m
values measured by the wireline logs at the base of the pipe.Site 1166 is situated on the Prydz Bay continental shelf on the southwestern flank of Four Ladies Bank, ~40 km southwest of Ocean Drilling Program (ODP) Site 742 (Leg 119). Prydz Bay is at the downstream end of a drainage system that originates in the Gamburtsev Mountains of central East Antarctica. The early development and growth of the Cenozoic Antarctic Ice Sheet is believed to have started in middle Eocene to early Oligocene time, but to date, drilling on the continent and the continental margin has not sampled a stratigraphic section that clearly spans and includes the transition period from preglacial to glacial conditions. Site 1166 was chosen to recover core from the Cenozoic sediments below the horizon reached at Site 742. This was intended to provide an age for the arrival of glaciers in Prydz Bay and a record of changes in paleoenvironments and biota with the onset of glaciation.
When the JOIDES Resolution arrived in Prydz Bay, the drill site had to be moved to an alternate site because pack ice covered much of western Prydz Bay where the primary site was located. An additional 3-nmi displacement of the site was required because a large tabular iceberg was floating directly over the alternate site. The target sedimentary section was thinner and likely a little younger near the base of the section at the final alternate site (i.e., Site 1166) than at the primary site. The drilling at Site 1166 was conducted safely and achieved the desired objective under the sometimes cold and stormy operating conditions.
Drilling at Site 1166 had to be halted temporarily about midway through because of a large storm with wind gusts exceeding 40 kt and swells >2 m, the maximum limit for shallow-water drilling. During the storm, an iceberg approached within 0.6 nmi, and the drill pipe had to be pulled out of the seafloor without a reentry cone in place. The ship was moved away from the site. Following the storm, the ship returned to the site and successfully reentered the hole to continue uninterrupted drilling to a total depth of 381.3 meters below sea floor (mbsf). Recovery at the site was 18.6%; low recovery was due partly to drilling through the upper section (135 mbsf) of diamictites, similar to those sampled at Site 742, and because of sandy fine-grain sediments in the lower part of the hole. Three full logging runs were obtained with excellent data from ~50 mbsf to the bottom of the hole.
The sedimentary section at Site 1166 comprises a diverse suite of strata that from the top down include glacial, early glacial, and preglacial rocks that consist of poorly sorted, sandy, and fine-grained sediments. The ages range from Holocene at the seafloor to Late Cretaceous at the bottom of the hole, with many disconformities throughout.
The sediments are divided into five lithostratigraphic units. Lithostratigraphic Unit I is of late Pliocene to Holocene age and comprises four subunits. The uppermost subunit (Subunit IA; 0.0-2.74 mbsf) is a biogenic-rich clay interval with pebble-sized clasts. This unit is interpreted as an ice-keel turbate, based in part on iceberg furrows that occur echo-sounder records in this area (O'Brien and Leitchenkov, 1997).
Below Subunit IA are two intervals of diamicton and one of poorly sorted clayey sandy silt. Subunits IB (2.79-106.36 mbsf) and ID (123.0-135.41 mbsf) are separated by a thin interval (Subunit IC; 113.30-117.22 mbsf) of biogenic-rich clayey silt. Subunit IB is predominantly clayey silt with rock pebbles and clasts that include common fibrous black organic matter. Minor (<2 cm thick) sand and granule beds are present. Subunit IC is interbedded dark gray sandy silt with lonestones and greenish gray diatom-bearing clayey silt with dispersed granules. Biogenic-rich intervals are slightly bioturbated. Contacts with the dark silt are sharp. Subunit ID has interbeds of dark gray clast-poor and clast-rich diamicton. Clast lithologies vary and include gneiss, granite, and diorite rocks. The diamictons suggest subglacial deposition; the sandy silt with ice-rafted debris (IRD) is a glaciomarine unit deposited during interglacial periods.
