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doi:10.2204/iodp.proc.308.108.2006 LithostratigraphyDrilling at Site U1324 penetrated Ursa Basin where the sediments above the Blue Unit are thicker than either to Site U1323 or Site U1322. Hole U1324B sampled the entire eastern levee deposits of the Southwest Pass Canyon channel-levee system and the overlying hemipelagic drape and distal turbidites of younger channel-levee systems (Fig. F4 in the “Site U1322” chapter). Sediment was recovered to 608 mbsf in Hole U1324B using APC and XCB coring with overall recovery >90%. The TD of 608 mbsf ties closely with seismic Reflector S60-1324, the top of the Ursa Canyon western levee. Therefore, sediment recovered at Site U1324 records the entire evolution of the eastern levee of Southwest Pass Canyon. Hole U1324C was spot-cored with the APC between 0 and 511.8 mbsf (Cores 308-U1324C-1H to 7H), but these cores were used primarily for geotechnical whole-round samples and were not used to define lithostratigraphic units. The sedimentary succession at Site U1324 is dominated by clay and mud in the upper 360 m and by interbedded silt, sand, and mud in the lower 250 m. Thus, we divided the succession into two lithostratigraphic units based on this distinction (Table T4). Both units were further divided into subunits based on the occurrence of intervals composed of contorted and faulted sediment and intervals composed of undeformed sediment. Cores 308-U1324B-4H to 44H were oriented, which allowed for detailed measurements of inclination and orientation of faults and dip of bedding planes (Table T5). Figure F2 summarizes the lithostratigraphic column, and Figures F3, F4, F5, F6, F7, F8, F9, F10, F11, F12, F13, F14, and F15 illustrate the range of features and variations in each of the lithostratigraphic units and subunits. Description of lithostratigraphic unitsUnit I
Lithostratigraphic Unit I extends from the seafloor to 364.7 mbsf and includes seven subunits (IA–IG). Lithostratigraphic Unit I is predominantly composed of terrigenous clay and mud with a marked paucity of silt and sand. Subunit divisions are based on the observation of distinct intervals composed of contorted and faulted sediment. Other than this distinction, the lithology throughout Unit I is very similar. The base of lithostratigraphic Unit I is defined as the top of a silt bed at 364.7 mbsf, marking a fundamental change in lithology to interbedded silt and very fine sand below, characteristic of lithostratigraphic Unit II. Subunit IA (0.0–43.9 mbsf)Lithostratigraphic Subunit IA is 43.9 m thick and is composed primarily of olive-green and reddish brown clay interbedded with centimeter- to decimeter-thick thin beds and laminae of black clay. Black clay is organic rich and shows transitions from black at the base to greenish gray at the top. The uppermost 0.38 m of this subunit is rich in nannofossils and foraminifers (Fig. F3). The base at 43.9 mbsf is marked by a fault that offsets parallel bedding surfaces (Table T5; Fig. F4A). Subunit IB (43.9–59.7 mbsf)Lithostratigraphic Subunit IB is composed of contorted and faulted greenish gray and reddish brown clay and centimeter- to decimeter-thick beds and laminae of black clay (Fig. F4). This lithology is similar to the lithology of Subunit IA but is deformed. Characteristics of faults were measured on the split core face and are summarized in Table T5. Generally, these are reverse faults with dips of ~30°. The top of this subunit corresponds closely to seismic Reflector S10 (Fig. F2). The seismic character of this subunit is chaotic with low-amplitude reflections and is distinct from the otherwise laterally continuous seismic reflections above and below this subunit (Fig. F4 in the “Site U1322” chapter). This is discussed further in “Core-seismic intergration.” Subunit IC (59.7–107.0 mbsf)Lithostratigraphic Subunit IC is composed of couplets of greenish gray and brownish gray laminae and beds up to 2–3 cm thick (Fig. F5). A minor component of silt occurs throughout this subunit as thin, discontinuous lenses and burrow fills a millimeter in diameter. Burrow fills are dominated by quartz grains with