Post-glacial biota from the inner part of southwest Joseph Bonaparte Gulf

Seventeen vibrocores from the inner part of Joseph Bonaparte Gulf off north western Australia penetrate a range of marine and marginal-marine sediments deposited in the post-glacial transgression and highstand. Ranging from gr avelly sand to fine silt, these sediments contain a diverse fossil biota do minated by molluscs and bryozoans, but also including ostracods and foramin ifers. Minor components include solitary corals, echinoids, soft coral and sponge spicules. wood debris and bone fragments. The biota can be divided i nto five major marine or marginal-marine environments (intertidal. lagoonal . estuarine, strandline and shelf) and one terrestrial (riverine) environme nt. The intertidal environment contains four sub-assemblages (mangroves, sa lt marsh, mud flat and sand flat) and the shelf environment six sub-assembl ages (hard substrate inner shelf, sandy substrate inner shelf. muddy substr ate inner shelf, epiphytic, inshore and oceanic). The most useful organisms for palaeoenvironmental reconstruction are bryozoans for differentiating v arious shallow-marine substrates, and foraminifers and ostracods for defini ng wafer depths, euryhaline, freshwater and oceanic influences. Palynomorph s were the only microfossils capable of providing control on terrestrial en vironments. The scarcity of marine plankton and the dominance of terrestria l palynomorphs in these marine sediments provides a salutary warning of the dangers of relying on plant microfossils alone when no independent environ mental data are available to test the interpretation. The mollusc and bryoz oan biota in the inner part of Joseph Bonaparte Gulf superficially resemble s the bryomol assemblage of cool-water shelves. This biotic assemblage is t he result of turbidity rather than water temperature. The turbidity suppres ses the photosynthetic, zooxanthellate and hermatypic organisms allowing mo lluscs. bryozoans and other apparently cool-water biotic elements to domina te.