English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Reinterpretation of the biostratigraphic record from Hole 462, drilled in the Nauru Basin during Leg 61, emphasizes the importance of larger foraminifers in dating heavily redeposited sequences. Younger ages derived from larger foraminifers are supported by the occurrence of some planktonic marker species (foraminifers and radiolarians) previously interpreted as downhole contaminants. A major implication of the new younger ages is that the main erosional events previously dated at 37 Ma, 32 Ma, 26 Ma, and 13 Ma now have an age of 30 Ma, 28 Ma, 26 Ma or younger, and 12 Ma, respectively. Erosional events at the end of the Eocene and within the early Oligocene can no longer be documented. INTRODUCTION Two holes were drilled at Site 462 in the Nauru Basin during Leg 61 in 1978 by the Glomar Challenger. Hole 462A was reentered four years later in 1982 during Leg 89 in order to reach deeper horizons within the basaltic complex. The aim of this chapter is to reinterpret the biostratigraphic record from Hole 462, which was strongly biased by severe reworking and displacements of almost all the age-diagnostic fossil groups (Larson, Schlanger, et al., 1981), in light of a synthesis of the available data from both holes. DISCUSSION The sedimentary sequence recovered at Site 462 (Holes 462 and 462A) overlying the basaltic complex is characterized by pelagic clay and more rarely by radiolarian ooze, the autochthonous sediments, which alternate with carbonate-rich layers. Pelagic clay is dominant in the lower portion of the sequence from 550 to 450 m sub-bottom (Cores 462-59 to 462-48), then it decreases upwards. In the Cenozoic, from 370 m to the top of the hole (Cores 462-39 to 462-1), the amount of pelagic clay and/or radiolarian ooze is about 10% of the total thickness. All the fossiliferous components, except the deep-dwelling agglutinated benthic foraminifers and possibly some radiolarians, have been transported. As a result, chronostratigraphy was jeopardized by reworking (Premoli Silva and Sliter, 1981; Premoli Silva and Violanti, 1981; Sanfilippo et al., 1981; Thierstein and Manivit, 1981). In the absence of age-diagnostic fossils from the autochthonous layers, the sediments were dated on the basis of the youngest components occurring in the various resedimented layers. However, under such conditions it is possible that even the youngest elements of a given fauna and/or flora could be not only displaced but also reMoberly, R., Schlanger, S. O., et al., Init. Repts. DSDP, 89: Washington (U.S. Govt. Printing Office). 2 Address: Dipartimento di Scienze della Terra, Universita di Milano, Via Mangiagalli, 34, 20133, Milan, Italy. worked. Consequently, the resulting dates could be much older than the actual age of deposition. Ages reported in the Site 462 report for Hole 462 samples and in several other chapters as well (Larson, Schlanger, et al., 1981) were based mainly on planktonic foraminifers and calcareous nannofossils. Discrepancies in the ages inferred from the different fossil groups are striking (see tables 4 and 5 in Larson, Schlanger, et al., 1981, pp. 52-53). Further discrepancies are evident if one compares the age inferred from the planktonics to those ages suggested by the shallow-water larger foraminifers (Premoli Silva and Brusa, 1981). Most of the larger foraminifers are known to possess a high level of biostratigraphic resolution. Evolutionary lineages have been recognized within numerous, rapidly evolving genera. Moreover, each step within a lineage is identified as a species. Lineages are well dated by detailed quantitative studies and calibrated against stratotypes (i. e., the Lepidorbitoides lineage). In some cases, their occurrence could be correlated with the planktonic foraminiferal zonation schemes (van Gorsel, 1978; Adams, 1967; Schaub, 1951, 1981; Brönnimann, 1954; Haak and Postuma, 1975). Most of the species have a short stratigraphic range, and are diagnostic of specific time intervals. Furthermore, correctly identified specimens of a given evolutionary stage are age-diagnostic, even when recovered from a noncontinuous sequence. Thus the ages inferred from most of the larger foraminifers are as reliable as those based on planktonic foraminifers, once the calibration of a lineage is established. At Site 462, although the shallow-water larger foraminifers are displaced in deeper water as are the calcareous plankton and most of the radiolarians, the taxa found give younger minimum ages than did previous age assignments that used planktonic organisms. The younger ages, based on the occurrence of some larger foraminifers from Hole 462, are as follows (from bottom to top) (see Fig. 1): • Cores 462-52 and 462-51 contain numerous specimens of Vaughanina cubensis, which is Maestrichtian species according to Brönnimann (1954). Following Brönnimann^ interpretation, Pseudorbitoides israelskyi among

A New Biostratigraphic Interpretation of the Sedimentary Record Recovered at Site 462, Leg 61, Nauru Basin, Western Equatorial Pacific