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Drilling at Broken Ridge and Ninetyeast Ridge during Ocean Drilling Program Leg 121 yielded thick sections of Upper Cretaceous nannofossil-bearing sediments including calcareous chalk, limestone with chert, and volcanic ash. The upper Campanian to Maestrichtian assemblages from Broken Ridge are austral in nature, containing many of the endemic Southern Ocean forms observed from the Falkland Plateau, Weddell Sea, and the Kerguelen Plateau. The absence of the Lithraphidites praequadratus-L. quadratus lineage and significant overlap of the first-appearance datum of Nephrolithus frequens and the last-appearance datum of Reinhardtites levis negate direct use of temperate zonation schemes for these sediments: the austral zonation is more effective but has lower stratigraphic resolution. The upper Campanian to Maestrichtian of Ninetyeast Ridge contains both austral and tropical forms in different horizons; the modified zonation of Sissingh (1977) is most effective in age-dating these sediments. The Turonian-Santonian calcareous nannofossil assemblages are relatively sparse and poorly preserved. The absence of diagnostic species in the Turonian-Santonian sections is due to poor preservation and the unfavorable paleoenvironment of the rapid deposition of the thick ash in this interval. Biostratigraphic resolution is significantly impaired in this interval. INTRODUCTION Broken Ridge and Ninetyeast Ridge, along with the Kerguelen Plateau, have tectonic histories derived from the history of the Kerguelen/Ninetyeast Ridge hot spot in the context of the evolution of the Indian Ocean. Broken Ridge and Kerguelen Plateau were once a conjugate oceanic platform that formed from intraplate volcanism in the Early Cretaceous or mid-Cretaceous. Ninetyeast Ridge was interpreted to be a hot-spot trace of mid-Cretaceous to Oligocene age that was produced when the Kerguelen hot spot was either under the Indian plate, at the Indian plate boundary, or under portions of the Antarctic plate that were later transferred to the Indian plate by ridge jumps (Luyendyk, 1977; Luyendyk and Rennick, 1977; Peirce, 1978; Duncan, this volume; Royer et al., this volume). Turonian to upper Maestrichtian nannofossil-bearing sediments were recovered from Sites 752-758 drilled on Broken Ridge and Ninetyeast Ridge during Ocean Drilling Program (ODP) Leg 1-21 (Fig. 1), during May-June 1988. Turonian-lowermost Campanian calcareous nannofossils were recovered at Site 755. Upper Campanian-upper Maestrichtian assemblages were recovered from Sites 752, 754, and 758. The Upper Cretaceous nannofossil assemblages from Broken Ridge and Ninetyeast Ridge are unusual and important in that they are mixtures of taxa that are characteristic of both the austral and temperate areas. This provides an opportunity for correlation of zonation schemes derived separately from both realms. The purpose of this report is to document the variations of the Cretaceous calcareous nannofossil assemblages from the Leg 121 sites and to provide a more detailed biostratigraphic zonation than that possible during the limited time for shipboard study. Numerous descriptive and stratigraphic studies have demonstrated significant paleobiogeographic effects on the distribution of Upper Cretaceous calcareous nannofossils. Worsley and Martini (1970) first recognized the provincial nature of the Maestrichtian nannofossils Micula murus (temperate) and Nephrolithus frequens ("polar"). Thierstein (1976) recognized that certain Cretaceous taxa have distinct distributions that are best explained by latitudinal preferences, leading him to define additional provincial Cretaceous taxa. Thierstein (1981) examined and quantitatively analyzed 243 upper Campanian through Maestrichtian assemblages and documented significant provinciality exhibiting strong paleolatitudinal variations. Two taxa were 1 Weissel, J., Peirce, J., Taylor, E., Alt, J., et al., 1991. Proc. ODP, Sci. Results, 121: College Station, TX (Ocean Drilling Program). 2 Dept, of Geology, University of Nebraska, Lincoln, NE 68588-0340, U.S.A. 60° 90° Figure 1. Location of Leg 121 sites. 120° E

Upper Cretaceous Calcareous Nannofossils from Broken Ridge and Ninetyeast Ridge, Indian Ocean