The geometry of carbonate platforms is determined by the nature and robustness of the carbonate sediment “factory,” and by interactions between this factory and relative sea level fluctuations. Traditionally, most researchers have emphasized the response of carbonate platforms to sea level change as the major determinant of platform anatomy (e.g., Kendall and Schlager, 1981; Crevello et al., 1989; Loucks and Sarg, 1993). Researchers have placed considerably less emphasis on the effects of changes in the character of the carbonate production system.
Two major carbonate depositional realms apparently exist in the modern ocean; the warm-water tropical realm and the cool-water (<20°C) temperate realm (Lees and Buller, 1972). Both realms seem to have persisted throughout the Phanerozoic (James and Clarke, 1997). The warm-water realm is typified by carbonate platforms that are characteristically rimmed by reefs or skeletal sand shoals, have considerable relief above surrounding basins, contain extensive low-energy shallow subtidal facies, and are flanked by extensive tidal flats (Wilson, 1975; Tucker and Wright, 1990; James and Kendall, 1992). These warm-water sediments are composed of a photozoan association (James, 1997) of light-dependent organisms and nonskeletal precipitates that have rapid production and accumulation rates. Ramps are present in this realm, but are not as common as rimmed platforms. By contrast, the cool-water realm is typified by open shelves and ramps, with generally high-energy facies in all inner to middle ramp and shelf settings, biogenic mounds only in slope or outer ramp environments, and considerable sediment transport into deep water (James, 1997). Sediments are derived from a heterozoan association of light-independent organisms that have comparatively low production and accumulation rates.
Modern warm-water carbonates have been well studied in the Florida-Bahama Banks, Persian Gulf, and Great Barrier Reef areas. Detailed seismic imagery of the Great Bahama Bank (e.g., Eberli and Ginsburg, 1989), the southwest Florida margin (e.g., Mullins et al., 1986; Brooks and Holmes, 1989), and the northeast Australia margin (e.g., Symonds et al., 1983; P. J. Davies et al., 1988, 1989) has revealed the internal structure of modern tropical platforms. These images are proving useful in interpreting older platforms formed in warm-water situations. By contrast, there are few published high-resolution seismic images of modern cool-water carbonates, and no previously published high-quality seismic reflection data from the largest temperate carbonate shelf in the modern world—the extensive carbonate platforms along the southern margin of Australia. The purpose of this paper is to illustrate the internal geometry of this cool-water shelf in the region of the western Great Australian Bight (GAB), to interpret the seismic stratigraphy in the context of onshore geology, and to compare this platform to other modern carbonate platforms.
There is little direct information about the subsurface of the western GAB shelf and slope. The Jerboa 1 exploration hole on the Eyre Terrace (Figure 1) penetrated a thin Cenozoic succession and recovered a condensed package of Mesozoic synrift rocks (Huebner, 1980). James and von der Borch (1991) used 1979-vintage seismic data to demonstrate that the southern Australian margin is a gentle incline made up of prograding clinoforms, apparently lacking reefal buildups. They inferred that clinoform reflector patterns resulted from off-shelf sediment transport of particulate carbonate sands, together with deep-water carbonate production dominated by bryozoan and sponge growth.
The analysis presented in this paper is based on the detailed seismic stratigraphic interpretation of a 2350 km grid of high-quality, regional two-dimensional (2-D) seismic reflection lines, 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 GAB (Figure 2). 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 data set (Figure 2). Thickness and depth estimates from two-way traveltime are based on seismic stacking velocities and on proprietary company interval velocity maps (JNOC, 1992).