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doi:10.2204/iodp.proc.339.107.2013 LithostratigraphyDrilling at Site U1389 penetrated a 990 m thick sedimentary section (Figs. F8, F9, F10). The shipboard lithostratigraphic program involved detailed visual assessment of grain size, sediment color, sedimentary structures, and bioturbation intensity to describe the facies and facies associations at Site U1389. Petrographic analyses of smear slides taken regularly from each hole (111 from Hole U1389A, 4 from Hole U1389B, 82 from Hole U1389C, 17 from Hole U1389D, and 136 from Hole U1389E) were used to provide detailed sediment description, identification of major components, and a more descriptive sediment classification. From Holes U1389A and U1389E, 99 samples were selected for powder X-ray diffraction (XRD) analysis of bulk mineralogy. Total carbonate content in these cores, based on shipboard analyses, ranges from 21 to 35 wt%. These results are consistent with abundances of biogenic carbonate 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 sedimentary succession at Site U1389 has been classified as one major lithologic unit with five subunits (IA–IE) (Figs. F9, F10). Unit I is a Holocene–late Pliocene (3.6 Ma) sequence of calcareous mud, silty mud with biogenic carbonate, sandy mud, and silty sand with biogenic carbonate. Overall, the succession is very uniform in character, with little variation in lithology or composition. The principal changes observed are in the relative number of silty mud and sandy mud beds, as well as in the number of bi-gradational, inversely graded, and normally graded sequences. Accordingly, the unit divisions for this site have been based mainly on facies changes rather than lithologic or compositional changes. The character of sediment physical properties, including 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 U1389, together with some of these additional properties, are summarized in Figure F11. Unit I descriptionBecause the five subunits in Unit I have relatively similar lithologies, texture, composition, and colors, this section is organized somewhat differently than the unit descriptions in other Expedition 339 site chapters. For Site U1389, this section begins by highlighting the distinctive characteristics of each subunit. The general characteristics shared by all subunits are described subsequently, following the same format used to describe individual subunits or units at other sites. Subunit IA
Overall, the dominant lithology in Subunit IA is calcareous mud (63%) (Fig. F10). The other lithologies include silty mud with biogenic carbonate (24%), sandy mud with biogenic carbonate (7%), and silty sand with biogenic carbonate (6%). Compositionally, a relatively equal contribution is made from siliciclastics (38%), detrital carbonate (34%), and biogenic carbonate (28%) (Table T2). Biogenic silica is a minor constituent (1%). Grain size in this subunit is also evenly spread between sand (31%), silt (30%), and clay (39%). Although the clay size fraction is most abundant, this subunit does have the highest sand abundance in Unit I. A downhole color change is prominent in the uppermost 2 m of Subunit IA, from olive-gray through dark greenish gray to dark gray. Below 15 mbsf, sediment becomes slightly more mottled in shades of green (greenish gray to dark greenish gray). In general, sediments with higher sand and/or carbonate contents have lighter colors. A distinct reddish mottled calcareous mud and a distinct greenish mottled calcareous mud also occur in discrete localities throughout the succession. Several coarser grained lithologies form graded beds within the calcareous muds. The numbers of these graded beds are as follows: silty mud with biogenic carbonate (n = 411; average thickness = 9 cm), sandy mud with biogenic carbonate (n = 166; average thickness = 4 cm), and silty sand with biogenic carbonate (n = 106; average thickness = 7 cm) (Table T3). These beds occur as normally graded, inversely graded, and bi-gradational sequences. The most complete bi-gradational sequences coarsen upward from calcareous mud through silty mud with biogenic carbonate to silty sand with biogenic carbonate, and then fine upward to silty mud with biogenic carbonate and into calcareous mud (e.g., Fig. F12). Some of the sequences are less complete, lacking the silty sand component. Contacts between lithologies within the bi-gradational units are gradational and bioturbated. Inversely graded sequences generally coarsen upward from calcareous mud to silty mud with biogenic carbonate and then to sandy mud. In more complete inversely graded sequences, the interval grades upward