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doi:10.2204/iodp.proc.339.104.2013 LithostratigraphyThe shipboard lithostratigraphic program at Site U1386 involved detailed visual logging of all archive sections, visual assessment of sediment color, petrographic analysis of smear slides and a limited number of thin sections, and selected X-ray diffraction (XRD) analyses of powdered bulk samples. Sediment in Hole U1386A, Hole U1386B below Core 339-U1386B-39X, and Hole U1386C below Core 339-U1386C-4R was sampled regularly for smear slides during visual core description. Relatively few smear slides were made from other intervals in Holes U1386B and U1386C, except to investigate lithologies and features of particular interest or difference from Hole U1386A. Fifty-two samples were selected from Holes U1386A–U1386C for powder XRD analysis in order to gain a general indication of bulk mineralogy. 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 from these cores, based on shipboard analyses, ranges from 10.9 to 40.7 wt%, with an average of 25.9 wt% (Table T2). These results are consistent with abundances of biogenic and detrital carbonate estimated from smear slides, so the lithologic names determined from smear slide analyses have been used without modification through this text, the accompanying summary diagrams, and the visual core description sheets. The sediment at Site U1386 is separated into two lithologic units, I and II (Figs. F5, F6). Unit I is a Holocene–Pleistocene sequence dominated by nannofossil mud, calcareous silty mud, and silty sand with biogenic carbonate. These three lithologies are generally organized as bi-gradational sequences (following the conceptual model of Gonthier et al., 1984), the most complete of which coarsens upward from nannofossil mud to calcareous silty mud to silty sand with biogenic carbonate and then fines upward through calcareous silty mud into nannofossil mud. Unit I is divided into three subunits (IA–IC) based on the relative abundance of the silty muds and silty sands. Subunits IA–IC have been defined in Holes U1386A and U1386B but not in Hole U1386C. Unit II is a Pleistocene–Pliocene sequence dominated by the same three lithologies but with a higher abundance of shelly sand and evidence for submarine mass flows. Because of their relatively similar major lithologies, lithologic Units I and II are distinguished on the basis of the patterns and inferred modes of sediment deposition, rather than on the basis of major compositional changes. 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 U1386, together with some of these additional properties, are summarized in Figure F7. Unit descriptionsUnit I
The sediments of Unit I are composed of varying mixtures of terrigenous and biogenic components (primarily clay minerals, nannofossils, detrital and biogenic carbonate, and quartz; see Site U1386 smear slides in “Core descriptions”) (Fig. F8). The three most common lithologies in Unit I are nannofossil mud, calcareous silty mud, and silty sand with biogenic carbonate (Figs. F5, F6; Table T3). Less common lithologies include calcareous mud, biogenic silty mud, and calcareous sandy silt. Unit I is divided into three subunits on the basis of the relative abundances of nannofossil mud versus calcareous silty mud and silty sand with biogenic carbonate (Fig. F6). Subunits IA and IC are relatively enriched in calcareous silty muds and silty sands with biogenic carbonate, whereas Subunit IB contains a higher proportion of nannofossil mud. Subunit IA
Lithologies and bedding: The major lithologies in Subunit IA are nannofossil mud (Figs. F5, F6, F9), calcareous silty mud, and silty sand with biogenic carbonate. The primary differences between these lithologies are changes in the abundance and grain size of the detrital component with correspondingly inverse variations in the abundance of biogenic carbonate. Subunit IA is distinguished from Subunit IB by the greater importance of silty sand with biogenic carbonate in Subunit IA. Minor lithologies are subtle variations on several of the major lithologies. These include sandy silt with biogenic carbonate (a slight grain-size variant of silty sand with biogenic carbonate) and calcareous mud (a slight compositional variant of nannofossil mud). In relatively long intervals of nannofossil mud (multiple meters or more in length), bedding is very indistinct; features that may be beds are distinguished primarily by subtle changes in color, bioturbation intensity, or silt/clay ratio. In intervals that contain several major lithologies, the most