Site 1129

Site 1129 was the third and proximal site of a planned three-site transect through a spectacular set of late Neogene clinoforms immediately seaward of the present-day shelf edge. Site 1129, located just seaward of the shelf edge in 202.4 m of water, intersects the most expanded record of the oldest part of the clinoform sequence. The principal objective of this transect is to collect detailed, high-resolution profiles through a late Neogene shelf-edge (high energy) to upper slope (low energy) succession deposited within a cool-water carbonate environment, to determine the response of this type of depositional system to Pliocene–Pleistocene sea-level fluctuations.

The succession recovered at Site 1129 is 604.2 m thick (Table 2) and was subdivided into three lithostratigraphic units. Unit I (0–149.8 mbsf) consists mainly of unlithified bryozoan floatstone to rudstone and bioclastic packstone to grainstone, with abundant bryozoan fragments. The floatstone and rudstone are pale yellow to light gray and have a poorly sorted, very fine to medium sand-sized bioclastic packstone matrix. The floatstone and rudstone contain abundant granule- to pebble-sized bryozoan fragments exhibiting highly diverse growth forms. The bioclastic packstone to grainstone is pale yellow, light gray, and light olive gray, and consists of very fine to fine sand-sized bioclasts with some coarse sand- to granule-sized bryozoan fragments. The unit is subdivided into five subunits, each forming a package grading upward from bioclastic packstone at the base to bryozoan floatstone and rudstone at the top, with each subunit representing an individual mound-building episode. Unit II (149.8–556.7 mbsf) consists of thick, light gray, light olive gray, and gray bioturbated bioclastic packstone and minor grainstone and wackestone with four nannofossil chalk intervals. Lithification increases from unlithified at the top of Unit II, to partially lithified with well-lithified chalk intervals at the base. The upper part of Unit II is dominated by very fine to fine sand-sized, generally well-sorted, massive bioclastic packstone with subordinate grainstone and wackestone. The lower part is characterized by muddy bioclastic mudstone, wackestone, and packstone with abundant well-defined burrows. Unit II therefore exhibits an overall coarsening-upward trend. Unit III (556.7–604.2 mbsf), with very poor recovery, consists of dolomitized fine sand-sized bioclastic grainstone and chert fragments. It is likely that the chert fragments represent thin beds and lenses of preferentially silicified wackestone/ooze. The boundary between Units II and III is very sharp, and represents a major hiatus.

Site 1129 contains two biostratigraphic units: (1) an expanded Quaternary section, more than 554 m thick, that is underlain by a thin and conformable Pliocene section, and (2) a middle–lower Miocene section. These units are separated at 556 mbsf by an unconformity representing ~12 m.y. The unconformity is marked by a bryozoan turbidite overlying indurated sediments and chert layers. Calcareous nannofossils are generally abundant and moderately well preserved in the upper part of the section, but below 371 mbsf the preservation is poor. Preservation and abundance of planktonic foraminifers degrade more rapidly downhole than the nannofossil assemblages. Below ~68 mbsf, most characteristic features of foraminifers are obscured by carbonate cement and recrystallization. Benthic foraminifers are generally abundant and well preserved in the upper part of Hole 1129C. At a depth of ~140 mbsf, preservation and abundance deteriorate markedly. Three main assemblages are recognized at Site 1129: (1) a distinctive, well-preserved Pleistocene assemblage found in bryozoan-rich accumulations (down to ~140 mbsf); (2) a Pleistocene assemblage (140–565 mbsf), which includes a variable redeposited neritic component; and (3) a sparse Miocene assemblage. The two Pleistocene assemblages indicate upper bathyal paleodepths, whereas the Miocene assemblage indicates an upper to middle bathyal paleodepth.

Long-core measurements of Hole 1129C sediments revealed an extensive interval of normal magnetization to a depth of ~340 mbsf. Sediments record reverse polarity magnetizations below that depth, with the boundary best defined by measurements on discrete samples. This reversal is interpreted as the Brunhes/Matuyama boundary. In Hole 1129D, a long interval of predominantly reverse-polarity magnetizations to a depth of ~540 mbsf is interpreted as the upper part of the Matuyama Chron (C1r). The Jaramillo Subchron (C1r1n) is recorded in both holes at depths of ~400–440 mbsf. Normal polarities are recorded again in Hole 1129D at depths of 540–550 mbsf, possibly representing the Olduvai Chron (C2n). This would indicate a variable sedimentation rate, with late Pleistocene sedimentation nearly triple the rate of the early Pleistocene. Rock magnetic characteristics, such as near saturation with inductions of 400 mT and median destructive fields of the IRM of 20–40 mT, suggest that magnetite and possibly magnetic sulfides are the principal remanence carriers. ARM/IRM indicate a high relative abundance of single domain grains, possibly biogenic magnetite.

