24. Late Paleogene and Early Neogene Foraminifers of Deep Sea Drilling Project Site 270, Ross Sea, Antarctica

Foraminifers from the upper Oligocene, lower Miocene, and Pleistocene at Site 270 in the southern Ross Sea, Antarctica, are discussed and illustrated. The total fauna comprises 74 genera and 163 species. A small number of taxa are recycled from Cretaceous and older Paleogene sediments. Four assemblage zones are proposed, the age of informal lithologic units discussed, and potentially important hiatuses identified. Microfaunas reflect a mixing of biogeographic affinities with one element exhibiting similarities to temperate late Paleogene assemblages in New Zealand, and another representing the early cold-water assemblages of the antarctic Neogene. INTRODUCTION Rich foraminiferal faunas have long been known from the Quaternary of the Ross Sea and other parts of Antarctica (Chapman, 1916; Heron-Allen and Earland, 1922; McKnight, 1962; Pflum, 1966; Kennett, 1968; Fillon, 1974; Anderson, 1975; Osterman and Kellogg, 1979; and Ward, 1979). Studies of Tertiary microfaunas have been hampered by lack of exposures on land and paucity of stratigraphic drill holes in the Ross Sea and coastal areas near the Transantarctic Mountains. However, in the last decade, drill cores provided by such projects as the Deep Sea Drilling Project (Leg 28), Dry Valley Drilling Project, Ross Ice Shelf Project, and the McMurdo Sound Sediment and Tectonic Studies have facilitated the study of late Neogene (D'Agostino, 1980; Webb and Wrenn, 1982) and Paleogene foraminifers (Webb, 1982a, b; Webb et al., 1982, in press). Microfaunas from very limited land outcrops of upper Neogene sediments have also been investigated (Webb, 1972, 1974; Webb et al., 1984). Only at Site 270 has the upper Paleogene and lower Neogene transition been recovered. This time interval is particularly significant in that it marks the intensification of cooling in the Ross Sea (Leckie and Webb, 1983). The present study documents the faunas of this transition, from cosmopolitan and temperate Paleogene faunas to more restricted polar assemblages of the Neogene. Another important objective of this paper is to establish the foraminiferal biostratigraphy at Site 270 with an emphasis on the problems of southern high-latitude biostratigraphic control during the mid-Tertiary and lack of reliable correlation beyond Antarctica. A companion paper (Leckie and Webb, 1983) addresses our paleoenvironmental interpretations of the upper Oligocene-lower Miocene sedimentary sequence at Site 270. LITHOSTRATIGRAPHIC FRAMEWORK DSDP Site 270 is located in the south-central Ross Sea on the western flank of a broad sedimentary basin on the eastern Ross Sea continental shelf (Figs. 1, 2). The Glomar Challenger penetrated a total section of 422.5 m at Site 270. Details of the drilling operations and a coring summary are published in Volume 28 of the Initial Reports (Hayes, Frakes, et al., 1975). A sedimentary succession of 412 m overlies a metasedimentary basement complex (marble and calc-silicate gneiss) that is tentatively correlated with the early Paleozoic Koettlitz Marble of the Transantarctic Mountains (Ford, 1975) (Fig. 2). The lowermost sedimentary unit, Unit 5, rests unconformably on the basement and is a subaerial breccia of supposed Tertiary age. Two thin, upper Oligocene shallow marine sandstones, a carbonaceous quartz sand (Unit 4), and a glauconitic greensand