common bedding style is the bi-gradational sequence, which ranges from a few decimeters to several meters in thickness. The most complete of these bi-gradational sequences coarsen upward from nannofossil mud to calcareous silty mud to silty sand with biogenic carbonate, and then fine upward through calcareous silty mud into nannofossil mud (Fig. F10). Some of these sequences are less complete, lacking the silty sand with biogenic carbonate; these sequences coarsen upward from nannofossil mud into calcareous silty mud and then fine upward into nannofossil mud. Several bi-gradational sequences in the upper ~30 m of Subunit IA are incomplete because subsequent erosion has removed part or all of the upper nannofossil mud, as indicated by the presence of a sharp to irregular upper contact. The contacts between all lithologies, and between subjacent beds of nannofossil mud, primarily are gradational and bioturbated. Exceptions are sharp upper contacts and some sharp and inclined bases of calcareous silty mud and/or silty sand with biogenic carbonate. Structures and texture: Few to no primary sedimentary structures were observed in Subunit IA, except for indistinct and discontinuous silty laminae and lenses in some beds. Bioturbation is the most obvious secondary sedimentary structure that is present throughout Subunit IA. The most common indicators of bioturbation are diffuse centimeter-scale mottling and millimeter-scale pyritic burrow fills. Black iron sulfide mottling is also common. Discrete burrows and recognizable ichnofossils are rare; those present occur in a few beds with discrete burrows of Chondrites. The bioturbation index ranges from sparse to slight, based on observation of beds with slight color changes. The sediment is fine grained through most of lithologic Subunit IA, with an average grain size of clay to silt. Exceptions are the intervals of silty sand with biogenic carbonate, in which the average grain size is very fine sand and the maximum grain size is medium to coarse sand. Composition: All lithologies in Subunit IA are dominated by terrigenous material (siliciclastic components of clay minerals, quartz, feldspars, mica, and volcanic glass, plus detrital carbonate) (Table T3; Fig. F8). The biogenic fraction is dominated by nannofossils, with rare to common foraminifers and rare pteropods and sponge spicules. Authigenic components are dominated by pyrite, dolomite, and glauconite. Abundances of terrigenous components, as estimated from smear slides (Fig. F8; Table T3), are 8%–50% siliciclastics (including quartz, feldspars, accessory minerals, and clay minerals), 8%–50% detrital carbonate, and 1%–5% glauconite. No discrete ash layers and no dropstones were observed. Abundances of biogenic components, as estimated from smear slides, are 20%–81% biogenic carbonate (primarily nannofossils, and foraminifers for the silty sand lithology) and 0%–1% biogenic silica (primarily diatoms and radiolarians). Total carbonate contents range from 19.94 to 38.24 wt% (average = 28.4 wt%) in Subunit IA (Table T2). Abundances of authigenic components, as estimated from smear slides, are 1%–5% pyrite (usually associated with burrows) and 1%–5% authigenic dolomite. Glauconite and dolomite can be abundant in the silty sand beds (Fig. F11). Whole and nearly whole macrofossils are very poorly represented in Subunit IA. The few specimens found include gastropods, bivalves, and a cold-water coral. Macrofossil fragments, however, are distributed through most of the subunit; recognizable fragments include gastropods, bivalves, and echinoderms (i.e., sea urchin spines). Examples of macrofossils are an almost complete cold-water coral (Sample 339-U1386A-1H-1, 5–6 cm; Fig. F12), coral branches (Samples 339-U1386A-9H-6, 122 cm, and 339-U1386B-8H-3, 144 cm; Fig. F13), a nearly complete pectinid valve (Sample 339-U1386A-2H-6, 70–72 cm; Fig. F14), bivalve shells (Samples 339-U1386A-3H-5, 87–89 cm [Fig. F15], and 339-U1386B-8H-CC, 28 cm), and gastropod shells (Samples 339-U1386B-5H-4, 8–9 cm [Fig. F16], and 10H-1, 53–54 cm). Color: The principal colors of the lithologies in Subunit IA, as noted during visual description of the core, range from olive-gray (5Y 4/2) to dark gray (5Y 4/1) and greenish gray to dark greenish gray. In general, sediments with higher sand and/or carbonate contents have lighter colors. Bulk mineralogy: Twelve bulk XRD samples were taken from Subunit IA. Diffraction peaks from quartz, calcite, and illite contribute 81% of the total peak intensities identified (Fig. F17; Table T4). Quartz accounts for 43.7% of the