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doi:10.2204/iodp.proc.339.103.2013 LithostratigraphyThe shipboard lithostratigraphic program at Site U1385 involved detailed visual logging of all archive sections for grain size, sediment color, sedimentary structures, and bioturbation intensity to describe the facies and facies associations. Sediments from Hole U1385A were sampled extensively for smear slides for petrographic analysis during visual core description. Relatively few smear slides were made from Holes U1385B–U1385E, and those were from lithologies and features of particular interest or different from Hole U1385A. Sixteen samples were selected from Holes U1385A and U1385B for X-ray diffraction (XRD) analyses of powdered bulk samples in order to gain a general indication of bulk mineralogy. Shipboard geochemical analyses of total carbonate contents from these cores range from 23 to 39 wt% in Hole U1385B (Fig. F4; Table T2) with carbonate values as high as 46 wt% reported from Hole U1385A. As is typical when comparing smear slide–based compositional estimates with direct geochemical measurements, these geochemical results indicate somewhat lower carbonate contents than were estimated from examining smear slides. Smear slide data typically overestimate the biogenic fraction compared with the fine-grained terrigenous fraction (i.e., detrital clay and silt), and for Site U1385 we found the smear slide estimates of carbonate content to be overestimated by a factor of 2. As a result, the geochemical results have been used to recalibrate our smear slide–based estimates of sediment carbonate content, and the lithologies determined for these sediments have been revised to reflect their decreased carbonate contents. The lithologic names used in this report and on the accompanying summary diagrams, except for the summary of component abundances based on smear slides (Fig. F5), have therefore been adjusted according to the conversion scheme shown in Table T3. It is important to note, however, that the individual core summaries and the database entries for Site U1385 still contain the lithologic names applied during shipboard visual core description (i.e., those based on the original smear slide estimates of carbonate content). The sediments at Site U1385 are defined as a single lithologic unit. Unit I is composed of a Holocene–Pleistocene sequence dominated by bioturbated nannofossil muds and nannofossil clays (both “marls” sensu Stow, 2005), which vary in the relative proportion of biogenic carbonate material (Fig. F6; Table T4). This variation in composition is evident in the visual color estimation, from lighter (more carbonate) to darker (less carbonate), which typically defines a marked color cyclicity. Relatively more terrigenous-dominated sediments are present in the upper quarter of Unit I, but their occurrence does not warrant the definition of any additional lithologic units or subunits. The character of sediment physical properties, including natural gamma radiation (NGR), magnetic susceptibility, and density, as well as sediment color spectra data, also record the distribution of this color/composition cyclicity (see “Physical properties”). Characteristics of the sedimentary sequence cored at Site U1385, together with some of these additional properties, are summarized in Figure F7. Unit I description
Lithologies and beddingThe major lithologies in Unit I are very uniform, falling entirely within the nannofossil mud and nannofossil clay classifications (Fig. F6; Table T4). The muds have a slightly greater proportion of silt-sized fraction than the clays. Significant differences exist, however, in carbonate content within this group (23–39 wt%), which yields distinctive color cyclicity. We therefore informally refer to dark and light nannofossil muds and clays in order to emphasize this distinction. Minor lithologies include those that are more biogenic rich, muddy nannofossil ooze and nannofossil silt, and those that are more terrigenous rich, mud with biogenic grains, sandy silt with nannofossils, and mud with nannofossils. The more terrigenous rich lithologies are only recognized in the upper 25% of lithologic Unit I (specifically in smear slides from Cores 339-U1385A-3H and 4H; Fig. F5; Table T4). Bedding is very indistinct and nonexistent in parts. The major lithologies are interstratified at the meter scale (typically 1–2 m) of dark- and light-colored sediment, whereas minor lithologies are interstratified at scales ranging from ~1 cm to ~1 m. The contacts between all