The offshore succession in the Great Australian Bight is divisible into two megasequences separated by a basinwide unconformity: (1) a Mesozoic [late Jurassic(?)-Cenomanian] (Stagg et al., 1990), siliciclastic-dominated, synrift to early postrift section; and (2) a Cenozoic (Paleocene to Holocene), carbonate-dominated section. The area of study for this paper is the upper, carbonate-dominated succession, comprising a sigmoidal series of sequences reaching a maximum thickness beneath the present-day outer shelf (Figure 5).
The extensive erosional unconformity at the top of the synrift and early postrift section forms an easily recognizable and mappable surface; seven unconformity-bounded seismic sequences overlie this surface (Figure 5). The ages assigned to this succession are tentative and based (1) on correlation of sequence 6B with the onshore Eucla Group (see following paragraphs); (2) on limited data from the Jerboa 1 well; (3) on the similarity in depositional style between the sequence 7 progradational wedge and Paleocene(?) to early middle Eocene progradational sequences recorded elsewhere along the Australian southern margin; and (4) on the division of the remainder of the sequences into a reasonable time-stratigraphic framework based on correlations with the Haq et al. (1987) sea level model and regional geology (Figure 6). Vibracores across the modern shelf have encountered Eucla Group limestone out to at least the middle shelf (Feary et al., 1993b; James et al., 1994), linking the eroded sea floor of the inner shelf to the seismic images.
This sequence is an east-west-oriented elongate sediment body occurring immediately seaward of a large basement high northeast of the seismic grid (Figure 7). The thickness of this wedge-shape body increases seaward to its thickest part (up to 230 m), and then abruptly downlaps onto the underlying unconformity (Figures 5, 8). In some places, a thin sediment apron is visible basinward of the clinoform front. Complex clinoform geometry (Figure 8) reflects sequential deposition of predominantly south-directed, high-frequency shelf-margin progradational wedges. A thin aggradational component at the top of the sequence represents a minor increase in accommodation space, probably resulting from sediment compaction and sag.
This relatively thin sequence (up to 195 m) underlies the present-day Eyre Terrace. Sequence 6A occurs seaward of, and at a lower elevation than, the sequence 6B carbonate shelf, but appears broadly coeval with that sequence (see Figure 5). Sequence 6A is composed of three overlapping sediment lobes (Figure 9) separated by unconformities. The lower sequence boundary onlaps landward against the sequence 7 siliciclastic wedge, and downlaps seaward onto the Cenomanian-middle Eocene unconformity. The upper sequence boundary is a prominent reflection onto which younger sequences downlap. Internal unconformities indicate that later lobes downlapped and onlapped against earlier lobes. With the exception of these sub-sequence boundaries, sequence 6A is characterized by a relatively coherent, continuous reflection character and few, generally small biogenic mounds (40-60 m thick and up to 1 km across).
Sequence 6B is the most extensive and geometrically most dramatic sequence within the offshore Cenozoic succession. It makes up the entire Cenozoic succession beneath the inner half of the present-day shelf, and correlates with the carbonate component of the Eucla Group exposed onshore (Figures 10, 11).
The sequence can be areally divided into three zones: (1) an inner shelf zone, (2) an escarpment zone, and (3) an outer shelf zone (Figure 10). The sequence overlies either crystalline basement or a relatively thin Mesozoic succession within the inner shelf zone (Figure 5). The escarpment zone is a thick (400-450 m), relatively narrow band where the top of this sequence abruptly dips from close to the present sea floor more than 250 m into the sediment pile (Figures 5, 12). The outer shelf zone is a thin (up to 170 m thick) apron that extends seaward below the present outer shelf from the base of the escarpment to near the position of the present shelf edge, where it wedges out by downlap onto the underlying unconformity. The basal sequence boundary unconformity is marked by reflections that downlap, in different parts of the basin, onto Precambrian basement, Mesozoic siliciclastics, and the sequence 7 progradational siliciclastic wedge. The upper sequence boundary in the outer shelf and escarpment zones is a dramatic onlap surface onto which sequences 2-5 onlap. Within the limits of seismic resolution, this sequence forms the sea floor of the modern inner shelf zone.
A broad clinoform structure is within the inner shelf zone, with older parts of the sequence occurring closer inshore. Clinoforms range from gently dipping, almost planar ramps in the oldest part of the sequence to more steeply dipping, oblique sigmoidal surfaces toward the escarpment. Reflections within the inner shelf zone are characteristically discontinuous and display considerable amplitude variation, contrasting with more continuous, moderate-amplitude reflections in the outer shelf zone, and high-amplitude, well-defined reflections in the escarpment zone. A combination of seismic geometry, seismic facies, and the relatively simple structure beneath the inner shelf and nearshore area permits a confident correlation between offshore sequences and the onshore stratigraphic succession (Figure 11).
