Site 1124
Hole 1124A
Position: 39°29.9014´S, 176°31.8938´W
Start hole: 1748 hr, 24 September 1998
End hole: 0305 hr, 25 September 1998
Time on hole: 9.28 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 3979.00
Distance between rig floor and sea level (m): 11.50
Water depth (drill pipe measurement from sea level, m): 3967.50
Total depth (from rig floor, mbrf): 3988.50
Total penetration (mbsf): 9.50
Coring totals: type: APC; number: 1; cored: 9.50 m; recovered: 100.11%
Formation: lithostratigraphic Subunit IA (0–9.5 mbsf): nannofossil silty clay
Hole 1124B
Position: 39°29.9014´S, 176°31.8938´W
Start hole: 0305 hr, 25 September 1998
End hole: 1745 hr, 26 September 1998
Time on hole: 38.67 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 3978.10
Distance between rig floor and sea level (m): 11.50
Water depth (drill pipe measurement from sea level, m): 3966.60
Total depth (from rig floor, mbrf): 3988.00
Total penetration (mbsf): 9.90
Coring totals: type: APC; number: 2; cored: 9.90 m; recovered: 99.90%
Formation: lithostratigraphic Subunit IA (0–9.88 mbsf): nannofossil silty clay
Hole 1124C
Position: 39°29.9014´S, 176°31.8938´W
Start hole: 1745 hr, 26 September 1998
End hole: 1630 hr, 30 September 1998
Time on hole: 94.75 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 3978.00
Distance between rig floor and sea level (m): 11.50
Water depth (drill pipe measurement from sea level, m): 3966.50
Total depth (from rig floor, mbrf): 4451.10
Total penetration (mbsf): 473.10
Coring totals: type: APC; number: 14; cored: 132.00 m; recovered: 102.12%
type: XC; number: 35; cored: 333.10 m; recovered: 89.76%
Formation: lithostratigraphic Subunit IA (8–60.74 mbsf): nannofossil silty clay
lithostratigraphic Subunit IB (60.74–178.4 mbsf): silty clay grading into nannofossil silty clay intercalated with clayey nannofossil ooze grading into nannofossil ooze
lithostratigraphic Subunit IC (178.4–294 mbsf): clay-bearing nannofossil chalk intercalated with clayey nannofossil chalk and nannofossil mudstone
lithostratigraphic Unit II (294–411.5 mbsf): nannofossil chalk with interbeds and laminae of clay-bearing nannofossil chalk that grades downcore through a nannofossil-bearing mudstone to a plain mudstone
lithostratigraphic Unit III (411.5–419.8 mbsf): clayey nannofossil chalk
lithostratigraphic Unit IV (419.8–429.09 mbsf): mudstone
lithostratigraphic Unit V (429.09–463.42 mbsf): nannofossil bearing mudstone
lithostratigraphic Unit VI (467.4–473.21 mbsf): nannofossil bearing mudstone
Hole 1124D
Position: 39°29.8841´S, 176°31.8917´W
Start hole: 1630 hr, 30 September 1998
End hole: 2330 hr, 1 October 1998
Time on hole: 31.00 hr
Seafloor (drill pipe measurement from rig floor, mbrf): 3978.00
Distance between rig floor and sea level (m): 11.50
Water depth (drill pipe measurement from sea level, m): 3966.50
Total depth (from rig floor, mbrf): 4133.60
Total penetration (mbsf): 155.60
Coring totals: type: APC; number: 14; cored: 133.00 m; recovered: 99.13%
Formation: lithostratigraphic Subunit IA (22.5–60.74 mbsf): nannofossil silty clay
lithostratigraphic Subunit IB (60.74–155.5 mbsf): silty clay grading into nannofossil silty clay intercalated with clayey nannofossil ooze grading into nannofossil ooze
Site 1124 is located ~600 km due east of New Zealand's North Island on the 250-km long ridge of Rekohu Drift. The main Rekohu sequence consisting principally of inferred Miocene drift sediments overlies older sediments beneath seismic reflector X and is thought to be onlapped by overbank turbidites from the Hikurangi Channel above diffuse reflector Y. By correlation with other sections, the Unit B sediments (between X and Y) at this site before drilling were believed to be calcareous pelagites. Unraveling the evolution of the Rekohu Drift is critical to understanding the development of Hikurangi Channel, and the injection of sediment into the DWBC. Rekohu Drift has clearly acted as an effective barrier to eastward dispersal of terrigenous sediment from Hikurangi Channel, which turns abruptly to the left (N) against the drift, during the Pliocene–Pleistocene. The channel is thought to have originally flowed to the north along the New Zealand margin into Kermadec Trench and then been diverted to flow eastward by a major slide off Hawkes Bay in the late Pliocene. It was hoped that Site 1124 would yield a mainly carbonate record of the Miocene paleohydrography of the DWBC, and (if it penetrated unconformity X) important information on the middle Cenozoic initiation of the system. The objectives of Site 1124 were thus to determine: (1) the Miocene evolution of the DWBC and associated water masses, (2) provenance of sediment in the DWBC system and, (3) the Neogene volcanic history of the North Island.
