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A preliminary biostratigraphic zonation of Site 558 (Holes 558 and 558A), lower Oligocene through Pleistocene, and Site 563, lower Oligocene through upper-middle Miocene, drilled on the west flank of the Mid-Atlantic Ridge is discussed. At Site 558 two holes were drilled by a combination of piston and rotary coring through a stratigraphic sequence from Oligocene through Pleistocene. Except for a gap of approximately 26 m in the upper-middle Miocene where Holes 558 and 558A do not overlap, the Cenozoic section recovered is the most complete and best preserved stratigraphic succession from this part of the Atlantic. At Site 563 one hole was drilled. Sediments were washed down to 156.5 m and then rotary cored to basement. The lower Oligocene through upper-middle Miocene sediments recovered are similar to those drilled at Site 558 both in lithology and fauna. The biostratigraphic succession at both sites is relatively undisturbed; no major gaps or reworking were noted and sedimentation accumulation rates are similar. Notably Bolboforma, a tiny spherical calcareous incertae sedis (algae?), was recovered at both sites at the same stratigraphic horizon in the Miocene. This form may have potential as a regional marker in deep-sea sediments. INTRODUCTION Sites 556 through 564 (ten holes) were drilled during Leg 82 in an area on the west flank of the Mid-Atlantic Ridge and southwest of the Azores Platform (Fig. 1). The primary objectives of drilling in this area were to sample ocean crust in order to study mantle heterogeneity. The paleontologic objectives were to confirm basement ages and to recover a complete section of Cenozoic pelagic sediments from this part of the Atlantic and, if possible, establish a biostratigraphic zonation and reference section. The biostratigraphic objectives were achieved; a nearly complete section of the lower Oligocene through Pleistocene was recovered through a combination of hydraulic piston coring and rotary coring in two holes at Site 558 (Fig. 2). Before this cruise, study of the sedimentation, stratigraphy, paleoenvironment, and geologic history of the area between 20-40°N in the mid-Atlantic was limited and based on spot cores and partial sections. On Leg 2 (Blow, 1970), spot cores of the Cretaceous to Pleistocene were taken at Sites 10 and 11. On Leg 37 (Miles, 1977), Sites 334, upper Miocene calcareous pelagic ooze and chalk were cored and, at Site 335, spot cores were taken through 450 m of Pleistocene through middle Miocene sediments. On Leg 49 (Poore, 1978), at Sites 411, 412, and 413, only Quaternary sediments were recovered (no coring was done at Site 414). In order to fill the gap in sampling and to provide data for study of the Cenozoic history and paleoenvironment of the area, it was decided that continuous coring Bougault, H., Cande, S. C , et al., Init. Repts. DSDP, 82: Washington (US. Govt. Printing Office) 2 Present address: Curtis and Echols, Geological Consultants, 800 Anderson, Houston, Texas 77401. should be done at as many sites as possible during Leg 82. Of the nine sites drilled, Sites 558 and 563, located close to Anomaly 13, were chosen for continuous coring. Sediments recovered at these sites are lower Oligocene to Pleistocene and indicate that during this interval the sites were above the calcite compensation depth (CCD). Site 558 is located at 37°46.2'N latitude and 37° 20.61' W longitude near the Pico Fracture Zone between Anomalies 13 and 15 on the flank of a basement high in water depth of 3754 m. Holes 558 and 558A were drilled at this site. Hole 558 was washed down to a depth of 158 m and sediments were continuously cored to a total penetration above basement of 408 m. After the first hole was successfully logged, Hole 558A was piston cored to a depth of 132 m, when mechanical problems in the inner core barrel forced termination before biostratigraphic overlap was accomplished. This gap of 26.5 m represents approximately 2 to 4 Ma of sedimentation (See Fig. 2 which shows the cored intervals, the foraminiferal planktonic zonation of the two holes, and the biostratigraphic gap at Site 558. It also shows for comparison the cored interval and zonation of Hole 563). At Site 563 (latitude 33°38.53'N; longitude 43° 46.04'W; water depth, 3786 m), one hole was drilled on Anomaly 13 on the west flank of the Mid-Atlantic Ridge about 60 miles south of the Hayes Fracture Zone (Fig. 1). Sediments were washed down to 156.5 m and then 22 rotary sediment cores were taken to basement at 364 m. Sediment for the study of foraminifers was not obtained from Core 563-23, which reached basement. The hydraulic piston coring program scheduled for this site to retrieve the younger section from 0-156.5 m had to be cancelled when the bit and core barrel were lost in the hole during the cutting of the third basalt core.

Lower Oligocene through Pleistocene Planktonic Foraminiferal Biostratigraphy, North Atlantic Sites 558 and 563 (Mid-Atlantic Ridge)