lithologies are bioturbated and gradational. Structure and textureNo primary sedimentary structures were observed. Bioturbation is the most obvious secondary sedimentary structure and is present throughout Unit I. The most common indicators are diffuse centimeter-scale mottling and millimeter-scale pyritic burrow fills. Black iron sulfide mottling is also common. Discrete burrows, including Zoophycos, Chondrites, and Planolites, and macroscopic pyritized burrows (Fig. F8) are also common. The bioturbation index ranges from slight to moderate. Small-scale subvertical microfaults (Fig. F9) and contorted beds are present at several depth intervals that are relatively consistent in two or more holes, recording syn- or postdepositional instability of the sediment column. The distribution of these features is summarized in Table T5, including features at ~39–44 meters composite depth (mcd) in Holes U1385B and U1385D, at ~66–74 mcd in Holes U1385B and U1385D, and at ~105–110 mcd in Holes U1385A, U1385B, and U1385E. One additional zone containing features that may be contorted strata is at ~34 mcd in Holes U1385A, U1385B, and U1385D. The sediment grain size is fine throughout, mostly clay and silty clay (mud). Although subtle textural variation is observed, from more clay rich to more silt rich, it is not yet clear how this varies with respect to composition, color, or other changes. CompositionThe principal components of all lithologies are biogenic and terrigenous. The biogenic fraction is dominated by nannofossils with rare to common foraminifers and rare pteropods, ostracods, diatoms, and sponge spicules. Terrigenous components are dominated by a clay and fine silt–sized fraction, which is indeterminate in smear slides, and by detrital carbonate of indeterminate nature. There are also minor amounts of quartz, mica, volcanic glass, dolomite (both detrital and authigenic), and an opaque mineral fraction. It was recognized later in Expedition 339 that it was difficult to distinguish between volcanic glass and biotite in smear slides, so some of the material identified in this site as volcanic glass may in fact be biotite. The abundances of terrigenous components, as estimated from smear slides (Fig. F5), are 0%–86% siliciclastics (including quartz, feldspars, accessory minerals, and clay minerals), 0%–35% detrital carbonate, and 0%–11% volcanic glass or biotite. No discrete ash layers and no dropstones were observed. The abundances of biogenic components are 7%–91% biogenic carbonate (primarily nannofossils) and 0%–6% biogenic silica (primarily diatom and sponge fragments). Because the geochemically measured total carbonate content only ranges from 23 to 39 wt% in these cores (see Fig. F4; Table T2), the smear slide values given should be treated as relative rather than absolute. Pyrite (usually associated with burrows) is present throughout the cored interval, constituting the most abundant and widespread authigenic sediment component observed. Pyrite, or more likely a precursor black iron sulfide, is present as millimeter-scale disseminated burrow fills and as discrete millimeter- to centimeter-scale burrow fills and nodules (Fig. F8). Authigenic dolomite, recognized as characteristically well-defined rhombic crystals, is present at low abundances in a limited number of smear slides. Macrofossils are very poorly represented at Site U1385. A single whole specimen of cold-water coral was found in interval 339-U1385B-4H-2, 80–82 cm (Fig. F10). Other macrofossils include shell fragments, including bivalves and gastropods. ColorThe principal colors of lithologies, as noted during visual description of the upper part of Unit I, range from light gray (N 7/ and 5Y 7/1) to gray (5Y 6/1) for the more biogenic rich, light nannofossil muds and clays. Lithologies that are more terrigenous rich, the dark nannofossil muds and clays, typically range from gray (5Y 5/1) to greenish gray (10Y 5/1). In the lower half of Unit I, all lithologies tend toward darker colors, including greenish gray (10Y 5/1, 5GY 5/1, and 5G 5/1) and dark greenish gray (10Y 4/1 and 5GY 4/1), so the light-to-dark color cyclicity is more muted. Bulk mineralogyXRD analyses were performed on 16 powdered bulk samples from Holes U1385A and U1385B to gain a general indication of the bulk mineralogy and to identify any trends with age or with depth in the sediment. The resulting scans are shown in Figure F11, and the mineral intensities are listed in Table T6. In general, the mineralogy is quite uniform downhole, although there are some variations