The Oligocene-Miocene boundary in the South Pacific

Abstract

Qualitative and quantitative planktonic foraminiferal trends have been examined across the Oligocene-Miocene transition in three Deep Sea Drilling Project sites (DSDP 289, 208, 206) in the South Pacific ranging from the equator to temperate regions. Primary objectives were to determine main changes in planktonic foraminiferal assemblages over the Oligocene-Miocene transition. Of available planktonic foraminiferal datums in the South Pacific sequences, Globoquadrina dehiscens is the most useful for designation and correlation of the Oligocene-Miocene boundary. The stratotype Aquitanian stage (earliest Miocene) in France contains Gq. dehiscens. Although the base of the Aquitanian stratotype is marked by an unconformity, biostratigraphic data elsewhere within the basin suggest that the evolutionary appearance of Gq. dehiscens occurred close to the time of the oldest stratotypic sediments. Therefore, the evolutionary appearance of Gq. dehiscens seems to represent one of the most reliable datums for interregional correlation of the Oligocene-Miocene boundary, one that occurs within the biostratigraphic range of Globorotalia (Fohsella) kugleri and marks the boundary between zones N4A and N4B. Globigerinoides first evolved during the late Oligocene and is not coincident with the Oligocene-Miocene boundary. Unlike the Eocene-Oligocene boundary, the Oligocene-Miocene boundary is not marked by a crisis in the Oligocene planktonic foraminiferal assemblages. Most Oligocene forms continue their range upward into the early Miocene, where most are replaced by typical Neogene forms. The only important Oligocene phylogenetic lineage to be essentially eliminated during the Oligocene-Miocene transition is Catapsydrax, the last important forms of which disappeared during earliest Miocene. Evolution of Gq. dehiscens effectively heralds the beginning of major evolutionary radiations in planktonic foraminifera that led to the distinctive Neogene phylogenies. Appearance of this form was followed by evolutionary radiation of Globigerinoides into a number of species; initial evolution of Globorotalia (Globoconella) incognita, which forms the earliest ancestral form of Globoconella; evolution of the Globigerina woodi group; Sphaeroidinellopsis disjuncta (the ancestral stock of the Sphaeroidinellopsis-Sphaeroidinella lineage); Globorotalia (Fohsella) peripheroronda from Gr. (F.) kugleri (representing the continued development toward the classic Fohsella forms); and typical forms of the Dentoglobigerina stock represented by Dentoglobigerina altispira. These evolutionary radiations are reflected by a general increase in simple species diversity through early Miocene. At all three sites, species diversity is lowest in the interval near the Oligocene-Miocene boundary. Among the three sites examined, diversity is highest in warm-subtropical Site 208, because at this latitude faunas include both tropical and temperate forms. The earliest Miocene is marked by high frequencies of Gr. (F) kugleri in tropical areas and Gq. dehiscens in warm-subtropical to temperate areas.