Silicoflagellates are siliceous marine planktonic microorganisms which have never been a significant contributor to pelagic sediments. Consequently only limited investigations have been made on these microorganic remains from the deep-sea deposits of the world.

Eleven taxa and one new species of silicoflagellates were recovered from the central North Pacific cores collected above the RV Thomas G. Thompson in 1968. The lower lithologic unit of a gravity core, TT28-25, was assigned a Miocene age after careful comparison of previous worldwide records. This is the first time that such a Miocene assemblage has been recognized from mid-latitude North Pacific sediments. From other cores, Quaternary assemblages were noted, including a limited occurrence of Mesocena cf. elliptica. The most recent extinction of this species seems to coincide with the last appearances of Dictyocha cf. ausonia and a proposed new species, D. subarctios. Their stratigraphic distribution was examined critically in connection with the results of paleomagnetic-reversal recor s and data on other microfossils from the area.

It is suggested that silicoflagellates could become a useful tool for deep-sea biostratigraphy in the middle latitudes of the North Pacific Ocean.

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