PRINCIPAL RESULTS

Site 1134

Site 1134 is located on the eastern Eyre Terrace upper slope in 701 m of water. This site was designed to intersect Cenozoic seismic Sequences 2, 3, and 4 and lobes 2 and 3 of Sequence 6A (Feary and James, 1998), and as much of the upper part of the Cretaceous section as time permitted. The principal objectives at this site were to (1) collect a detailed record of Paleogene– early Neogene temperate to subtropical mid-latitude sedimentation in an upper-slope environment and (2) recover a record of marine flooding of the evolving rift basin in the Cenomanian. The target depth at this site was just below a high-amplitude reflector of probable Cenomanian age, estimated before drilling to be 520 mbsf (on the basis of stacking velocities).

Site 1134 contained a 397.1-m-thick record (Table 2) of middle Eocene–middle Pleistocene sedimentation that was subdivided into six units. Unit I (0–33 mbsf) consists of calcareous nannofossil ooze with varying amounts of planktonic foraminifers. In general, whitish matrix-supported intervals, consisting of planktonic foraminifers, sponge spicules, bioclasts, benthic foraminifers, radiolarians, and minor tunicate spicules in the >63-Ám fraction, alternate with darker, grainier, light gray intervals that contain echinoid spines as well as rare glauconite, pyrite, and unidentified black grains. Unit II (33–66 mbsf) is characterized by soft-sediment deformation interpreted as slumping. Two slumped intervals were identified, with the lower interval defining the boundary between Units II and III. Unit II consists of white to light gray, calcareous nannofossil ooze and calcareous nannofossil foraminifer ooze, as well as unlithified wackestones, packstones, floatstones, and rudstones. The light gray wackestones to rudstones contain a wide variety of coarse components such as various bryozoans (robust and delicate branching, arborescent, and fenestrate growth forms), bioclasts, sponge and tunicate spicules, as well as pellets. Pebble-sized lumps of calcareous nannofossil ooze, which are interpreted to be reworked clasts, are observed in packstones and floatstones. Unit III (66–152 mbsf) consists of calcareous nannofossil ooze and calcareous nannofossil foraminifer ooze. In the lower part of the unit, there are short-term alternations between unlithified and lithified sediment that are too rapid to be solely related to sediment compaction. The lower limit of alternating lithification corresponds to the base of Sequence 3 and thus may reflect condensed sedimentation related to this sequence boundary. The lower limit of Unit III is marked by a change from dominantly pelagic sediments to partially lithified wackestones of Unit IV. Unit IV (152–214.3 mbsf) consists of unlithified to partially lithified wackestone/packstone, foraminifer chalk, and very minor packstone. The style of deposition is uniform throughout the unit, characterized by meter-scale alternations of light gray foraminifer chalk and light gray to gray wackestone/packstone. The unlithified to partially lithified wackestone/packstone contains similar components to the chalk, but it is richer in glauconite and dominated by bioclasts. Some of these layers have a sharp base and are normally graded, and thus are interpreted as turbidites. The lower boundary of Unit IV has been placed at the top of a white foraminifer chalk. In Unit V (214.3–368.2 mbsf), two main lithologies were recovered: (1) strongly bioturbated calcareous nannofossil chalk and (2) fragments of porcellanite and chert. The matrix of white chalk is dominated by calcareous nannofossils with common sponge spicules as well as trace amounts of bioclasts and dolomite, whereas the coarse fraction consists of planktonic and benthic foraminifers. The gray and green porcellanite are silicified chalk. Only 30 cm of the lowermost Unit VI (368.2–397.1 mbsf) was recovered. The sediments within this unit are coarse sand-sized, brown limonitic sandstones with quartz, limonite, glauconite, mica, and abundant skeletal grains, as well as minor planktonic foraminifers.

Calcareous nannofossils and planktonic foraminifers indicate that sediments recovered at Site 1134 range from Quaternary to middle Eocene in age. Two hiatuses are indicated: the first occurs within the Pliocene (nannofossil Zones NN18–NN13 missing at ~57 mbsf) and the second in the middle–upper Miocene (nannofossil Zones NN9–NN7 missing at ~114 mbsf). Planktonic foraminifers also indicate hiatuses at approximately the same depths. A third hiatus may be present in the Eocene section as nannofossil Zone NP17 was not recognized; however, poor core recovery makes confirmation of this hiatus difficult. Calcareous nannofossils are moderately well preserved above ~200 mbsf, with decreasing preservation below this level. Planktonic foraminifers are moderately well preserved throughout most of Hole 1134A, with preservation deteriorating only in the lowermost 30 m of the hole. Benthic foraminifers are generally rare compared to planktonic foraminifers, decreasing in abundance in the lower 130 m of Hole 1134A. Preservation of benthic foraminifers is comparable to that of the planktonic foraminifers. The five main assemblages of benthic foraminifers can be correlated with coevel assemblages at Site 1126 and indicate middle bathyal paleodepths.

