Abstract:
Bryozoan reef-mounds are characteristic features occurring within mid-ramp, slope paleoenvironments in the geological record, especially in Paleozoic carbonate sequences, but until now have not been recorded from the modern ocean. Recent scientific drilling in the Great Australian Bight (ODP Leg 182) has confirmed the existence of shallow subsurface bryozoan reef-mounds in water depths of 200-350 m. These structures, with up to 65 m of synoptic relief, occur both as single mounds and as mound complexes. They are unlithified, have a floatstone/rudstone texture, and are rich in flat-robust branching, fenestrate, delicate branching, arborescent, and articulated zooidal bryozoan growth forms. The muddy matrix is composed of foraminifers, serpulids, fecal pellets, irregular bioclasts, sponge spicules and calcareous nannofossils. 14C AMS dates indicate that the most recent mounds, whose tops are 7-10 m below the modern seafloor, flourished during the last glacial lowstand but perished during transgression. This history reflects changing oceanographic current patterns; strong upwelling during lowstands, reduced upwelling and lowered trophic resources during highstands. Large benthic foraminifers restricted to the mounds confirm overall mesotrophic growth conditions. The mounds are similar in geometry, scale, general composition and paleoenvironments to older structures, but lack obvious microbial influence and extensive synsedimentary cementation. Such differences reflect either short-term local conditions or long-term temporal changes in ocean chemistry and biology.