The following interpretation of the Cenozoic sequences of the western GAB in terms of four depositional phases (Figures 6, 19) is based on the character of the seismic sequences as outlined, the global sea level model, and our current understanding (principally resulting from onshore studies) of southern Australian Cenozoic stratigraphy.
After the Cenomanian-Paleocene erosional hiatus throughout southern Australia (Stagg et al., 1990), the early Cenozoic is marked by deposition of terrigenous clastic sands in environments that range from fluvial to paralic to fully marine. We interpret the sequence 7 seaward-prograding marginal wedge as a lowstand manifestation of this event, deposited adjacent to the rift edge within accommodation space created by compaction subsidence of the Mesozoic Eyre subbasin rift succession, with sedimentation largely constrained to progradation beneath erosional base level (presumably wave base) during successive lowstands. Complex downstepping clinoform geometries within this sequence (Figure 8) almost certainly resulted from eustatic sea level fluctuations superimposed on the overall progradational trend.
We infer that the youngest sequence 7 sediments are the thin, latest early or earliest middle Eocene mudstones intersected at the base of the Tertiary succession in Jerboa 1 (Bein and Taylor, 1981; Stagg et al., 1990). We interpret this unit as a thin foredelta apron extending seaward from the toe of the clinoform front at the limit of seismic resolution that was deposited during the final sequence 7 transgression. Thus, sequence 7 sediments are of Paleocene to latest early or earliest middle Eocene age, and correlate with the Hampton Sandstone recognized onshore (where it is of middle Eocene age) and offshore in the eastern GAB (where it is called the Pidinga Formation) (Fraser and Tilbury, 1979; Stagg et al., 1990). In some places, reflectors are disrupted by volcanic intrusions that in most cases apparently are related to leaky basement faults that fed extrusions onto the top sequence 7 paleosurface. The upper sequence boundary is onlapped by overlying sequences, but shows no evidence of erosion, whereas onshore the top of this succession is locally an erosional unconformity.
The transition from a siliciclastic depositional regime to a carbonate depositional environment resulted from intrusion of oceanic waters from the west as the gulf between Australia and Antarctica continued to open. This intrusion was accompanied by a predictably dramatic change in seismic sequence character. Instead of the areally restricted sequence 7 progradational wedge, depending upon the source and rate of terrigenous sediment supply, the offshore region was subject to carbonate sedimentation over a much wider area. In shallower waters, the extensive sequence 6B carbonate platform developed, apparently coeval with the deposition of the sequence 6A lobes in deeper water (Figures 5, 9, 10).
The carbonate platform component (sequence 6B) has two distinct phases of growth. The lower phase has clear ramp geometry with biogenic mounds disrupting otherwise gently prograding clinoforms. These fit well with our understanding of cool-water carbonate ramp geometries (Ahr, 1973; Burchette and Wright, 1992) and are offshore equivalents of the cool-water Wilson Bluff and Abrakurrie limestones of the Eucla Group onshore. The upper phase has all the attributes of a flat-topped, rimmed platform (Handford and Loucks, 1993); reflectors across most of the platform are subhorizontal and terminate abruptly against a series of stacked transparent to domed reflections that form a massive zone in front of which are seaward-dipping reflections that downlap onto earlier ramp-phase strata.
Deposition of sequence 6B was largely controlled by the balance between relative sea level movements and organic growth capability as dictated by prevailing environmental conditions. Initially, deposition of the broad ramp sequence beneath the present-day innermost shelf (and probably extending some distance inland) followed a major rise in relative sea level and the incursion of normal or near-normal cool to cold marine waters. The obliquely progradational phases represented sequential episodes of relative sea level rise in a situation where there was increased growth potential, causing a progressive seaward shift in the depositional maximum. These rising sea level episodes, which together form an overall relative sea level rise, were interrupted by episodes of relative sea level fall represented by the unconformities and onlap geometries within this sequence. We interpret the formation of the steep escarpment zone, reflecting a dramatic increase in organic growth potential, to be the result of warmer marine conditions (Feary and James, 1995).
Sequence 6A is physically separated from sequence 6B and lies in deep water as a series of gently seaward-dipping reflectors forming broad lobate bodies. At Jerboa 1, this sequence consists of calcilutites and marls (Huebner, 1980) that indicate deposition in progressively deeper water upward (McGowran et al., 1997). We interpret this as a multi-lobed, deep-water slope sediment apron (Cook and Mullins, 1983; Coniglio and Dix, 1992). The zone between the two sequences was likely a bypass slope (sensu McIlreath and James, 1979) across which sediment, derived from the developing platform, moved to accumulate in a depression in front of the sequence 7 terrigenous clastic wedge.
