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doi:10.2204/iodp.proc.339.108.2013

Lithostratigraphy

Drilling at Site U1390 recovered a 351 m thick section of sediment (Figs. F3, F4). The shipboard lithostratigraphic program at Site U1390 involved detailed visual logging of all archive sections, visual assessment of sediment color, petrographic analysis of smear slides, and X-ray diffraction (XRD) analyses of powdered bulk samples. We sampled regularly for smear slides during visual core description in Hole U1390A (n = 96); these results were used to provide detailed sediment description, to identify major components, and to apply a more detailed descriptive sediment classification. Smear slides were selected from specific intervals in Holes U1390B (n = 29) and U1390C (n = 4) to investigate lithologies and features of specific interest and to identify any differences from Hole U1390A. Thirty-eight samples were selected from Hole U1390A for powder XRD analysis in order to obtain a general indication of bulk mineral composition (Fig. F5; Table T2). Ten of these samples were processed to identify clay mineralogy (Fig. F6).

The age at the bottom of Hole U1390A is between 1.2 and 1.6 Ma (see “Biostratigraphy”). Two minor hiatuses are recognized at 228.68 and 293.68 mbsf, with durations of 0.26 to >0.6 m.y. and of 0.9 to 1.2 m.y., respectively.

The sedimentary succession at Site U1390 is classified as one major lithologic unit divided into two subunits (IA and IB) (Fig. F4; Table T3). Overall, Unit I is dominated by calcareous mud and silty mud with biogenic carbonate. Sandy mud with biogenic carbonate is a subordinate lithology.

Total carbonate content based on carbonate analyses in Hole U1390A ranges from 21 to 34.5 wt% with an average of 27 wt% (Table T4). These results are consistent with abundances of biogenic carbonate and detrital carbonate estimated from smear slides. Thus, the lithologic names determined from smear slide analyses have been used without modification through this text, the summary diagrams, and the visual core description sheets.

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 U1390, together with some of these additional properties, are summarized in Figure F7.

Unit I description

• Intervals: 339-U1390A-1H-1, 0 cm, through 38X-CC, 37 cm; 339-U1390B-1H-1, 0 cm, through 21H-CC, 6 cm; 339-U1390C-1H-1, 0 cm, through 19H-CC, 35 cm

• Depths: Hole U1390A = 0–351.26 mbsf (bottom of hole [BOH]), Hole U1390B = 0–189.64 mbsf (BOH), Hole U1390C = 0–175 mbsf (BOH)

• Age: Holocene–Pleistocene

Unit I sediment is composed of varying amounts of terrigenous and biogenic components (mainly clay minerals, nannofossils, detrital and biogenic carbonate, and quartz) (Table T5). Calcareous mud, silty mud with biogenic carbonate, sandy mud with biogenic carbonate, and silty sand with biogenic carbonate are common lithologies in Unit I.

Unit I is divided into Subunits IA and IB on the basis of the composition of the muddy deposits (Table T5) and the thickness of the sandy deposits (i.e., silty sand with biogenic carbonate and sandy mud with biogenic carbonate). The lithology of Subunit IA is dominated by mud. In contrast, sand beds in Subunit IB are thicker than those in Subunit IA.

Subunit IA

• Intervals: 339-U1390A-1H-1, 0 cm, through 32X-CC, 29 cm; 339-U1390B-1H-1, 0 cm, through 21H-CC, 6 cm; 339-U1390C-1H-1, 0 cm, through 19H-CC, 35 cm

• Depths: Hole U1390A = 0–293.78 mbsf; Hole U1390B = 0–189.64 mbsf (BOH), Hole U1390C = 0–175 mbsf (BOH)

