Valanginian to Barremian Benthic Foraminifera from ODP Site 766 (Leg 123, Indian Ocean)

A Valanginian to Barremian bathyal foraminiferal assemblage (118 taxa belonging to 51 genera) is documented from ODP Site 766, drilled near the foot of the Exmouth Plateau off northwest Australia. The majority of taxa are cosmopolitan species previously described from the boreal assemblages of northern Europe and the northern margins of the Tethys, but 32 taxa could not be assigned to previously described species and are left in open nomenclature. The chronostratigraphy of this section is based upon nannofossils, palynomorphs, and radiolarians. The foraminiferal succession is divided into four assemblages based on the first and last occurrences of characteristic benthic foraminifera: 1) An upper Valanginian Lenticulina ouachensis Textularia bettenstaedti assemblage with Ammodiscus tenuissimus, Bulbobaculites sp., and Aaptotoichus clavellatus. The last occurrences of these taxa are observed near the transition between the Valanginian and Hauterivian. 2) A lower Hauterivian assemblage is characterized by Lenticulina heiermanni and Saracenaria forticosta. 3) An upper Hauterivian assemblage contains Planularia crepidularis, Patellina subcretacea, and Reinholdella hofkeri. A major faunal discontinuity is observed at the top of this assemblage between Cores 766A-32R and 766A-30R, probably associated with a depositional gap between the late Hauterivian and the Barremian. 4) The overlying Barremian assemblage is characterized by Gavelinella barremiana, Glomospira spp., and Pseudogaudryinella sp. This record enables long-distance comparisons of the stratigraphic ranges of benthic foraminiferal species in the austral bioprovince with their reported ranges in the North Atlantic region and the northern Tethys. Although the assemblages at Site 766 consist of ca. 74% cosmopolitan species, they differ from Tethyan assemblages in the rarity or absence of index taxa such as Praedorothia spp., Lenticulina nodosa and L. eichenbergi, as well as ornamented, palmate morphotypes such as Citharina spp., Flabellina spp., and Frondicularia spp. They differ from the typical boreal assemblages by the lack of diverse agglutinated taxa. As a result, benthic foraminiferal zonations established for the Lower Cretaceous of the northern Tethys and Tethyan DSDP sites could not be applied at Site 766. Taxonomic and biostratigraphic differences at Site 766 support the existence of a taxonomically distinct Early Cretaceous austral bioprovince. INTRODUCTION The biogeographic distribution of various marine organisms in Lower Cretaceous sequences has enabled paleontologists to recognize the existence of two main bioprovinces during Early Cretaceous time: a boreal bioprovince in high northern latitudes and a Tethyan bioprovince in low tropical latitudes. The presence of a third main bioprovince in the southern hemisphere the austral bioprovince, which was the southern hemisphere equivalent to the cool boreal bioprovince is, however, rarely mentioned in the literature as research has traditionally focused on boreal and tropical assemblages. For a long time, the Cretaceous foraminiferal microfaunas from Australia and New Zealand were viewed as unusual and endemic. Even now, relatively little is known about the microfaunas from the austral bioprovince and about their relationship to those of the other two bioprovinces during the Early Cretaceous. The recovery of Lower Cretaceous sedimentary sequences from ODP sites in the Indian Ocean has, fortunately, provided new material for studying the distribution of foraminifera in the high southern latitudes. At ODP Site 766 diverse, well-preserved, bathyal assemblages have been recovered from sediments as old as Valanginian, in contrast to the predominantly marginal assemblages previously described from epeiric basins in Australia and the Indian Ocean. The foraminifera from ODP Site 766, therefore, offer a unique insight into the composition of deep-water benthic assemblages during the Early Cretaceous. These assemblages also provide the opportunity to retrace the evolution of one of the oldest continental margins in the world from its early rifting phase early in the Cretaceous, when the supercontinent Gondwana fragmented and the Indian Ocean started to open, to its mature position in the Albian at the edge of a vast ocean. The main objective of this work is to conduct a detailed taxonomic and biostratigraphic survey of the lesser-known Valanginian to Barremian benthic foraminifera at Site 766 and to compare them with better-documented assemblages from the Tethyan and boreal bioprovinces. Benthic foraminifera are particularly useful for stratigraphy during this interval because planktonic species are absent. They are also important for paleogeographic and paleoceanographic reconstructions and for worldwide stratigraphic correlations (e.g. Bartenstein 1979).