Unit I has lonestones throughout. For Subunits IB and ID, the diamictons suggest subglacial deposition or deposition of a proglacial morainal bank. Sandy silt intervals with shell fragments and microfossils (Subunit IC) suggest current reworking and glaciomarine sedimentation during times of significant glacial retreat. The lonestones signify deposition of IRD. In seismic reflection data, Unit I comprises the topset beds of the seaward-prograding sedimentary section of the outer continental shelf. The contact between lithostratigraphic Units I and II is an abrupt unconformity and was recovered (interval 188-1166A-15R, 8-11 cm [135.63 mbsf]). X-ray diffraction (XRD) data above and below the unconformity show shifts in relative abundances of mica, illite, kaolinite, hornblende, and plagioclase. The shifts suggest increased amounts of weathered terrigenous material within Unit I, just above the unconformity.
Unit II (135.63-156.62 mbsf) is an upper Eocene to lower Oligocene diatom-bearing claystone with thin interbedded sands and lonestones. Carbonate contents range from 0.4 to 3.3 wt%. Sands are poorly sorted and bioturbation is moderate. Rare fibrous black organic clasts are present within the sand beds. The bottom of Unit II has rhythmically interbedded centimeter-thick sand and dark claystone. XRD shows gibbsite and kaolinite in Unit II, suggesting erosion of chemically weathered material produced during soil formation. Unit II is a marine sequence that records ice rafting of pebbles during a marine transgression. At Site 742 (Leg 119), 40 km away, similar or younger-age sediments that were interpreted as proximal glacially influenced proglacial or subglacial deposits were sampled. The contact between lithostratigraphic Units II and III is abrupt and was also recovered (interval 188-1166A-17R, 77-78 cm).
Unit III (156.62-267.17 mbsf) consists of massive and deformed sands with a silty clay matrix. The unit is of late Eocene age based on initial palynological studies (O'Brien et al., in press). Calcium carbonate content ranges from 0.4 to 8.4 wt%. The sands have a uniform fabric, are poorly sorted, and lack internal structure. Pebbles of quartzite and rare fibrous black organic fragments are widely dispersed throughout. Two calcite-cemented intervals are present in the sands. The lower part of Unit III is deformed and folded by soft-sediment deformation of sandy beds and includes black organic-rich material with pieces of wood. The coarse-grained sands record deposition on an alluvial plain or delta, and the deformed beds record some reworking of material from underlying organic-rich horizons. A similar-looking sequence of carbonaceous material was drilled in the bottom 2 m at Site 742 and was interpreted as fluvial or possibly lacustrine. At Site 742, however, a sequence comparable to the homogeneous coarse sands was not recovered. The coarse sands of Unit III may record a preglacial alluvial plain or a braided delta of a glacial outwash system. The contact between lithostratigraphic Units III and IV was not sampled.
Unit IV (276.44-314.91 mbsf) comprises black highly carbonaceous clay and fine sandy silt with organic-rich laminae and rare to moderate bioturbation. The sandy silt contains abundant mica and some pyrite. The organic-rich laminae have organic carbon (OC) values as high as 9 wt% and contain common occurrences of authigenic sulfides. Calcium carbonate content ranges from 0.3 to 3.7 wt%. Unit IV records deposition in a restricted marine or lagoonal environment. Initial palynological studies indicate a Turonian age for Unit IV (O'Brien et al., in press).
Unit V (342.80-342.96 mbsf) consists of a small sample of undated finely laminated gray claystone that was captured in the core catcher within a thick no-recovery zone at the bottom of the hole. From resistivity and velocity log data, the unit may have a relatively large clay content. The claystone is likely preglacial in origin. The interpretation is based on tentative lithologic and seismic stratigraphic correlation with ODP Site 741, 110 km away, from which Lower Cretaceous gray claystone was recovered (Barron, Larson, et al., 1989). Palynomorphs indicate a Turonian age (86-91 Ma) (O'Brien et al., in press).
