Site 1121
Hole 1121A
Position: 50°53.87581´S, 176°59.86176´E
Start hole: 2225 hr, 1 September 1998
End hole: 1555 hr, 2 September 1998
Time on hole: 17.50 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 4503.10
Distance between rig floor and sea level (m): 11.10
Water depth (drill pipe measurement from sea level, m): 4492.00
Total depth (from rig floor, mbrf): 4511.50
Total penetration (mbsf): 8.40
Coring totals: type: APC; number: 1; cored: 8.40 m; recovered: 99.64%
Formation: lithostratigraphic Subunit IA (0–8.40 mbsf): yellow to yellowish brown silty or sandy clay and grayish brown and light yellowish brown silt, silty sand, and sand beds
Hole 1121B
Position: 50°53.87581´S, 176°59.86176´E
Start hole: 1555 hr, 2 September 1998
End hole: 1045 hr, 4 September 1998
Time on hole: 42.83 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 4499.00
Distance between rig floor and sea level (m): 11.10
Water depth (drill pipe measurement from sea level, m): 4487.90
Total depth (from rig floor, mbrf): 4638.70
Total penetration (mbsf): 139.70
Coring totals: type: APC; number: 3; cored: 23.00 m; recovered: 129.04%
type: XCB; number: 14; cored: 4.60 m; recovered: 49.85%
Formation: lithostratigraphic Subunit IA (0–15.2 mbsf): yellow to yellowish brown silty or sandy clay and grayish brown and light yellowish brown silt, silty sand, and sand beds
lithostratigraphic Subunit IB (15.2–32.7 mbsf): pale yellow and light yellowish brown clay with fragments of chert layers and nodules
lithostratigraphic Subunit IIA (32.7–35.07 mbsf): white nannofossil ooze with a subordinate content of diatoms, radiolarians, sponge spicules, and silicoflagellates
lithostratigraphic Subunit IIB (35.07–119.4 mbsf): light greenish nannofossil diatom ooze
lithostratigraphic Subunit IIC (119.4–132.7 mbsf): light greenish gray nannofossil ooze
lithostratigraphic Subunit IID (132.7–139.7 mbsf): greenish gray nannofossil-bearing clay
Site 1121 is located on the Campbell Drift under the combined effect of the DWBC and ACC in the Southwest Pacific. Drilling at this site showed that, contrary to the generally accepted view, the sediment accumulation defined by topography and seismic reflection profiles is not a Neogene contourite drift.
The sediment body drilled lies along the foot of the 3000-m-high eastern escarpment of Campbell Plateau and is ~800 km long. The underlying crust dates from the rifting of Campbell Plateau from Marie Byrd Land at 85 Ma. Immediately overlying that crust is up to 1.1 s (~1000 m) of sediment presumed to be a wedge of nonmarine clastics followed by latest Cretaceous and Paleogene marine sediments and Neogene contourites. In particular, Carter and McCave (1997) surmised that the uppermost sequence of closely spaced reflectors (<160 m thick) was of Pliocene–Pleistocene age. A short gravity core from the site has yielded an abundant assemblage of upper Pleistocene and reworked older foraminifers. This sequence, presumed to carry a record of ACC and DWBC history, was the drilling target. The objectives were to obtain well-dated records of isotopic properties of oxygen and carbon that would show the shifts of the Subantarctic Front, the onset of and changes in the inflow of Circumpacific Deep Water, and, through grain-size analysis, document changes in the vigor of the deep circulation.
Two holes that recovered a sedimentary section spanning the last 63 m.y. were cored with the APC/XCB at Site 1121 to a maximum depth of 139.7 mbsf (Table 3). Hole 1121A consists of one single mudline core. Seventeen cores were taken with the APC/XCB at Hole 1121B from 0 to 139.7 mbsf with 62.9% recovery when operations were terminated.
The cored section is divided into two units, each subdivided (Fig. 9). Subunit IA (0–15 mbsf) is mottled silty and sandy clay with conspicuous manganese nodules. Subunit IB (15–32.7 mbsf) is a relatively uniform yellow siliceous clay with cherts as nodules and (broken) layers. This unit is possibly of Neogene age, and represents a residual deposit with a very slow net sedimentation rate (~1 m/m.y.). It is the residue of significant erosion by the ACC/DWBC some time after the Paleocene. Unit II includes four subunits dominated by varying mixtures of siliceous and nannofossil-bearing ooze with up to 50% carbonate. The unit is extensively bioturbated and contains common chert below 71 mbsf. The lithologic features indicate deep marine deposition of pelagic biogenic material close to the CCD in the order of 15–30 m/m.y. The inferred erosion in the upper part of the section is corroborated by the ratio of shear strength to overburden pressure which, with values over 0.5 in the upper 20 mbsf, is indicative of overconsolidation. Chert in the lower part of the section precludes similar measurements in the Paleocene Unit II which is probably also overcompacted.
The uppermost 3 mbsf of the section contains a condensed sequence dated by using diatoms as late Pleistocene over early Pleistocene over late Pliocene in the top meter, underlain by 2 m of early late Pliocene. Foraminifers and coccoliths in the top meter are of middle to late Pleistocene age. The remainder of the upper 32 mbsf composing Unit I yielded a few upper Neogene nannofossils and reworked (?)Paleocene radiolarians. From 32 mbsf to the bottom of the hole (Unit II) the sequence provides for the first time in the southwest Pacific a continuous record of the late early to late Paleocene. This includes all major microfossil groups including nannofossils, diatoms, radiolarians, and benthic foraminifers. Radiolarian zones RP4, 5, and 6, nannofossil zones NP4, 5, 6, and 8, benthic foraminiferal zone CD1, and diatom Hemiaulus incurvus Zone are present in the record. This has provided tight control on an equally good magnetostratigraphic record in the Paleocene section where magnetochrons C27r, C27n, C26n, and C25r are recognized. The upper condensed section also contains a number of reversals but, apart from the tentative recognition of the Gauss Chron, the biostratigraphic control is not adequate to place them somewhere within the Oligocene to Pliocene, where they probably reside. The combined age data allow specification of sedimentation rates in the order of 15–30 m/m.y. in the Paleocene and ~1 m/m.y. in the Neogene.
The profiles of interstitial water constituents show local fluctuations suggesting dominantly lithologic control rather than a simple diffusion process of diagenetic fluids. The primary chemical reactions may be silica diagenesis, dissolution of carbonate, and possibly ion-exchange reactions of clay minerals. In particular, relatively high concentrations of dissolved silica in the interval between ~35 and 120 mbsf are related to the dissolution of biosiliceous sediments.