in relative peak intensities, especially illite, that may indicate changes in mineral content. The primary minerals identified include quartz, calcite, dolomite, K-feldspar, plagioclase, and the clay minerals chlorite, illite, and kaolinite. Quartz and calcite are the dominant peaks, with quartz generally the larger. In two samples (from 41 and 52 mbsf), however, the calcite peak is nearly twice as large as the quartz peak. The dolomite peak at ~29.45°2θ is a relatively large peak, although this is probably due to its high crystallinity rather than to its relative abundance. The two feldspar peaks are present throughout the cored sediments and indicate that these minerals make up a minor component of the mineral grains. The clays chlorite, illite, and kaolinite are present in all samples, and the presence of kaolinite was confirmed by heating. The chlorite peak is somewhat smaller than the kaolinite and illite peaks. Whereas the diffraction patterns are remarkably uniform, there appears to be some variation in the patterns from different samples in the region from about 5° to 8°2θ (Fig. F12), where the baseline intensity appears to change from sample to sample, although the peaks at 6.25°and 8.78°2θ remain fairly constant. The changing baseline intensity levels in this interval may indicate the presence of poorly crystalline mixed-layer clay minerals in the sediment. An attempt was made to quantify the amount of this poorly crystalline material by measuring the intensity of the pattern at an angle of 7°2θ, with the assumption that the higher background levels indicate a larger percentage of this clay material in the sediments. Figure F13 shows that this intensity generally increases with depth in the sediment. In order to explore the clay mineral composition in more detail, each pressed-powder sample was glycolated and rescanned. For each sample, the diffraction intensity at 7.65°2θ after glycolation was less than the diffraction intensity at that position without glycolation and there was often a broad peak at about 5°2θ, indicating that the glycolation of expandable (smectitic) interlayers had caused peak shifts to lower 2θ values (Fig. F14). The magnitude of the intensity decrease at 7.65°2θ following glycolation was also used to indicate the relative abundance of smectitic and/or interlayered clays downcore at this site (Fig. F15). Although sample-to-sample variability is high, the general trend shows that the intensity decrease increases downcore, which supports the interpretation of higher abundances of smectites and/or smectite-bearing mixed-layer clays at depth. DiscussionThe entire section cored at Site U1385 is very typical of a hemipelagic continental margin succession deposited under normal marine conditions with a fully oxygenated water column. Sedimentation rates (average = 10 cm/k.y.), as reported in “Biostratigraphy,” are quite normal for such an environment. The dominant lithologies are calcareous muds and clays (also known as marls), with variation in biogenic content clearly evident as color variation from lighter (more calcareous) to darker (more terrigenous). These lithologic variations are expressed clearly in the NGR, magnetic susceptibility, and density records (see “Physical properties”), as well as in the automated color spectral data. In particular, changes in reflectance (L*) appear to correlate well with changes in the relative abundance of detrital and biogenic sediment components. Following the accepted definition of hemipelagite as fine-grained sediment with the silt-size component >40% of the terrigenous fraction and typically with a mixed biogenic-terrigenous composition (Stow and Tabrez, 1998), we would confirm the succession at Site U1385 as hemipelagic. It was deposited slowly and continuously by a combination of vertical (pelagic) settling and slow lateral advection across the continental margin. The sediment shows only very slight postdepositional disturbance caused by slope instability, as evidenced by localized minor faulting and slumping, commensurate with its deposition on a broad knoll between relatively large channels. The sediment also shows continuous bioturbation throughout, which will have caused a certain degree of vertical mixing. Common Zoophycos ichnofossils attest to potential sediment mixing of >10–20 cm vertically. Other observations to highlight with regard to the planned postexpedition study of this site as a marine reference section for Quaternary climate variation would be
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