into silty sand. Basal contacts are gradational or bioturbated. The upper contacts are often sharp or irregular, if not disturbed by bioturbation. These two sequence types are considered as typical contourite facies and are the most common sequences in Subunit IA (n = 177). The normally graded sequences in Subunit IA (n = 51) fine upward, generally from silty sand with biogenic carbonate to silty mud with biogenic carbonate and then to calcareous mud (Fig. F13; Table T3). The sequences have sharp, erosional, or irregular basal contacts with the underlying calcareous mud. However, in places the basal contacts become unclear due to bioturbation. Some of these sequences are less complete, lacking the silty sand. Subunit IB
The dominant lithology in Subunit IB is calcareous mud, which is more abundant in Subunit IB (82%) than in any other subunit (Table T4; Fig. F10). Other lithologies include silty mud with biogenic carbonate (11%), sandy mud with biogenic carbonate (4%), and silty sand with biogenic carbonate (3%). Other than a small relative increase in the average abundance of siliciclastics (from 38% to 45%), little compositional difference can be determined between Subunits IA and IB (average abundances of detrital carbonate and biogenic carbonate is 31% and 24%, respectively, in Subunit IB) (Table T2). The clay size fraction is more abundant (47%) and the sand size fraction is less abundant (20%) in Subunit IB than in Subunit IA, whereas the silt abundance remains similar. Even after accounting for the difference in subunit thicknesses, the number and average thickness of silty mud beds (n = 15; average thickness = 9 cm) are less than those observed in Subunit IA. The number and thickness of sandy mud beds (n = 15; average thickness = 4 cm) and of silty sand beds (n = 9; average thickness = 7 cm) are also lower than in Subunit IA (Table T3). This distinct change is also observed in the decrease in bi-gradational and inversely graded sequences (contourite facies; n = 9) and normally graded sequences (n = 10). Subunit IC
The dominant lithology in Subunit IC is calcareous mud (69%) (Table T4; Fig. F10). Minor lithologies include silty mud with biogenic carbonate (18%) and sandy mud with biogenic carbonate (13%), whereas silty sand with biogenic carbonate is absent. The average abundance of siliciclastic components in smear slides (56%) is the highest observed in any subunit, with correspondingly lower abundances of detrital carbonate (27%), and biogenic carbonate (22%) (Table T2). Subunit IC is slightly enriched in the clay size fraction (51%) and depleted in sand (17%), whereas the abundance of silt remains similar to that observed in Subunits IA and IB. The number and thickness of silty mud beds (n = 40; average thickness = 103 cm) and sandy mud beds (n = 37; average thickness = 58 cm) increase in Subunit IC relative to Subunit IB. Silty sand beds are rare, with only three thin to medium beds occurring (3, 17, and 20 cm thick) (Table T3). This distinct facies change is also observed in the sequence types, with bi-gradational (n = 19) and normally graded (n = 17) beds becoming more abundant. Subunit ID
As with Subunits IA– IC, this subunit is dominated by calcareous mud. However, the abundance of calcareous mud decreases slightly from Subunit IC to Subunit ID (from 69% to 63%) (Table T4; Fig. F10). The abundances of silty mud with biogenic carbonate (19%) and sandy mud with biogenic carbonate (16%) increase slightly from Subunit IC, with the abundance of sandy mud with biogenic carbonate reaching its maximum for Unit I. Silty sand with biogenic carbonate is a minor contributor to Subunit ID (2%), but even this low abundance is noteworthy compared to the absence of this lithology in Subunit IC. The average abundances of siliciclastic sediment components (50%), detrital carbonate (27%), and biogenic carbonate (24%) are similar to their abundances in Subunits IA–IC (Table T2), as are the abundances of the clay (48%), silt (29%), and sand (17%) size fractions. The facies change identified in Subunit ID is a relative increase in the number and thickness of silty mud beds (n = 64; average thickness = 150 cm) and sandy mud beds (n = 74; average thickness = 128 cm). The number of silty sand beds again remains low (n = 7; average thickness = 14 cm; Table T3). The increase in silty and sandy beds also corresponds with an increase in the number of bi-gradational (n = 50) and normally graded (n = 23) sequences, which are more abundant than in Subunits IB and IC. This increase in silty and sandy beds, as well as in graded beds, is interpreted to record a relative increase in current activity (Figs. F9, F10). Subunit IE