total intensity, calcite for 23.0%, and illite for 14.7%. Quartz and calcite contributions in Subunit IA are somewhat less than their average values for the entire succession at Site U1386, whereas the illite contribution in Subunit IA is considerably higher than its average value for the site (10.5%). K-feldspar, plagioclase, and kaolinite each account for 1%–5% of the sample, and minor amounts (<1%) of hornblende, augite, or pyrite were recognized based on peaks thought to be diagnostic. Additional observations based on the XRD patterns are that the ratio of plagioclase to K-feldspar peak intensities is 3.1 in Subunit IA and that dolomite is an average of 20% of the calcite peak intensity. These are both higher than the whole-site average values of 2.5 and 15%, respectively. Subunit IB
Lithologies and bedding: Subunit IB is distinguished from Subunit IA by the reduced abundance of coarser grained lithologies, as shown in Figures F5 and F6. As a result, the major lithologies in Subunit IB are nannofossil mud and calcareous silty mud. Silty sand with biogenic carbonate is a minor lithology. As was described for Subunit IA, relatively long intervals of nannofossil mud in Subunit IB (several meters or more in length) are characterized by very indistinct bedding. In intervals that contain both major lithologies, the most common bedding style is a less complete bi-gradational sequence, which coarsens upward from nannofossil mud into calcareous silty mud and then fines upward into nannofossil mud (Fig. F18). Because of the scarcity of silty sand with biogenic carbonate, more complete bi-gradational sequences are rare. One example of a normally graded bed (interval 339-U1386A-13H-5, 100–118 cm), with a sharp irregular base, does not appear to be part of a bi-gradational sequence and its depositional origin is unclear. The contacts between all lithologies and between subjacent beds of nannofossil mud are primarily gradational and bioturbated. Exceptions are a few beds of calcareous silty mud and/or silty sand with biogenic carbonate with irregular bases. Structures and texture: No primary sedimentary structures were observed in Subunit IB, except for indistinct and discontinuous silty laminations and lenses in some beds. Bioturbation is the most obvious secondary sedimentary structure and is present throughout the observed section. Characteristics of the bioturbation are similar to those of the bioturbation in Subunit IA. The bioturbation index in Subunit IB ranges from sparse to slight. The sediment is fine-grained through most of Subunit IB, with an average grain size of clay to silt. Exceptions are the very few beds of silty sand with biogenic carbonate in Hole U1386A, where the average grain size is very fine sand and the maximum grain size is medium to coarse sand. Composition: As is true for Subunit IA, all lithologies in Subunit IB are dominated by terrigenous material (siliciclastic components of clay minerals, quartz, feldspars, and mica, plus detrital carbonate) (Table T3; Fig. F8). The biogenic fraction is dominated by nannofossils, with rare to common foraminifers and rare pteropods and sponge spicules. Authigenic components include pyrite, dolomite, and glauconite. Abundances of terrigenous, biogenic, and authigenic components, as estimated from smear slides (Fig. F8; Table T3), are similar to those of Subunit IA. No discrete ash layers and no dropstones were observed. Total carbonate contents range from 17.03 to 40.71 wt% (average = 27.8 wt%) (Table T2). Whole and nearly whole macrofossils are very poorly represented in Subunit IB. The few specimens found include recognizable coral fragments and a gastropod (Sample 339-U1386B-21X-1, 113–115 cm; Fig. F19). Macrofossil fragments, however, are distributed through most of the subunit, including fragments of bivalves, echinoderms, corals, and Arenaria. Color: The principal colors of lithologies in Subunit IB, as noted during visual description of the core, range from dark gray (5Y 4/1) to greenish gray (10Y 5/1) to dark greenish gray (10Y 4/1) and very dark gray (2.5Y 3/1). In general, sediments with higher sand and/or carbonate contents have lighter colors. Bulk mineralogy: Fourteen bulk XRD samples were analyzed from Subunit IB. Diffraction intensities from the minerals quartz, calcite, and illite make up 85.2% of the total mineral diffraction intensities identified (Fig. F17; Table T4). Similar to Subunit IA, quartz accounts for 44.4% of the total intensity, but calcite contributions (31.9%) are somewhat higher and illite contributions (8.9%) are somewhat lower than both the whole-site averages and their respective contributions in Subunit IA. Minor mineral contributions are similar to those in Subunit IA, although the plagioclase/K-feldspar ratio is lower than in Subunit IA and the whole-site average, and dolomite averages 10.8% of the calcite peak intensity. Subunit IC
