Foraminiferal biofacies on the mid-latitude Lincoln Shelf, South Australia: oceanographic and sedimentological implications

Abstract

Foraminiferal assemblages on the southern mid-latitude Lincoln Shelf comprise mixtures, more strongly in shallow waters, of Holocene and relict Pleistocene specimens. Over 200 benthic and 15 planktonic species were recorded, and a higher diversity was found in the central and deeper parts of the shelf and slope. Cluster analysis identified five assemblages or biofacies. Nearshore assemblage A (<50 m) and inner mid-shelf assemblage B (50-90 m) are dominated by relict and Recent shallow-water species. The outer mid-shelf assemblage C (90-120 m) contains more cibicidids and is transitional between the shallow- and deep-water assemblages. Outer shelf assemblage D (120-170 m) is characterized by cibicidids and anomalinids and by a higher planktonic and benthic diversity, and it can be subdivided into three sub-assemblages D1-D3. Unlike others, assemblage D was not recognized from the most western transect samples. The outer shelf to slope assemblage E (170-400 m) is typically deep-water, having forms like Hoeglundina elegans, Pullenia bulloides and Melonis affinis. The western, Great Australian Bight sector has the larger epifaunal benthic Sorites-Marginopora group, while the infauna is higher in the eastern, Neptune sector of the shelf. We attribute these contrasts to the influence of the warm Leeuwin Current and to the mixing between gulf, shelf and oceanic waters, respectively. A former lagoonal environment is largely responsible for the accumulation of relict tests during lower sea-level periods of the late Pleistocene when climate was more arid. The lack of Recent sediments preserved on the inner shelf is considered to be due to strong wave abrasion causing sediment starvation. Richer foraminiferal assemblages from the outer shelf to upper slope parallel an increasingly calcareous sediment. Faunal evidence indicates a warmer, nearly oligotrophic condition on Lincoln Shelf, compared to the mesotrophy on the adjacent Lacepede Shelf. This difference may be due to the separation of these two shelves by Kangaroo Island acting as a local oceanographic (environmental) and biogeographic barrier. © 1996 Elsevier Science B.V. All rights reserved.