Long-core measurements at Site 1134 indicate that the intensity of magnetization is extremely weak in the uppermost 100 mbsf, similar to that at Site 1126. However, discrete measurements revealed that a stable remanence is present, and the Brunhes/Matuyama boundary was identified in both holes. Magnetization intensity decreases to a minimum near 110 mbsf in a manner characteristic of the dissolution of fine magnetic particles. There is a marked increase in intensity to 120 mbsf, which coincides with the first appearance of chert at Site 1134. A brief magnetostratigraphy was determined within the Anomaly 5 sequence. Whole-core measurements and nonmagnetic coring experiments were continued, but the intensity of magnetization was so weak that further analysis is required from shore-based work.

Construction of a composite section for Site 1134 indicates that recovery of Holocene–middle Miocene sediments was complete to a depth of 151.7 mcd. The composite section was constructed primarily using 400 nm color reflectance, the 700:400 nm color reflectance ratio, and GRA bulk density data from Holes 1134A and 1134B. The mcd scale at Site 1134 is expanded by ~6% relative to the mbsf scale. The upper 35 m of sediment (lithostratigraphic Unit I) exhibits cyclic oscillations in color reflectance, which are distinctively correlatable between holes. Lithostratigraphic Unit II (34.7–68.9 mcd) contains two slumped intervals that yield excellent correlative horizons at their tops and bases. Lithostratigraphic Unit III (>68.9 mcd) is more difficult to correlate because of dissimilarities in the records of each hole, which may be a result of differential lithification and diagenesis.

Methane concentrations at Site 1134 were low with maximum concentrations of 7.7 ppm and average concentrations less than 5 ppm. Calcium carbonate contents range from 58 to 94 wt%, averaging 80–92 wt%.

Site 1134 was characterized by a very steep gradient in pore-water salinity, reaching values of 97 by a depth of 65.90 mbsf. This salinity gradient is similar to that measured at Sites 1130 and 1126 and is consistent with the idea that the brine has a common level beneath all sites and is controlled by the level of the hydrostatic head during sea-level lowstands. Because of the low initial concentration and accumulation of organic material at Site 1134, oxidation rates of organic material and carbonate diagenesis are reduced compared to the more proximal sites. Significant geochemical reactions appear to take place only in the upper 40 mbsf at Site 1134. In this interval, there is a decrease in the concentration of metastable minerals such as aragonite and HMC and in the appearance of small concentrations of dolomite. As a result, there is only a small increase in strontium with depth an increase that is quickly swamped by increases associated with greater salinity. The concentration of iron in interstitial waters from Site 1134 is higher than at shallow water hydrogen sulfide-dominated sites, reaching concentrations >60 mM.

Sediment physical properties measurements closely reflect lithologic variations and provide essential data for core-log correlation. These data were subdivided into four units based on variations in measured parameters. Unit I (0–33 mbsf) is characterized by a sharp decrease in porosity (50%–30%) and increase in bulk density (1.7–2.0 g/cm3) and P-wave velocity (1.6–1.8 km/s) that corresponds to the nannofossil ooze succession of lithostratigraphic Unit I. The top of Unit 2 is defined by an abrupt increase in bulk density and NGR and correlates with the transition from nannofossil ooze to unlithified bioclastic wackestone. Unit 2 (33–66 mbsf) is typified by high variability in P-wave velocity, bulk density, and NGR that correlates with intervals of soft sediment deformation interpreted as slumps. Unit 3 (66–145 mbsf) is characterized by a gradual decrease in bulk density and P-wave velocity, and a sharp decrease in NGR (10–2 cps). The lower part of the unit shows a marked increase in magnetic susceptibility (negative values to ~10910– 6 SI units) and porosity. The base of Unit 3 is defined by an increase in NGR and a sharp decrease in magnetic susceptibility. Unit 4 (145–368 mbsf) shows highly variable P-wave velocity, bulk density, and porosity, which results from alternation of nannofossil oozes and wackestones/packstones in the upper part, and alternations between chalk and chert in the lower part of the unit.

Three logging strings were successfully deployed in Hole 1134A. Overall, downhole measurement trends are similar to those at Site 1126C, but gamma-ray values at Site 1134 display markedly higher values than at Site 1126C. The sedimentary succession at Site 1134 was divided into six logging units based on trends in the data that reflect lithologic alternations between calcareous ooze, wackestones, grainstones, and thin chert interbeds. Positive density and porosity separation and relatively high PEF values indicate the presence of dolomite toward the top of Unit 1. Narrow separation of density and porosity and relatively low PEF values indicate the presence of quartz in the carbonates of Unit 2. The presence of blackened grains in recovered cores coincides with higher gamma-ray values, indicating a possible phosphatic and/or glauconitic composition. Downhole logs also identify an interval of unusually low potassium concentrations in Unit 3, the significance of which is still to be established. Density, PEF, caliper, and resistivity were the only logs acquired in the sandstones at the base of the succession (Unit 6). High density and high PEF peaks are consistent with the small amount of limonitic sandstone recovered from this horizon, which may correspond to Unit 8 at Hole 1126C. Seven checkshot stations were recorded, with individual stations situated near the estimated depths of significant reflectors.

 

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