RESULTS

183 samples had abundant diatoms, mainly consisting of pelagic and neritic species with few benthic species. The usefulness of the diatom stratigraphy is limited because marker species are rare in many samples, and many other samples are barren of diatoms, particularly between 26.7 and 389 mcd.

Three intervals contain age-diagnostic species:

1. Two samples at 472.18 and 469.98 mcd contain low abundances of Coscinodiscus yabei (only at 472.18 mcd), Actinocyclus moronensis, Coscinodiscus lewisianus, and Denticulopsis punctata f. hustedtii. According to Barron (1985b), the ranges of the last three species overlap in the C. lewisianus diatom zone of the middle Miocene, whereas C. yabei has its lowest occurrence at or slightly above the C. lewisianus Zone. Because these species only occur in two samples and there is a 15-m barren interval above their occurrences, it is not possible to be certain of their true ranges at Site 1143. Therefore, we assign this narrow interval to middle Miocene.
2. The lowest occurrence of Nitzschia marina at 389.42 mcd indicates that the age of this sample is no older than the Subzone Nitzschia porteri B (Barron, 1985b), which is of late Miocene age. The next higher zonal marker, Nitzschia miocenica, was not found at Site 1143; therefore, we cannot constrain the age above this sample.
3. The interval from 26.7 to 0 mcd contains Pseudoeunotia doliolus, and the assemblage is dominated by Azpeitia nodulifer, Cyclotella spp., and Thalassionema nitzschioides. This interval may be correlated to the Nitzschia reinholdii to P. doliolus Zones, of late Pliocene to Holocene age, based on the range of P. doliolus (Barron, 1985b).