Tectonic Setting

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 that 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. 1). 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. Cenozoic sedimentation resulted in an extensive, relatively thin (up to 800 m; Feary and James, 1998) 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, with geohistory analysis of the Jerboa-1 exploration well indicating minimal Tertiary subsidence (Hegarty et al., 1988). Slight regional tilting (<1°?) during the middle Miocene resulted in uplift and exposure of the Nullarbor Plain and restriction of Neogene sedimentation to the modern outer shelf and upper slope.

Cenozoic Stratigraphy of the Eucla Basin

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. Inland, the Eucla Basin succession thins and feathers out against Precambrian basement; it then gradually thickens southward to its thickest point beneath the modern shelf edge (Fig. 2). The Eucla Basin succession is entirely carbonate, 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 (Late Jurassic?—Cenomanian; Stagg et al., 1990) siliciclastic-dominated syn- to early postrift section, and a Cenozoic (Paleocene—Holocene) predominantly carbonate-dominated section. These two sections are separated by a major, basinwide unconformity. The subject of the bulk of this drilling leg is the upper succession, which makes up an overall sigmoid-shaped series of sequences reaching a maximum thickness beneath the present-day outer shelf (Fig. 2). The stratigraphy of the lower, Mesozoic succession can be derived from the sequence intersected in the Jerboa-1 exploration well (Fig. 1); 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. 2) (Feary and James, 1998). 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 are likely to preserve a detailed record of cool-water faunal community relationships and potentially to provide an analog for cool-water mounds recognized in the rock record, but for which no modern analogs have previously been identified.

The ages assigned to this succession precruise were extremely tentative and are based on (1) correlation of Sequence 6B with the onshore Eucla Group (Fig. 2); (2) the similarity in depositional style between the Sequence 7 progradational wedge and Paleocene?—early 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 stratigraphic framework (based on Feary and James, 1998):


Sequence 7: Paleocene—middle Eocene progradational siliciclastic wedge deposited in a depositional sag, representing initial transgressive sedimentation;

Sequence 6A: middle-late Eocene to early-middle Miocene deep-water carbonates forming a multilobed sediment apron;

Sequence 6B: cool-water ramp carbonates with biogenic mounds (middle-late Eocene to Oligocene), overlain by an upper, warm-water, flat-topped platform rimmed by the early?-middle Miocene "Little Barrier Reef" (Feary and James, 1995);

Sequence 5: small late-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 late Miocene aggradational deep-water carbonate ramp sequence;

Sequence 3: latest Miocene and early Pliocene highstand aggradational deep-water carbonate ramp sequence;

Sequence 2: thick succession of highstand, Pliocene—Pleistocene, 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 Quaternary deep-water drape.


Existing Data

Present knowledge of the western Great Australian Bight margin is based on extensive, high-quality seismic reflection data, together with a single oil exploration drill hole, which provide little information about the Cenozoic succession. The original Leg 182 drilling proposals (James and Feary, 1993; Feary et al., 1994) were based on detailed seismic stratigraphic interpretation (Feary and James, 1998) of a grid of 2350 km of high-quality, regional 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 (GI) gun seismic data as 0.5-nmi-spaced grids centered on each site, together with tie-lines between sites. These data permitted minor refinements of some site locations to avoid potential safety concerns.

Jerboa-1 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). Jerboa-1 penetrated 1738 m of a Cenozoic and Cretaceous sedimentary section before bottoming in Precambrian metabasalt basement and did not encounter any significant hydrocarbon shows. The top 232 m 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%).



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