Lithologies and bedding: Subunit IC is distinguished from Subunit IB by the increased abundance of coarser lithologies downhole. As a result, the major lithologies in Subunit IC are the same as those in Subunit IA (nannofossil mud, calcareous silty mud, and silty sand with biogenic carbonate). As in Subunit IA, the primary differences between these lithologies are changes in the abundance and grain size of the detrital component, with correspondingly inverse variations in the abundance of biogenic carbonate. Subunit IC contains a wider range of minor lithologies than Subunits IA and IB. These minor lithologies include biogenic mud, calcareous sandy silt, calcareous silty sand, and pebbly sand (Fig. F20). As is observed elsewhere in Unit I, relatively long intervals of nannofossil mud in Subunit IC (several meters or more in length) exhibit very indistinct bedding, defined primarily by subtle changes in color, bioturbation intensity, or silt/clay ratio. In intervals that contain multiple major lithologies, bi-gradational sequences are common; both more complete sequences (nannofossil mud–calcareous silty mud–silty sand with biogenic carbonate–calcareous silty mud–nannofossil mud) and less complete sequences (nannofossil mud–calcareous silty mud–nannofossil mud) are present. A few beds located near the base of Subunit IC have sharp erosional bases, fine upward from silty sand with biogenic carbonate to calcareous silty mud, and exhibit bioturbated tops. One of these beds, enriched in rock fragments and glauconite, is illustrated in Figure F21. The pebbly sand comprises subrounded to rounded pebbles in a matrix of very fine to fine sand; however, this bed was recovered only in a core catcher (Sample 339-U1386B-45X-CC), so its texture may be an artifact of drilling. The pebbles are sandstone with calcareous cement (Table T5). The contacts between most lithologies and between subjacent beds of nannofossil mud are primarily gradational or bioturbated. Exceptions are sharp bases, irregular bases, and some sharp and inclined bases on silty sand or sandy silt beds. Structures and texture: No primary sedimentary structures were observed in Subunit IC, except for indistinct and discontinuous silty laminations and lenses in some beds. Bioturbation is the most obvious secondary sedimentary structure in this subunit and is present throughout the observed section. Characteristics of the bioturbation are similar to those of the bioturbation in the rest of Unit I. The bioturbation index in Subunit IC ranges from sparse to slight. The sediment is fine grained through most of Subunit IC, with an average grain size of clay to silt. Exceptions are the intervals of sandy silt, silty sand, and pebbly sand, where the average grain size ranges from coarse silt to fine sand and the maximum grain size ranges from medium sand to pebble. Composition: As is true for the rest of Unit I, all lithologies in Subunit IC are dominated by terrigenous material (siliciclastic components of clay minerals, quartz, and mica plus detrital carbonate) (Table T3; Fig. F8). The biogenic fraction is dominated by nannofossils with rare to common foraminifers and rare pteropods and sponge spicules. Diatom fragments are also present at several levels. Authigenic components are dominated by pyrite, dolomite, and glauconite. Abundances of terrigenous, biogenic, and authigenic components, as estimated from smear slides (Fig. F8; Table T3), generally are similar to their abundances in Subunits IA and IB. The primary difference is the presence of diatom fragments, at abundances <20%, in several smear slides from Subunit IC. No discrete ash layers and no dropstones were observed. Total carbonate contents range from 14.88 to 32.86 wt% (average = 24.33 wt%) (Table T2). Whole and nearly whole macrofossils are very poorly represented in Subunit IC. The few specimens found include recognizable coral fragments in Section 339-U1386B-35X-6. Macrofossil fragments, however, are distributed through most of the subunit; recognizable fragments include bivalves, echinoderms, and corals. Color: The principal colors of the lithologies, as noted during visual description of the core, range from gray (5Y 5/1) to greenish gray (5GY 5/1, 5Y 4/1, and 10Y 5/1) to dark greenish gray (10Y 4/1). In general, sediments with higher sand and/or carbonate contents have lighter colors. Bulk mineralogy: Twenty bulk XRD samples were analyzed from Subunit IC. Diffraction intensities from quartz, calcite, and illite account, on average, for 82.9% of the total identified mineral diffraction intensities (Fig. F17; Table T4). Quartz accounts for 50.1% of the intensities observed, which is more than in Subunits IA and IB, whereas the calcite contribution (23.4%) is similar to Subunit IA and the illite contribution (9.4%) is elevated but not as high as in Subunit IA. Minor minerals observed are the same as in Subunit IA, but the plagioclase/K-feldspar ratio is 2.35 and dolomite is 17.3% of calcite peak intensity in Subunit IC. Unit II
Lithologies and beddingThe dominant lithologies in Unit II are the same as the major lithologies in Unit I—silty sand with biogenic carbonate, calcareous silty mud, and nannofossil mud—although the relative importance of nannofossil mud decreases significantly downhole through Unit II (Figs. F5, F6). Because of their relatively similar major lithologies, Units I and II are distinguished on the basis of the patterns and inferred modes of sediment deposition rather than on the basis of major compositional changes. Minor lithologies include biogenic mud, a compositional variant of nannofossil mud, and calcareous sandy silt, a compositional and grain size intermediate between silty sand with biogenic carbonate and calcareous silty mud. In Unit II, major lithologies are organized into sequences that begin with a sharp or erosional base, grade upward from silty sand to silty mud, and exhibit a bioturbated or gradational upper contact. This pattern is especially abundant in Cores 339-U1386B-46X through 47X and Core 339-U1386C-7R through Section 8R-2. The remainder of Unit II is composed of this type of normally graded sequence interstratified with thick beds of nannofossil mud (Fig. F22). An additional bedding style in Unit II is represented by at least three thick, massive beds that contain angular or contorted centimeter- to decimeter-scale intraclasts of nannofossil mud embedded in a shelly silty sand matrix. Each of these beds has a sharp to erosional base and a sharp upper contact. The intraclasts are randomly dispersed in the matrix and are not graded by size. Three of these beds are found in Hole U1386C: from interval 339-U1386C-12R-1, 130 cm, to 13R-2, 83 cm (at least 666 cm thick); from interval 13R-3, 144 cm, to 14R-1, 16 cm (at least 181 cm thick); and from interval 15R-4, 142 cm, to 15R-5, 97 cm (122 cm thick) (Fig. F23). Note that Unit II lithologies generally were lithified enough to require cutting by saw rather than by wire. Structures and textureParallel lamination, low-angle inclined lamination, and normally graded bedding are the only primary sedimentary structures reported from Unit II. The lamination is present in a few of the normally graded beds and in almost all silty mud beds. Bioturbation is a relatively common secondary sedimentary structure in Unit II, especially in the nannofossil muds and the calcareous silty muds. In those lithologies, bioturbation intensity generally is sparse to slight. The sandy silt and silty sand, however, have lower bioturbation intensities, with bioturbation effects generally not observable in the sandy portions of the graded beds. Where bioturbation is present, the most common indicators are diffuse centimeter-scale mottling and millimeter-scale pyritic burrow fills. Discrete burrows and identifiable ichnofauna are rare. Small-scale subvertical faults (Fig. F24), fault sets, and contorted beds are present at several levels in Unit II. Dewatering structures are associated with at least one of these subvertical faults. The contorted beds are characterized by soft-sediment deformation, with convolute bedding and excellent examples of small-scale recumbent folds (Fig. F23A). The orientation of shell fragments in parts of these beds illustrates the pattern of soft-sediment deformation (Fig. F23A, F23B). Angular intraclasts of nannofossil mud are randomly dispersed in the matrix of shelly silty sand (Fig. F23C) and range from 5 mm to 4 cm in diameter. Millimeter-scale intraclasts are observed at the bases of several normally graded beds (interval 339-U1386C-6R-1, 55 cm, and 7R-1, 26 and 96 cm). The average sediment grain size in Unit II is slightly larger than the average in Unit I because of the increased abundance of sandy silt and silty sand beds. The maximum grain sizes in Unit II also are larger than those in Unit I, especially when