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 D.A. FEARY ET AL. LEG 182 SYNTHESIS: EXPOSED SECRETS OF THE GAB 2packstone 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 mid-to 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 H 2 S and CH 4 disseminated gas hydrates might be present within the Great Australian Bight succession.

Origin of the Late Neogene Roe Plains and their calcarenite veneer: implications for sedimentology and tectonics in the Great Australian Bight

“…Images further reveal that the shelf comprises two sectors: (i) an inner part, composed of seismic sequences 6 and 7 (depositional phases A and B, Eocene -Middle Miocene), that has been differentially uplifted and is now subject to erosion; and (ii) an outer part, formed of seismic sequences 1 -5 (depositional phases C and D) that is accretionary and post-Middle Miocene in age ( Figure 3). Basic sequence stratigraphy of the upper slope has been confirmed by ODP drilling during Leg 182 (Feary et al 2000(Feary et al , 2004.…”

Global synchronous changes in the carbon isotopic composition of carbonate sediments unrelated to changes in the global carbon cycle

“…Although changes in the percentage of aragonite arise both as a result of variations in the origin of the sediment (oceanic as opposed to platform-derived) and the diagenetic conversion of aragonite to calcite, which normally takes places during marine burial (25,26), it is suggested that the variations in the ␦ 13 C of the sediments mainly result from the admixing of platform and oceanic-derived sediments. The conclusion, that there is an absence of a diagenetic influence on the ␦ 13 C, stems from the fact that all of the sediments are dominated by calcium carbonate (Ͼ90%) and the geochemical pore fluid profiles at these locations are controlled by diffusion (27)(28)(29)(30). These sediment-pore water systems can therefore be considered to act as closed systems.…”

“…Although this hypothesis cannot be proven at all of the sites, it has been established by using seismic data that the western margin of GBB has prograded toward the west Ϸ20 km, over the past [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25]. At the other locations, where studies have been less detailed, it is clear based on the sediment description that there is a decrease in the proportion of shallow water-derived sediments with increasing depth (28,30), a change consistent with a change in sea level and proximity to the shallow-water source. A further inf luencing factor may be that the change to more positive ␦ 13 C values in the periplatform sediments corresponds approximately to the onset of Milankovitch-induced glaciations.…”

“…The proportion of aragonite and calcite in the sediments was determined by x-ray diffraction using a model that assumed that the sample was composed only of aragonite, LMC, high-Mg calcite, and dolomite (58). The ages of the samples were calculated by using published age vs. depth relationships (27)(28)(29)(30).…”

Cenozoic post-breakup compressional deformation and exhumation of the southern Australian margin

“…1). The basin is largely unexplored, with only 13 wells drilled during offshore hydrocarbon exploration, though Ocean Drilling Program (ODP) Leg 182 drilled 27 holes at 9 sites near the Jerboa-1 well in the western part of the basin (Feary et al, 2000). It comprises a series of depocentres which include the Eyre, Duntroon, Ceduna and Recherche sub-basins, that formed during middle-late Jurassic to early Cretaceous N-S to NW-SE directed extension (Totterdell and Bradshaw, 2004).…”