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doi:10.2204/iodp.proc.307.105.2006 LithostratigraphySite U1318 is located upslope of Challenger Mound in water depths of 408.8–409.9 m and represents the shallowest sedimentary package of the slope transect. Principal objectives of this site were to recover sediments from the low-amplitude seismic unit (P2) (Van Rooij et al., 2003) and to constrain the age of the unconformities identified as erosional surfaces in the seismic profile. Hole U1318A was drilled continuously to 142.2 mbsf, and 15 cores were taken with a recovery of 90.2%. Hole U1318B was cored to 244.6 mbsf with a total of 27 cores and a recovery of 87.1%. In Hole U1318C, after establishing the mudline in Core 307-1318C-1H, the drill string washed down to 70.0 mbsf and core recovery continued to 154.5 mbsf with 10 cores and a recovery of 96.3%. The sediments from Site U1318 are divided into three units (Table T2) based on visual description of the sedimentary features and composition, biostratigraphic constraints, and correlation to the magnetic susceptibility and carbonate content data. They are the upper Pleistocene clay (Unit 1), the lower Pleistocene thin sandy layers (Unit 2), and the lower Miocene (to possible Pliocene) silty clay as much as 155 m thick (Unit 3) (Fig. F3). Significant changes in magnetic susceptibility and carbonate content are recognized at a major unconformity located just above a bivalve bed at 86.7, 86.2, and 84.2 mbsf in Holes U1318A, U1318B, and U1318C, respectively. The unconformity defines the boundary between Units 2 and 3. Lithostratigraphic unitsUnit 1
Unit 1 is dominated by an unlithified clayey succession divided into three subunits with distinct boundaries. Boundaries among three subunits correspond to shifts in the magnetic susceptibility record (see “Paleomagnetism”). Subunit 1A
Subunit 1A is dominated by dark grayish brown to dark gray silty clays, which are partly interbedded with centimeter-thick fine-sand layers occasionally grading upward. The sediments are dominantly massive but contain some laminated intervals. Centimeter- to millimeter-thick laminae consist of light and dark greenish gray clay with very fine sand layers representing small-scale fining-upward cycles. Bioturbation in the laminated horizons is rare. In contrast, massive horizons are moderately to strongly bioturbated. Dropstones are frequently found in this subunit. Fossils are generally rare and dispersed. Mollusk shells, up to 3 cm, rarely occur in the upper 15 m. Black spots, aggregations of fine iron sulfide grains, are common. The base of the subunit is defined by an erosive boundary overlain by a 24 cm thick graded fine sand to clayey silt layer (Fig. F4A). The same sedimentary package is identified in Section 307-U1316B-5H-2, 2 cm. Subunit 1B
The lithology of Subunit 1B comprises dark grayish brown to very dark gray silty clays that are similar to facies of Subunit 1A. In contrast to Subunit 1A, dropstones are much less frequent and laminated horizons are more frequent in this subunit. Lamination occurs on a centimeter to millimeter scale (Fig. F4B). Fossils are rare throughout the subunit. Several white fine-sand layers, only 1–2 mm thick, are found at ~58 and ~57 mbsf in Holes U1318A and U1318B, respectively. These layers consist of reworked volcanic glasses. Unfortunately, the lower boundary of this subunit was not recovered because of onboard sampling in Hole U1318A and the core boundary between Cores 307-U1318B-7H and 8H. Subunit 1C
The lithology of Subunit 1C is characterized by alternating dark grayish brown fine sandy clay and very dark grayish brown silty clay. This subunit can be distinguished by coarser grain sizes in comparison to Subunits 1A and 1B. Laminations of fine sand and silty clay are dominant in the middle part of Subunit 1C from 64 to 77 mbsf. Lamination thickness varies from millimeter to centimeter scale. Dropstones are absent. The base of the subunit is defined by a sharp, erosive boundary at 82 mbsf in Section 307-U1318B-10H-1, 100 cm. Unit 2
Unit 2 is dominated by olive-gray, medium–fine sand interbedded with dark yellowish brown silty clay. A gradual increase in sand indicates a coarsening-upward trend throughout this unit. A majority of the sand beds are normal graded with sharp lower and upper boundaries. Lithoclasts, up to 3 cm in diameter, are found in both sand and clay horizons. Sand beds commonly contain well-preserved mollusk and other unidentified shell fragments. Notably, a 6 cm long gastropod retained its pale yellow color in Section 307-U1318A-9H-6, 64 cm (Fig. F4C). The base of this subunit consists of a conglomerate, 5–10 cm thick, with black pebbles and granules. Black angular to subangular gravels consist of a mixture of apatite, calcite, and quartz based on X-ray diffraction (XRD) analyses. The boundary is defined just above the bivalve bed in the uppermost part of Unit 3 and is associated with a major unconformity between Tertiary (Pliocene or Miocene) and Pleistocene sediments. Detailed calcareous nannofossil data revealed that the conglomerate of this unit is 0.96–1.22 Ma and that the well-sorted fine sand just beneath the bivalve bed is older than 3.6 Ma (see “Biostratigraphy”). The relationship between the conglomerate and the underlying bivalve bed is best expressed in Section 307-U1318B-10H-4, whereas in Hole U1318A, the same feature is disturbed by the section boundary between 10H-2 and 10H-1 (Fig. F5). Unit 3
Unit 3 is divided into three subunits based on the differences observed within the carbonate content (Fig. F3) (see “Geochemistry and microbiology”). Subunit 3A generally consists of silty clay, fine sands, and clayey silt with carbonate contents of 27–50 wt%. The uppermost 10 cm interval of Subunit 3A is a prominent bivalve bed that is unconformably overlain by the basal conglomerate of Unit 2. Subunit 3B is homogeneous greenish gray silty clay with a carbonate content of 10–22 wt%. The bottom section of the core, Subunit 3C, consists of greenish gray silty clays to fine sands with some dolomitic horizons. Carbonate content is 21–35 wt%, except for strongly lithified horizons, which have carbonate contents of ~70 wt%. Sediments are moderately lithified throughout Unit 3, except for several well lithified horizons in Subunit 3C. Subunit 3A
