Mobil Explor. and Prod. Serv., United States

Hydraulic piston coring at five of six sites obtained a nearly complete sequence of undisturbed Cenozoic samples; magnetostratigraphy investigated by close sampling representing time intervals of about 10 (super 4) yr. Most of the Cenozoic nannofossil and many of the foraminiferal zonal boundaries were accurately determined and magnetostratigraphically calibrated at those five Leg 73 boreholes. Their numerical ages have been computed assuming a linear spreading rate and a radiometric age of 66.5 m.y. for the Cretaceous-Tertiary boundary. Data confirm previous determinations of the Pleistocene-Pliocene boundary at 1.8 (1.7) m.y. B.P. and of the Pliocene-Miocene boundary at 5.1 (5.0) m.y. B.P. The Miocene-Oligocene boundary is placed within chron C-6 C, with a magnetostratigraphic age of 23.8-24.0 (22.7-22.9) m.y. The Oligocene-Eocene age is also very precisely located within chron C-13 R, with a magnetostratigraphic age of 37.1-37.2 (35.5-35.6) m.y. The Eocene-Paleocene boundary should be located within an uncored interval of chron C-24, with a magnetostratigraphic age of 59.0 (55.4) + or - 0.2 m.y. A general accord of the magnetostratigraphic ages from Leg 73 sites and the radiometric ages published in the literature validates the assumption of a linear sea-floor-spreading rate during the Cenozoic.--Modified journal abstract.