Site 1122
Hole 1122A
Position: 46°34.78088´S, 177°23.60962´W
Start hole: 2254 hr, 5 September 1998
End hole: 0415 hr, 7 September 1998
Time on hole: 29.35 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 4446.20
Distance between rig floor and sea level (m): 11.20
Water depth (drill pipe measurement from sea level, m): 4435.00
Total depth (from rig floor, mbrf): 4570.10
Total penetration (mbsf): 123.90
Coring totals: type: APC; number: 8; cored: 75.80 m; recovered: 96.97%
type: XCB; number: 5; cored: 48.10 m; recovered: 29.02%
Formation: lithostratigraphic Subunit IA (0–22.7 mbsf): light brownish gray silty clay
lithostratigraphic Subunit IB ( 22.71–109.35 mbsf): greenish gray silty clay with interbedded dark greenish gray and greenish gray sand and fine sand turbidites
lithostratigraphic Subunit IC (109.35–123.90 mbsf): dark greenish gray silt and very fine sand turbidites, which are intercalated in grayish green and greenish gray silty clay
Hole 1122B
Position: 46°34.78088´S, 177°23.60962´W
Start hole: 0415 hr, 7 September 1998
End hole: 0625 hr, 7 September 1998
Time on hole: 2.17 hr
Seafloor (drill pipe measurement from rig floor, m mbrf): 4441.00
Distance between rig floor and sea level (m): 11.20
Water depth (drill pipe measurement from sea level, m): 4429.80
Total depth (from rig floor, mbrf): 4450.50
Total penetration (mbsf): 9.50
Coring totals: type: APC; number: 1; cored: 9.50 m; recovered: 103.26%
Formation: lithostratigraphic Subunit IA (0–9.5 mbsf): light brownish gray silty clay
Hole 1122C
Position: 46°34.78020´S, 177°23.62152´W
Start hole: 0625 hr, 7 September 1998
End hole: 0800 hr, 12 September 1998
Time on hole: 121.58 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 4443.00
Distance between rig floor and sea level (m): 11.20
Water depth (drill pipe measurement from sea level, m): 4431.80
Total depth (from rig floor, mbrf): 5070.40
Total penetration (mbsf): 627.40
Coring totals: type: APC; number: 13; cored: 103.70; recovered: 103.33%
type: XCB; number: 55, cored: 523.70 m; recovered: 46.65%
Formation: lithostratigraphic Subunit IA (0–22.7 mbsf): light brownish gray silty clay
lithostratigraphic Subunit IB ( 22.71–109.35 mbsf): greenish gray silty clay with interbedded dark greenish gray and greenish gray sand and fine sand turbidites
lithostratigraphic Subunit IC (109.35–261.7 mbsf): dark greenish gray silt and very fine sand turbidites, which are intercalated in grayish green and greenish gray silty clay
lithostratigraphic Subunit ID (261.7–389.9 mbsf): greenish gray and gray silty clay interbedded with dark greenish gray very fine sand and silt turbidites
lithostratigraphic Subunit IIA (389.9–472.3 mbsf): greenish gray to light greenish gray silty clay with interbeds of dark gray or dark greenish gray fine sand and silt beds
lithostratigraphic Subunit IIB (472.3–550.4 mbsf): dark greenish gray to greenish gray silty clay interspersed with light brownish gray to gray, nannofossil-rich layers
lithostratigraphic Subunit IIIA (550.4–580.62 mbsf): green to greenish gray clayey silt to silty clay with interbeds of greenish to greenish gray fine sand and silt beds and white to gray nannofossil-bearing foraminifer sands
lithostratigraphic Subunit IIIB (580.62–617.85 mbsf): greenish gray fine sand with intraclasts of silty clay and abundant wood fragments (debris flow) and greenish gray to dark greenish gray fine sand-bearing siltstones and light greenish gray foraminifer-bearing nannofossil chalks
Site 1122 is located ~830 km east of South Island, in a water depth of 4430 m on the north bank levee of the abyssal Bounty Fan. The fan is located in the most seaward axial deep of the Bounty Trough, a rift basin formed in the Late Cretaceous during the separation of New Zealand and Antarctica across the newly forming mid-Pacific Rise. The Bounty Channel feeds sediment along the axis of the trough and into the path of the Pacific DWBC, which flows north along 80-m.y.-old oceanic crust of the Southwest Pacific abyssal plain. Subsidence calculations indicate that Site 1122 was situated at depths of ~4700 m in the early Miocene, subsequently shoaling to ~4400 m today because of late Cenozoic sediment deposition. The seismic line through the fan shows that the channel here has over 300 m of incision, with a higher north bank levee, of which the upper parts are composed of a spectacular series of climbing sediment waves, deposited from turbidity current overspill.
