The Great Australian Bight is located on the central part of the southern continental margin of Australia. The Eucla Shelf, 1300 km in lateral extent and up to 260 km wide, is the site of the bryozoan mounds (Fig. F1). This divergent passive continental margin formed by the separation of the Australian continent from the Antarctica in the mid-Cretaceous and by the subsequent northward drift (Veevers et al., 1991). This continental margin has been a province of cool-water carbonate deposition since Eocene time, overlying Mesozoic basal siliciclastic sequences (McGowran et al., 1997; Feary and James, 1998; Feary, Hine, Malone, et al., 2000). The shelf edge to upper slope of the Eucla Shelf is marked by a huge sediment wedge, consisting of prograding clinoforms of entirely Quaternary age (cf. Feary and James, 1998; Feary, Hine, Malone, et al., 2000), so the shelf margin resembles a distally steepened ramp (cf. Read, 1985). This sediment wedge is >500 m thick, giving an accumulation rate exceeding 40 cm/k.y. (Hine et al., 1999). This is equivalent to the rate of modern shallow-water tropical carbonates and twice as high as the rate of modern slope deposition on the Bahama Banks (cf. Eberli, Swart, Malone, et al., 1997).
Bryozoan reef mounds were drilled at Sites 1129, 1131, and 1132 at modern water depths of 200-350 m, which coincide with the facies transition zone from shallower coarse-grained bryozoan-rich shelf sediments to deeper fine-grained spiculitic mud (James et al., 1994, 2001). Seismic images indicate that these mounds have a depositional relief of ~50 m or more and markedly elongate features, extending to several kilometers parallel to the slope and to several hundred of meters normal to the slope (Feary and James, 1995, 1998; Feary, Hine, Malone, et al., 2000; James et al., 2000). They occur both as isolated mounds and as mound complexes that grade laterally into surrounding evenly bedded sediments. Drilling into these mounds has shown that the mounds have an unlithified floatstone to rudstone texture and are constructed primarily of a diverse suite of up to pebble-sized bryozoans (Feary, Hine, Malone, et al., 2000; James et al., 2000). All bryozoan growth forms are present in varying degrees (cf. Bone and James, 1993). The poorly sorted muddy "matrix" consists mainly of benthic and planktonic foraminifers, fecal pellets, singlets of articulated zooidal bryozoans, sponge and tunicate spicules, and calcareous nannofossils with a lime mudstone to packstone texture.
In this study, we determined the stable isotopic values of bryozoan skeletons collected from the upper parts of Holes 1129C, 1131A, and 1132B, in which the bryozoan reef mounds were situated. Site 1129 at 202 mean water depth (mwd) is located slightly down the shelf slope (Fig. F1) and includes bryozoan mound complexes in the upper 150 m of the Pleistocene succession. The upper 150 m of Hole 1129C consists mainly of bryozoan floatstone to rudstone and bioclastic packstone to grainstone, with abundant bryozoan fragments (Feary, Hine, Malone, et al., 2000) (Fig. F2). Unlithified boundstones form thick intervals (up to 10 m thick) from 41 to 62.5 meters below seafloor (mbsf) and from 72.3 to 86.3 mbsf and are also found as thin intervals (<1 m thick) from 5.9 to 19.3 mbsf and from 132.9 to 142.6 mbsf (Fig. F2). Several packets, each of which begins with basal bioclastic packstone and/or wackestone that grade upward into bryozoan boundstone, are found in this hole (Feary, Hine, Malone, et al., 2000). These single packets are ~18-41 m in thickness. Site 1131, at 332 mwd, is situated off Site 1129 on the upper slope (Fig. F1) and contains bryozoan mound complexes in the uppermost part of the Pleistocene succession (Feary, Hine, Malone, et al., 2000; James et al., 2000). The upper 40 m of Hole 1131A consists mainly of bryozoan floatstone to rudstone and massive homogeneous bioclastic packstone with a wackestone layer (Feary, Hine, Malone, et al., 2000) (Fig. F3). Some intervals (up to 4.8 m thick) of unlithified boundstones are present from 4.8 to 24.8 mbsf (Fig. F3). Site 1132, at 218 mwd, is immediately seaward of the shelf (Fig. F1). The upper 135 m of Hole 1132B chiefly comprises bryozoan floatstone to rudstone and bioclastic packstone to wackestone with abundant bryozoan fragments (Feary, Hine, Malone, et al., 2000) (Fig. F4). Unlithified boundstones are present, forming many thick intervals (up to 5 m thick) associated with some thin layers (decimeter scale), corresponding to major bryozoan mound complexes in the upper 90 m of the Pleistocene succession (Feary, Hine, Malone, et al., 2000).