Mixed foraminiferal biofacies on the mesotrophic, mid-latitude Lacepede Shelf, South Australia

Abstract

The Lacepede Shelf on the southern Australian passive continental margin narrows from 200 km off the River Murray to 50 km to the southeast (the Bonney Shelf) and 70 km to the west. It is mostly deeper than 40 m. Sediments are Holocene and late Pleistocene quartzose elastics and bryozoan / mollusc carbonates. Dredged samples from four transects yield over 200 foraminiferal species. Test preservation in any single taxon ranges from fresh to brown (or relict). Plankton is rare on the shelf, but increases to 10% or more in 100 m and deeper waters. Relict benthics, representing products of previous low sealevels, are concentrated off the River Murray mouth. Mixed deeparid shallow-water assemblages are common in mid- to inner-shelf sites and also on the eastern Bonney Shelf, where there is seasonal wind-driven upwelling. A cluster analysis determined five major assemblages: I, slope and deeper water (>130 m); II, mid to outer shelf (60-130 m);III, up-welling center, "mixing A" (<82 m on Bonney Shelf); IV, "mixing B" (150-338 m on Bonney Shelf; 35-70 m on other transects); and V, shallow relict (<55 m off River Murray mouth). These mixed thanatocoenoses have been generated in the past 300 Ka during sealevel fluctuations over tens of meters. The rarity of fresh tests on the inner to middle shelves indicates a low deposition rate of recent foraminifera due to high energy, particularly wave abrasion. It is anticipated that some fossil assemblages may similarly be strongly mixed over 10 3-104 years.