Three holes that recovered a sedimentary section spanning the last 16 m.y. were cored with the APC/XCB at Site 1122 to a maximum depth of 627.4 mbsf (Table 4). Hole 1122A was cored with the APC to 75.8 mbsf and deepened with the XCB to 124.0 mbsf. One failed mudline core was taken at Hole 1122B. Thirteen cores were taken with the APC at Hole 1122C from 0 to 103.7 mbsf. The hole was deepened with the XCB to 627.4 mbsf. In an attempt to log Hole 1122C the triple combination was deployed but was unable to pass the bit by more than 12 m. It was decided to recover the logging tool in rapidly deteriorating weather conditions and to terminate operations at the site because of the heavy seas and high winds.
Drilling confirmed that the upper ~300 m of the sediment pile is composed of rhythmic late Pleistocene sand turbidites (Fig. 10), deposited at a high rate of more than 40 cm/k.y. Recovery ranged from good when the turbidite sands were separated by regular muds and cored by APC, to very poor where they were inferred to be dominated by sand and cored XCB. Between 300 and 450 mbsf, the turbidite sequence changed into late Pliocene to middle Pleistocene current-worked sands, and muds that are inferred to have been deposited under the influence of the DWBC, possibly reinforced by the ACC. A substantial 8-m.y.-long condensed sequence or unconformity exists at ~470–500 mbsf, below which current-influenced sands and muds of late Miocene age (10–18 Ma), with a sedimentation rate of ~5 cm/k.y., were penetrated. Accompanying microfaunas show a shift to less diverse foraminifer assemblages, suggestive of the incursion of colder waters and also of the appearance of Subantarctic diatom floras. The lower part of the Miocene sediments contains abundant coarser grained sand, carbonized wood fragments, and transported shallow-water foraminifers, consistent with the onlap of these sediments onto the angular unconformity observed on the seismic profile. Because of drilling difficulties, the hole terminated just above this unconformity, though poor core recovery and the unfortunate lack of a sonic log makes it difficult to be certain how far above.
Site 1122 yielded an excellent high-resolution record of the input of middle upper Pleistocene sediment into the deep western boundary current and probably ACC system. The site contained some significant surprises. Preliminary analysis shows that (1) turbidite deposition on the fan levee continued through both glacial and interglacial periods, although the frequency was higher in glacials; (2) the major reflecting horizons seen on the seismic profile do not correspond to oxygen isotope stage stratigraphy, and (3) an unexpectedly young age (~0.7 Ma) for the change from DWBC-influenced sedimentation to New Zealand-derived turbidite/fan levee-influenced sedimentation. Seven major tephra horizons were located at Site 1122, and, together with the excellent paleomagnetic record and close micropaleontologic controls, these will provide a tight chronostratigraphic framework for more detailed studies.
A complete magnetostratigraphy was determined in the uppermost 440 mbsf of the APC/XCB section at Site 1122 after AF demagnetization at 20 mT (Fig. 10). All chrons from the Brunhes (C1n) to the onset of C2r.1n (Reunion) at 2.14 Ma could be identified. In the upper Miocene section, below the major unconformity, Chrons C5r.1n and C5r.2n were determined.
Physical sediment properties were determined both by high-resolution MST core logging and by index property measurements. Natural gamma-ray intensity indicates clay mineral concentration and varies strongly between sand and clay layers. Magnetic susceptibility, gamma-ray attenuation porosity evaluator (GRAPE) density, and digital reflectance data measured with the Minolta Spectrophotometer reveal cyclicities, which were used for stratigraphic correlation. Detailed hole-to-hole comparisons demonstrated nearly complete recovery of the sedimentary sequence down to 83 mcd with a gap in the continuous record at 73 mcd.
The primary controlling factor on the interstitial water chemistry at Site 1122 is sulfate reduction and methane genesis, which governs alkalinity, phosphate, and ammonia concentration. In contrast to the complete utilization of sulfate in the upper part of the core, the reappearance of increasing sulfate levels in the middle of the section represents an approach to the original sulfate concentration during sediment deposition, possibly because of a lack of sufficient metabolizable organic matter and to low sedimentation rates. Other significant chemical profiles are magnesium and calcium, from which we may deduce the lateral transport of magnesium-rich fluid during the dissolution of carbonate. The general chemical zonations of interstitial waters at Site 1122 correspond to those of lithostratigraphic units and paleontological age divisions‹in particular, the sharp reduction of methane at 260 mbsf, which coincides with the base of the highly pyritized turbidites of the mud wave sequence. The calcium carbonate concentration varies between 0 and 77% indicating strong environmental changes during sediment deposition. Organic carbon contents averages 0.24% and shows also strong variations with the sedimentary facies. The organic carbonate concentration is low for marine environments and may reflect the dilution of the organic matter either by marine carbonate or terrigenous detritus.