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 an d, to a lesser extent. allochthonous accumulations across the grain-si ze spectrum, from mud to boulder size. The bryozoan skeletons are grou ped, on the basis of architecture and style of disarticulation/fragmen tation, into nine morphotypes. Erect rigid forms comprise (1) fenestra te, (2) foliose, (3) flat, robust branching and (4) delicate, branchin g morphotypes. Large Adeona sp. skeletons are recognised as a separate type of fenestrate form because of their distinctive shape, mineralog y and restricted environment. Erect flexible forms are either (5) arti culated branching or (6) articulated zooidal. Other important morphoty pes are (7) encrusting, (8) nodular/arborescent and (9) vagrant. Diffe rences in skeletal architecture result in distinct particle types. Enc rusting morphotypes on soft organic substrates and the wide variety of erect rigid forms are broken into fragments. Erect flexible forms dis integrate upon death into definitive sand and silt size grains. Encrus ting morphotypes on hard substrates, nodular/arborescent and vagrant f orms remain whole, and when large, accumulate in place. Morphotypes ar e mineralogically distinct and so bryozoan sediment composition is env ironmentally 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 i n composition, with minor aragonite particles.