Site 1126

Site 1126 is located on the eastern Eyre Terrace upper slope in 783.8 m of water. This site was designed to intersect Cenozoic seismic Sequences 2, 3, 4, and lobes 1 and 3 of Sequence 6A, and to intersect as much of the upper part of the Cretaceous section as time permitted. The target depth at this site was a high-amplitude reflector of probable Cenomanian age, estimated before drilling (on the basis of stacking velocities) to be at 525 mbsf. Because this was also the first ODP site in this basin, it provided the opportunity to establish a basic stratigraphy for the Cenozoic sequences that could then be refined at later sites.

Five lithostratigraphic units were delineated at Site 1126. Unit I (0—60.2 mbsf) is a calcareous ooze with a gradational alternation between two major sediment types: (1) white gray nannofossil-rich matrix-supported ooze and (2) light gray grain-supported ooze rich in planktonic foraminifers. Unit II (60.2—116.8 mbsf) is characterized by several intervals of slumped calcareous ooze. Sediment composition in these bodies is the same as in the undisturbed sediments. The first downhole occurrence of silicified layers (porcellanite) subdivides Unit II into an upper Subunit IIA and a lower Subunit IIB. As in Unit I, intervals that are not affected by slumping display a light/dark alternation. In the lower part of Unit II, these cycles are increasingly asymmetric, with the dark portions becoming dominant. Unit III (116.8—165.5 mbsf) consists of calcareous ooze with interbeds of silicified layers in the upper part of the unit. The lower part of the unit is porcellanite free and displays the same type of cyclicity as sediments in the overlying units. Unit IV (165.5—396.6 mbsf) coincides with an interval of low core recovery and thus cannot be described in detail. It consists of an alternation of calcareous ooze to chalk intervals and silicified pelagic limestones with some planktonic foraminifers and bioclasts. Unit V (396.6—455.9 mbsf) consists of black to green sandstones, silty sandstones, clayey siltstones, and minor granule conglomerates.

Nannofossil and planktonic foraminifer biostratigraphies show that Quaternary through middle Eocene sequences were recovered. Sandstones of probable Cenomanian age (Unit V) recovered at the base of the drilled interval are barren of marine microfossils. Sedimentation rates were relatively high in the Quaternary (31 m/m.y.) and lower Pliocene (24 m/m.y.) and slow in the remainder of the Neogene and Paleogene sections, where rates alternate between 7—8 m/m.y. and 11—15 m/m.y. with faster rates in the upper Miocene, lower Miocene, and upper Oligocene. Two intervals of slow sedimentation are present at the Miocene/Pliocene and early/middle Miocene boundaries. Benthic foraminiferal assemblages show a change in benthic environments from lower bathyal to middle bathyal in the upper part of the middle Eocene nannofossil zone, NP16. Slumps between ~55 and 110 mbsf are marked by a mixture of late Miocene and early Pliocene planktonic taxa found within a latest early Pliocene assemblage. The slump is also flagged by a mixture of a displaced upper bathyal assemblage of benthic foraminifers found within an in situ middle bathyal assemblage.

The results of paleomagnetic long-core measurements of cores from Hole 1126B and 1126C were disappointing. The NRM was dominated by a vertically downward coring contamination that was largely removed by 20 mT demagnetization, whereupon the signal was almost uniformly reduced to the noise level of the instrument. In addition, there were anomalous peaks in intensity near the tops of most cores and of several sections. These were so large that we interpret them as contamination. Only a single core gave a sequence of reversals that could be interpreted magnetostratigraphically. A comparison between whole-core and archive-half core measurements showed that the two measurements are not significantly different from each other, thereby settling a long-debated question. The nannofossil chalks gave stronger signals, but poor recovery precluded determination of a useful magnetostratigraphy. In the sandstone recovered toward the base of Hole 1126D (Unit V), normal magnetizations were observed. Their high inclination, giving a paleolatitude of ~50°S, is in accordance with northerly motion of the Australian plate as it moved away from the Antarctic-Australia spreading center. Rock magnetic properties of representative samples from the various sedimentary units differ in a manner that is consistent with the NRM variation. In the nannofossil chalk and in some of the more strongly magnetized nannofossil oozes, the magnetic carrier appears to be single-domain magnetite consistent with a magnetotactic bacterial origin, whereas a different and harder phase dominates in the uppermost nannofossil ooze sediments.

The upper 154.0 m of sediment at Site 1126 was double cored. Construction of the composite section from Holes 1126B and 1126C indicates that a complete record of the sedimentary section was not recovered. Correlation between cores was hindered by significant differential core distortion, particularly at the very top of each core where an expanded record was indicated. Correlations were further hindered by a decrease in core recovery below 100 mbsf, resulting from the presence of multiple thick chert layers. Below 60 mcd, large data gaps and the presence of numerous slumps made correlations difficult and highly tentative. The composite section indicates gaps in the record at ~26—27, 64—65, 73—74, 86—87, 109—112, 124—125, and 130—137 mcd.