minor amounts of carbonate fragments and sponge spicules, as determined by smear slide analyses (Fig. F14). Subunit ID (107.0–151.0 mbsf)Lithostratigraphic Subunit ID is composed of faulted and contorted reddish brown and greenish gray clay couplets (Fig. F6). Bed dips are typically ~20°, and faults are mostly normal faults with small offset. Resistivity-at-the-bit (RAB) images of the formation in this subunit reveal dipping beds (5°–55°) throughout the interval (see “Downhole measurements”). The base of this subunit occurs just above the prominent seismic Reflector S30. Subunit IE (151.0–264.8 mbsf)Lithostratigraphic Subunit IE is a thick interval composed of 1–2 cm thick couplets of dark gray to light gray clay laminae and thin beds (Fig. F7). At the base of each couplet, dark gray clay has a slightly higher content of silt and organic matter than light gray clay; the couplets are normally graded. Black clay layers are commonly irregular and mottled rather than forming continuous laminae (Fig. F7). This subunit contains scattered white silt specks (burrow fills), burrows highlighted by black iron sulfides, and rare silt laminae. An interval enriched in foraminifers and nannofossils occurs between 165.0 and 167.2 mbsf, which corresponds closely with seismic Reflector S30. Subunit IF (264.8–286.1 mbsf)Lithostratigraphic Subunit IF is composed of faulted and contorted couplets of green and brownish green clay. Beds are steeply dipping (up to 70°) with small-offset faults and folding (Fig. F8). RAB images confirm the occurrence of deformed sediments in this subunit (see “Downhole measurements”). Subunit IG (286.1–364.7 mbsf)Lithostratigraphic Subunit IG is composed of greenish gray, reddish brown, and black mottled clay (Fig. F9) commonly arranged in couplets 2–7 cm thick with black clay grading to brownish gray and greenish gray clay. The black clay has a slightly higher component of silt than the greenish gray and brownish gray clay, and each couplet is normally graded. Black color is possibly caused by precipitation of iron sulfides and red/brown color by precipitation of iron oxides. A single bed of massive gray medium sand occurs from 305.7 to 306.8 mbsf, but otherwise this subunit is dominated by clay (Fig. F9). Unit II
Lithostratigraphic Unit II extends from 364.7 to 600.8 mbsf and includes four subunits (IIA–IID). The top of this unit is defined at the top of a normally graded thin bed of silt at 364.7 mbsf that marks a fundamental change in lithology in Hole U1324B. The unit predominantly includes interbedded silt and very fine sand with beds and laminae of mud and clay. Subunit divisions were distinguished by distinct intervals of contorted sediment. Subunit IIA (364.7–445.5 mbsf)Lithostratigraphic Subunit IIA is composed of bioturbated and mottled laminae and beds of greenish gray and reddish brown mud interbedded with normally graded reddish brown silt and very fine sand (Fig. F10). Laminae and bed thicknesses range from 1 to 45 cm. X-ray diffraction (XRD) analyses of green and red clay show that the samples are dominated by quartz and contain calcite, dolomite, feldspars, mica (illite), kaolinite, and chlorite; the X-ray diffractograms are similar in the two types of clay (Fig. F15). Carbon analyses show total carbon content of 2.4 and 2.5 wt% for the red and green clay, respectively. Thus, the variations from reddish brown, greenish gray, and blackish gray appear to be related primarily to precipitation of iron oxides or sulfides, depending on the redox conditions within the sediment. Subunit IIB (445.5–481.9 mbsf)Lithostratigraphic Subunit IIB is composed of contorted interbeds of brown and gray silty sand and contorted beds of green and red clay (Fig. F11). Rare burrows filled with sandy silt occur throughout this subunit. Deformed beds are well expressed in Cores 308-U1324B-57X through 59X. Subunit IIC (481.9–578.9 mbsf)Lithostratigraphic Subunit IIC is composed of interbedded greenish gray and reddish brown mud with thin beds and light gray silt laminae (Fig. F12). Silt beds are normally graded with sharp bases and gradational tops to mud. A few silt beds have scoured bases. Very