Diatoms, radiolarians, foraminifers, and calcareous nannofossils were examined at Site 1166. Diatoms are present in limited intervals of the core and provide the primary biostratigraphic age control. Three distinct diatom assemblages were noted—Quaternary, Pliocene, and late Eocene-early Oligocene age. Extant Quaternary diatoms occur at 2.12-2.92 mbsf (age = <0.66 Ma). Quaternary to upper Pliocene diatoms occur in diamicts to at least 106.37 mbsf. Two layers of diatomaceous clay (~113.95-114.10 and ~114.50-115.15 mbsf) occur, containing the upper Pliocene diatom Thalassiosira kolbei (1.8-2.2 Ma) and the Thalassiosira vulnifica to Thalassiosira striata-T. vulnifica Zones (2.2-3.2 Ma), respectively. Radiolarians suggest an age of >2.4 Ma for the lower bed. The boundary between lithostratigraphic Units I and II is a major disconformity (~30 m.y.). Diatoms in Unit II between 135.73 and 153.48 mbsf are of early Oligocene-late Eocene age (~33-37 Ma). Diatoms were not recovered below Unit II, but several specimens of pollen, spores, dinoflagellates, and wood fragments were noted in lower intervals of the hole. Planktonic foraminifers are common above ~90 mbsf, and their ages generally agree with those for diatoms and radiolarians. Calcareous nannofossils are rare.
Paleomagnetic stratigraphy is difficult because of limited core recovery, but a clear pattern of magnetic polarity intervals is recorded where the recovery is relatively high. A correlation to the geomagnetic polarity time scale (GPTS) is in progress using key biostratigraphic datums. Downhole variations in the concentration-dependent and magnetic mineralogy-dependent parameters show that the main lithostratigraphic units have alternating high and low magnetic mineral concentrations and distinct magnetic signatures. Iron sulfide minerals are present below ~140 mbsf.
Interstitial water profiles document downhole sediment diagenesis and diffusional exchange with bottom seawater. From 0 to 150 mbsf, the oxidation of organic matter reduces sulfate values from 28 to 8 mM and ammonium increases from 177 to 1277 mM. From 0 to 75 mbsf, alkalinity decreases from 4.5 to 1 mM, silica decreases from 800 to 200 mM, potassium decreases from 12 to 2 mM, and calcium increases from 10 to 22 mM. The profiles suggest diagenetic silicate reactions are occurring within the sulfate reduction zone. Between 150 and 300 mbsf, calcium and magnesium show minor changes in relative concentration (15 and 24 mM, respectively) suggesting diffusional processes are dominant.
OC contents of the sediments based on 14 samples (selected by dark color) vary according to lithostratigraphic unit. The diamictite (Subunit IB) has OC values of 0.4-1.4 wt%; the massive sand (Unit III) has OC values of 0.2-0.5 wt%, except for one bed near the base of the fluvial/deltaic sand section of Unit III that contains 9.2 wt% OC; the carbonaceous claystone (Unit IV) has 1.5-5.2 wt% OC. Inorganic carbon was low (<0.1 wt%) throughout most of the recovered section. Gas analyses showed only background levels of methane (4-10 ppmv), and no other hydrocarbons were detected. Most samples are enriched in carbon relative to nitrogen, which suggests the input of land-plant organic matter, especially for samples with >1 wt% OC. Rock-Eval pyrolysis analysis shows that the pyrolyzable fraction of the OC is low (hydrogen index values of 50 mg of hydrocarbon per gram of carbon or less), consistent with degraded plant material as the source of the carbon in the more carbonaceous (>2 wt%) samples. Samples with lower carbon contents (<1.4 wt%) may contain a recycled higher thermal maturity component. This recycled organic component is suggested by Rock-Eval Tmax values that approach 490°C as OC decreases toward values of 0.5 wt%. The diamictites (Unit I) have a greater proportion of recycled organic matter than the carbonaceous units (base of Units III and IV), which contain mostly first-cycle organic matter.
The majority of the sedimentary section has porosities between 20% and 40%, with the exception of Unit II, where the average porosity is 50%. P-wave velocities change abruptly at most lithostratigraphic boundaries. The measured shear strengths show that the sediments, especially in Unit I, are overconsolidated, with an overconsolidation ratio of ~2 below 70 mbsf. The overconsolidation record implies at least one or two periods when sediments were either compacted by a 250- to 300-m-thick sediment column, now eroded away, or were loaded by 330- to 420-m-thick nonbuoyant ice during prior glaciations.