High concentrations of methane and hydrogen sulfide through much of the section are a major feature at Site 1129. Methane and H2S concentrations at this site fall between the levels found at Sites 1127 and Site 1131. Calcium carbonate content is uniformly high, primarily in the range of 86–94 wt%. Organic carbon concentrations generally vary from 0.4 to 0.7 wt%.

Site 1129, like many other sites drilled during Leg 182, is influenced by the presence of high-salinity pore fluids within and below the cored interval. Within the cored interval, the presence of pore waters with sodium/chloride ratios significantly higher than seawater indicates the influence of fluids that have been involved in the precipitation and dissolution of halite. Site 1129 was characterized by high rates of sulfate reduction of organic material and consequent production of alkalinity. The absence of significant iron concentrations resulted in a pH less than 6. Although high concentrations of hydrogen sulfide and high alkalinity values were measured at relatively shallow depths, significant sulfate reduction did not occur until below 100 mbsf. This suggests that hydrogen sulfide is diffusing from the underlying sediments into the overlying oxic sediments, where it is oxidized to sulfate. Rates of carbonate alteration, reflected in the strontium concentration (>2 mM), were the highest yet observed during Leg 182. Dolomite is ubiquitous below a depth of 70 mbsf and reached values as high as 24 wt%.

Physical properties measurements at Site 1129 correlate well with lithologic changes observed in the sedimentary section and provide an important data set for core-log correlation. In general, Site 1129 is characterized by significant variations in NGR and GRA bulk density. In general, variations are less marked in P-wave velocity, porosity, and other physical properties data. A well-defined cyclicity is seen in the NGR record and can be correlated with the downhole gamma-ray logs and NGR records from Site 1127 and 1131. Physical properties data were divided into three major units. Unit 1 (0–40 mbsf) is characterized by a rapid increase in NGR, as observed at most other Leg 182 sites. The base of this unit coincides with the top of the three bryozoan mound sequences, which form the lower part of lithostratigraphic Unit I. P-wave velocity and bulk density also increase with depth toward the base of this unit. Unit 2 (40–258 mbsf) is characterized by a uniformly high NGR with some cyclic variations and by a slight increase in GRA bulk density and P-wave velocity. A significant offset to higher values of P-wave velocity and bulk density can be seen at 90 mbsf in Unit 2. Unit 3 (258–558 mbsf) is characterized by increasing P-wave velocity (1650–2450 m/s) and bulk density and decreasing porosity, which reflects the increasing induration of the succession. Below 558 mbsf, core recovery was too low for physical properties characterization of the deeper part of the sedimentary section.

Hole 1129D was successfully logged with three logging-tool strings. The triple combo produced very good results, except for the interval between 360 and 460 mbsf, where the porosity log was affected by excessive borehole rugosity. The log patterns are very similar to the logs from Hole 1131A; the uranium gamma-ray log showed distinct cyclicity in the lower part of the Pleistocene sequence, with 18–20 individual cycles, each measuring 10–20 m in thickness. Chalk layers and firmgrounds in the Pleistocene succession, as indicated by the conventional logs, are clearly imaged by FMS data. The FMS data also provide a detailed picture of the thickness distribution of thinly bedded cherts (~20 cm) and carbonates (1–2 m) in the middle upper Miocene succession, where recovery was very poor.

The high-resolution site survey seismic data at Site 1129 clearly images the bryozoan mound complex within lithostratigraphic Unit I and shows that this or similar mound complexes have existed at and immediately below the shelf edge throughout the Pleistocene. A combination of the results from Sites 1129 and 1131, now correlated with the recognition of a distinctive motif on seismic data, confirms that development of extensive bryozoan mound complexes has been one of the characteristic features of Pleistocene cool-water carbonate sedimentation across a large area of the western Great Australian Bight.


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