Cenozoic Planktonic Foraminiferal Zonation and Basinal

Abstract

Foraminiferal analyses of Cenozoic strata of Central Valley, Luzon, and southern Iloilo, Panay, indicate a general planktonic zonation for the Middle and Later Cenozoic which is similar to that recognized in equivalent marine strata of other tropical areas of the world. There is generally parallel basinal development in Central Valley, Luzon, and Iloilo, Panay, beginning in the Late Oligocene-Early Miocene interval. The cycle commenced with shelf-type conditions and orbitoidal facies giving way upward and with time to increasingly deeper-water facies of the bathyal zone. Maximum water depths of at least 1,000-2,000 meters were attained during the Miocene as indicated by Nonion barleeanus, Nonion pompilioides, Pullenia bulloides, Osangularia bengalensis, Laticarinina pauperata, and others. The parallel history in the two areas stops in the Late Miocene when Central Valley underwent prominent basin-filling and a transition to shallow-water paralic facies. In Iloilo, deep basinal conditions prevailed into the Middle Pliocene before basin-filling occurre . The more complete planktonic sequence is that of Iloilo. Late Oligocene time is suggested by Globigerina concinna ciperoensis in at least one part of the basin. Early Miocene time (Aquitanian) is represented by a lower Globigerina dissimilis zone and an upper Globoquadrina tripartite rohri zone. Middle Miocene time (Burdigalian) is indicated by a lower Globorotalia fohsi fohsi population giving way upsection to first left-coiling populations of Globorotalia menardii praemenardii and then to right-coiling populations of the same species. From bottom to top, Upper Miocene zones include a basal Globoquadrina dehiscens advena zone, a Globoquadrina altispira altispira zone (top of occurrence), a Globigerina nepenthes zone (top of occurrence), and at the top a Globoquadrina altispira globos -Globoquadrina dehiscens dehiscens zone. Lower Pliocene populations are characterized by Sphaeroidinella dehiscens dehiscens and Globigerina eggeri (dextral). Middle Pliocene populations lack these but include an abundance of Globorotalia truncatulinoides and Globorotalia menardii menardii. Pulleniatina obliquiloculata is common to abundant in the Middle and Lower Pliocene sections and rare in the uppermost Miocene. It is dominantly right-coiling in the Middle and most of the Lower Pliocene, left-coiling at the base of the Pliocene, right-coiling and rare in the uppermost Miocene, and mostly left-coiling and very rare below. Upper Pliocene and Pleistocene beds lack definitive planktonic zonation, probably because of the filling of the basins and the advent of paralic facies in the Pleistocene.