The sediments of Subunit 3A are mostly composed of greenish gray to light greenish gray silty clays interbedded with well-sorted greenish gray fine sand and greenish gray to gray silt that have erosive boundaries at their bases. The frequency of sandy and silty beds increases upward. This subunit is characterized by high grain-size variability and carbonate contents. Carbonate contents increase from ~30 wt% at the bottom to ~45 wt% at the middle of the subunit (Fig. F3). Shell fragments and other bioclasts sporadically occur throughout the subunit. A prominent bivalve bed ~10 cm thick overlies well-sorted fine sand at the uppermost horizon of Subunit 3A. Bone bivalves are characterized by their black “steinkern” (internal cast), whereas in others, the shell belt is still preserved. The cast material was identified as a mixture of apatite, calcite, and quartz by XRD analysis. Thick-walled shells rarely occur within silty clay sediments in the upper part of Subunit 3A. Burrows are clearly visible because of the variable lithologic components. The base of Subunit 3A is defined by a sharp erosive boundary at 127.3 mbsf in Hole U1317C, yet was not recovered in Holes U1318A and U1318B. Subunit 3B
Subunit 3B is a very homogeneous greenish gray silty clay with carbonate contents of ~20 wt% in the upper half and ~10 wt% in the lower. The uppermost and lowermost few meters of Subunit 3B comprise sands with some bioclasts. Bioturbation is abundant throughout the subunit (Fig. F4D). Some burrows are infilled with iron sulfide concretions. Subunit 3C
Subunit 3C is characterized by a greenish gray silty clay to fine sand with carbonate contents from ~25 wt% at the top to ~35 wt% at the bottom. A density decrease of fine sand beds indicates a fining-upward trend from the middle to upper part of the succession. Fossils are generally rare and dispersed. Bioturbation is abundant to moderate. There are some extremely lithified beds with carbonate contents of ~70 wt% in the middle of the subunit. Two of these are dolomite-bearing horizons at 223.7 and ~228 mbsf, which are markedly more lithified and lighter in color. The thin section from this horizon shows dense appearance of dolomite. DiscussionThis discussion is focused on (1) the facies change in Unit 1, (2) the major unconformity between Units 2 and 3, and (3) the extremely lithified horizons found in lower part of Unit 3. Lamination and bioturbation in Unit 1The alternating laminated and bioturbated massive intervals in the upper Pleistocene Unit 1 reflects periodic variations in the depositional environment. Further studies will reveal whether these variations reflect differences in seafloor oxygenation, and depositional ratios, which may be related to glacial–interglacial cycles. The ice-rafted dropstones suggest a positive correlation of dropstone density with bioturbated massive intervals, although the correlation is not always straightforward. Phosphatic bivalve bedThe major unconformity that separates the lower Pleistocene clay from the underlying Tertiary (Pliocene–Miocene) strata is defined at the phosphatic bivalve bed at ~86 mbsf at Site U1318. Figure F5 summarizes a detailed sedimentary succession with biostratigraphic ages above and below the bivalve bed. The bivalves have not been further identified. Efforts to classify them in detail and evaluate their ecological significance will be part of postcruise research. Calcareous nannofossil data indicate that well-sorted fine sand just beneath the shell bed is older than 3.6 Ma. One sample including both Pleistocene and Miocene species indicates sediment mixing by bioturbation. Burrows incised into this fine sand are filled with brownish sands derived from the Pleistocene dark grayish brown fine sand. In contrast, the basal conglomerate just above the shell bed is 0.96–1.22 Ma. These findings mark a major hiatus just below or above the shell bed. Precipitation of phosphatic minerals associated with the bivalves and angular to subangular phosphatic gravels of the basal conglomerate suggest some changes in oceanographic conditions. Future research, such as measurements of stable isotopes and minor elements, will elucidate the history of this sedimentary succession. LithificationThe composition of indurated zones encountered in the premound strata at Sites U1316 and U1318 is indicated by smear slide and thin section analysis. These zones are composed primarily of silt-sized quartz and feldspar grains and clay-sized material cemented by a calcite and dolomite microspar composed of micrometer- to decimicrometer-sized crystals. In Cores 307-U1318C-24H and 25H, lithified zones overlie and underlie unlithified silty clays composed dominantly of coccoliths. Coccoliths are uncommon to absent in the lithified zones. We speculate that lithification occurred when coccolithic muds underwent aggrading neomorphism, destroying their original texture and forming microspar. Surface energy–driven recrystallization (Ostwalt ripening) is likely the driving mechanism for the recrystallization, but we can offer no reasons at this time as to why particular zones underwent this process as well as partial dolomitization. When the stratigraphic column is projected on the seismic profile, the boundary between Subunits 3B and 3C can be roughly correlated to the boundary between seismic Units P2 and P1 (Fig. F9). An unconformity suggested by a seismic study of Van Rooij et al. (2003) is not evident in the preliminary lithostratigraphic and biostratigraphic investigation during Expedition 307 (Fig. F2). The P2/P1 boundary may instead correspond to the interval of change in lithification degree. |
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