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Eight holes were drilled at five sites in the Guaymas Basin in the central part of the Gulf of California during Deep Sea Drilling Project (DSDP) Leg 64. Holes 477,477A, 477B, 481, and 481A were drilled in the two spreading rifts in the central part of the basin, and Hole 478 was drilled in the flat basin floor. The sediment dates from the Pleistocene to Holocene epoch. Sites 479 and 481 are on the continental slope within the depths of the present oxygen-minimum zone. The six basal cores from Hole 479 are from the late Pliocene epoch, and an unconformity seems to occur between the upper Pliocene and the overlying Pleistocene sequence. Hole 480 was drilled with a hydraulic piston corer and contains an upper Pleistocene to Holocene sequence of well-laminated diatomaceous mud. Almost all specimens of Neogloboquadrina pachyderma are dextrally coiled in the upper Pliocene to Holocene sequence. Tropical and subtropical species occur in many samples but are not abundant. Such temperate forms as Globigerina bulloides bulloides, G. bulloides quadrilatera, G. bulloides umbilicata, Globigerinita glutinata, Neogloboquadrina dutertrei forma A, and N. pachyderma forma A and forma B are predominant in the Pleistocene to Holocene sequence. The late Pliocene fauna from the basal part of the Hole 479 is abundant in Globigerinoides and includes Globigerinoides bollii, G. obliquus, and sinistrally coiled Pulleniatina obliquiloculata. The upper Pleistocene to Holocene sequence is characterized by the abundant and continuous occurrence of Globigerina bulloides quadrilatera and G. bulloides umbilicata. Evidence for three cooler intervals occurring in the middle to late Pleistocene epoch is apparent, but the climatic fluctuation indicated by planktonic foraminifers differs from that of the northeast Pacific Ocean, probably because of the geography, geomorphology, and the special paleoceanographic conditions of the Gulf. The proto-Gulf sequence defined on the reflection profiles includes the uppermost Pliocene. The angular unconformity between the proto-Gulf sequence and the younger sediment was formed in the early Pleistocene epoch, during the opening of the Gulf, rather than at the beginning of the present spreading phase. Many of the planktonic foraminiferal species and morphotypes are illustrated in the plates. One species, Turborotalita guaymasensis n. sp., is new. INTRODUCTION Leg 64 included two distinct drilling areas: off the tip of Baja California and the Guaymas Basin in the central part of the Gulf of California. We studied planktonic foraminifers from the upper Pliocene to Holocene sections in the Guaymas Basin (Fig. 1). The primary purposes of drilling were to investigate the nature of the ocean crust currently generated in the Gulf, where sedimentation rates are very high (Moore, 1973), and to study the paleoceanography and biostratigraphy of the laminated diatomaceous sediment on the continental slope in the central Gulf. The Guaymas Basin includes two spreading rifts separated by a 20-km transform fault. Sites 477 and 481 are in the south and north rifts, respectively. The sediment includes Quaternary diatom ooze and turbidites intruded by basalt, dolerite, and gabbro sills. Site 478 is on the basin floor —northwest of the south rift—over postulated young crust. Its sequence is similar to that of Sites 477 and 481. Sites 479 and 480 are on the northern continental slope of the Guaymas Basin within the present oxygen-minimum layer and over the presumed proto-Gulf sediment and crust (Moore, 1973). Hole 479 penetrated 444 meters of diatomaceous ooze to laminated mudstone. SediCurray, J., Moore, D. G., et al., Init. Repts. DSDP, 64: Washington (U.S. Govt. Printing Office). 2 For the geographic and hydrographic setting, see Matoba and Yamaguchi (this volume). ment from the lowest part of the hole contains a late Pliocene planktonic foraminiferal fauna. Hole 480 duplicated the upper 152 meters of Hole 479 and was drilled with the newly developed hydraulic piston corer. Bradshaw (1959), Bandy (1961), and Parker (1973) reported on the Recent distribution of planktonic fopminifers in the Gulf. Miocene to Pleistocene foraminifers from deposits exposed on islands and on the continental slope in the Gulf are included in the papers of Natland (1950) and Moore (1973), respectively. Smith (1970) and Ingle (1973c, 1974) studied planktonic and benthic foraminifers of upper Miocene to Pliocene land sections and wells in the northern extensions of the Gulf. METHODS We examined 258 sediment samples from Holes 477 to 481A in the Guaymas Basin; planktonic foraminifers occur in 129 samples. Each sample contained about 10 cm of sediment. We disaggregated the samples by soaking them in warm water and by adding varying amounts of hydrogen peroxide, depending on the consolidation of the sediment. We washed the disaggregated samples through a 250-mesh (0.063mm) screen and allowed them to dry. Diatomaceous sediment samples, especially those from a few top cores of each hole, contained abundant diatom remains, which flocculated as they dried. We boiled these samples in a concentrated solution of sodium hydroxide to dissolve diatom silica. Before extracting the foraminifers, each sample was dry sieved through a 115-mesh (0.125-mm) screen, and the coarser fraction was analyzed for foraminifers. Where planktonic foraminifers were abundant, the samples were split into a workable size and 200 to 300

Late Pliocene to Holocene Planktonic Foraminifers of the Guaymas Basin, Gulf of California, Sites 477 through 481