The biostratigraphy for Hole 1138A is similar to that for Holes 747A and 748B of Leg 120, described by McCartney and Harwood (1992). Site 747 was the northernmost of the sites drilled during Leg 120 and is ~200 km south of Site 1138, whereas Site 748 is ~500 km farther to the southeast. For all three Leg 120 sites, the Distephanus speculum speculum Zone and the D. speculum speculum forma pseudofibula plexus range zone are identified above a lengthy unzoned interval in the lower to upper Miocene. The unzoned interval does not contain silicoflagellates that are consistent and distinct enough to allow zonation.
The zonation for Site 1138 is illustrated in Table T1 and should be compared to the biostratigraphy from Sites 747 and 748 (see McCartney and Harwood, 1992). Comparison might also be made to the Quaternary occurrences listed by Bukry (1991) for ODP Leg 119.
Silicoflagellates were generally abundant in the Pliocene and Pleistocene samples from Hole 1138A, which were dominated by six-sided distephanid morphologies with a single specimen of Dictyocha ssp. There was considerable range of size among the six-sided Distephanus, which is reflected in the taxonomic separation into Distephanus speculum (the smaller specimens) and Distephanus boliviensis (the larger specimens). There also were five-, seven-, and nine-sided variants and multiwindowed forms. Distephanus pulchra was abundant in Samples 183-1138A-4R-CC and 5R-CC, making up >10% of the silicoflagellate specimens.
The Pliocene and Pleistocene and several meters of the upper Miocene are included in the D. speculum speculum Zone, which was found in twelve samples, from Samples 183-1138A-1R-CC through 12R-2, 100-101 cm. This interval contained abundant D. speculum speculum and D. boliviensis. The lowermost three samples from this interval, from Samples 183-1138A-11R-CC through 12R-2, 100-101 cm, contained Dictyocha fibula fibula.
The D. speculum speculum forma pseudofibula plexus was found in six samples, from Samples 183-1138A-12R-3, 100-101 cm, through 13R-4, 100-101 cm. The plexid morphologies were most abundant in the lowest of these samples, where there was also dominant Bachmannocena diodon, but never made up a majority of the silicoflagellates. In the uppermost two samples of the plexid interval, the plexid morphologies were outnumbered by D. speculum speculum.
Silicoflagellates were generally rare in samples from below the "pseudofibula plexus" interval. An exception was Sample 183-1138A-15R-CC, where there were abundant Bachmannocena circulus. A similar occurrence of abundant B. circulus and B. diodon below the plexus interval was found in Hole 747A (McCartney and Harwood, 1992). In the McCartney and Harwood study, the lower to upper Miocene interval below the "pseudofibula plexus" was left unzoned because of a lack of consistent silicoflagellates; this practice is also followed in the present study.
The lowermost two samples examined in this study, Samples 183-1138A-28R-CC and 29R-CC, are tentatively placed in the Naviculopsis biapiculata Zone, despite the lack of the nominative taxon, based on the presence of Naviculopsis eobiapiculata and Distephanus raupii.
Silicoflagellates were generally abundant in the lower and middle Miocene samples and are present but not abundant in lower Oligocene samples from Hole 1140A (Table T2). The flora is generally dominated by Distephanus crux, D. speculum, and Corbisema triacantha. Much of the studied interval lacked key biostratigraphic indicators and is unzoned.
The biostratigraphy for Hole 1140A is similar to that found for Holes 747A, 748B, and 751A of Leg 120 (McCartney and Harwood, 1992). The uppermost sample examined through Sample 183-1140A-1R-1, 25-26 cm, contained two specimens of Dictyocha aculeata and is thus placed in the Pleistocene Dictyocha aculeata Zone (see McCartney et al., 1995). This sample also includes a specimen of D. crux, suggesting some reworking of Miocene(?) materials into this sediment; similar reworking in this sample has also been found in the diatoms (K. Roessig, pers. comm., 2001; see Arney et al., this volume).
The zonation for Samples 183-1140A-1R-2, 108-109 cm, through 2R-CC is uncertain. The presence of a few specimens of D. speculum speculum f. notabilis suggests a possible placement in the D. speculum speculum f. pseudofibula plexus Zone, but the diatom study (Arney et al., this volume) shows an earlier age. An unusual silicoflagellate, described below as D. speculum speculum forma cylindrus, was found from Samples 183-1140A-1R-2, 108-109 cm, through 2R-1, 25-26 cm. This silicoflagellate has an apical ring of size similar to the basal ring and sometimes has unusually prominent apical spines situated at the corners of the apical ring.
Samples 183-1140A-3R-CC through 7R-CC are placed in the Distephanus raupii-Corbisema triacantha Zone (see McCartney and Harwood, 1992) on the basis of the absence of D. raupii and the presence of C. triacantha in low abundance. Samples 183-1140A-8R-CC through 22R-CC are unzoned; silicoflagellates were abundant, but there was an absence of biostratigraphically useful taxa such as Naviculopsis biapiculata and Corbisema archangelskiana (see Hole 747A, McCartney and Harwood, 1992).
The diatom study (Arney et al., this volume) suggests an early Oligocene age for Samples 183-1140A-20R-CC through 22R-CC, but the lack of biostratigraphically important silicoflagellates, such as Corbisema archangelskiana and Naviculopsis trispinosa (see McCartney and Harwood, 1992) prevented the clear determination of a silicoflagellate zone.