Upper Cretaceous calcareous nannofossils from Broken Ridge and Ninetyeast Ridge

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.