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doi:10.2204/iodp.proc.308.106.2006

Lithostratigraphy

Site U1322 has a thinner sediment package above the Blue Unit than either Site U1323 or U1324. Sediment was recovered to 234.5 mbsf. The succession was divided into two lithostratigraphic units (Units I and II) on the basis of visual core descriptions from Hole U1322B (Fig. F6; Table T6). The total depth of this hole ties closely to seismic Reflector S60-1322. The boundary between lithostratigraphic Units I and II (125.8 mbsf) occurs just above seismic Reflector S30 (Fig. F6). Lithostratigraphic Unit I is dominated by clay locally interbedded with silt and is further divided into four subunits based on intervals of deformed and undeformed sediment. Lithostratigraphic Unit II is composed of mud and characterized by the alternation of deformed and undeformed sediment.

Description of lithostratigraphic units

Unit I

• Interval: 308-U1322B-1H-1, 0 cm, through 15H-1, 100 cm
• Depth: 0–125.8 mbsf
• Age: Holocene/late Pleistocene
• Lithology: clay

Lithostratigraphic Unit I includes four subunits (IA–ID). This unit is predominantly composed of clay with a 2 m thick interval of foraminifer-nannnofossil-bearing clay at the top. Subunit division is based on the observation of distinct intervals of deformed sediment (Subunits IB and ID). The ratio of deformed intervals to total thickness in this unit is ~47%. The base of lithostratigrapic Unit I is defined at the base of a highly deformed interval and ties to just above seismic Reflector S30.

Subunit IA (0–37.4 mbsf)

Lithostratigraphic Subunit IA is 37.43 m thick and is characterized by 1–3 cm thick olive-green and reddish brown couplets of clay accompanied by black mottled bands (Fig. F7). The clay couplets have sharp bases and show color gradation upward from dark green/black near the base to reddish brown at the top. Grain-size grading associated with color change could not be determined by core or smear slide examination. Black mottled bands are rich in organic matter, the color most likely caused by iron sulfide precipitation. The uppermost 2 m of this subunit is rich in foraminifers and nannofossils. The base of lithostratigraphic Subunit IA at 37.43 mbsf is marked by a sharp contact between flat bedding above and deformed bedding below.

Subunit IB (37.4–58.6 mbsf)

Lithostratigraphic Subunit IB is composed of centimeter-thick beds and laminae of deformed greenish gray and reddish brown clay. Two strongly folded and steeply dipping intervals occur in this subunit and are marked by sharply defined basal surfaces at 41.25 and 52.45 mbsf, respectively. The interval between 41.25 and 52.45 mbsf is characterized by brittle deformation including small-scale (centimeter-scale vertical offset) reverse and normal faults (Fig. F8).

Subunit IC (58.6–91.3 mbsf)

Lithostratigraphic Subunit IC is composed of couplets of greenish gray and brownish gray laminae and thin beds of clay with thicknesses ranging 1–5 cm (Fig. F9). A minor silt component occurs throughout this subunit in laminae, discontinuous lenses, and millimeter-sized burrow fills. The silt-sized grains are predominantly quartz with variable amounts of mica, calcite, dolomite, and other minor constituents.

Subunit ID (91.3–125.8 mbsf)

Lithostratigraphic Subunit ID is composed of deformed reddish brown and greenish gray clay couplets (Fig. F10). The subunit has a tightly folded zone in the middle that grades downward into a homogeneous clay interval. The homogeneous intervals contain greenish gray and/or brownish gray mud without any sedimentary structures or faults. Reddish mud clasts incorporated in the homogeneous clay are present at the base of this subunit.

Unit II

• Interval: 308-U1322B-15H-1, 100 cm, through 29H-CC, base
• Depth: 125.8–234.5 mbsf
• Age: late Pleistocene
• Lithology: clay and mud

The top of Lithostratigraphic Unit II corresponds to a sharp contact at the base of the thickest deformed interval in Lithostratigraphic Unit I at 125.8 mbsf (Fig. F10) and coincides with the change from a deformed interval above to a laminated and undeformed interval below. This unit is made up of alternating intervals of meter-scale deformed, laminated clay or mud. The deformed intervals are composed of interbeds of brownish and greenish gray mud and are characterized by dipping beds, small-scale faults, recumbent folds, and mud clasts (Fig. F11). Some intervals contain homogeneous mud with a few centimeter- to decimeter-scale mud clasts. These homogeneous layers are more frequent at the base of deformed (folded and faulted) intervals (Fig. F12). We distinguished nine deformed intervals separated by undeformed mud layers at their boundaries. The deformed intervals have thicknesses varying from 2 to 5 m (intervals 2-1 to 2-9 in Fig. F6; Table T7). The ratio of the deformed intervals to total thickness of lithostratigraphic Unit II is 61% if the homogeneous mud intervals are included as part of the deformed intervals.

Coherent intervals between deformed intervals consist of dark greenish gray and reddish gray laminated mud with local occurrences of black mottled mud as laminae and specks. Three intervals in the upper part of lithostratigraphic Unit II show distinct increases in silt content (Fig. F6), which occurs as thin laminae and specks (burrow fills) throughout these intervals. Silt laminae occur at frequencies of about 5/m; most laminae are discontinuous and disrupted by bioturbation.

X-ray diffraction analysis

Fourteen clay and mud samples from Hole U1322B were analyzed by X-ray diffraction (XRD) to evaluate downhole variations in bulk mineralogy (Fig. F13). The diffractograms illustrate that the mineralogy consists of a mixture of quartz, calcite, dolomite, feldspars, and various phyllosilicates (Fig. F14). In this preliminary study we did not attempt to quantify the mineralogy of the clays and restricted the analysis to bulk rock powders.

