西太平洋晚第三纪钙质超微化石及其化石带的划分

NEOGENE CALCAREOUS NANNOFOSSILS FROMWESTERN PACIFIC OCEAN AND DIVISIONOF NANNOFOSSIL ZONATION

本文研究了菲律宾海东、北部“大洋钻探工程”125航次782A相786A二个钻孔晚第三纪的钙质超微化石。根据Okada和Bukry分带。本区自下而上划分为13个带(或亚带)。讨论了渐新世和中新世、中新世和上新世及第三纪和第四纪的界线。本区存在3个沉积间断:晚渐新世与中中新世之间:中中新世与晚中新世之间以及晚中新世与早上新世之间。修正了Cyclicargolithus abiseclus的时限分布,提出了 Cycliargolithus floridanus floridanus和cychcargoluhas flondanus abisectus是Cyclicargolithus floridanus的2个生态亚种。

The study area named Benin basin is located in the north of Philippine sea. The Benin area is made up of a complex series of arcs and basin formed since the start of westward subduction of Pacific lithosphere in the Eocene. This paper addresses the nannofossil stratigraphy of Miocene - Pliocene sediments, and these materials were collected from two holes, 782A and 786A, Ocean Drilling Program, Leg 125. More than 200 samples have been examined by light microscopy, and 10 critical samples have been selected for detailed study using scanning electron microscopy. Based on Okada and Bukry's zonation, 13 zones (or subzones) can be divided, and Zones CH1- 3, Zone CN8 and Subzone CN10a are missing. This indicate, therefore, that three hiatuses were detected as follows: between upper Oligocene/middle Miocene, within late Miocene, and between late Miocene/early Pliocene. The end of the acme in the occurrence of Cyclicargolithus abisectus marks the subzone CN1a/CN 1b boundary according to Bukry's (1973, 1975) zonation. During the present study, Cyclicargolithus abisectus attains its acme in zone CP19. but then continue to be common until subzone CN5a. Common as well as some samples of subzone CH5a, it final-ly disappear with Cyclicargolithus floridanus near the top of this subzone. Reticulofenestra bisecta for the subzone COl9b/CN1a boundary according to Bukry's zonation, and the LO of this taxon was located at normally C6C of the geomagnetic polarity time scale (23.7 Ma) based on Berggren et al. (1985). But the LO of Reticulofenestra bisecta was observed on the top of nomaly C6C (23. 5 Ma) in the present study, higher than that of Berggren et al. (1985) and higher than the LO of Sphenolithus ciperoensis. As a result, the LO of the species may be used to recognize the subzone CN1a/CN1b boundary. Therefore, we believe that the LO of Reticulofenestra bisecta is probably a reasonably nannofossil marker to ap-proximate the Oligocene/Miocene boundary based on the present study and other updated information. Cyclicargolithus floridunus is very similar to Cyclicargolithiu abisectus in terms of structure under either the light microscope or SEM, and the main difference between the two species is probably due to size. In addition, the two species also have similar long range distribution, and extend through subzone CN5a. The stratigraphic data presented here strongly suggest that Cyclicargolithus abisectus is a large-sized variant of Cyclicar-golithus. Therefore, it could possibly be an ecologic variant of Cyclicargolithus floridanus.