The two factors that determined the nature and distribution of sequence 6A were the presence of an extensive carbonate platform upslope (sequence 6B), from which carbonate detritus was derived, and the existence downslope of a relatively flat-lying paleobathymetric terrace that provided a suitable depositional environment. This paleobathymetric terrace, which broadly coincides with the seaward half of the present-day Eyre Terrace, was formed by compaction and sag of the Eyre subbasin; the sequence 6A carbonate lobes were able to occupy the portion of this terrace not already infilled by the sequence 7 siliciclastic wedge (Figures 7, 9). Although the distribution of the sequence 6B carbonate platform shows that shelf deposition extended farther west, and that presumably fine material was also derived and moved offshore in that area too, the absence of a suitable paleobathymetric terrace meant that the sequence 6A lobes did not extend into that area. Furthermore, even though compaction and sag of the Mesozoic sequence initially formed the paleobathymetric terrace, the presence of individual subsequences spanning both basement highs and lows indicates that sag did not exert as close a control on sedimentation as it did for the sequence 7 siliciclastic wedge sequence. The scarcity of mounds within sequence 6A indicates that conditions for mound development in this deeper muddy environment were marginal compared to the more favorable conditions that caused greater mound growth on the sequence 6B shelf. Location of these shelf mounds on sequence or subsequence boundaries implies either that initial mound growth required firm, stable, and perhaps cemented surfaces, or that these surfaces represent times of diminished fine-sediment supply that encouraged mound development. Irrespective of where in the eustatic sea level cycle these buildups grew, they are clearly deep-water mounds; so little is known about such mounds that it is not possible at present to make more detailed inferences concerning the controls on their growth and distribution.
The tectonic tilting that uplifted the onshore portion of the Eucla basin succession and resulted in restriction of carbonate deposition to seaward of the sequence 6B escarpment zone is tentatively dated at middle Miocene (Lowry, 1970); this age is in accord with our interpretation. By uplifting the sequence 6B carbonate platform, this tilting dramatically reduced the area available for shallow-water carbonate accumulation and terminated this early phase of carbonate platform development. The seaward margin of this platform has been subject to wave erosion from the time of tilting until the present day, resulting in the present broad, essentially flat, inner continental shelf.
Tectonic tilting combined with the late middle Miocene global sea level fall (Haq et al., 1987; Figure 6) exposed middle Miocene and older innermost shelf strata. Sequences 5, 4, and 3 all accumulated seaward of the sequence 6B steep platform margin, and there is no record of them overlying any more landward parts of the platform. Sequence 5 is of very local distribution, occurs adjacent to the base of the escarpment (Figure 5), and has internal reflectors indicating upward and outward accretion from the base of the escarpment. Localization of this sequence to the central and eastern part of the area probably is a result of the steepness of the escarpment; the sequence is present only where the escarpment is steeper than 2.2-2.4°, and does not occur farther west where the escarpment progressively decreases to a low angle (<0.6°) ramp. We interpret this feature as a lowstand wedge (Figure 20) that formed as sea level fell dramatically in late middle Miocene, reaching its lowest level at the end of the middle Miocene. Such an interpretation implies that the Little Barrier Reef at that time was a shoreline cliff, subaerially exposed much like the seaward cliffs of the Nullarbor Plain are today, and subject to meteoric diagenesis. The geometry of sequence 5 is remarkably similar to the debris aprons occurring below the Pleistocene escarpments of modern reef platforms today that consist of the products of debris accumulation and lowstand reef growth (James and Ginsburg, 1979; Grammer et al., 1993).
Sequence 4 is confined to relatively deep water and is mainly stratified, but with some mound complexes. This sequence is difficult to decipher because it was not sampled in Jerboa 1, and its distribution, geometry, and stratal relationships provide few unambiguous indications of depositional conditions. The aggradational geometry and widespread distribution indicate, however, that there were no areal or vertical constraints on sediment accumulation. The presence of broad, low-relief mounds points to a return to cooler water deposition following the warm subtropical or tropical interval responsible for the Little Barrier Reef. Sequence 4 best corresponds to the late Miocene lowstands (Figures 6, 20).
Sequence 3 has a distinctive ramp geometry, abuts the Little Barrier Reef escarpment, is dominantly aggradational, and can be divided into a stratified lower portion and biogenic mound-rich upper portion. None of the mounds in this succession have the attributes of coral reefs. All of the mounds are located inboard and do not form a barrier of any sort. This cool-water ramp likely represents increased accommodation during the latest Miocene and early Pliocene relative highstands (Figure 20).
The tops of both sequence 4 and sequence 3 are truncated by a major erosional unconformity marked by abrupt reflector truncation. The location of this erosional surface in deep water implies marine erosion controlled by current flow.
This extensive late Neogene platform buries all older sequences seaward of the sequence 6B escarpment, laps down onto the outer shelf, and directly underlies the late Quaternary surficial cool-water sediment veneer (Figures 5, 17). We interpret this sequence as a cool-water deposit correlated with exposed marine late Pliocene and Pleistocene highstand deposits throughout southern Australia (Figure 6). An innermost shelf, feather-edge component of this platform is exposed along the seaward margin of the Nullarbor Plain as the Roe Calcarenite.
This late Neogene sequence probably is composed of sediments similar to those of the modern shelf and upper slope (Feary et al., 1993b; James et al., 1994). The upper surface is truncated by both the high energy of the present environment and by erosion and nondeposition during the repeated high-amplitude, short-period sea level fluctuations of the Pleistocene. These dynamics, although preventing much shelf sediment accumulation, did result in carbonate detritus being moved seaward and deposited below wave base at or beyond the shelf edge as a shelf-margin wedge. In an environment of minimal tectonic subsidence, this deposition resulted in essentially horizontal progradation of the shelf edge over most of the area (although slightly oblique progradation over the Eyre subbasin indicates that there was minor continued compaction and base-level sag). Oceanographic conditions immediately beyond the shelf edge were suitable for abundant organic growth to produce the linear trend of deep paleoshelf edge mounds that were progressively buried by the advancing progradational sediments. Occurrences of onlapping and downlapping reflections within sequence 2 resulted from minor sea level fluctuations.