fine lower sand occurs in thin, discontinuous light gray and tan laminae. Figure F12E shows a photomicrograph of silt from this subunit that is dominated by quartz with abundant mica grains. Drilling mud was observed to penetrate silt/mud interbeds, and it lined the outside margins of the core sections. Subunit IID (578.9–600.8 mbsf)Lithostratigraphic Subunit IID is composed of contorted interbeds of silty sand with greenish gray mud (Fig. F13). Folds and tilted beds are observed in cores but XCB disturbance and biscuiting mask the original structures (Fig. F13). RAB images of this subunit reveal a striking three-dimensional (3-D) view of a fold with the fold axis trending north-south (see “Downhole measurements”). Interpretation of lithostratigraphyLithostratigraphic Unit I (0–364.7 mbsf) is interpreted to record a succession of rapidly deposited levee turbidite clay and mud with discrete intervals of MTDs. Lithostratigraphic Subunit IA is interpreted to be hemipelagic drape and very distal turbidites from the Old and Young Timbalier Canyon channel-levee systems to the west (Winker and Shipp, 2002). The subtly graded couplets of light and dark gray clay in each subunit throughout lithostratigraphic Unit I may record fine-grained turbidity current overspill on the east levee of the Southwest Pass Canyon. Sedimentation rates for lithostratigraphic Unit I are estimated to be ~0.4–0.6 cm/y (see “Biostratigraphy”). Considering that the average thickness of clay laminae couplets is ~2 cm, the overspill events recorded at Site U1324 would occur, on average, every 4 y. Similar color bands and laminations in mud and clay sediment from the Amazon Fan levees were interpreted to record turbidity current overspill events occurring every 1–3 y (Piper and Deptuck, 1997; Pirmez and Isram, 2003). Lithostratigraphic Subunits IB, ID, and IF are only different compared to the other subunits within lithostratigraphic Unit I by being composed of tilted, contorted, and faulted beds. This suggests that these intervals have been remobilized downslope and they are thus interpreted as MTDs. We infer that the relatively mild deformation observed in these subunits indicates that they have remained relatively intact during transport and probably have not moved significant distances from their original deposition location. The base of lithostratigraphic Unit I and top of lithostratigraphic Unit II at 364.7 mbsf marks a distinct lithologic change separating mud and clay above from interbedded sand, silt, and mud below. This boundary ties closely to seismic Reflector S40-1324, which also marks a fundamental change in seismic facies (Fig. F2). In lithostratigraphic Unit I, the seismic facies is predominantly characterized by intervals of continuous subparallel reflections and by transparent intervals. The seismic facies of lithostratigraphic Unit II is very chaotic and discontinuous. This major boundary also marks distinct character changes in the gamma ray and resistivity logs (Fig. F2; see “Downhole measurements”). Above this boundary, both logs do not show major variations. Below this boundary, resistivity varies frequently and ties well with the thin beds and laminae of silt and sand observed in lithostratigraphic Unit II. Gamma radiation indicates the presence of more silt and sand below this boundary as well. A marked change in the evolution of the Southwest Pass Canyon is interpreted to be recorded at the lithostratigraphic Unit I/II boundary. Before a single confining channel was established in the Southwest Pass Canyon system, there were probably numerous low-relief channels that were unable to completely confine the turbidity currents, leading to abundant overspill of sand and silt. Thus, the sedimentary succession of lithostratigraphic Unit II is dominated by thin beds of silt and sand interbedded with mud representing overbank deposits. The sands and silts tie to discontinuous reflectors and lenses in seismic data and are not regionally extensive. Lithostratigraphic Subunits IIB and IID are interpreted to be MTDs consistent with a dynamic environment with high sedimentation rate and extensive overspill. It is possible