33. LATE CRETACEOUS TO PLEISTOCENE FORAMINIFERA FROM THE SOUTHEAST PACIFIC BASIN, DSDP LEG 35

During DSDP Leg 35, Sites 322, 323, 324, and 325 were drilled in the Southeast Pacific Basin, in the Antarctic region. Late Cretaceous to Pleistocene planktonic and benthonic foraminifera were investigated from the recovered sediments. The Pliocene and Pleistocene assemblages yielded a nearly monospecific planktonic fauna of Neogloboquadrina pachyderma. Only a few, probably displaced, benthonic forms were found. The late and middle Miocene sediments were barren of foraminifera. A lower bathyal to abyssal fauna of arenaceous foraminifera existed throughout the region during the early Miocene. In the shallower continental slope area (Site 325) this fauna was accompanied by a sparse assemblage of deepwater calcareous benthonic species and an isolated occurrence of Globorotalia zealandica incognita. The age assignment of this species is foraminiferal Zone N6-N7; Burdigalian. The lower boundary of this fauna could be Oligocene, inasmuch as the observed benthonic species, especially Cyclammina, range from Oligocene to Miocene. A new fauna of arenaceous foraminifera, found only at Site 323, occurs below this unit. The fauna occurs in sediments from Paleogene to Late Cretaceous age. This section of brown claystone contains a 5-meter thick layer of well-preserve d calcareous sediments. It is of early Paleocene age (Danian) from the Globigerina edita Zone and it falls in the nannoplankton Cruciplacolithus tenuis to Chiasmolithus danicus zones. The oldest sediments recovered from Site 323 were deposited during the Late Cretaceous in a fairly shallow environment. Some calcareous nannoplankton were preserved, however, the calcareous foraminifera were dissolved indicating that deposition was near the carbonate compensation depth (CCD). The Danian foraminiferal fauna indicates a depositional depth of upper to middle bathyal, at about 2000 meters. The level of the CCD must have changed considerably at this time. Another fluctuation in the CCD occurred at the end of the Danian as is represented by a zone without carbonate deposition. A hiatus probably exists between the Paleocene and ?Oligoceneearly Miocene sediments. More recent arenaceous assemblages indicate lower bathyal to abyssal conditions. The sea floor of the abyssal plain has subsided to a depth of about 3000-4000 meters, and the continental rise at Site 325 to about 2000-3000 meters. Even arenaceous benthonic foraminifera are missing from sediments younger than early Miocene, and deposition consisted almost exclusively of siliceous fossils.