Site 1120
Hole 1120A
Position: 50°3.80333´S, 173°22.30042´E
Start hole: 0045 hr, 28 August 1998
End hole: 0600 hr, 28 August 1998
Time on hole: 5.25 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 553.90
Distance between rig floor and sea level (m): 11.00
Water depth (drill pipe measurement from sea level, m): 542.90
Total depth (from rig floor, mbrf): 558.50
Total penetration (mbsf): 4.60
Coring totals: type: APC; number: 1; cored: 4.60 m; recovered: 100%
Formation: lithostratigraphic Unit I: 0–4.6 mbsf; foraminifer nannofossil ooze and nannofossil foraminifer ooze
Hole 1120B
Position: 50°3.80333´S, 173°22.30042´E
Start hole: 0600 hr, 28 August 1998
End hole: 2020 hr, 28 August 1998
Time on hole: 14.33 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 555.20
Distance between rig floor and sea level (m): 11.00
Water depth (drill pipe measurement from sea level, m): 544.20
Total depth (from rig floor, mbrf): 743.20
Total penetration (mbsf): 188.00
Coring totals: type: APC; number: 8; cored: 68.30 m; recovered: 92.55%
type: XCB; number: 13; cored: 119.70 m; recovered: 69.59%
Formation: lithostratigraphic Unit I: 0–4.6 mbsf; foraminifer nannofossil ooze and nannofossil foraminifer ooze
lithostratigraphic Unit II: 4.6–11.5 mbsf; foraminifer nannofossil ooze
lithostratigraphic Unit III: 11.5–54.9 mbsf; foraminifer nannofossil ooze
lithostratigraphic Unit IV: 54.9–184.55 mbsf; nannofossil ooze
Hole 1120C
Position: 50°3.81546´S, 173°22.29950´E
Start hole: 2020 hr, 28 August 1998
End hole: 0613 hr, 29 August 1998
Time on hole: 33.88 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 556.90
Distance between rig floor and sea level (m): 11.00
Water depth (drill pipe measurement from sea level, m): 545.90
Total depth (from rig floor, mbrf): 601.50
Total penetration (mbsf): 44.60
Coring totals: type: APC; number: 5; cored: 44.60 m; recovered: 100.78%
Formation: lithostratigraphic Unit I: 0–4.6 mbsf; foraminifer nannofossil ooze and nannofossil foraminifer ooze
lithostratigraphic Unit II: 4.6–11.5 mbsf; foraminifer nannofossil ooze
Hole 1120D
Position: 50°3.82210´S, 173°22.30042´E
Start hole: 0613 hr, 29 August 1998
End hole: 0600 hr, 1 September 1998
Time on hole: 47.78 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 556.90
Distance between rig floor and sea level (m): 11.00
Water depth (drill pipe measurement from sea level, m): 545.90
Total depth (from rig floor, mbrf): 777.60
Total penetration (mbsf): 220.70
Total length of drilled intervals (m): 157.40
Coring totals: type: XCB; number: 9; cored: 63.30 m; recovered: 104.53%
Formation: lithostratigraphic Unit IV: 157.4–220.7 mbsf; nannofossil ooze
Site 1120 is located ~650 km southeast of Stewart Island on the eastern flank of Pukaki Rise, near the middle of the Campbell Plateau, in 543 m deep water. The seismic succession is punctuated by seven conspicuous reflectors (from top to bottom = reflectors A–F), which probably represent unconformities. Few basement highs remained on the Campbell Plateau to shed terrigenous sediment after late Cretaceous rifting of the eastern plateau margin and subsequent erosion, and the entire succession above reflector E (probably Paleocene) is apparently biopelagic carbonate. Times of water-mass change, perhaps accompanied by current activity and sublevation, are indicated by changes in seismic character that take place across younger reflectors D to A, the target of Site 1120 drilling.
Site 1120 was drilled to establish the age of the major unconformities in the Campbell Plateau sequence and to determine the evolution of the shallow parts of the ACC where it sweeps past and over the southeast corner of the Plateau. The analysis of seafloor magnetic anomalies suggests that the ACC had its inception in the Oligocene, at ~32 Ma, when Australia separated from Antarctica and allowed partial circumpolar flow. At this time, the Campbell Plateau was situated immediately down-current from the opening southern ocean and was therefore exposed to vigorous current activity. It was anticipated that reflector B would be of middle to late Miocene age and correlate with either or both the sharp cooling of Antarctica recorded isotopically at 15–14 Ma, and a phase of known volcanism and minor faulting in eastern South Island, New Zealand, between ~13–10 Ma.
