Carbonate sediments and sedimentary rocks often contain a particularly sensitive record of paleoceanographic and biostratigraphic evolution. Before Leg 182, drilling on carbonate platforms had concentrated on tropical and subtropical environments. In contrast, Leg 182 investigated continental margin cool-water carbonates. These biogenic sediments, formed where seawater temperatures rarely rise above 20°C, commonly mantle continental margins in middle and high latitudes and are untapped storehouses of information regarding the evolution of global climates, eustacy, and marine biology.
The southern Australian continental margin reaches its greatest width in the Great Australian Bight (Fig. F1), making it an ideal location to study cool-water carbonate facies and evolution. The Great Australian Bight has been the site of cool-water carbonate sedimentation since Eocene time, resulting in an almost 1-km-thick succession, and it is now the largest area on the globe composed of such sediments. In addition, slight tectonic tilting during the late middle Miocene caused subaerial exposure of Eocene-middle Miocene strata in an extensive shallow basin. Although these sediments form a more compressed and less continuous section than the section offshore, prior to this drilling leg they provided the only basis for development of actualistic models for the formation and development of cool-water carbonates. Leg 182 drilling in the Great Australian Bight provided essential and original information to contrast the sedimentologic, paleontologic, paleoceanographic, and climatic records between warm- and cool-water realms. Postcruise analysis will also allow the development of well-constrained models that can be used in the interpretation of older Mesozoic and Paleozoic continental margin carbonate systems, as well as an isotopic and biostratigraphic record of Southern Ocean development.
The southern margin of the Australian continent is a divergent, passive continental margin that formed during the protracted period of extension and rifting that led to the separation of Australia and Antarctica in the Cretaceous and evolved during the subsequent northward drift of the Australian continent. The initial extension phase before breakup in the mid-Cretaceous (96 Ma), together with the subsequent period of slow spreading until the middle Eocene (49 Ma), resulted in deep continental margin basins filled with as much as 12 km of mainly terrigenous clastic sediments (Willcox et al., 1988; Davies et al., 1989). These basins broadly correspond to the sites of modern upper slope terraces (e.g., the Eyre Terrace at 400-1600 m depth in the western Great Australian Bight; Fig. F1). The onset of faster spreading in the middle Eocene also corresponded with the establishment of fully marine conditions and the initiation of carbonate sedimentation in the widening "gulf" between Australia and Antarctica. Carbonate sedimentation continued throughout the remainder of the Cenozoic as the gulf evolved first into a broad, open seaway and then into the modern Southern Ocean. Ce-nozoic sedimentation resulted in an extensive, relatively thin (almost 1 km thick) (Feary and James, 1998, reprinted as Chap. 2) Eucla Basin succession deposited in a predominantly platform-sag to platform-edge tectonic regime (Stagg et al., 1990).
Throughout the Cenozoic, the western Great Australian Bight portion of Australia's southern continental margin has been particularly stable. Interpretation of geohistory data from the Jerboa-1 exploration well indicates minimal Tertiary subsidence (Hegarty et al., 1988). Slight regional tilting (<1°?) during the late middle Miocene resulted in uplift and exposure of the Nullarbor Plain and restriction of Neogene sedimentation to the modern outer shelf and upper slope.
The Eucla Basin extends inland as far as 350 km from the present coastline and seaward some 200 km to the modern shelf edge and upper slope. The Eucla Basin succession thins and feathers out inland against Precambrian basement and gradually thickens southward to its thickest point beneath the modern shelf edge (Fig. F2). The Eucla Basin succession is entirely carbonate dominated, apart from the basal siliciclastic sequence both offshore (Sequence 7) and onshore (Hampton Sandstone), and a thin, transgressive, paleovalley-filling and strandline succession of terrigenous clastics on the inland margins of the basin.
The succession is basically divisible into two mega-sequences: a Mesozoic (Upper? Jurassic-Cenomanian; Stagg et al., 1990), siliciclastic-dominated synrift to early postrift section, and a Cenozoic (Eocene-Holocene) carbonate-dominated section. These two sections are separated by a major basinwide unconformity. The main focus of Leg 182 is the upper succession of sediments that forms an overall sigmoid-shaped series of sequences reaching a maximum thickness beneath the present-day outer shelf (Fig. F2). The stratigraphy of the lower, Mesozoic succession can be derived from the sequence intersected in the Jerboa-1 exploration well (Fig. F1); however, little information on the upper, Cenozoic section was obtained from this hole.
