With the exception of the short interval in the latest early Miocene-early middle Miocene, the post-Paleocene faunal record across Australia’s southern margin indicates a prevalence of cool to subtropical surface water conditions (James and Bone, 1991; McGowran et al., 1997) corresponding to sea surface temperatures of generally less than 20°C. Although there is also faunal evidence for intermittent intervals of warm subtropical to tropical conditions during the Oligocene and late Quaternary (Shafik, 1992; Almond et al., 1993), these warmer intervals apparently resulted from northward movement of the Subtropical Convergence Zone, and possibly also involved increased flow of warm Leeuwin Current waters into the western GAB; however, the brevity of these intervals has meant that they had little impact on the overall sedimentary record.
Based on correlation with the onshore sequence, we infer that the escarpment zone containing reefs is the offshore equivalent of the Nullarbor Limestone. The escarpment was deposited in the global warm period during the latest early Miocene-early middle Miocene (Savin et al., 1985). It is unclear whether input of a warm Leeuwin Current watermass accentuated the effects of global warming in the western GAB. We suggest that these reefs are warm subtropical or tropical reefs that grew close to sea level in waters of perhaps 18-22°C.
With the documentation of the Eucla Shelf presented here, there are now seismically imaged examples of four major Cenozoic carbonate platforms: the Great Bahama Bank, West Florida Shelf, northeast Australia margin, and Eucla Shelf. Each platform has its own individual characteristics that prevent its definition as an ideal example (the Great Bahama Bank on a leeward margin, the erosive currents of the West Florida Shelf, the northward drift from cooler to warmer depositional realms of northeast Australian platforms, and tectonism in the midst of growth of the Eucla Shelf). Nevertheless, they respectively represent carbonate platform deposition in situations that are wholly tropical, subtropical, warm-water with an initial cool-water phase, and cool-water with a warm-water phase. The differing seismic geometries within these platforms provide insights into the variability of response to the different factors controlling platform evolution in a range of temperature regimes.
The leeward margin of the Great Bahama Bank is a thick accumulation of markedly prograding seismic sequences (Eberli and Ginsburg, 1987, 1989), but of a different character than the prograding clinoforms of the Eucla basin. Great Bahama Bank sequences, beginning in the late Oligocene, are either (1) simple sigmoid sequences that have no obvious reef structures and are thought to be continuous slopes of platform-derived sediments extending into the basin (i.e., ramps) or (2) complex sigmoid-oblique sequences that are interpreted as a platform margin of reefs or carbonate sands separating subhorizontal lagoonal and steeply dipping forereef facies. Eberli and Ginsburg (1989) suggested that these geometries characterize warm-water highstands (complex sequence) and low-stands (simple sequence) in this tropical area. These correspond to the same sequences that we interpret as cool-water ramp and warm-water reef platform, respectively, illustrating the difficulties in classifying the nature of carbonate platforms on geometry alone.
The latest middle Miocene is represented by a strong reflector interpreted as an exposure surface. Earlier Oligocene-Miocene sequences are aggradational and progradational, grading upward from simple sequences to complex sequences. The late Miocene-Holocene sequences, although still progradational, have a greater aggradational component than underlying sequences, implying increased accommodation space. This is opposite to the Eucla Shelf situation, illustrating the strong local control of relative sea level as a function of the relationship between subsidence and rates of sediment production.
The West Florida Shelf, located in a subtropical setting with incursions of fresh water from the Mississippi and periodic current-induced upwelling, is characterized by ramplike architecture throughout the Cenozoic (Mullins et al., 1988; Gardulski et al., 1991). This platform was affected by large-scale gravity collapse in the latest early Miocene, followed by rapid infilling by prograding clinoforms. Similar to the Eucla Shelf, but on a smaller scale, the early middle Miocene is characterized by the presence of local carbonate reefs occurring sporadically along the outer shelf, perhaps attesting to global oceanic warming during this highstand, as well as to local effects. Deposition during and following the latest middle Miocene was dramatically affected by strong flow of the Loop Current, as a consequence of oceanic closure of the central American seaway. Sea level fall in the late Miocene resulted in a widespread karst unconformity, which is now overlain by Pliocene-Pleistocene slope-front-fill clinoforms.
The history of the West Florida Shelf particularly illustrates the importance of oceanographic effects on platform deposition because the intensification of current flow resulted in the transformation from prograding clinoform deposition fundamentally controlled by sea level fluctuations to current-controlled pelagic ramp deposition. We interpret the erosional truncation of Eucla basin sequences 3 and 4 reflectors during the Pliocene to be the result of a similar along-slope current; however, in this case the oceanographic effect was short lived, and later Pliocene-Pleistocene deposition was again fundamentally controlled by sea level fluctuations.
Northeast Australia Platforms
Analysis of extensive seismic data over the carbonate platforms of northeast Australia enabled the identification of rifting, subsidence, plate motion and collision, and paleo-oceanographic and sea level fluctuations as the factors controlling the development of these platforms (P. J. Davies et al., 1989). Ocean Drilling Program drill holes on the Great Barrier Reef margin (Davies et al., 1991) resulted in a more detailed understanding of the factors controlling Pleistocene progradation and aggradation on this margin. Feary et al. (1993a) showed that sequence geometry at the Great Barrier Reef shelf edge was controlled by the interaction between sediment supply (controlled by formation of the outer barrier reef) and depositional base-level variations (controlled by sea level fluctuations), with the progradational phase reflecting relatively high rates of sediment supply from the warm-water, tropical carbonate shelf. Although probably representing a considerably greater time interval, the shelf-edge component of the Pliocene-Pleistocene sequence 2 in the Eucla basin succession displays remarkably similar progradational clinoform geometry. We similarly speculate that this geometry was controlled by the interaction between eustatic sea level fluctuations (acting on a stable, essentially nonsubsiding shelf margin) and sediment supply derived from the extensive cool-water carbonate shelf of the GAB. The similarities between these two areas indicate that water temperature is not one of the factors controlling shelf-edge depositional geometry, although with the proviso that rates of deposition may vary considerably between warm- and cool-water realms.
Earlier studies of cool-water carbonates have been largely on the microscale or mesoscale (Nelson, 1988; Boreen and James; 1995; James and Clarke, 1997) and focusing on sediment composition, facies dynamics, and local sequence analysis. This study provides the macroscale perspective for these sediments that is so important if they are to be recognized elsewhere, and if they are to be useful for interpreting ancient limestones. This analysis of the Eucla Shelf platform indicates that seawater temperature and sea level are fundamental in determining the nature of any carbonate platform. The attributes most affected are geometry of the internal packages and rates at which the structure grew. The images of cool-water sequences from the Eucla Shelf are remarkably similar to the numerous illustrated examples of carbonate ramps from the rock record (Burchette and Wright, 1992). These similar geometries indicate that Cenozoic cool-water ramps are good analogs for many ramps in the older rock record, but with the caveat that not all ancient ramps are cool-water in origin.