Leg 182 Synthesis: Exposed Secrets of the Great Australian Bight

Abstract

During Ocean Drilling Program (ODP) Leg 182, nine sites were drilled across the southern Australian margin in the Great Australian Bight, with the objective of obtaining a more detailed understanding of cool-water carbonate depositional processes and global environmental change in mid-latitude settings. Drilling results provide insights into the temporal and spatial aspects of cool-water carbonate deposition in shelf edge and slope environments. Drilling showed that the spectacular prograding clinoform sequence forming the upper slope and outermost shelf of the Great Australian Bight margin was rapidly deposited during the Pleistocene. Meter-scale lithologic cycles rapidly accumulated in response to orbitally forced sea level fluctuations. Skeletal elements within these wackestone to packstone, coarsening-upward cycles consist of tunicate spicules, brown bioclasts, bryozoan fragments, and red coralline algal debris—a heterozoan assemblage typical of coolwater carbonates. The high accumulation rates, comparable to rates in warm-water carbonate environments, reflect partitioning of sedimentation between the shelf and slope as high wave energy from the Southern Ocean interacted with sea level fluctuations to generate vigorous off-shelf transport. Mound features visible on seismic reflection data on and underlying the uppermost slope throughout the Pleistocene of the central and western Great Australian Bight are in situ bryozoan reef mounds. These mounds consist of diverse suites of bryozoans, together with coralline algae, echinoid spines, and benthic foraminifers, in a mudstone to 1Feary, D.A., Hine, A.C., James, N.P., and Malone, M.J., 2004. Leg 182 synthesis: exposed secrets of the Great Australian Bight. In Hine, A.C., Feary, D.A., and Malone, M.J. (Eds.), Proc. ODP, Sci. Results, 182, 1–30 [Online]. Available from World Wide Web: . [Cited YYYYMM-DD] 2Board on Earth Sciences and Resources, National Research Council, Washington DC 20001, USA. dfeary@nas.edu 3Department of Marine Science, University of South Florida, 140 7th Avenue South, St. Petersburg FL 33711, USA. 4Department of Geological Sciences, Queens University, Kingston ON K7L 3N6, Canada. 5Integrated Ocean Drilling Program and Department of Geology and Geophysics, Texas A&M University, 1000 Discovery Drive, College Station TX 77845-9547, USA. Initial receipt: 9 April 2003 Acceptance: 12 April 2004 Web publication: 10 June 2004 Ms 182SR-017 D.A. FEARY ET AL. LEG 182 SYNTHESIS: EXPOSED SECRETS OF THE GAB 2 packstone matrix. Seismic and isotopic data indicate that mounds developed cyclically in response to glacial–interglacial productivity cycles. Increased upwelling during sea level lowstands promoted active mound growth, in contrast to the thin mud accumulations that draped inactive mounds during highstands. Such mounds have not previously been described from the “modern” ocean and provide unlithified analogs for similar features that occur in the midto late Paleozoic rock record. The interaction of high-salinity (up to 106‰) interstitial brines with abundant organic matter within the upper parts of the sedimentary succession produced high concentrations of methane (up to 50%) and hydrogen sulfide (up to 15%). This unusual chemical environment lead to extensive carbonate recrystallization and dissolution of highmagnesium calcite and the precipitation of low-magnesium calcite and dolomite. In certain environments, therefore, cool-water carbonates may be at least as diagenetically active as their warm-water counterparts. The brines probably formed in shallow evaporative pools and lagoons on the shelf during sea level lowstands, seeped into the underlying sediments, and flowed toward the upper slope. Thermodynamic considerations suggest that H2S and CH4 disseminated gas hydrates might be present within the Great Australian Bight succession.