The gently inclined ramp geometry of reflections within the oldest parts of sequence 6B, beneath the modern inner shelf, correlates with the Wilson Bluff Limestone onshore. These shallowly dipping reflections (gradients of less than 0.7°) indicate predominantly aggradational deposition with only a slight basinward progradational component. This portion of the sequence contains relatively few biogenic mounds; however, where present, mounds preferentially overlie higher amplitude reflections that in many cases appear to represent unconformities.
Several surfaces beneath the modern inner shelf appear to be unconformities, one of which is likely to correlate with the unconformity at the base of the Abrakurrie Limestone; however, the disruption of seismic reflectors by biogenic mounds and the wide (~25 km) seismic line spacing preclude the confident identification of any particular unconformity. On the basis of overall reflector geometry, it is likely that the Abrakurrie Limestone basal unconformity corresponds to the transition from essentially planar ramp to more oblique sigmoidal clinoform reflector geometries exhibiting more pronounced basinward progradation with more abundant mounds. A combination of the broad inner shelf (~50 km) and the relatively thin sequence (~400 ms) prevents showing seismic data illustrating this relationship in a figure.
The transition to more oblique sigmoidal geometry with significant basinward progradation culminated with the formation of the escarpment zone beneath the modern middle shelf. We correlate this transition from ramp to rimmed-platform geometry with the transition from cooler water carbonates of the Wilson Bluff and Abrakurrie limestones to warmer water carbonates of the Nullarbor Limestone (Feary and James, 1995). A zone approximately 8-10 km wide immediately landward of the escarpment top is marked by an abundance of relatively large reefs with high-amplitude capping reflections indicating more strongly cemented surfaces. As a consequence of the greater seismic reflectivity associated with these high-amplitude surfaces, there is considerable fade-out of reflections beneath this zone (marked by discontinuous, low-amplitude reflections with considerable seismic noise), and minor velocity pull-up. This reefal escarpment has been interpreted as the Great Australian Bight “Little Barrier Reef” (Feary and James, 1995). The Little Barrier Reef is best developed and steepest (2-3.5°) in central parts of the basin (e.g., Figure 12), and flattens out to a more shallow-dipping ramp (<0.6°) toward both the east and the west. The outer shelf zone is the basinward extension of this youngest part of sequence 6B, consisting of evenly spaced reflections with a minor carbonate mound component, although in one case an unusually large mound (100 m thick; 2.3 km across) extends well up into the overlying sequence.
The combined offshore and onshore components of sequence 6B thus comprise an areally extensive (some 350,000 km2) carbonate platform that developed through a large portion of the early-middle Cenozoic (over approximately 28 m.y.), but nevertheless attained a thickness of only 450-500 m.
This sequence is a small sediment wedge with restricted distribution lying at the foot of the steepest part of the progradational carbonate shelf escarpment zone in the eastern part of the area (Figures 12, 13). Sequence 5 reflections onlap against the steepest segment of the sequence 6B escarpment and downlap onto the prominent unconformity at the top of the sequence 6B outer shelf zone (Figure 5). The upper boundary of this sequence, in turn, is onlapped by reflectors of sequences 2-4 (Figure 12). In most places, the internal character of this sequence is a coherent pattern of approximately constant thickness, low-to moderate-amplitude, continuous reflections (Figure 14). On some lines, reflection relationships indicate that there were two sedimentation pulses separated by an unconformity. The earlier phase is comprised of subhorizontal reflectors, whereas the later phase is more steeply dipping and downlaps onto the early phase.
Sequence 4 is a thin interval (<160 m thick) characterized by relatively abundant biogenic mounds on many lines, occurring beneath much of the present-day outer shelf, Eyre Terrace, and uppermost slope (Figure 15). Sequence 4 is thickest in the central part of its distribution, beneath the present-day shelf edge, and has a broadly sigmoidal shape in dip section. The lower sequence boundary in the inner, landward portion is an onlap surface with reflections onlapping against the sequence 6B outer shelf zone and, where present, the sequence 5 debris apron. Sequence 4 reflections over the remainder of its distribution downlap onto the basal sequence boundary. The upper sequence boundary is onlapped by sequence 2 reflections, and both onlapped and downlapped by sequence 3 reflections. This upper sequence boundary is a high-amplitude reflection toward the landward extent of this sequence in the central part of the area. We speculate that this reflection may represent a hard-ground surface. Truncation of sequence 4 reflections toward the seaward margin of this sequence resulted from erosion during the hiatus between deposition of sequences 2 and 3. Stratal patterns indicate that initial deposition occurred along a relatively narrow zone immediately seaward of the toe of the sequence 6B outer shelf zone sediment apron. Subsequently, deposition spread landward across this outer shelf zone, and then extended both farther seaward and toward the west. The distribution of mounds is variable both along and between lines. Higher concentrations of mounds appear to define a discontinuous linear zone of elongate mound complexes that may mark a paleoshelf edge (Figure 15). The size of mounds varies from the limit of resolution (~25 m thick x 450 m across) up to 50 m thick x 1.2 km wide. Extensive coalesced mound complexes landward of the sigmoid crest are up to 55 m thick x 5 km across in dip section.