including the larger intraclasts of nannofossil mud, the shell fragments, and the pebbly sand (Core 339-U1386C-7R-2). CompositionBecause the major lithologies in Unit II are the same as the major lithologies in Unit I, the compositions of these two units are relatively similar (Fig. F8). The important terrigenous components continue to be siliciclastics (clay minerals, quartz, feldspars, mica, and accessory minerals) and detrital carbonate, and the important authigenic components continue to be pyrite, dolomite, and glauconite. Biogenic components in the finer grained lithologies continue to be dominated by nannofossils, but shell fragments, including fragments of displaced shallower water taxa, are an important component in the sandier lithologies. Total carbonate contents range from 10.96 to 33.56 wt% (average = 21.39 wt%) (Table T2). Wood fragments are also relatively common in the silty muds, silty sands, and sandy silts between 415 and 425 mbsf. A 1 cm thick wood-rich layer is present in interval 339-U1386C-8R-1, 40.5–41.5 cm. Relatively complete macrofossils are rare in Unit II. Several are present, however, including an articulated bivalve (Sample 339-U1386C-13R-2, 53–54 cm; Fig. F25) and a gastropod shell (Sample 339-U1386C-12R-4, 77–78 cm; Fig. F26). In order to compare the composition of the sand fraction from Units I and II, a series of 16 samples of sand separated from core catcher samples were resin-impregnated and made into thin sections. Results of this petrographic study are shown in Table T5. ColorThe principal colors of the lithologies in Unit II, as noted during visual description of the core, range from greenish gray (5GY 5/1 and 5Y 5/1) to dark greenish gray (10Y 4/1) and very dark greenish gray (10Y 3/1 and 5GY 3/1). In general, sediments with higher carbonate and/or sand content have lighter colors. Bulk mineralogySix bulk XRD samples were analyzed in Unit II. Bulk mineralogies are somewhat similar to those of Unit I, and quartz, calcite, and illite account for 85.2% of the total identified mineral diffraction intensities. Quartz contributes 50.1% of the total peak intensity, which is more than the whole-site average. Calcite contributes 25.9%, which matches the whole-site average, and illite contributes 9.2%, which is slightly less than the whole-site average (Fig. F17; Table T4). Minor minerals in Unit II are similar to those in Unit I, the plagioclase/K-feldspar intensity ratio is 2.55, and dolomite is 10.7% of the calcite intensity. DiscussionGiven the setting of Site U1386, several lines of evidence for current transport and changing flow speeds support the interpretation of Unit I as a sequence of contourite deposits. Among these lines of evidence are
Subunits IA (~110 m thick) and IC (~205 m thick) are typical examples of a mixed sandy and muddy contourite succession (Fig. F10), whereas Subunit IB (~105 m thick) is a typical example of a muddy/silty contourite succession (Fig. F18). Subunits IA and IC also contain a few turbidites, but these represent a very minor portion of the section. In comparison to the sediments of Unit I, Unit II is a relatively coarser grained succession of normally graded or massive deposits. The normally graded beds have sharp to erosional bases, generally are a few centimeters to a few decimeters thick, and have gradational bioturbated upper contacts. These characteristics support the interpretation that the beds are turbidites. Between 485 and 508 mbsf, most of the turbidites are sandy and composed of shelly sand, but silty turbidites also are present. At least three very thick massive beds in Unit II have sharp bases, contain intraclasts of nannofossil mud in a matrix of shelly sand, include zones with contorted bedding, and exhibit sharp upper contacts. Based on these characteristics, these beds are interpreted as the deposits of higher concentration sediment mass gravity flows (debris flows), and therefore are considered to be debrites. The interpretation that Unit II records a significant component of downslope transport, whereas Unit I mainly records alongslope transport, indicates at least one significant change in the depositional regime at Site U1386 since ~5.54 Ma (Fig. F27; ages based on preliminary biostratigraphy; see “Biostratigraphy”). From ~5.54 Ma to sometime between 1.2 and 1.6 Ma (Unit I/II boundary based on calcareous nannofossil biostratigraphy), deposition at this site included downslope sediment mass gravity flows, both as turbidity currents and as higher concentration debris flows. Sediment was sourced from upslope regions that likely