As with the overlying succession, this subunit is dominated by calcareous mud, although there is relatively less mud (50%) and more silty mud with biogenic carbonate (34%) than in any other subunit (Table T4; Fig. F10). The abundance of sandy mud with biogenic carbonate is the same as in Subunit ID, whereas silty sand with biogenic carbonate is absent from Subunit IE. The average abundances of siliciclastic components (50%), detrital carbonate (24%), and biogenic carbonate (23%) are similar to those in the overlying units (Table T2), as are the average abundances of the clay (50%), silt (31%), and sand (19%) size fractions. The observed facies changes identified in Subunit IE are a decrease in the number and thickness of silty mud beds (n = 43; average thickness = 144 cm), a significant decrease in the number and thickness of sandy mud beds (n = 22; average thickness = 76 cm), and the absence of silty sand beds (Table T3). This decrease in the importance of silty and sandy beds also corresponds with a decrease in the number of bi-gradational sequences (n = 22) and the absence of normally graded sequences. Lithologic characteristicsThe sedimentary succession at Site U1389 has been classified as one major lithologic unit with five subunits (IA–IE) (Figs. F9, F10). Unit I is a ~990 m thick Holocene–Pliocene (~3.6 Ma) sequence of calcareous mud (61%), silty mud with biogenic carbonate (24%), sandy mud with biogenic carbonate (11%), and silty sand with biogenic carbonate (4%) (Table T4). The silty and sandy lithologies are generally present as bi-gradational, normally graded, or inversely graded sequences within the calcareous mud. The division of Unit I into five subunits was based on subtle changes in the relative abundances of the different lithologies and in the facies characteristics of the sandy and silty lithologies. TextureOn average, sediment is fine grained through most of Unit I; the average grain size is clay in 45% of the section and silt in 32% of the section (Table T2). The intervals of sandy mud and silty sand with biogenic carbonate have an average grain size from very fine to fine sand (24%), with a maximum grain size of medium sand. Silty and sandy units generally are poorly sorted, with detrital siliciclastic grains that are subrounded to rounded. Detrital carbonate grains are generally subrounded to subangular, many with abraded margins indicative of reworking. CompositionAll lithologies in Unit I are similar in composition, with only subtle changes in abundances between the subunits (Tables T2, T4). Accordingly, the composition of Unit I is described here, and the subtle differences are identified for each subunit below. Unit I is dominated by terrigenous components, including siliciclastics (44%, including quartz, feldspars, heavy minerals, clay minerals, and volcanic glass), and detrital carbonate (30%). Biogenic components (25%) are dominated by calcareous nannofossils, with rare to common foraminifers (Table T2). Biogenic silica components are rare (<1%) but when present include sponge spicules and fragmented radiolarians. No discrete ash or dust layers and no dropstones were observed. Authigenic components, such as pyrite (usually classified as opaque mineral grains) and dolomite, are also present in minor amounts (<1%) throughout Unit I. Some dolomite grains are subangular, which may indicate a detrital origin. Glauconite grains are also present throughout Unit I (<1%). Both glauconite and dolomite are generally more abundant in the silty sand beds. Bulk mineralogyInterpretations of bulk mineralogy are based on XRD scans of pressed powders without any rescans after glycolation. The principal minerals identified include quartz, plagioclase, calcite, dolomite, and clays (Fig. F14; Tables T5, T6). The peak intensity of quartz in Unit I varies between 23,000 and 32,000 counts, with the highest counts in Subunits IA and ID. The peak intensity of calcite varies between 12,700 and 17,000 counts in Unit I, with the highest intensity in Subunit IE and no clear trend with lithology. The intensity of the illite diffraction peak ranges between 3,000 and 4,300 counts, with the greatest intensities in Subunits IA and IE. Other clay minerals (chlorite, kaolinite, and smectite) show lower intensities and very little variation throughout Unit I. Plagioclase tends to be slightly more abundant in Subunits IA and ID. Sediment in Subunits IA and IB contains slightly more hornblende than in the other units. Overall, Subunit IA shows the greatest variability in mineral peak intensities, followed by Subunits ID and IE, but the subunits have a relatively uniform composition throughout. Faunal elements and