The extensive erosional unconformity at the top of the synrift section forms an easily recognizable and mappable surface. Seven unconformity-bounded seismic sequences have been recognized overlying this unconformity (Fig. F2) (Feary and James, 1998, reprinted as Chap. 2). One of the most striking elements of this seismic stratigraphic analysis is the identification of numerous mound-shaped structures, interpreted as biogenic mounds (Feary and James, 1995), that are present throughout the Cenozoic succession. These structures may preserve a detailed record of cool-water faunal community relationships and potentially could provide an analog for cool-water mounds recognized in the rock record, but for which no modern analogs have been identified.
The ages assigned to this succession were extremely tentative prior to drilling and are based on (1) correlation of Sequence 6B with the onshore Eucla Group (Fig. F2), (2) the similarity in depositional style between the Sequence 7 progradational wedge and Paleocene?-lower Eocene progradational sequences elsewhere along Australia's southern margin, and (3) the division of the remainder of the sequences into a reasonable time-stratigraphic framework. On this basis, the offshore sequences can be placed in the following seismic stratigraphic framework (based on Feary and James, 1998; reprinted as Chap. 2):
- Sequence 7: Paleocene-middle Eocene progradational siliciclastic wedge deposited in a depositional sag, representing initial transgressive sedimentation;
- Sequence 6A: middle upper Eocene to lower middle Miocene deep-water carbonates forming a multilobed sediment apron, broadly contemporaneous with Sequence 6B;
- Sequence 6B: cool-water ramp carbonates with biogenic mounds (middle upper Eocene-Oligocene), passing up into an upper, warm-water, flat-topped platform rimmed by the lower? middle Miocene "Little Barrier Reef" (Feary and James, 1995);
- Sequence 5: small upper middle Miocene lowstand sediment wedge with restricted distribution, lying at the foot of the steepest part of the progradational carbonate shelf escarpment zone;
- Sequence 4: extensive upper Miocene aggradational deep-water carbonate ramp sequence;
- Sequence 3: uppermost Miocene and lower Pliocene highstand aggradational deep-water carbonate ramp sequence;
- Sequence 2: thick succession of highstand Pliocene-Quaternary cool-water carbonates with spectacular clinoform ramp geometry that forms most of the modern outer shelf and contains large deep-water biogenic mounds; and
- Sequence 1: thin uppermost Quaternary deep-water drape.
Before Leg 182 drilling, knowledge of the western Great Australian Bight margin was based on extensive high-quality seismic reflection data and a single oil exploration drill hole. Together, these provided little information about the Cenozoic succession. The original Leg 182 drilling proposals (Feary et al., 1994, 1995) were based on detailed seismic stratigraphic interpretation (Feary and James, 1998, reprinted as Chap. 2) of a grid of 2350 km of high-quality, regional two-dimensional (2-D) seismic reflection lines. These lines were collected and processed by the Japan National Oil Corporation (JNOC) in 1990 and 1991 over an area of 155,000 km2 on the continental shelf and upper slope of the western Great Australian Bight. An additional 1380 km of moderate-quality, regional 2-D seismic lines, collected by Esso Australia in 1979 and reprocessed by JNOC, were also used to fill gaps in the JNOC dataset. The 1996 seismic site survey cruise (Feary, 1995) collected high-resolution, 80-channel generator-injector gun seismic data as 0.5-nmi-spaced grids centered on each site, together with tie-lines between sites (e.g., Fig. F30 in the "Site 1126" chapter). These data permitted minor refinements of some site locations to avoid potential safety concerns.
The Jerboa-1 exploration well was drilled by Esso/Hematite in 1980 as a wildcat oil exploration well in a water depth of 761 m above a prominent tilted basement fault block located in the southern half-graben of the Eyre Sub-basin (Bein and Taylor, 1981) (Fig. F1). Jerboa-1 penetrated 1738 m of Cenozoic and Cretaceous sedimentary section before bottoming in Precambrian metabasalt basement, and did not encounter any significant hydrocarbon shows. The top 232 m of the hole was washed down and cased so that only 145 m of Tertiary section was actually drilled and logged. No cores were cut in this interval, so lithologic and biostratigraphic inferences are based on cuttings and downhole logs. Thermal modeling and vitrinite data (Stagg et al., 1990) indicate that the entire sedimentary section at Jerboa-1 is thermally immature (Rv = <0.65%).