• Age: Holocene–Pleistocene

Lithologies and bedding

The major lithologies in Subunit IA are calcareous mud, silty mud with biogenic carbonate, sandy mud with biogenic carbonate, and silty sand with biogenic carbonate (Fig. F4). An important feature of these lithologies is that increased grain size corresponds to increased detrital content and decreased abundance of biogenic carbonate. The downhole variations in the number of beds of these major lithologies, except for calcareous mud, are shown in Figure F4. Within Subunit IA in Hole U1390A, overall downhole changes in the number of silty sand, sandy mud, and silty mud beds appear to show three cycles in Cores 339-U1390A-1H through 13X, 13X through 19X, and 19X through 32X. A similar cyclic change is also recognized at the previous sites (Unit I at Sites U1386, U1387, and U1389). These cycles might be controlled by the changes of sediment supply, bottom water current velocity, relative sea level, or neotectonic activity.

Almost all the intervals composed of calcareous mud are characterized by indistinct bedding (Fig. F8). Thus, features that might be beds are distinguished by subtle changes in color, bioturbation intensity, or silt/clay ratio. The coarser sediments (i.e., silty sand with biogenic carbonate, sandy mud with biogenic carbonate, and silty mud with biogenic carbonate) are intercalated in the calcareous mud and show two distinctive bedding patterns, bi-gradational grading and normal grading (Figs. F9, F10). The bi-gradational sequence is typically characterized by coarsening upward from calcareous mud to silty mud with biogenic carbonate to sandy mud with biogenic carbonate (or silty sand with biogenic carbonate) and subsequent fining upward from silty mud with biogenic carbonate to calcareous mud. Some of the bi-gradational sequences in Subunit IA lack sandy intervals (i.e., muddy sand with biogenic carbonate and silty sand with biogenic carbonate). Thicknesses of the bi-gradational sequences are commonly a few decimeters to several meters. In contrast, the normally graded sequence is characterized by fining upward, generally from sandy mud with biogenic carbonate or silty sand with biogenic carbonate to silty mud with biogenic carbonate to calcareous mud. The thickness of the normally graded sequences is generally <1 m. Inversely graded sequences are rare (Fig. F11) and show lithologic changes from calcareous mud to silty mud with biogenic carbonate to sandy mud with biogenic carbonate with a sharp upper contact.

Contacts between all lithologies, and between subjacent beds of calcareous mud, are mainly gradational or bioturbated but do also occur as sharp basal contacts, particularly with the normal graded sequences.

No distinctive features suggesting tectonic influence (e.g., microfaults, slumping, or sediment gravity flow deposits) are recognized in Subunit IA.

Structures and texture

For the most part, primary sedimentary structures were not observed in Subunit IA, except for faint lamination in parts of the calcareous mud (e.g., Sections 339-U1390B-2H-2 through 2H-5; Fig. F12). The preserved laminations suggest especially rapid sedimentation rates for the calcareous mud in these intervals.

Bioturbation is the most evident postdepositional sedimentary structure in Subunit IA and is present throughout the observed section. Most of the bioturbation and individual burrows are recognized as centimeter to millimeter scale, dispersed, and gray to black color mottling and pyritic burrow fills. In some intervals, the relative frequency of the mottling seems to be greater and the mottles darker than those of the previous sites (U1386–U1389). Some of the black mottling appears to be associated with high values of natural remanent magnetization (NRM) intensity in shipboard paleomagnetic measurements (see “Paleomagnetism”). The bioturbation intensity is slight to sparse on the basis of the observation of beds with slight color changes.

