In this paper the calcareous nannofossils biostratigraphy of middle Miocene to Pleistocene sediments recovered in the California margin during Leg 167 has been presented. The area covers a large latitudinal transect and is characterized by strongly variable paleobiogeographic conditions during this time interval that make it difficult to recognize most of the biostratigraphic intervals of the standard zonations of Martini (1971) and Okada and Bukry (1980), as pointed out before by Wise (1973) and Bukry (1981). To recover a biostratigraphic resolution and detailed intersite correlations, the regional reliability of 50 biohorizons were checked by considering their mode of occurrence, ranking, and spacing.
For the Pleistocene interval, the following six reliable biohorizons have been identified: LO of P. lacunosa, FO of Gephyrocapsa sp. 3, LO and FO of large Gephyrocapsa, FO of G. oceanica s.l., and LO of R. asanoi. The AE of small Gephyrocapsa spp. and the LOs of H. sellii and C. macintyrei do not seem to be reliable in the studied area. For the Pliocene interval, the following nine reliable biohorizons have been identified: LO of D. pentaradiatus, LO of D. surculus, LO of D. tamalis, LO and FCO of D. asymmetricus, PB and PE of D. pentaradiatus, LO of R. pseudoumbilicus, and LO of A. delicatus. The LOs of Discoaster brouweri and D. triradiatus and the FCO of P. lacunosa seem to be moderately reliable. The AB of Discoaster triradiatus, the LO of Sphenolithus spp., the FO of H. sellii, the LO of A. primus, the FOs of C. rugosus and C. acutus, and the LO of T. rugosus do not seem to be reliable in the studied area.
At the transition between the early Pliocene (Zanclean) and mid-Pliocene (Piacenzian) a set of biohorizons were detected (H. sellii FO, P. lacunosa FCO, D. pentaradiatus PB, A delicatus LO, and D. pentaradiatus PB) showing the same relative order and similar ages as those observed in the Mediterranean area. It therefore seems conceivable to infer similar paleoceanographic conditions between the distant California margin and the Mediterranean region during this time interval.
For the late Miocene interval, the following eight reliable biohorizons have been identified: LO of D. quinqueramus, PB and PE of R. pseudoumbilicus, FO of A. primus, FO and LO of M. convallis, LO of C. calyculus, and FO of Catinaster spp. The following two biohorizons have been considered unreliable: FO of D. berggrenii and LO of D. hamatus. The following nine biohorizons are not detected in the study area: FO and LO of N. amplificus, FO of D. loeblichii, FO of D. neorectus, FO of D. hamatus, FO of C. calyculus, LCO of C. miopelagicus, FO of D. bellus, and FO of C. coalitus.
Sediments of middle Miocene age have been recovered only at low-latitude Site 1010 and therefore the reliability of the six biohorizons identified (D. kugleri FO and LO, C. macintyrei FO, C. floridanus LCO, C. premacintyrei LO, and S. heteromorphus LO) cannot be fully evaluated.
As a result of this analysis, a set of biozones for the California margin has been proposed that has been correlated with the GCS and calibrated to the GPTS. This integrated time frame has been used for dating the successions recovered during Leg 167. The biozones proposed for the Pleistocene seem to have a global value and are proposed as an alternative to the standard zonation of Okada and Bukry (1980).