North American stratigraphic principles as developed by geologists mapping on continents can be applied to deep-sea sediments. The ability of the Deep Sea Drilling Project to obtain long cored intervals at many ocean basin sites can provide sufficient data for the definition and recognition of rock-stratigraphic units (e.g., formations).

Deep-sea sediments should be divided and reported in terms of distinctive lithologic units, which may be assigned rock-stratigraphic names if there is reasonably good evidence of their continuity. This practice is preferable to current stratigraphic practices in oceanography of designating sediments in chronostratigraphic terms. The generally accepted North American usage of formation as a mappable lithologic unit devoid of any time connotation is recommended over the European usage of formation which commonly is actually a chronostratigraphic unit correlated across major facies changes.

Legs 8 and 9 of the Deep Sea Drilling Project recovered 3,000 m of core in 15 sites (Sites 69-84), spread over a distance of 7,500 km along the equatorial Pacific. Two leg 8 sites (74, 75) and all Leg 9 sites (77-84) were cored to basalts. Sites 70 and 77 were divided into several lithologic units that served as a standard of reference for Legs 8 and 9 sites respectively. The consistent stratigraphic sequence (homotaxial) and areal distribution of these lithologic units led participants on Legs 8 and 9 jointly to adopt four oceanic formation names: the Line Islands Oceanic Formation, Marquesas Oceanic Formation, Clipperton Oceanic Formation, and San Blas Oceanic Formation. These formations are lithologically distinct, Tertiary, diachronous units that can be traced at least 4,500 km. T e most useful and objective criteria to define these formations are color differences, which are accompanied by mineralogic, textural, and biotic changes. To a lesser degree bedding features further aid in the characterization of these formations. The same stratigraphic principles used on land can be applied to oceanic sediments. New stratigraphic principles are not recommended.

Stratigraphic cross sections and paleogeographic facies maps show that the Tertiary depositional history of the central and eastern equatorial Pacific was complex. Two broad sedimentation regimes prevailed; an easterly volcanogenic one and a westerly pelagic one. The upward stratigraphic progression from the highly calcareous Marquesas Oceanic Formation to the more siliceous Clipperton Oceanic Formation supports a general model of sedimentation in which deposition took place in progressively deeper water as sea-floor spreading moved the sites westward. Alternatively, similar lithologic patterns could be achieved by simple sea-floor subsidence in a regionally progressive manner, oceanic currents and upwelling changing position with time, etc. In detail, however, the lithologic patterns are not adequately resolved by such simple models, and other as yet poorly understood factors probably are operative.

Paleoequator reconstructions based on isopach and depositional axis isopach maps of chronostratigraphic units support the earlier hypothesis of Morgan, that in the middle Tertiary the north-northwestward migration of the Pacific plate took a more west-northwestward trend. The spatial and temporal distributions of these isopachs with a north-northwestward trend during the early Tertiary and a west-northwestward trend during the late Tertiary suggest that this change took place sometime in the early to middle Miocene (15-25 m.y. ago).

Description of deep-sea sediments in terms of lithologic units and the establishing, tracing, and dating of oceanic formations can help provide a stratigraphic framework within which our interpretations must lie. This can result in an improved basis for understanding the interrelations between rock-stratigraphic and chronostratigraphic units; lithologic and biologic sedimentation changes through time and space; sea-floor spreading and its effect on sedimentation patterns; depositional processes; subaerial dispersal patterns; and in better communication between oceanographers and continental stratigraphers.