Composition

Mineral compositions based on smear slide observations show no clear differences between the two subunits (Fig. F13). The results show that almost all lithologies at Site U1390 are dominated by siliciclastic minerals including clay minerals, quartz, feldspars, mica, dolomite, and detrital carbonate (Fig. F14; Table T5). The biogenic fraction is mainly composed of nannofossils, with a few foraminifers present. A few beds are also characterized by rare to abundant diatoms, some radiolarians, and few to common sponge spicule fragments (e.g., Sections 339-U1390A-19X-3 and 339-U1390B-18H-3; Fig. F15). Authigenic components are dominated by dolomite and pyrite (Fig. F16). Abundances of terrigenous components, as estimated from smear slides (Table T5), are 45%–63% (average = 51%) siliciclastics (including quartz, heavy minerals, opaques, micas, and feldspars) and 13%–35% (average = 24%) detrital carbonate. No discrete volcanic ash layers and no dropstones were observed. Abundances of biogenic components, as estimated from smear slides, are 10%–38% (average = 23%) biogenic carbonate (primarily nannofossils, plus foraminifers for the sandy mud and silty sand lithology) and 1%–22% (average = 0.4%) biogenic silica (primarily diatoms and radiolarians). Siliceous microfossils (≤20%) were estimated only in intervals 339-U1390A-19X-3, 108 cm (163.94 mbsf) and 19X-4, 10 cm (164.15 mbsf). Total carbonate contents (assuming all inorganic carbon to be CaCO₃) range from 21.15 to 34.51 wt% (average 27.04 wt%; Table T4). Abundances of other minerals, as estimated from smear slides, are <5% pyrite (usually classified as opaque mineral grains) and <5% authigenic dolomite (mostly observed by its rhombic shape). A limited number of glauconite grains are also present (interval 339-U1390A-1H-2, 80 cm; 2.3 mbsf).

Macrofossil fragments and sparse nearly whole specimens are visible throughout Subunit IA at Site U1390. Recognizable fragments include gastropods, bivalves, echinoids, cold-water corals, and Arenaria (Fig. F17).

Color

A downhole color change from olive-gray (5Y 4/2) to dark gray (5Y 4/1) occurs at Sections 339-U1390A-3H-1 (~13 mbsf), 339-U1390B-1H-6 (8.2 mbsf) and 339-U1390C-2H-4 (9.03 mbsf). Subsequently, below 32 mbsf, sediments become slightly more greenish (dark greenish gray) (10Y 4/1). Very dark greenish gray intervals are moderately well developed, which is consistent with relatively low values of L*.

The principal colors of the lithologies in Subunit IA, as noted during visual description of the core, range from dark greenish gray (56.36% of Subunit IA) to dark gray (13.94%) to greenish gray (12.12%) and very dark greenish gray (8.28%). The remaining colors of Subunit IA are olive-gray (3.84%), gray (2.22%), greenish gray (1.21%), reddish gray (1.01%), dark olive-gray (0.81%), and dark grayish brown (0.20%).

Bulk mineralogies

The bulk mineral composition of 18 sediment samples in Subunit IA was analyzed by XRD. Diffraction peaks from silicate minerals such as quartz, plagioclase, and illite and carbonate minerals such as calcite and dolomite contribute most of the measured total diffraction peak intensity (Fig. F5; Table T2).

Subunit IB

• Interval: 339-U1390A-32X-CC, 29 cm, through 38X-CC, 37 cm

• Depth: 293.78–351.26 mbsf (BOH)

• Age: Pleistocene

Lithologies and bedding

Subunit IB comprises the lower part of Unit I and is characterized by more thick sands than were observed Subunit IA. In particular, bi-gradational contourite sequences from several to at least 7 m thick are well preserved in Subunit IB (Fig. F18). One bi-gradational sequence ~10 m thick was recovered in Cores 339-U1390A-33X and 34X, if we assume that both cores recovered parts of the same bed. The observation that thick sands are present in Subunit 1B is consistent with downhole logging results (see “Downhole measurements”).

No distinctive features that suggest tectonic influence (e.g., microfaults, slumping, or sediment gravity flow deposits) are recognized in Subunit IB.

Structures and texture

For the most part, primary sedimentary structures were not observed in Subunit IB, although bioturbation occurs as a common postdepositional sedimentary structure throughout. Individual burrows are recognized as centimeter to millimeter scale, dispersed, gray to black color mottling and pyritic burrow fills. The bioturbation intensity is slight to sparse based on the observation of beds with slight color changes.