With the exception of samples taken from Core 308-U1322B-1H, the overall composition of the samples is remarkably similar, although variations in peak intensities indicate changes in mineral proportions. The larger variations in abundance are observed in the quartz, calcite, and dolomite peaks. The abundance of calcite to dolomite is estimated by taking the ratio of counts per second from the principal peak of each mineral (Fig. F15). At 0.47 mbsf, the foraminifer-nannofossil-bearing clay is dominated by the calcite peak, with a calcite/​dolomite peak ratio of ~35, dropping rapidly to ~0.8 in the mottled clay below at 2.14 mbsf. At ~36 mbsf, the calcite/​dolomite ratio increases to ~2, with negligible difference between adjacent samples of clay of different colors. Samples taken from various clay and mud intervals below 100 mbsf suggest that dolomite is consistently more abundant, although the calcite/​dolomite ratio varies significantly between 0.3 and 0.9. Abundances of calcite and dolomite in the XRD samples are consistent with smear slide observations that show a significant component of calcite in most samples (Fig. F13) and with coulometric analysis (see “Geochemistry and microbiology”) that shows significant amounts (10–20 wt%) of inorganic carbon throughout the sediments.

The calcite encountered was characterized based on smear slide observations as composed of a mixture of detrital grains and biogenic components (nannofossils and foraminifers). The dolomite grains are most likely detrital. Thus, it is possible that variations in the calcite/​dolomite ratio are in part related to variations in the input of detrital sediment versus biogenic components. Additional shore-based studies with a more representative set of samples are necessary to resolve any smaller-scale variations.

Interpretation of lithostratigraphy

Lithostratigraphic Unit I is interpreted to contain a succession of clay turbidites deposited in a distal levee environment. Two MTDs occurred within this levee assemblage. Lithostratigraphic Subunit IA is interpreted as hemipelagic drape and very distal turbidites. Lithostratigraphic Subunits IB and ID do not have distinct grain size differences compared to the other subunits within Unit I, but the presence of deformed beds indicate that these intervals have been remobilized and are thus interpreted as MTDs. The homogeneous interval at the base of lithostratigraphic Subunit ID contains mud clasts, which suggest that it is part of the same MTD.

The lithology of lithostratigraphic Unit I (0–125.8 mbsf) at Site U1322 is similar to the lithology of lithostratigraphic Unit I at Site U1324. Seismic Reflectors S10 and S20, corresponding to the upper boundary of lithostratigraphic Subunit ID, can be traced between the two sites. Therefore, lithostratigraphic Unit I at Site U1322 is correlative with lithostratigraphic Unit I at Site U1324. Lithostratigraphic Subunit IA at Site U1322 is slightly thinner than at Site U1324 and also appears to be slightly finer grained (higher clay content). The thickness and apparent grain size changes are consistent with turbidity current overspill from a channel to the west.

The well-preserved layers observed in MTDs in Subunits IB and ID imply mild deformation during transport, meaning that they probably have not moved significant distances from their original location. The MTDs in Site U1324 are also characterized by mild deformation such as dipping and folding of beds, also indicating short transport distance (see the “Site U1324” chapter). On the other hand, the occurrence of a homogeneous interval at the base of lithostratigraphic Subunit ID indicates strong deformation and homogenization during transport and emplacement. Therefore, the style of deformation of lithostratigraphic Subunit ID at Site U1322 compared to the correlative Subunit ID at Site U1324 indicates that the former was transported farther than the latter.

The base of lithostratigraphic Unit I and top of lithostratigraphic Unit II at 125.8 mbsf marks a lithologic difference from highly deformed and homogenized mud above to coherently laminated mud below. This boundary ties closely to seismic Reflector S30, which marks a significant contrast in seismic facies (Fig. F6). This major boundary also separates distinct log signatures in formation resistivity (Fig. F43; see “Downhole measurements”). Within lithostratigraphic Unit II the interpreted breaks at the top and base of packages of MTDs correspond closely to sharp excursions in resistivity. Many of the deformed intervals have higher resistivities than the laminated mud intervals, but the reverse is also observed. However, postcruise analysis of sediment velocity will resolve this issue.

Lithostratigraphic Unit II is characterized by alternating intervals of deformed sediment, homogeneous mud intervals, and coherent laminated mud intervals (Fig. F6). As observed for lithostratigraphic Subunits IB and ID, deformed intervals and homogenized intervals do not have distinct grain size differences compared to coherent intervals. However, the presence of deformed beds indicates that they have been remobilized and they are thus interpreted as MTDs. As the homogenized intervals occur at the base of deformed intervals, they are also interpreted as MTDs. The homogeneous nature of these intervals and the presence of mud clasts suggest emplacement by debris flows, whereas the folded and faulted intervals suggest emplacement by slumps and slides.

Summary interpretation

Lithostratigraphic Unit I, composed of clay and muddy MTDs, can be correlated to lithostratigraphic Unit I at Site U1324 because of the continuous seismic reflectors and similar lithostratigraphic pattern. Channel-levee systems, west of Site U1324, are thought to be the sediment source. Overspills of mud and clay developed the levee assemblage.

Lithostratigraphic Unit II, composed of thinner interbedded muds and muddy MTDs, is characterized by a stacked set of MTDs at Site U1322. The total ratio of MTDs to total thickness is 61% in lithostratigraphic Unit II.

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