LATEST OLIGOCENE TO EARLIEST PLIOCENE BENTHIC FORAMINIFERAL BIOFACIESOF THE NORTHEASTERN GULF-OF-MEXICO

We examined uppermost Oligocene to lowermost Pliocene sections from fo ur northeastern Gulf of Mexico boreholes for quantitative benthic fora miniferal faunal changes, stratigraphic ranges, paleobathymetry, organ ic carbon content, and planktonic foraminiferal relative abundances. T he Eureka boreholes provide a depth transect in the De Soto Canyon are a from the upper to lower bathyal zone: E68-136 (557m present depth, a pproximately paleodepth), E66-73 (857m present depth, 860-1000m paleod epth), E68-151A (1326m present depth, approximately 1300m paleodepth), and E68-141A (1599m present depth, approximately 1600m paleodepth). A number of taxa last appeared in the late Oligocene to early Miocene ( Biochrons P22-N5) at E68-136; several of these disappearances constitu te global last occurrences. A global benthic foraminiferal taxonomic t urnover that began in the latest early Miocene in other parts of the o cean was restricted to the middle Miocene at E68-136 (Biochrons N9-N12 ), although faunal abundance changes began in late early Miocene Bioch ron N8. At middle bathyal borehole E66-73, ten taxa last occurred in B iochrons N8-N10, which is consistent with the timing of the taxonomic turnover in the Pacific and Atlantic. Depth-related faunal trends are examined and compared with previously published distributional data, r esulting in revised paleobathymetric ranges of 12 taxa. Detailed age-p aleodepth reconstructions reveal several stratigraphically and bathyme trically significant predominance biofacies in the northeast Gulf of M exico: 1) Uvigerina pigmea dominated the middle-upper bathyal late Neo gene; 2) Lenticulina spp. dominated the late Oligocene- middle Miocene bathyal zone; 3) Oridorsalis spp., Gyroidinoides spp., and Globocassi dulina subglobosa dominated the late Neogene lower bathyal zone; and 4 ) Uvigerina proboscidea was important in die late Neogene in the middl e to upper bathyal zones. Four distinct bathymetric migrations are map ped, and 34 additional taxa are shown to have distinct paleobathymetri c distributions. Planktonic foraminiferal biostratigraphic control all ows us to evaluate the stratigraphic usefulness of benthic foraminifer al ranges. We revise the stratigraphic ranges of 12 bathyal benthic fo raminiferal taxa, requiring re-correlation of the benthic foraminifera l zonal boundaries of Berggren and Miller (1989).