Hole 1124A consists of a failed mudline core. The bit was raised by 5 m and Hole 1124B was spudded. The core barrel was retrieved with a shattered liner and no core. The second APC core achieved incomplete stroke and required retrieval of the BHA with the stuck core barrel to the surface. Hole 1124C was washed to 8.0 mbsf where XCB coring was initiated. The hole was cored with the XCB from 8.0 to 27.2 mbsf and deepened with the APC to 159.2 mbsf and with the XCB to 473.1 mbsf (Table 6). The hole was logged from total depth to 78 mbsf with the triple combination, the FMS-sonic, and the GHMT. Hole 1124D was drilled ahead to 22.6 mbsf with the XCB and deepened with the APC to refusal at 155.6 mbsf.
An excellent and complete spliced record was obtained for 17.7 to 174 mcd from Holes 1124C and 1124D. The uppermost 11.7 mcd is also complete in Cores 181-1124A-1H and 1124B-1H and 2H, but coring difficulties caused by ashes lost the small section between 11.7 and 17.7 mcd. The B/M boundary is located at 33 mcd (Fig. 12), however, and it will be straightforward to estimate the resulting time gap in the middle Pleistocene, which may be only ~100 k.y. The coring problems suggest considerable lateral variability in ash thickness and cementation over the short distances between holes (~20 m).
The sequence here has been divided into six lithostratigraphic units. Units I and II, which compose the drift sequence, occupy the top 412 mbsf. These units are pale greenish gray ooze and chalk showing cyclicity in color, GRAPE, and magnetic susceptibility. Ash beds are increasingly common upsection from the late Miocene to Holocene sediments, which encompass the top 200 mbsf. The drift sequence is broken by an unconformity with a hiatus of ~4 m.y. within the lower Miocene (23–19 Ma). Beneath this is a thick (110 m) upper Oligocene section overlying the Marshall Paraconformity at 412 mbsf, across which there is a 5 m.y. hiatus (32–27 Ma). Beneath this are four thin units of contrasting character: lower Oligocene nannofossil chalk (Unit III), middle Eocene brown to dark brown mudstone (Unit IV), Paleocene nannofossil chalk with zeolitic interbeds (Unit V), and Upper Cretaceous cherty zeolitic mudstone with nannofossils (Unit VI), the first three being separated by two hiatuses and the last two by the K/T boundary. Unfortunately the boundary section itself is missing between cores though it shows clearly on the FMS and other downhole logs as a resistivity high and magnetic susceptibility low. Carbonate contents are variable, averaging 36% but ranging from 0% to 88%. The lowest values are found in the middle and upper Miocene, between 100 and 300 mbsf. The organic carbon content is normal for deep-sea sediment, averaging 0.31%. The Eocene brown mudstone (Unit IV) which superficially resembles the facies of the Waipawa Black Shale on land, also has a low organic carbon content of 0.26 to 0.44 wt%.
Despite poor preservation of all groups in the Oligocene and Miocene, 66 microfossil datum points (49 above the middle Oligocene) have been recognized, giving a reasonably well-constrained age-depth curve. Planktonic foraminifers are too poorly preserved‹only thick-shelled species survive‹for assessment of warm/cold assemblages. However the diatoms represent a warm subtropical flora. The upper Neogene, but not upper Oligocene/lower Miocene, contains reworked Eocene forms. As other indicators suggest the deep current was flowing vigorously, this may point to opening of a source to the south supplying Bounty Trough or scouring of Chatham Rise. Corrosion of foraminifers clearly sets in after the Marshall Paraconformity compared with specimens in underlying strata, evidence of a new bottom-water source.
