Occurrence and abundance of diatoms at DSDP Holes 72-515B and 72-516F

Samples from the upper Oligocene and lower Miocene of Holes 515B (Brazil Basin) and 516F (Rio Grande Rise) were examined for fossil marine diatom content. The preservation of the diatoms was poor and the species diversity low in both holes. However, it was possible to zone portions of the intervals studied using the zonation proposed by Gombos and Ciesielski (1983), which is based, as far as possible, on common and robust species. Thus, the interval in Hole 515B represented by Cores 515B-15 and 515B-16 is assigned to the Coscinodiscus rhombicus Zone and the interval represented by Cores 515B-17 through 515B-44 is assigned to the Rocella gelida Zone. The C. rhombicus Zone is early Miocene in age and the R. gelida Zone is late Oligocene to early Miocene in age. In Hole 516F the interval represented by Cores 516F-6 through 516F-10 is assigned to the R. gelida Zone Gate Oligocene to early Miocene), and the interval represented by Cores 516F-11 through 516F-15 is assigned to the Triceratium groningensis Zone (late Oligocene). Two new fossil diatom taxa are defined herein: Coscinodiscus lewisianus Greville f. concavus n. f. and Rocella semigelida n. sp.

Silicoflagellate abundance in ODP Holes 183-1138A and 183-1140A

The biostratigraphic distribution and abundance of lower Oligocene and Miocene to Pleistocene silicoflagellates are documented from Ocean Drilling Program Leg 183 Holes 1138A and 1140A, on the Kerguelen Plateau. The Distephanus speculum speculum forma pseudofibula plexus is found in the upper Miocene in Hole 1138A, but other important biostratigraphic markers are not available. Diversity and abundance of silicoflagellates vary considerably in Hole 1138A, with silicoflagellates more abundant in the Pliocene and Pleistocene and some intervals of the Miocene barren of silicoflagellates or containing only limited numbers of specimens. The silicoflagellates of Hole 1140A include a new skeletal morphology, described here as Distephanus speculum speculum forma cylindrus. Silicoflagellates were generally abundant throughout the lower and middle Miocene in Hole 1140A.