Wireline logging was carried out in Hole 1166A with excellent results. Three runs were made using the triple combination (triple combo), sonic/geological high-sensitivity magnetic tool (GHMT), and Formation MicroScanner (FMS) tool from 33 mbsf to the bottom of the hole at 385 mbsf. Six logging units are recognized, and each stratigraphic unit has a distinctive signature and appearance, especially in the resistivity, sonic velocity, and FMS data. The deeper parts of the hole (logging Units 4b, 5a, 5b, and 6, equivalent to lithostratigraphic Units III [lower part], IV, and V) consist of preglacial to early glacial sediments that have large gamma-ray fluctuations indicative of heavy mineral K, Th, and U contents associated in part with the high OC values here. These units also have lower velocity, density, and resistivity than the thick overlying deltaic sands of lithostratigraphic Unit III. All log traces show abrupt shifts at the logging Units 3/4 and 2/3 boundaries (equivalent to lithostratigraphic Units II/III and I/II boundaries) and are recognized as unconformities. The diamictons with interbedded glaciomarine clays and silts of lithostratigraphic Unit I have generally high and variable magnetic susceptibilities that suggest high variability in magnetite concentrations. FMS images clearly show the variability in the lithostratigraphic units, the presence of lonestones, and the deformation of the organic-rich silt-sand horizons (lithostratigraphic Unit III). The resistivity and velocity logs, along with seismic reflection profiles, suggest the alluvial sands (Unit III) to glaciomarine diatom-bearing claystones (Unit II) transition represents a marine transgression.
Logging while drilling (LWD)
was done in Hole 1166B to test the Power Pulse and compensated dual resistivity
(CDR) tools and record spectral gamma-ray and resistivity data in the uppermost
42 m of the sediment. This interval could not covered by wireline logging
because of the pipe position. Resistivity values increase in linear segments
from near zero at the seafloor to the 3.5 m
values measured by the wireline logs at the base of the pipe.
A primary objective of Leg 188 was achieved when a set of cores that record intervals in the history of Antarctic paleoenvironments for the Prydz Bay region extending back through the early stage of glaciation to preglacial times was recovered at Site 1166. Drilling during Leg 119 in Prydz Bay recovered a record of early proximal glaciation at Sites 739 and 742 but did not capture the transition to warmer climates as would be indicated by the presence of local vegetation. Correlation of Site 1166 to Site 742 (~40 km apart) by comparison of downhole logs and regional seismic stratigraphy shows that Units I and II at Site 1166 are equivalent to (or older than) similar units at Site 742. Below the level of Unit II, however, Site 1166 samples are stratigraphically lower and record a more temperate alluvial facies than seen at Site 742. The lower part of Unit III (i.e., the deformed organic-rich sands and silts) may have been sampled in the lowermost 2 m of core at Site 742, but confirmation of this awaits further comparison of the two drill sites. If the organic units are the same, then a thick section of sands (Unit III) is missing at Site 742. The deepest unit at Site 1166 (Unit V) lies below a regional seismic unconformity that can be traced to Site 741, ~110 km away, where similar gray claystones like those of Unit V were also sampled. The age of the claystone at Site 741 is Early Cretaceous, which is preglacial.
The paleoenvironmental record inferred from the cores at Site 1166 shows a systematic uphole change from preglacial warm to full-glacial cold climates, such as that envisioned for the Prydz Bay region. The rich carbonaceous strata (undated Unit IV), which overlie Unit V, record a time of more temperate climatic conditions when vegetation existed on Antarctica. The sands (undated Unit III) of the alluvial plain environment may represent the transition into the progressively colder climates that are recorded in the proglacial (late Eocene to early Oligocene Unit II), glaciomarine (Unit II and late Pliocene and younger Unit I), and subglacial (Unit I) sediments. The ages of Units III and IV are late Eocene and Turonian, respectively (O'Brien et al., in press).
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