Magnetostratigraphy was particularly good for the spliced interval of Holes 1124C and 1124D (30–170 mcd). All subchrons of the Matuyama, Gauss, Gilbert, and C3r-C4r inclusive were recognized in the upper part of the record. However, a strong magnetic overprint was encountered between 180 and 280 mbsf preventing unambiguous polarity determination over that interval. Magnetic intensities were very weak in the Oligocene, but increased in the Paleocene and around the Cretaceous/Tertiary (K/T) boundary where C29r was identified spanning the K/T boundary interval. Shore-based research should allow polarity and environmental magnetism to be determined for the whole record at Site 1124.
The Upper Cretaceous-Paleocene siliceous zeolitic mudstones were deposited at an average rate of 5 m/m.y., and the rates for the succeeding two unconformity-bounded Eocene and lower Oligocene sections are indeterminate. Above the Marshall Paraconformity, the thick upper Oligocene–lower Miocene (~27–23 Ma) section accumulated at ~27 m/m.y. The middle and upper Miocene sections accumulated steadily at 10 m/m.y., slowing down (or possibly a brief hiatus) around 9.5 to 8.5 Ma. A sharp increase in sedimentation at 2 Ma and continuing at ~38 m/m.y. possibly records the Hikurangi channel switching toward the drift and contributing fine tails of turbidity currents to it. This provides a possible age for the emplacement of the very large Ruatoria slide off eastern New Zealand.
Physical properties are very uniform in two intervals, 20–178 and 178–280 mbsf, corresponding to lithostratigraphic Subunits IB and IC, respectively. The early drift and sub-drift sequence shows downhole increases in density and compaction. The brown mudstone unit, however, is of lower density than the chalk above and below. Temperature measurements yield a gradient of 51.9°C/km and an estimated heat-flow of 0.049 W/m2.
Good logging results were obtained with all tools except the NMRS magnetic intensity instrument. All show signals of useful dynamic range, save magnetic susceptibility between 318 and 419 mbsf, which is of very low amplitude, paralleling the core values. These values correlate very well between core and log over the whole hole. The brown mudstone unit stands out sharply in gamma, porosity, seismic velocity, photoelectric effect and magnetic susceptibility. A 30-cm-thick layer at the correct position for the K/T boundary is evident in resistivity, magnetic susceptibility, and on the Formation MicroScanner display. The logs are vital for filling in the major features of a 17-m zero recovery interval just above 300 mbsf. The integrated travel times based on the sonic log suggest that reflector Y is most likely the top of the brown mudstones.
The organic carbon concentrations average 0.3% and are in the normal range for deep-sea sediments. Carbonate contents show a high variability with values between 0.1% and 88.3%, and thus reflect a varying combination of fluctuating biological productivity, dilution by non-carbonate hemipelagic sedimentary components, and postdepositional carbonate dissolution forced by oxidation of organic matter.
The dominant chemical reactions that control the interstitial water element concentrations include organic matter degradation, carbonate dissolution/ precipitation, silica dissolution, chert formation, and reactions with clay minerals. The element profiles of alkalinity, phosphate, and ammonia are typical of a situation without active sulfate reduction, reflecting organic matter oxidation and carbonate precipitation. The behavior of Ca, Mg, and Sr in the bottom of the section reflects a chemical reaction other than carbonate diagenesis. The decrease of Sr is similar to the pattern of Li, which is related to Si utilization to form the chert in the lowermost part of the core. The low Si concentration in the middle of the section is attributed to poor preservation of biogenic siliceous sediments, probably caused by low paleoproductivity. The general chemical zonation of interstitial waters at Site 1124 can be related to the lithostratigraphic units, paleontological age divisions, and hiatuses.
This site will provide well dated and characterized material for paleoceanographic studies of the deeper levels of flow entering the southwest Pacific Ocean, though in places only bulk carbonate isotopic data may be possible.