ichnofossilsMacrofossil fragments and some nearly whole specimens occur throughout Unit I. Recognizable fragments include gastropods, bivalves, echinoderms, cold-water corals, and Arenaria (Figs. F15, F16, F17). Bioturbation and burrows are also common throughout, usually indicated by diffuse centimeter-scale mottling and millimeter-scale pyritic burrow fills. Black iron sulfide mottling is also common in the calcareous mud. Discrete burrows and recognizable ichnofossils are rare, but include Chondrites, Zoophycos, Planolites, and Spirophyton (Figs. F18, F19, F20). The bioturbation index for all of Unit I ranges from sparse to slight. DiscussionOverall, the sedimentary succession recovered at Site U1389 is very uniform in character, with little variation in lithology or composition. The principal changes observed are in the relative abundance of silty mud and sandy mud beds, as well as in the number of bi-gradational, inversely graded, and normally graded sequences. Accordingly, the subunits at this site have been defined primarily on the basis of facies changes, rather than lithologic or compositional changes. Unit I is interpreted as the record of a contourite-dominated depositional system, at least for Subunits IA–IC, based on several lines of evidence. Most compelling is the fact that the site presently is located on a bathymetric high surrounded by deeply erosional contourite channels (García et al., 2009) on three sides and a bathymetric depression to the northwest (Fig. F1). Furthermore, the site lies directly under the influence of the lower core of MOW and has no obvious connection with downslope (turbidite) channels. There seems to be little evidence of a direct turbidity current input and almost no possibility that turbidity currents would have deposited thick, graded beds at Site U1389, at least in the recent past (i.e., Subunits IA– IC). Lithologic characteristics further support a contourite interpretation, including
Despite their grading and typically sharp basal contacts, the normally graded sequences are interpreted as truncated contourite deposits, rather than turbidites, based on the following criteria (after Faugères and Mulder, 2011):
Truncation of the idealized bi-gradational contourite sequences is common as a result of changes in current velocity, producing incomplete sequences and locally sharp or irregular erosional contacts (Faugères and Mulder, 2011; Stow and Faugères, 2008). Inversely graded sequences occur where there is long-term increase in mean current velocity followed by a rapid decrease in velocity. By contrast, normally graded sequences occur where current velocity has increased sufficiently to cause erosion into the underlying silty mud contourite, leaving a sharp-based sandy contourite directly overlying a muddy contourite. Long-term fluctuations in flow velocities may produce “stacked sequences,” where several partial or truncated sequences directly overlie each other. This characteristic is well developed at Site U1389. Subunit IB is more difficult to understand, largely because core recovery in this interval (~30%) is markedly lower than in the overlying succession. Two possible interpretations are (1) the recovered sediment, which is mainly mud rich, is representative of all of Subunit IB, so that Subunit IB may record a phase when bottom currents were less intense, depositing only muddy and silty contourites interbedded with some hemiplegic material, or (2) the 70% unrecovered section is, in fact, relatively sand/silt rich, indicative of relatively higher current velocities. The slight relative increase in siliciclastic components in the sediments of Subunit IB (Table T2) may also indicate that the sediment in this subunit has a more complex composition, with a more diverse sediment supply under higher current velocities. Downhole logging results strongly support the interpretation that the unrecovered sediment is relatively sand/silt rich. This interpretation is further supported by marked changes in interstitial water geochemistry, from which we would infer a lateral fluid conduit at approximately this level, aided by the presence of the coarser grained, but unrecovered, lithologies. Preliminary correlation with other sites suggests that Subunits IA– IC at Site U1389 correspond to Subunits IA–IC at Site U1386 and the informal subdivisions of 1A–1C at Site U1387. Subunit ID is missing from Sites U1386 and U1387 because of the presence of an extensive hiatus. Subunit IE is equivalent in age with Unit II at Sites U1386 and U1387, although the lithologies and inferred transport processes are quite different. |
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