Clockwise surface-current motion in the North Pacific deflects colder isotherms (California Current) south along the Pacific Coast of North America. This cool-water mass contains a planktonic foraminiferal fauna characterized by Globigerina pachyderma (sinistral coiling north of Lat. 45°N.), G. bulloides, G. quinqueloba, and Globigerinita uvula. A similar Globigerina bulloides fauna dominates planktonic assemblages

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in middle Miocene through Holocene sediments exposed along the Pacific Coast north of Lat. 28°N. Therefore, the sequence of tropical planktonic species commonly used to zone Tertiary sediments is difficult to utilize at the leading edge of the northern East Pacific. Nevertheless, variations in percentages and coiling directions of temperate and cool-water species, together with restricted occurrences of tropical species, allow paleo-oceanographic definition of that part of the stratigraphic record currently accepted as the Pliocene-Miocene boundary, specifically the interval from 13-10 × 10⁶ yrs. ago.

Bathyal upper Miocene sediments exposed north of Lat. 30°N. are characterized by a subarctic to cool-temperate planktonic fauna marked by a zone of sinistral-coiling specimens of Globigerina pachyderma. A warm-temperate to subtropical fauna characterized by Globigerina eggeri, G. conglomerata, Globorotalia inflata, G. menardi tumida, G. crassaformis, G. hirsuta, Globigerinoides ruber, G. triloba, G. conglobatus, and Sphaeroidinella dehiscens occurs in sediments deposited at Lat. 34°N. (Repetto Formation of southern California) approximately 10-9 × 10⁶ yrs. ago. This tropical to warm-temperate facies can be traced within a wedge of bathyal marine sediments extending north from Lat. 8°N. (Charco Azul Formation of Panama) to Lat. 47°N. (Quinault Format on of Washington). North from the equator the number of subtropical and tropical species characterizing the biofacies decreases; Pulleniatina obliquiloculata is absent within the biofacies north of Lat. 25°N.; Sphaeroidinella dehiscens, Globigerina conglomerata, Globigerinoides triloba, G. conglobatus, and Globorotalia menardi tumida are not present north of Lat. 35°N. At Lat. 47°N. the biofacies is marked only by dextral-coiling specimens of Globigerina pachyderma and rare occurrences of Globorotalia crassaformis and G. hirsuta.

The initial appearances of Globorotalia crassaformis, G. inflata, and Sphaeroidinella dehiscens are currently used to mark the Pliocene-Miocene boundary in tropical latitudes. Consequently, their first appearance possibly can be used as a correlative Pliocene-Miocene boundary in the temperate eastern Pacific. However, expansion of warm isotherms from the equatorial region during the early Pliocene can not be considered instantaneous. Indices of the tropical to warm-temperate biofacies appeared later at progressively higher latitudes. A choice must be made between utilization of a time-transgressive biologic datum or a radiometric datum for an epoch boundary. Given a framework of absolute dates, appearance and oscillation of planktonic foraminiferal faunas can be evaluated in terms of aleo-oceanographic parameters rather than emphasized as criteria for establishing highly controversial epoch boundaries.

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