Cenozoic nannofossils from the Exmouth Plateau, eastern Indian Ocean

Six sites (759-764) were drilled on the Exmouth Plateau during Ocean Drilling Program Leg 122. Nannofossilrich Cenozoic sediments were recovered at all six sites, reflecting the open-ocean conditions that prevailed over the Exmouth Plateau during the Cenozoic. Calcareous nannofossils are abundant, diverse (250 different species identified), and generally well preserved throughout the composite lower Paleocene to Quaternary section. The diversity and preservation of nannofossils permits a high degree of stratigraphic resolution at each site. Site 762 on the central part of the Exmouth Plateau contains an almost unbroken Cenozoic record (only Miocene Zones NN3, NN8, and NN10 are missing). This site may prove to be a useful Cenozoic biostratigraphic and biomagnetochronologic reference section for the eastern Indian Ocean.

Calcareous nannofossils in ODP Sites 135-840 and 135-841

Ocean Drilling Program Leg 135 drilled at two sites on the Tonga Ridge. Calcareous nannofossils recovered at Site 840 on the Tonga Ridge date the sedimentary sequence as late Pleistocene or Holocene (CN15) through late Miocene (CN9) in age. A hiatus occurs in the mid Pliocene. Site 841 in the Tonga Trench yielded a sedimentary sequence with nannofossils from middle Pleistocene Subzone CN14b through the middle or late Eocene Subzones CP14a-CP15b overlying a rhyolitic volcanic basement. Part of the Eocene interval contains the shallow-water nannofossil taxa Braarudosphaera, Micrantholithus, and Pemma. A major unconformity separates lower Oligocene Zone CP 16 from lower middle Miocene Zone CN4 strata.

Paleogene calcareous nannofossil stratigraphy of ODP Leg 120 sites

Calcareous nannofossils are abundant in the Paleogene sediments recovered during Ocean Drilling Program Leg 120. Although no continuous Paleogene section was obtained, Sites 747 through 751 complemented each other so as to provide a virtually complete composite stratigraphic section. The calcareous nannofossil biostratigraphy at Sites 747, 748, and 749 is discussed. Correlation of calcareous nannofossil biozones and magnetozones at these sites suggests some diachrony with low-latitude areas, as well as on a regional basis. Changes in calcareous nannofossil diversity throughout the Paleogene are analyzed and interpreted as reflecting major paleoclimatic events.

Calcareous nannofossil stratigraphy and datums of sediments from ODP Leg 198 sites

A relatively complete lower Paleocene to lower Oligocene sequence was recovered from the Southern High of Shatsky Rise at Sites 1209, 1210, and 1211. The sequence consists of nannofossil ooze and clay-rich nannofossil ooze. Samples from these sites have been the target of intensive calcareous nannofossil biostratigraphic investigations. Calcareous nannofossils are moderately preserved in most of the recovered sequence, which extends from nannofossil Zones CP1 to CP16. Most traditional zonal markers are present; however, the rarity and poor preservation of key species in the uppermost Paleocene and lower Eocene inhibits zonal subdivision of part of this sequence.

(Appendix) Range chart of calcareous nannofossils in ODP Hole 129-802A

Only Site 802 has recorded appreciable Cenozoic carbonate sediments during Ocean Drilling Program Leg 129 in the central Mariana Basin of the western Pacific Ocean. Calcareous nannofossils provide the best biostratigraphic information for the 360-m Tertiary section, which consists primarily of volcaniclastic turbidites interbedded with calcareous claystone and chalk. Many samples contain significant amounts of nannofossils reworked from older sediments. An unconformity appears to be present between Cores 129-802A-32R and -33R, with upper Oligocene-lower Miocene sediments above and lower Eocene-upper Paleocene sediments below the unconformity. The sediments below the unconformity contain abundant reworked Cretaceous nannofossils. Only one sample from Site 801 yielded nannofossils, and those consist of a mixture of Campanian-Maastrichtian and Paleogene forms.

Abundance of calcareous nannofossils across the K/T boundary at ODP Hole 113-690C (Table 1)

A biostratigraphically continuous, but intensely bioturbated, Cretaceous/Tertiary boundary sequence was cored during Ocean Drilling Program (ODP) Leg 113 on Maud Rise (65°S) in the Weddell Sea off East Antarctica. This interval is the first recovered by ODP/DSDP in the South Atlantic sector of the Southern Ocean and offers a unique opportunity to study the nannofossil sequences leading up to and beyond the terminal Cretaceous event at a high southern latitude. The K/T boundary lies just within Chron 29R and is placed at ODP Sample 113-690C-15X-4, 41.5 cm. An iridium anomaly was independently noted at about this level as well. Upper Maestrichtian-lower Paleocene sediments consist mostly of light-colored nannofossil chalks. Dark brown sediments at the base of the Danian (Zone CPla) are characterized by an increased clay content attributed to a drop in calcareous microplankton productivity following the terminal Cretaceous event. Although delineation of the boundary is hampered by intense bioturbation, the sharp color contrast between overlying clay-rich, dark brown chalks of the Tertiary and light cream colored chalks of the Cretaceous aids in the selection of the K/T horizon. Several dark colored burrows sampled at intervals as far as 1.3 m below the boundary and within the light colored Cretaceous chalk were found to contain up to 17% Tertiary nannofossils. Calcareous nannofossils from the boundary interval were divided into three groups for quantitative study. The three groups, "Cretaceous," "Tertiary," and "Survivor," exhibit a sequential change across the boundary with the Cretaceous forms giving way to a Survivor-dominated assemblage beginning at the boundary followed shortly thereafter by the appearance of the Tertiary taxa, Cruciplacolithus and Hornibrookina. The species, H. edwardsii, comprises nearly 50% of the assemblage just above the Zone CPla/CPlb boundary, an abundance not reported elsewhere at this level. Calculation of individual species abundances reveals several additional differences between this K/T boundary interval and those studied from middle and low latitude sections. The percentage of Thoracosphaera is much lower at the boundary in this section and a small form, Prediscosphaera stoveri, is extremely abundant in Cretaceous sediments just below the boundary.