Latest Oligocene to earliest Pliocene benthic foraminiferal biofacies of the northeastern Gulf of Mexico

We examined uppermost Oligocene to lowermost Pliocene sections from four northeastern Gulf of Mexico boreholes for quantitative benthic foraminiferal faunal changes, stratigraphic ranges, paleobathymetry, organic carbon content, and planktonic foraminiferal relative abundances. The Eureka boreholes provide a depth transect in the De Soto Canyon area from the upper to lower bathyal zone: E68-136 (557m present depth, -600m paleodepth), E66-73 (857m present depth, 860-lOOOm paleodepth), E68-151A (1326m present depth, -1300m paleodepth), and E68-14 1A (1599m present depth, -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 constitute global last occurrences. A global benthic foraminiferal taxonomic turnover that began in the latest early Miocene in other parts of the ocean was restricted to the middle Miocene at E68-136 (Biochrons N9-N12), although faunal abundance changes began in late early Miocene Biochron N8. At middle bathyal borehole E66-73, ten taxa last occurred in Biochrons 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, resulting in revised paleobathymetric ranges of 12 taxa. Detailed age-paleodepth reconstructions reveal several stratigraphically and bathymetrically significant predominance biofacies in the northeast Gulf of Mexico: 1) Uvigerina pigmea dominated the middle-upper bathyal late Neogene; 2) Lenticulina spp. dominated the late Oligocenemiddle Miocene bathyal zone; 3) Oridorsalis spp., Gyroidinoides spp., and Globocassidulina subglobosa dominated the late Neogene lower bathyal zone; and 4) Uvigerina proboscidea was important in the late Neogene in the middle to upper bathyal zones. Four distinct bathymetric migrations are mapped, and 34 additional taxa are shown to have distinct paleobathymetric distributions. Planktonic foraminiferal biostratigraphic control allows us to evaluate the stratigraphic usefulness of benthic foraminiferal ranges. We revise the stratigraphic ranges of 12 bathyal benthic foraminiferal taxa, requiring re-correlation of the benthic foraminiferal zonal boundaries of Berggren and Miller (1989). INTRODUCTION Previous studies have shown that one of the largest benthic foraminiferal faunal changes of the Cenozoic occurred throughout the deep sea (>200m) in the late early to middle Miocene (Berggren 1972; Schnitker 1979, 1986; Woodruff and Douglas 1981; Boersma 1986; Thomas 1985, 1986a, 1886b, 1989, 1992; Woodruff 1985; Kurihara and Kennett 1986; Miller and Katz 1987; Thomas and Vincent 1987; Woodruff and Savin 1989; Miller et al. 1992; Thomas 1992; for an alternative view, see Boltovskoy and Boltovskoy 1988; Boltovskoy et al. 1992). These authors documented that major changes in taxonomic composition, percentages, and absolute abundances of benthic foraminifera began in the late early Miocene and culminated in the middle Miocene. Although this event has been well documented at open ocean locations, it remains poorly documented in marginal seas such as the Gulf of Mexico. The Eureka boreholes examined in this study (northeast Gulf of Mexico) yield faunal abundance changes and stratigraphic ranges that reflect this global middle Miocene benthic foraminiferal event. Benthic foraminiferal faunas have been recognized for their potential to assess paleobathymetry (e.g. Natland 1933; Bandy 1960). While Bandy (1960) promoted the concept that benthic foraminifera have distinct upper and lower depth limits, Streeter (1973) and Schnitker (1974) established that deep-water (>200m; bathyal-abyssal) benthic foraminifera are correlated to water mass properties that may vary independently of depth. Numerous subsequent studies have documented that deep-water benthic foraminiferal distributions are associated with physiochemical properties other than depth, and that depth alone does not control the vertical distribution of benthic foraminifera. For example, vertical distributions are correlated with water masses (e.g. Lohmann 1978; Corliss 1979; Schnitker 1979; Murray 1984; for Gulf of Mexico examples, see Pflum and 88? 86? 84? TEXT-FIGURE 1 Eureka bo ehole location map in the Gulf of Mexico. Contours are in meters. micropaleontology, vol. 39, no. 4, pp.367-403, plates 1-6, text-figures 1-32, tables 1-5, 1993 367 This content downloaded from 157.55.39.152 on Sat, 26 Nov 2016 04:11:28 UTC All use subject to http://about.jstor.org/terms M. E. Katz and K. G. Miller: Latest Oligocene to Earliest Pliocene benthicforaminiferal biofacies of the northeastern Gulf of Mexico TABLE 1 Age model parameters Datum Age (Ma) Depth (feet below sea level) E68-136 E66-73 E68-151 FO C. acutus & 5.