that the triggering mechanisms for the MTDs in lithostratigraphic Unit I and those in lithostratigraphic II are different. The MTDs of lithostratigraphic Unit I were potentially related to low effective stresses developed in the thick levee assemblage above the Blue Unit. The MTDs of lithostratigraphic Unit II were probably associated with the dynamic depositional environment of the young Southwest Pass Canyon channel-levee system. Core-seismic integrationComparison of 3-D seismic survey Line 150 (Fig. F4 in the “Site U1322” chapter) with Site U1324 lithostratigraphy suggests a strong correlation between seismic facies and lithostratigraphic units. Lithostratigraphic Unit I extends from the seafloor to just below seismic Reflector S40-1324 at 364.7 mbsf and is characterized overall by intervals of laterally continuous, high-amplitude reflectors and intervals of acoustically transparent and/or chaotic and discontinuous reflectors. Lithostratigraphic Subunits IB, ID, and IF are characterized by faulted and contorted bedding and tie closely to the transparent/chaotic seismic facies (Fig. F2; see also Fig. F4 in the “Site U1322” chapter). Such acoustically transparent/chaotic intervals are interpreted to be MTDs that were not transported very far but rather slumped and faulted during a short transport. The strong regional seismic Reflector S30 at the base of a thick transparent/chaotic interval ties to just below the base of lithostratigraphic Subunit ID. It is interpreted that this subunit represents a regional-scale MTD (Fig. F4 in the “Site U1322” chapter). This MTD can be correlated to Sites U1323 and U1322 in the seismic section. Lithostratigraphic Unit II extends from the base of lithostratigraphic Unit I just below seismic Reflector S40-1324 to the TD at 608 mbsf and is characterized by intervals of high-amplitude chaotic facies and high-amplitude continuous reflections. Seismic Reflectors S50-1324 and S60-1324 occur within this unit. Lithostratigraphic Subunits IIB and IID are characterized by faulted and contorted sediment and tie closely with the intervals of high-amplitude chaotic zones (Fig. F2). These subunits are interpreted to be MTDs but are not seismically transparent like the MTDs of lithostratigraphic Unit I. The high-amplitude signature may be related to the abundance of thin bedded silt, sand, and mud observed throughout lithostratigraphic Unit II. These subunits are not regionally extensive and do not correlate to Sites U1323 or U1322. Lithostratigraphic Subunits IIA and IIC tie closely to high-amplitude continuous reflectors between seismic Reflectors S40-1324 and S60-1324 (Fig. F2). These subunits are not characterized by faulted and contorted sediment and contain interbedded silt, sand, and mud and thin laterally to the east and are truncated by MTDs (Fig. F4 in the “Site U1322” chapter). Summary interpretationSite U1324B recovered a thick sedimentary succession overlying the Blue Unit and records the evolution of the eastern levee of the Southwest Pass Canyon channel-levee system. Lithostratigraphic Unit I is composed of clay, mud, and three MTDs. Lithostratigraphic Unit II is composed of interbedded silt, sand, and mud and two MTDs. The boundary between these units reflects a fundamental change in the development of the Southwest Pass Canyon channel-levee system. Below this boundary, a transitional period characterized by relatively unconfined deposition of sand, silt, and mud reflects a young developing channel-levee system. Above this boundary, the Southwest Pass Canyon channel-levee system was firmly established west of Site U1324 and was effective in confining sands and silts to a main channel corridor. Overspill of mud and clay developed the thick levee assemblage recorded in lithostratigraphic Unit I. Core-seismic integration suggests that acoustically transparent intervals are regional MTDs composed of faulted and contorted mud and clay. However, close examination reveals that these MTDs contain levee clay and mud that are only mildly deformed and tilted and were not transported very far from their in situ position. |
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