The Eucla basin extends inland up to 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 (Figure 1); the succession gradually thickens southward to its thickest beneath the modern shelf edge (Figure 5). Onshore carbonates have been subaerially exposed since the middle Miocene, and the modern surface of the Nullarbor Plain is a vast, flat karst plateau with little vegetation.
Precambrian crystalline basement is overlain by a thin, siliciclastic, Cretaceous cover sequence (of Neocomian-Aptian, Albian-Cenomanian, and Coniacian-Campanian age) that is locally thick where it fills basement depressions (Lowry, 1970). The oldest Cenozoic unit is the thin Hampton Sandstone that is estuarine to fluvial at the base, and contains marine fossils indicating a late middle Eocene age toward the top (Benbow, 1990; Clarke et al., 1996). The Hampton Sandstone is overlain by the thick Wilson Bluff Limestone (Figure 6), a medium- to thick-bedded, white, muddy, burrowed unit of middle and late Eocene age containing abundant bryozoans, scattered echinoid tests and spines, brachiopods, bivalves, sponge spicules, planktonic foraminifers, chert, and minor glauconite (Lowry, 1970). This unit progressively onlapped farther inland, reaching its maximum extent to the margins of the Eucla Platform (Figure 1) in the late Eocene. The maximum thickness onshore is approximately 300 m in the center of the basin at the present shoreline.
The Abrakurrie Limestone (Figure 6) is a coarse-grained bryozoan calcarenite that onlaps the Wilson Bluff Limestone (James and Bone, 1991), is distinctly cyclic, and contains numerous hard-grounds (James and Bone, 1992). The Abrakurrie is up to 100 m thick at the coast, where the area of deposition is 450 km wide, and extends up to 130 km inland to a feather edge. The Abrakurrie Limestone is of middle Oligocene to early Miocene age (Li et al., 1996).
The Nullarbor Limestone (Figure 6) is a hard, fossiliferous, muddy limestone that ranges in age from late early Miocene to early middle Miocene (Lindsay and Harris, 1975; Benbow and Lindsay, 1988). This limestone has a distinctively warmer water aspect than the underlying Abrakurrie Limestone, as is reflected by ubiquitous coralline algae, numerous large benthic foraminifers, more abundant mollusks, and local concentrations of zooxanthellate corals (Lowry, 1970). The middle Miocene highstand, which resulted in deposition of the Nullarbor Limestone, was higher than the Abrakurrie Limestone highstand, and as a result the Nullarbor Limestone overlies the Abrakurrie Limestone near the coast and the Wilson Bluff Limestone inland. At the landward margin of the Eucla Platform, the Nullarbor Limestone passes into a complex mosaic of strandline terrigenous and carbonate facies (Benbow, 1990). The top of the Nullarbor Limestone is truncated by modern or Neogene erosion.
The late Miocene and early Pliocene was a period of surface and subsurface karstification of the exposed Eucla Group limestones, and extensive ferricrete and silcrete formation inland from the Eucla basin. The central part of the Nullarbor Plain near the modern shoreline is veneered with the late Pliocene (McGowran et al., 1997) Roe Calcarenite, a thin (1-2 m) limestone particularly rich in a diverse assemblage of shallow-water gastropods, bivalves, and large foraminifers (Ludbrook, 1969).