Four holes that recovered a sedimentary section spanning the last 23 m.y. were cored with the APC/XCB at Site 1120 to a maximum depth of 220.7 mbsf (Table 2). Hole 1120A consists of one single mudline core. Twenty-one cores were taken with the APC/XCB at Hole 1120B from 0 to 188.0 mbsf before operations were put on standby because of excessive heave. After waiting on weather, five cores were taken with the APC at Hole 1120C to 44.6 mbsf before a broken wireline and deteriorating weather conditions ended operations. Hole 1120D was drilled to 157.4 mbsf and cored with the XCB to 220.7 mbsf when operations had to be terminated because of the excessive heave.
Site 1120 penetrated a succession of calcareous biogenic oozes that downsection become less siliciclastic, less foraminifer rich, and contain less distinct bedding. Bioturbation ranges from moderate to heavy throughout the core, with identifiable traces of Zoophycos, Palaeophycus, and Planolites. The lithology is visually monotonous and featureless for most of the section. However, using subtle changes in composition determined from smear slides, four units were identified (Fig. 8). The uppermost lithology (Unit I, 0–4.6 mbsf) comprises glacial-interglacial cycles in alternating beds of foraminifer-oozes and nannofossil-foraminifer oozes. Unit II (4.6–11.5 mbsf) is separated from Unit I by an unconformity and comprises a foraminifer nannofossil ooze with glauconite and rare pyrite concretions. Unit III (11.5–54.9 mbsf) is a foraminifer nannofossil ooze similar to Unit II. The lowermost unit (54.9–221.12 mbsf) is a foraminifer-bearing nannofossil ooze. Foraminifers are less abundant in this unit than in overlying beds, and neither glauconite nor siliceous sponge spicules were observed.
The section is punctuated by a number of significant paraconformities, the first of which lay just at 4.5 mbsf, separating stratified cycles of darker and lighter white ooze of late Pleistocene age from slightly compacted, brownish foraminiferal ooze of middle Pleistocene age below. Another paraconformity may occur at around 13 mbsf, corresponding to a reflection on the site survey 3.5-kHz profile, and a change in microfaunas to latest Miocene (5–6 Ma). Between ~13 and 170 mbsf occurs an expanded section of middle and late Miocene age (13.6–6 Ma), apparently without breaks across minor seismic discontinuities at 61 and 98 mbsf. Another probable unconformity, marked by reflector B on seismic reflection profiles, is located at ~170 mbsf, where nannofossil evidence suggests a biostratigraphic break may occur.
The stratigraphic thickness is very low in the Pleistocene and Pliocene, and the presence of major hiatuses indicates that intensified water mass movements occurred across the Campbell Plateau during this time. The relatively complete middle to late Miocene section that underlies corresponds to sedimentation rates of 1–2 cm/k.y. over the period ~5.5 to 16 Ma, and indicates quieter, undisturbed biopelagic conditions. The lowest part of the core is of early Miocene age (18–23 Ma), and has a slower sedimentation rate, consistent with current influence. The occurrence of the usually long-ranging Sphenolithus heteromorphus in only one core (between 167 and 177 mbsf) suggests that up to 2–3 m.y. of late early Miocene and early middle Miocene sediment may be either missing at an unconformity or represented by condensed sedimentation.
Benthic foraminifers indicate that the site has been bathyal throughout the Neogene–Quaternary. Evidence from all planktonic microfossil groups consistently indicates that during the Pleistocene the surface waters and intermediate water masses above the Campbell Plateau were cold. In the Miocene section, changes in the composition of calcareous microfossils reflect cyclic alternations between warmer and colder conditions. The planktonic foraminifers and calcareous nannofossils show a general trend toward colder faunas and floras from the early-middle to the middle-late Miocene.
Physical sediment properties were determined both by high-resolution MST core logging and by index property measurements. Magnetic susceptibility, GRAPE density, natural gamma-ray intensity, and digital reflectance data measured with the Minolta Spectrophotometer reveal cyclicities, which were used for stratigraphic correlation in the top 50 mbsf. Detailed hole-to-hole comparisons demonstrated nearly complete recovery of the sedimentary sequence down to 50 meters composite depth (mcd), with a gap in the continuous record at 19 mcd.
A shipboard paleomagnetic reversal chronology could not be determined because of a combination of low intensities and magnetic overprints.
The profiles of interstitial water constituents are controlled by simple diagenetic diffusion processes, which show slightly increasing alkalinity, chloride, calcium, lithium, and silica concentrations and decreasing potassium and magnesium trends with depth, and no signatures of sulfate reduction. This results from the uniform lithology (carbonate oozes) throughout the hole. The dominant chemical reactions were dissolution of carbonate, silica diagenesis, the possible precipitation of dolomite, and ion-exchange reactions in clay minerals.