Only low concentrations of methane and ethane were detected at Site 1126. Methane ranges from 2.2 to 13.6 ppm. Ethane is present in five samples between 186.5 and 236 mbsf (1.1—2.7 ppm) but is not detected at greater depths. The carbonate content is uniformly high (80—91.2 wt%) in the upper 116 mbsf but becomes highly variable (30.6—93.2 wt%) from 116 to 254.2 mbsf. Because of poor core recovery, no samples are available in the interval from 254.2 to 348.6 mbsf. From 348.6 to 389.6 mbsf, the carbonate content returns to high levels within a narrow range (79.5—90.0 wt%), dropping again to very low values in sandstones (0.4—0.6 wt%) from 406.4 to 454.5 mbsf. Total organic carbon is low throughout the cored interval except for the bottommost sample of Hole 1126D, which has 1.33 wt% organic carbon. All other organic carbon values are less than 0.8 wt% and most are less than 0.2 wt%

Site 1126 is characterized by a large increase in salinity that manifests itself as shallowly as 10 mbsf, reaching a maximum of 106 (about three times normal salinity) by a depth of ~100 mbsf. Most of the other geochemical parameters exhibit nonconservative behavior with respect to chloride, including calcium which shows a net excess, and magnesium, sulfate, and alkalinity, all of which show net losses. The pore waters have a sodium/chloride ratio close to that of seawater. We believe that the origin of these saline fluids may relate to fossil brines that penetrated the sediments during a previous sea-level lowstand.

Physical properties data closely reflect the location of sequence boundaries and changes in lithology and mineralogy. These data were subdivided into five units on the basis of shifts in measured parameters. Physical properties (PP) Unit 1 is characterized by high natural gamma radiation (NGR) with a trend of increasing P-wave velocity and bulk-density measurements to the bottom of the unit. The base of PP Unit 1 roughly coincides with the late Pliocene/Pleistocene boundary. PP Unit 2 is characterized by relatively high natural gamma radiation that dramatically decreases toward the base of the unit. This decrease occurs because of the downhole disappearance of aragonite. P-wave velocity increases throughout PP Unit 2, as does thermal conductivity. The base of the unit coincides with the late Pliocene/early Miocene boundary. PP Unit 3 is characterized by low variability in P-wave velocity and NGR. The base of this unit was not recovered in the sediments, but can be defined at 185 mbsf using downhole logs. PP Unit 4 is characterized by alternations of high-velocity porcellanite and lower velocity oozes. This high contrast of sediment induration was probably the major cause of the low recovery in this unit. The top of PP Unit 5 coincides with a major hiatus at the base of the Tertiary. PP Unit 5 shows much greater NGR values and higher P-wave velocities reflecting the transition from PP Unit 4 ooze into PP Unit 5 sandstone. This indicates that PP Unit 5 sediments were affected by diagenesis. Thermal conductivity co-varies with water content, bulk density, and P-wave velocity.

Hole 1126D was logged with three tool strings: the triple combination logging tool, Sonic/Geologic High Resolution Magnetic Tool (GHMT) combination, and the well seismic tool (WST). The FMS tool was not run with the Sonic as usual because of (1) excessive heave caused by dual swell directions that was not compensated for by the wireline heave compensator and (2) a large hole diameter as indicated by the caliper on the triple combo. The triple combo was deployed from 439 mbsf to well above the mudline. The Sonic/GHMT tool string was deployed from 430 mbsf to above the mudline. The WST was used for recording nine checkshot stations between 420 and 130 mbsf. Several zones of washed out hole affected readings made by excentric tools (e.g., neutron porosity and density), whereas readings measured by hole-centered tools (e.g., magnetic susceptibility, sonic, and resistivity logs) were little influenced by hole conditions. The magnetic susceptibility data were useful for subdividing the measured section into 10 provisional logging units, some of which correlate with the seismic stratigraphic units. In the interval below 160 mbsf where core recovery was poor, changes in several logs provide the detail and continuity necessary to further subdivide the section. The spectral gamma-ray log measured by the triple combo proved very useful for correlating the part of the section measured through pipe (upper 116 mbsf) with core measurements. In the lower siliciclastic part of the section, photoelectric effect (PEF) values indicate the presence of iron minerals.

The checkshot survey showed that the actual time-depth conversion curve fell within the narrow envelope of time-depth curves derived from stacking velocities, so depth estimates for other sites based on stacking velocity curves can be viewed with more confidence. The preliminary biostratigraphic data permitted ages of the regional Cenozoic seismic sequences to be defined, showing that Sequence 2 corresponds to the Pleistocene, Sequence 3 is of Pliocene—late Miocene age, Sequence 4 is of latest Miocene age, and Sequence 6A is of Eocene—Oligocene age.


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