Resumen de Benthonic Foraminifera of the bathyal Zone from Oligocene through Quaternary
Oligocene - Quaternary benthonic foraminifera were studied in 191 samples from sites 208 (leg 21) and 357 (leg 39) which were drilled by the Deep-Sea Drilling Project in the South Pacific and South Atlantic Oceans, respectively. In addition, the previously studied benthonic foraminifera from sites 289 (leg 30, Equatorial Pacific), 329 (leg 36, South Atlantic), 214, 216, 217 (leg 22, Indian Ocean) and 253 254 (leg 26, Indian Ocean) were checked and compared with those obtained during legs 21 and 39. All these sites lie at depths varying between 1253 and 3010 m and apparently have remained within the middle bathyal zone since the Oligocene. It is believed that the sediments studied are undisturbed and do not contain significant reworked elements. In total 260 foraminifers were identified down to species, subspecies or forma level. Several identifications were not quite certain or species were interpreted in a broad sense; consequently these names are acompanied by “?”, “cf.”, “aff.” or by “ex gr.” and “s.l.”, respectively. About 25 foraminifers were left unidentified owing to their quantitative scarcity or other reasons. The faunas of all sites are similar quantitatively and exhibit strong dominance. Cassidulina subglobosa, s. 1., Cibicides kullenbergi, Eggerella bradyi, s. 1., Gyrcidina soldanii, Oridorsalis umbonatus and Pullenia osloensis are dominant at almost all the sites throughout the whole Oligocene through Quaternary sequence. Cibicides wuellerstorfi dominates also at all the sites but from the Quaternary through Middle Miocene only. There are other abundant species, different at distinct sites. Using the stratigraphic subdivisions established by previous authors utilizing planktonic foraminifers and other microfossils, the time range of benthonic foraminifers were established. The following ten foraminifers were found to be suitable as guide fossils: «Bulava indica», Bulimina jarvisi, B. macilenta, B. miolaevis, Cassidulina cuneata, C. subglobosa horizontalis, Cibicides wuellerstorfi, Planulina marialana gigas, Pyrgo murrhina and Stilostomella tuckerae. By means of them it is possible to subdivide the Miocene section into three parts and to separate the Miocene from the Pliocene and the Pliocene from the Quaternary. However, it is not possible to separate the Miocene from Oligocene, or to recognize smaller stratigraphic units (zones, horizons). Planulina marialana gigas and Cibicides wuellerstorfi represent the evolutionary trend. The Lower Miocene/Moddle Miocene boundary is the critical point were (going upwards) Cibicides wuellerstorfi begins to predominate strongly over Planulina marialana gigas.
Late Cenozoic benthonic foraminifers of the bathyal zone are rather poor guide fossils because they have too large time range. Those with a shorter time range are few and quantitatively rare. In addition, their extraction is a very time consuming job as at bathyal depths benthonic foraminifers comprise less than 0.5% of the total planktonic/benthonic thanatocoenosis, and taxonomy is extremely confused which makes it difficult to use studies published by other researchers.