Composition

The features of the main lithologies in Subunit IB are the same as those in Subunit IA. Abundances of terrigenous components in Subunit IB (Hole U1390A) are also similar to those in Subunit IA (Table T5), with 40%–70% (average = 57%) siliciclastics (including quartz, heavy minerals, opaques, micas, and feldspars) and 20%–30% (average = 22%) detrital carbonate. No discrete volcanic ash layers and no dropstones were observed. Abundances of biogenic components are 10%–30% (average = 21%) biogenic carbonate (primarily nannofossils, plus foraminifers for the sandy mud and silty sand lithology), and biogenic silica is absent.

Color

The principal colors of the lithologies in Subunit IB range from greenish gray (69.05% of Subunit IB) to light greenish gray (21.43%). The remaining colors of Subunit IB are gray (5.95%), reddish gray (1.19%), light greenish gray (1.19%), and dark gray (1.19%).

Bulk mineralogy

The bulk mineral composition of six sediment samples from Subunit IB was analyzed by XRD. Diffraction peaks from silicate minerals such as quartz, plagioclase, and illite and carbonate minerals such as calcite and dolomite contribute most of the measured total diffraction peak intensity (Fig. F5; Table T2).

Discussion

Contourite depositional system

Given the physical setting of Site U1390, several lines of evidence support the interpretation of Unit I as a sequence of contourite deposits and thus evidence for current transport and changing current velocities. Among these lines of evidence are that

• This site is located on the sheeted drift related to the Guadalquivir contourite channel;

• Bi-gradational graded successions are abundant;

• Interbedded normal graded sequences with sharp base contacts and inversely graded sequences with sharp top contacts are occasionally observed; and

• Obvious primary sedimentary structures, commonly observed in sediment gravity flow deposits, have not been recognized in these sediments.

Subunit IA (293.68 m thick) is a typical example of a muddy/silty contourite succession, similar to that defined by Stow and Faugères (2008) as a conceptual contourite facies model (Figs. F4, F8, F9, F10, F11), whereas Subunit IB (over 57.58 m thick) is a typical example of a sandy contourite succession (Figs. F4, F18). Very thick bi-gradational sequences are interpreted to indicate rapid sedimentation caused by high sediment supply from the Guadalquivir Channel. Some faint laminations observed at the top of Hole U1390B (Fig. F12) also support the interpretation of rapid sedimentation. The differences observed between Subunits IA and IB are most likely associated with a change from a sand-dominated CDS to a mud-dominated CDS.

Relationship between the observed hiatuses and the lithologic features

Two hiatuses are recognized on the basis of the micropaleontological data. In Hole U1390A, one hiatus is at 228.68 mbsf, which extends from 0.26 to >0.6 Ma, and the other is at 293.68 mbsf, which extends from 0.9 to 1.2 Ma (see “Biostratigraphy”). The older hiatus apparently corresponds to the boundary between Subunits IA and IB. It is difficult to observe the contact clearly because the boundary is in the core catcher of Core 339-U1390A-32X. However, the total thicknesses of the sandy deposits (i.e., silty sand with biogenic carbonate and sandy mud with biogenic carbonate) within each core show clear differences across the Subunit 1A/1B boundary (Fig. F4). Thus, it is suggested that some of the controlling factors for the deposition of the contourites (e.g., bottom water current velocity, sediment supply, or relative sea level) changed during the hiatus.

In contrast, the younger hiatus can be located precisely at the downhole color change from darker greenish gray to lighter greenish gray at interval 339-U1390A-26X-2, 19 cm (228.7 mbsf), based on nannofossil dates of 0.26 Ma at 228.65 mbsf and >0.6 Ma at 228.73 mbsf (Fig. F19; see “Biostratigraphy”). No major lithologic change is observed across this hiatus, as calcareous mud is present both above and below that level. The only compositional change is a decrease in the abundance of the siliciclastic fraction, from 38% above the hiatus to 55% below (Table T5). Further detailed examination (e.g., composition or grain size) will be necessary to better understand the hiatus in relation to the changes in depositional environment.

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