ABSTRACT
Sediments recovered in the Great Australian Bight during Leg 182 record carbonate deposition in a middle- and high-latitude setting against the background of an evolving Southern Ocean and northward drift of the Australian continent. Approximately 3.5 km of sediments was recovered from nine sites, in water depths ranging from 202 to 3875 m. Most drilling took place on the shelf edge and upper slope, in 202–784 m water depth, through a mainly carbonate succession.
Two distinct groups of strata, Eocene–middle Miocene and Pleistocene in age, form the upper Cenozoic component of the continental margin, separated by a thin, upper middle Miocene–Pliocene interval characterized by slumps, sediment gravity-flow deposits, or unconformities. Such erosion, corrosion, and/or mass wasting and redeposition processes reflect periods of margin instability, seismicity, or lowered sea level.
The older succession consists of Eocene shallow-water terrigenous sands and carbonates that deepen upward into Oligocene and early-middle Miocene pelagic ooze and chalk. The carbonate component was poorly recovered because of irregular selective silicification, with available cored material indicating a sequence dominated by nannofossil chalk with abundant sponge spicules and characterized by stained hardgrounds and numerous omission surfaces. The high-quality downhole logs collected through all poorly recovered intervals will enable more detailed postcruise lithological analysis for this part of the succession. In contrast, the middle Eocene–early Oligocene succession was well recovered on the continental rise at Site 1128 in 3875 m water depth, and it contains an excellent expanded (>350 m) siliceous biogenic record of proto Circum-Antarctic Current evolution.
The younger Pleistocene package is a spectacular, extraordinarily thick (> 500 m), seaward-dipping wedge of carbonate sediment on the shelf-edge/uppermost slope that prograded seaward onto the Eyre Terrace and downlaps onto older sediments. Rates of accumulation exceed 40 cm/k.y., equivalent to many shallow-water tropical carbonates and twice the rate of Bahamian slope sedimentation. These green and gray carbonate strata are surprisingly uniform in composition, made up of fine carbonate sand and silt composed of skeletal fragments, mainly delicate bryozoans, ostracodes, benthic and planktonic foraminifer tests, tunicate sclerites, nannofossils, and siliceous sponge spicules. The facies transition upslope into shallower water is marked by the presence of numerous spectacular bryozoan-rich buildups. These mounds, in water depths of ~200–350 m, are characterized by numerous and diverse bryozoans, with intervening bioclastic sand and mud. These are among the first modern analogs of similar mounds that were an important component of carbonate depositional regimes in earlier Phanerozoic time.
One of the surprising discoveries of Leg 182 was the presence of a brine, varying in salinity between 80 and 105, within and underlying seven drill sites. The Cl– distribution at three sites (1127, 1129, and 1131) suggests that the top of the brine has an essentially uniform depth below sea level and, therefore, crosscuts sequence boundaries. Although the origin of the brine has not yet been established, pore fluids in the Pleistocene portion of the sediments from the shallow water sites possess a Na+/Cl– ratio greater than that of seawater, suggesting that fluids in these sediments were involved in the dissolution of NaCl. It is likely that further postcruise study of the nature and distribution of this brine will shed light on lowstand shelf evaporative processes and the nature of fluid flow within cool-water carbonate margin sediments. Because of high sedimentation rates and their location at the edge of a broad continental shelf, the shallow water sites initially contained a high concentration of organic material. The brine underlying and within the Pleistocene succession contains up to three times normal sulfate concentrations and, with sufficient organic material, was therefore able to produce significant amounts of hydrogen sulfide. In addition, the relatively low iron concentration in these carbonate sediments means that the H2S was not sequestered as iron sulfides. Consequently, H2S concentrations were able to reach high levels, >150,000 ppm at one site. The oxidation of organic material also had an important influence on carbonate recrystallization, which is occurring at higher rates than previously thought possible for cool-water carbonates.