36.3 mbsf in Hole U1388B (Sample 339-U1388B-5X-CC, 31–32 cm) produce high-intensity peaks at characteristic angles for plagioclase, K-feldspar, and illite. Hornblende has peak intensities similar to those from other sites, and a sample at 138.7 mbsf in Hole U1388B has a fairly large intensity for a pyroxene (augite) peak. The sample processed for clay minerals at 2.65 mbsf in Hole U1388A is sandy and contains no clay minerals. Unglycolated samples from other depths show peaks for illite, kaolinite, and chlorite, as well as an elevated baseline between the chlorite and illite peaks. When glycolated, the baseline in this interval decreases and a more intense peak appears at lower angles on the diffractogram, indicating that poorly crystalline clays have expanded to the characteristic d-spacing of 17.8 Å, typical of smectite expanded with glycol. DiscussionUnit I contains a significant number of sandy beds, with thicknesses up to several meters, as well as many intervals of mud with biogenic carbonate that exceed 15 m thick. The latter provide relatively good age control (see “Biostratigraphy”). Full understanding of sedimentation at this site is hampered by partial recovery. Based on the recovered material, however, both contourites and turbidites have been identified in relatively similar numbers, and both contourites and turbidites are distributed through the entire length of Unit I. These relatively similar abundances and stratigraphic distributions indicate that both alongslope and downslope processes were important for delivering sediment to Site U1388 throughout the time indicated by the cored record. Some details in the record at Site U1388 do suggest that the relative importance of alongslope vs. downslope flow changed through time. For example, the number of contourite beds increases in Cores 339-U1388B-14X and 15X, possibly caused by changes in current strength at that time. Also, some intervals containing lighter colored sand laminae may indicate sediment reworking by currents and removal of the fine fraction. The presence of two apparent mass transport units (a debrite and a slump) reinforce the importance of downslope transport. Given the evidence for sustained activity of both alongslope currents and downslope processes, the possibility existed for numerous interactions between downslope and alongslope currents during deposition of Unit I. This range of interactions may help explain the irregular development of sharp contacts at the bases or tops of sandy sequences, as well as the deposition of inversely graded beds. Some normally graded and sharp-based (base-cut-out) beds could be also contourites, as in Site U1389 (see “Lithostratigraphy” for Site U1389 [Expedition 339 Scientists, 2013d]), and further detailed studies are necessary to differentiate them from turbidite beds. Several of the intervals investigated by optical and XRD techniques have relatively high percentages of rock-forming minerals other than quartz, suggesting an immature mineral assemblage and the possibility of relatively direct sediment supply to Site U1388 through downslope processes. |
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