Table T5. Cenozoic calcareous nannoplankton occurrences in the Medford corehole, Leg 174AXS. (See table note.)

33.0 Heliolithus riedelii (common) and Discoaster mohleri (rare), occurrences that indicate Zone NN8. Other taxa include,, Chiasmolithus bidens, C. consuetus, Fasciculithus tympaniformis, Neochiastozygus junctus, Toweius eminens, and Zygodiscus plectopons. Braarudosphaera spp. and Micrantholithus spp. are unusually common.
41.0 An assemblage dominated by the pentatiths of Braarudosphaera spp. and Micrantholithus spp. The occurrence of D. mohleri indicates Zone NP7. However, the assemblage is little diversified; in particular species of Heliolithus are absent. The zonal assignment based on a single sample is thus tentative.
48.0 An assemblage very similar to that at 41.0 m. The co-occurrence of Fasciculithus tympaniformis, Heliolithus kleinpelli, and Discoaster mohleri allow a confident assignment to Zone NP7.
55.5–60.1 A high diversity assemblage very different from those at 41.0 and above, with few pentaliths (of Braarudosphaera bigelowii) and with Chiasmolithus danicus, Cruciplacolithus tenuis, and Ellipsollithus macellus, indicative of the lower part of Zone NN4.
60.6 Very rare, poorly preserved coccoliths with Chiasmolithus danicus and Cruciplacolithus danicus, which confer to this level a minimum NP3 zonal age. The possibility that this level belongs to lower Zone NP4 as well cannot be excluded. Cretaceous coccoliths are reworked at this level.
72.0 Abundant Cretaceous species with Arkhangelskiella cymbiformis, Eiffeilithus turriseiffelii, Micula spp., Watznaueria barnesae, and rare Danian species. The occurrence of Biantholithus sparsus and Cruciplacolithus primus indicate Subzone NP1b species are being mixed into this interval.

Stratigraphic interpretation
48.0–55.5 There is a clear stratigraphic gap between Zone NP7 and lower Zone NP4. The sampling is insufficient to resolve the hiatus which may be ~3 m.y.
57.0–72.0 The sampling is also insufficient to resolve the stratigraphic succession where one or several stratigraphic gaps are suspected.

48–55.5 The marked change in the composition of the assemblages may be related to shallowing. Although the ecology of pentalith-bearing species is not well established, they seem to be more common in shallower waters. The sedimentary gap may be associated with shallowing.

Notes: The upper three stratigraphic levels (3.0, 6.6, and 11.0 ft) were barren, probably because of secondary silicification. Level 60.6 ft contained only very rare coccoliths. All other stratigraphic levels contained common to abundant, well to moderately well preserved calcareous nannofossils.