BRYOZOANS AS CARBONATE SEDIMENT PRODUCERS ON THE COOL-WATER LACEPEDE SHELF, SOUTHERN AUSTRALIA

Modern sediments on the Lacepede Shelf and adjacent slope are typical mid-latitude, cool-water, palimpsest deposits, dominated by bryozoan, mollusc and quartz particles. Bryozoan bioclasts form autochthonous and, to a lesser extent. allochthonous accumulations across the grain-size spectrum, from mud to boulder size. The bryozoan skeletons are grouped, on the basis of architecture and style of disarticulation/fragmentation, into nine morphotypes. Erect rigid forms comprise (1) fenestrate, (2) foliose, (3) flat, robust branching and (4) delicate, branching morphotypes. Large Adeona sp. skeletons are recognised as a separate type of fenestrate form because of their distinctive shape, mineralogy and restricted environment. Erect flexible forms are either (5) articulated branching or (6) articulated zooidal. Other important morphotypes are (7) encrusting, (8) nodular/arborescent and (9) vagrant. Differences in skeletal architecture result in distinct particle types. Encrusting morphotypes on soft organic substrates and the wide variety of erect rigid forms are broken into fragments. Erect flexible forms disintegrate upon death into definitive sand and silt size grains. Encrusting morphotypes on hard substrates, nodular/arborescent and vagrant forms remain whole, and when large, accumulate in place. Morphotypes are mineralogically distinct and so bryozoan sediment composition is environmentally controlled. Shallow-water (< 130 m), high-energy bryozoan deposits are intermediate Mg-calcite (4-12 mol% MgCO3) and aragonite bryozoan grains. Deeper-water, outer-shelf, shelf-edge and upper slope bryozoan sediments are predominantly low to intermediate Mg-calcite in composition, with minor aragonite particles.