extended onto the continental shelf, as evidenced by the abundance of shallower water fauna in the shelly sands in the middle of Unit II. Unfortunately, the depositional history of this site between ~5.54 and ~1.2 Ma cannot be reconstructed in detail because of the presence of an extensive hiatus (3.2–1.9 Ma) and gaps in core recovery. However, the sediment record represented by Unit II indicates that downslope processes dominated deposition at this site, both prior to and immediately following the hiatus. Turbidite processes around this time were considered previously in the Faro Drift region by Roque et al. (2012), but debrites have been identified here for the first time. Beginning at the transition from Unit II to I, sometime between 1.2 and 1.6 Ma, clearly recognizable bottom current processes are inferred to have dominated sediment transport and deposition (Fig. F27). The onset of clear bottom current influence at Site U1386 is recorded by interbedding of contourites with sandy and silty turbidites in the lowermost 30 m of Subunit IC. This interstratification indicates that episodes of downslope transport continued even as alongslope transport became more important. Occasional sediment input by downslope processes continued through the rest of Subunit IC, as evidenced by the rock fragment- and glauconite-rich turbidites at ~218 mbsf in Hole U1386A and ~253 mbsf in Hole U1836B. After this transitional interval, alongslope transport processes appear to have strengthened significantly, as recorded by thick sandy contourite beds in the overlying remainder of Subunit IC (Fig. F27). These are the thickest sandy contourites in Unit I, suggesting a sustained time of strong and persistent flow at this site. Deposition at Site U1386 subsequently shifted to a muddy/silty mode, as recorded by the muddy/silty contourites of Subunit IB. This shift suggests a weakening of alongslope transport at this site, with one or more of the following as possible causes:
A large (2–4 km wide) channel previously described by Llave et al. (2001, 2007) and Hernández-Molina et al. (2006) was located just to the south of Site U1386 during deposition of Subunits IA and IC, but this channel was not present (or at least nearby) during Subunit IB deposition. From their more proximal location, downslope flows in these channels could have supplied sand material during deposition of the coarser grained Subunits 1A and 1C. Based on preliminary biostratigraphic information, the episode of decreased sand deposition represented by Subunit IB persisted from >580 ka to at least 470 ka, (subject to uncertainties in the present age-depth model for Site U1386). Alongslope depositional processes appear to have strengthened sometime between 470 and 270 ka at Site U1386, producing the change from the predominantly muddy/silty contourites of Subunit IB to the sandier contourites of Subunit IA. The sandy contourites of Subunit IA also are thinner than those of Subunit IC, suggesting that flow was weaker or more variable or sand supply was reduced since ~580–700 ka relative to conditions prior to that time (Fig. F27). Physical property data apparently resolve cyclicity in Subunit IA at inferred glacial–interglacial timescales (see “Physical properties”), suggesting that variations in bottom current strength were important at Site U1386 during that time. The apparent Pleistocene cyclicity might also indicate that Site U1386 could be considered a mixed system, with downslope processes (turbidity currents) dominant during sea level lowstands and alongslope processes (contour currents) dominant during highstands. Equally, however, tectonic activity might be considered a prime control on downslope deposition. Further work and comparison with other sites is required in order to resolve these controls. The complexities of changes in sediment source and/or current flow are seen in the XRD data that show higher illite percentages, higher dolomite/calcite ratio, and a possibly higher plagioclase/K-feldspar ratio in Subunit IA. This may indicate a sediment source or current flow change when compared with older subunits. These differences are greater between Subunits IA and IB than between IA and IC. Another highlighted observation is the importance of authigenic dolomite and glauconite throughout the succession. What is their potential source? What are the implications for transport through currents? What is the importance of numerous shallow-water shells, corals, and glauconite? How were they transported to this site? |
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