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During Leg 63, eleven holes were drilled at seven sites off California and Mexico (Fig. 1). A primary objective of Leg 63 was to obtain a series of upper Cenozoic stratigraphic sections between southern California and southern Baja California. This north-to-south transect of sections has improved correlation of California microfossil assemblages with the low-latitude planktonic microfossil zones and provides a means for studying fluctuations in the cool-water California Current. The most valuable sections are from offshore southern California (Sites 467 to 469), northern Baja California (Site 470), and southern Baja California (Site 472). The occurrences of coccoliths, radiolarians, diatoms, and planktonic foraminifers and the resulting biostratigraphic zonation of these sections are summarized in this chapter. For detailed reports, the reader is referred to the individual chapters by Bukry (coccoliths and silicoflagellates), Wolfart (radiolarians), Barron (diatoms), and Poore (planktonic foraminifers) in this volume. Similarly, the reader is referred to the individual site reports for discussion of the largely unfossiliferous hemipelagic sections recovered at Sites 471 and 473. The biostratigraphic time scale adopted for Leg 63 is shown in Figure 2. This scale is modified slightly from that of Ryan and others (1974) and correlated to the paleomagnetic time scale of La Brecque et al. (1977), as updated by Mankinen and Dalrymple (1979). Middlelatitude coccolith zones are included in the Miocene after Bukry (this volume), and middleto high-latitude radiolarian zones are added in the Pliocene and Quaternary after Kling (1973). Correlation of the diatom zones follows Barron (1980), except for minor modification (Barron, this volume). The Pliocene/Quaternary boundary is that of Haq et al. (1977), the Miocene/Pliocene boundary is from Cita (1975) and Van Couvering et al. (1976), and the middle/upper Miocene boundary is after Berggren and Van Couvering (1974) and Theyer and Hammond (1974). This time scale represents an integration of lowlatitude zonations (planktonic foraminifers, coccoliths, and radiolarians) with middleto high-latitude zonations (diatoms, coccoliths, and radiolarians, in part) and serves as a model that can be tested by comparing the various microfossil biostratigraphies in the Leg 63 sections. Coccoliths are the most consistently occurring planktonic microfossils in Leg 63 sediments and provide bio-

Biostratigraphic Summary, Deep Sea Drilling Project Leg 63