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Cores of rocks ranging in age from Pleistocene to latest Jurassic from 17 sites in the northwest Pacific Ocean have yielded new information on the biostratigraphic relations of foraminifers, radiolarians, and nannoplankton. Areas sampled by the Glomar Challenger on leg 6 of the Deep Sea Drilling Project were the Horizon Ridge (Guyot), the Shatsky Plateau and surrounding abyssal floor, the Philippine Sea, and the Caroline Ridge.
Calcareous nannofossils are present in most of the recovered cores, including cores of abyssal brown clay, and are the most commonly preserved of the 3 groups. Diverse foraminiferal assemblages are associated with calcareous rocks of all ages whereas radiolarians are commonly poorly preserved or absent in highly calcareous strata. Radiolarians are more markedly affected by biogeographic variability than are the calcareous microfossils; biostratigraphy developed in equatorial regions cannot be applied in the Shatsky Plateau region (north of 30°). However, volcanic ash-rich areas of the Philippine Sea produce unusual Miocene nannoplankton similar to those in ash-rich sediments from the Caribbean, and foraminifers are sparse or absent and commonly small in size.
Biostratigraphic boundaries determined by the 3 microfossil groups are generally in accord. Stage and epoch boundaries based on nannoplankton may occur higher in cores than determinations based on Foraminifera and Radiolaria. The various effects of ecology, preservation, and drilling techniques on the development of zonal scheme based on the 3 major groups of microfossils require careful evaluation.
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