Sequence 3 is an areally extensive unit (Figure 16) underlying much of the present-day outer shelf. The sequence is thin in the west (<50 m), and reaches its maximum thickness (235 m) toward the middle of the area. Internal reflection geometry is dominantly aggradational with only a slight progradational element. The lower sequence boundary is an onlap surface against the sequence 6B escarpment at the landward edge, and a downlap surface onto the underlying sequence 4 farther seaward (Figures 5, 12). The upper sequence boundary is a marked erosional unconformity, with truncated reflections both in the upper parts of the sequence and on the seaward margin attesting to significant erosion (see Figure 17). The upper sequence boundary is also a high-amplitude reflection in the central part of the area where no erosion is apparent. As with the sequence 4 upper boundary, we speculate that this high-amplitude reflection represents a well-lithified surface.
Over much of its distribution, sequence 3 can be divided into two subsequences; however, our inability to trace the subsequence boundary into the remainder of the area precludes formal subdivision. The lower subsequence is more extensive toward the west of the basin, whereas the upper subsequence extends farther both seaward and landward in the central and eastern parts of the area, indicating an eastward shift in the locus of sedimentation during the course of sequence 3 deposition. The lower subsequence is characterized by low- to moderate-amplitude, essentially continuous aggradational reflections that feather out seaward, and contains few mounds. By contrast, extensive mound growth in the upper subsequence has resulted in markedly discontinuous reflections. Prolific biogenic mound development within the landward part of the upper subsequence in the central and eastern parts of the area has produced a coalesced mound complex 90-110 m thick and up to 30 km wide across the shelf. This complex is much thinner and more restricted toward the west, where it is composed of individual mounds ranging from 90 to 110 m thick and 1.2 to 1.5 km across toward the landward edge, compared to 30 to 45 m thick and 0.4 to 1 km across farther seaward.
This sequence is a spectacular sigmoidal unit that forms a thin succession over the outer shelf (70-90 m), reaches peak thickness at the present shelf edge (350-400 m), and thins as a wedge farther seaward beneath the modern slope (Figures 5, 17, 18). The lower sequence boundary is a concordant to low-angle downlap surface in the more landward parts of this sequence, with the exception of the reflectors that onlap against the sequence 6B escarpment or the sequence 5 debris apron. Farther seaward, onlapping and downlapping reflectors represent infilling of the eroded upper surface of underlying sequences; sequence 2 basal reflectors are generally concordant with the lower sequence boundary where there was no apparent erosion. The upper sequence boundary over the width of the present-day outer shelf, within the limits of seismic resolution, is the modern sea floor. Markedly progradational internal reflectors within the upper arm of the sigmoid, beneath the modern outer shelf, are truncated at the modern sea floor, indicating that this sequence boundary is an erosional surface. Farther seaward, the upper sequence boundary also is marked by low-angle truncated reflectors.
Reflectors in the thickest part of the sequence, beneath the modern outermost shelf, shelf edge, and uppermost slope, possess a marked sigmoidal clinoform geometry (Figure 17). Complex reflector onlap and erosional truncation patterns within this clinoform package reflect hiatus or erosional episodes; however, the density of seismic lines is insufficient to permit subsequences defined by these surfaces to be mapped around the seismic grid. Abundant mounds form a broadly linear belt on dip sections, extending from immediately seaward of the modern shelf edge to the base of the sequence further landward (Figures 5, 17); we infer that this trend marks the position of the paleoshelf edge throughout sequence 2 deposition.
Sequence 2 also contains individual mounds within the upper arm of the sigmoid, beneath the modern outer shelf. No mounds have been identified within the deeper water segment, beneath the modern slope; this lower arm of the sigmoid is characterized by continuous, regular, moderate-amplitude reflections. The stratal geometry of the sequence as a whole indicates contraction of the depositional area over the duration of sequence 2, resulting in the eventual concentration of sedimentation on the present shelf edge.
Sequence 1 is an extremely thin sequence (up to 50 m thick) that mantles deeper parts of the margin (below 150-200 m water depth) and is inferred to be wholly muddy carbonate facies; seismic pulse interference restricts any interpretation within this sequence.