Cretaceous Planktonic Foraminifers and Depositional Environments from the Ontong Java Plateau with Emphasis on Sites

Cretaceous planktonic foraminifers from Ocean Drilling Program (ODP) Leg 130 consist of two age groups: one dominantly Aptian in age and the other dominantly Maastrichtian. The oldest dated sediments from Ontong Java Plateau occur at ODP Site 807 and are assigned to the early Aptian Globigerinelloides blowi Zone. The assemblage correlates closely with Aptian foraminifers from Deep Sea Drilling Project (DSDP) Site 289 and a younger Aptian assemblage from DSDP Site 288 where drilling ended before reaching basement. Planktonic foraminifers from ODP Site 803 suggest a late Aptian to early Albian age and are overlain by radiolarians of late Albian age. These ages are in conflict with a radiometric age of about 90 Ma for basement at the site. The dominantly Maastrichtian group occurs at Site 807 and consists of poorly preserved, mostly small-sized specimens that range in age from perhaps the latest Campanian to the Cretaceous/Tertiary boundary. Age assignments are made difficult by pervasive dissolution and resedimentation that extends into the Paleogene. Deposition of the Aptian limestone at Site 807 took place at lower bathyal depths beneath the foraminiferal lysocline but above the calcite compensation depth (CCD). Deposition then continued beneath the CCD until the late Campanian or Maastrichtian when carbonate sedimentation resumed. Within the Maastrichtian, deposition again took place at lower bathyal depths at or above the rapidly dropping CCD until the Cretaceous/Tertiary boundary when the CCD apparently rose. Small planktonic foraminifers accumulated in winnowed and redeposited fine-grained sediments whereas larger planktonic foraminifers were deposited largely in mass-flow deposits that increase in number upsection through the Maastrichtian. The increased pulses of larger planktonic foraminifers followed by the abrupt occurrence of laminated sediments, an abundance of small, globular planktonic foraminifers, and an increase in radiolarians in the latest Maastrichtian likely were the result of northward plate motion into more productive waters and different current systems. At Site 803, deposition beneath the CCD continued from the earliest sediments above the basalt until the Eocene. Carbonate sedimentation during the late Albian to early Santonian, however, is recorded atop the plateau at Site 288. The extensive hiatuses in the Cretaceous sequence from the Ontong Java Plateau correlate in part with times of widespread hiatuses in the Pacific Basin. Evidence of pervasive mechanical erosion during the Cenomanian to Santonian and the Campanian to Maastrichtian on the top of the plateau perhaps were caused by the longitudinal position of the plateau relative to western boundary currents and eddy effects above the plateau. INTRODUCTION Cretaceous planktonic foraminifers were recovered from Sites 803 and 807 drilled on the northern part of the Ontong Java Plateau (Fig. 1) during Ocean Drilling Program (ODP) Leg 130. These and similar planktonic foraminifers described from Sites 288 and 289 of Deep Sea Drilling Project (DSDP) Leg 30 (Sliter, 1992) occur in the sedimentary sequence that directly overlies basement and thus provide a minimum age for the cessation of volcanism and a means to interpret the initial depositional environment. The timing of volcanism that formed the immense Ontong Java Plateau (>1.5 × 10 km) recently has come under scrutiny as it pertains to both the plume initiation model of large oceanic basalt plateaus (Richards et al., 1991) and the eruptive rate of the oceanic flood basalt events that apparently occurred within a few million years (Tarduno et al., 1991; Whitechurch et al., 1992). In addition, the massive and sudden outpouring of submarine basalts during the emplacement of large oceanic plateaus may be linked to global changes in sea level and climate (Caldeira and Rampino, 1990,1991; Arthur etal., 1990; Larson, 1991a, 1991b; Tarduno etal., 1991).Thus, establishing the temporal record of volcanism at Ontong Java Plateau, the world's largest oceanic plateau, offers perhaps the best means to 1 Berger, W.H., Kroenke, L.W., Mayer, L.A., et al., 1993. Proc. ODP, Sci. Results, 130: College Station, TX (Ocean Drilling Program). 2 U.S. Geological Survey, 345 Middlefield Road MS 915, Menlo Park, CA 94025, U.S.A. 3 Department of Geology and Geography, University of Massachusetts, Morrill Science Center, Amherst, MA 01003, U.S.A. investigate the environmental consequences of a major submarine volcanic event. The Cretaceous foraminifers and associated biogenic material also provide a means to assess the initial depositional environment on the Ontong Java Plateau. Two aspects are of particular interest. First, an estimation of the water depth of the earliest sediments following the cessation of volcanism is needed. This is important in determining the preand post-emplacement vertical motion history of the plateau. Second, and equally important, the sediments record a rare glimpse of southern mid-latitude paleoceanographic conditions in the Cretaceous Pacific Ocean as the plateau moved northward with the Pacific Plate from a paleolatitude of about 40°S in the Aptian, to about 17°S in the Maastrichtian (Gordon, 1990). Here we present the Cretaceous planktonic foraminiferal biostratigraphy and environmental interpretation for Leg 130. Results from Sites 803 and 807 are compared with those from a reexamination of samples from DSDP Sites 288 and 289. Primary emphasis is placed on the oldest samples from the four sites and the younger section from Site 807. Stratigraphic distributions of Cretaceous foraminifers in Hole 807C are shown in Tables 1 and 2, selected species are illustrated in Plates 1-4, and a faunal reference list is given in the Appendix. METHODS A total of 91 samples from Leg 130 were examined in thin section in addition to a few isolated specimens that were derived from crushed limestone or washed samples. In addition, seven samples from upper Aptian limestone of Site 289 were crushed and examined for whole specimens of planktonic foraminifers. Samples for thin sections were selected randomly throughout most of the cored intervals. Most of the