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8. Cenozoic Ostracodes from Hole 628A, Odp Leg 101, Bahamas1

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8. Cenozoic Ostracodes from Hole 628A, Odp Leg 101, Bahamas1 Austin, J. A., Jr., Schlager, W., et al., 1988 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 101 8. CENOZOIC OSTRACODES FROM HOLE 628A, ODP LEG 101, BAHAMAS1 Claude Guernet2 and Eric Fourcade2 ABSTRACT The Oligocene to Pliocene section from Hole 628A supplied about 100 species of Tertiary ostracodes. Deep-sea psy- chrospheric? species (Bradleya cf. dictyon, Agrenocythere cf. gosnoldia, Cardobairdia spp., Henryhowella sp., Cyther- opteron spp., etc.) are present throughout the section. Starting in the Miocene, neritic species (Hulingsina sp., Puriana spp., Caudites spp., Loxoconcha fischeri, Cytherelloidea sp., etc.) dominate. Redeposition of these species from the continental shelf seems to be penecontemporaneous with sedimentation. Variations in the assemblages indicate bio­ stratigraphic position. Species having an ecologic or stratigraphic importance are discussed and illustrated. INTRODUCTION Species of the first two groups are the most interesting for paleobathymetric reconstructions. However, note that (to the Five of the sites drilled during Leg 101 penetrated Oligocene limits of our current knowledge of ostracode ecology) it is diffi­ (or older) sediments. However, only a small part of the Tertiary cult to discriminate between the infraneritic (circalittoral) zone series was recovered at two sites (Holes 634A and 635A). Two and the epibathyal one. In fact, below the lower limit of the in­ other Leg 101 holes were difficult to interpret. Hole 627B was fralittoral (50-100 m water depth, in clear, well-lighted water), difficult because downhole contamination occurred during drill­ the main zone of biological turnover is the thermocline separat­ ing; Hole 626D, because of downslope contamination by neritic ing the thermosphere from the psychrosphere.3 The depth of the sediments. Therefore, we present only results from Tertiary os­ thermocline varies geographically, especially according to lati­ tracodes of Hole 628A. tude, and probably changed during Cenozoic time. Around the Hole 628A was located in a water depth of 966 m north of Lit­ Florida-Hatteras slope (the Straits of Florida and the Blake Pla­ tle Bahama Bank (latitude 27°31.85'N; longitude, 78°18.95'N), teau), the thermocline is a major barrier between shallow- and about 15 km from the neritic platform (Fig. 1). Today, this plat­ deep-water species, as shown by Cronin (1983). form belongs to the epibathyal zone. Drilling penetrated 298 m of In spite of these constraints, this change in ostracode assem­ unconsolidated or slightly lithified sediments and reached the blages from Oligocene to late Pliocene (Table 1) allows us to upper Paleocene or the lower Eocene. Ostracodes occur above conclude the following: the base of the lower Oligocene. The lower and middle Miocene are missing from two sections (Austin, Schlager, et al., 1986). 1. The Oligocene from Hole 628A yielded rare ostracodes (60 specimens), which belong almost exclusively to common or PALEOENVIRONMENTAL INTERPRETATION deep-sea genera, including Cytherella, Krithe, Agrenocythere, We identified about 100 ostracode species. Those species that Henryhowella, Cardobairdia and "Bairdia," the most typical el­ occur in at least three samples and are represented by more than ements. We interpreted the occurrence of occasional abraded or five specimens (having carapaces or single valves) are noted in broken valves of neritic genera (Jugosocythereis, Pokornyella, Table 1. Most specimens cannot be assigned precisely to known and Paracytheridea) as transportation of sediment from the con­ species because many belong to faunas from deeper-water envi­ tinental shelf. This conclusion is also supported by the presence ronments that have not been studied much. However, the speci­ of lepidocycline foraminifers in our samples (Fourcade and But- mens can be divided into three ecological groups: terlin, this volume). 2. From the middle Miocene (NI 1 part/N 13 part) to the up­ 1. Species of genera exclusively or mainly occurring in the per Pliocene (the Pleistocene is represented in only one sample, modern neritic environment, such as Cytherelloidea, Huling• by a single valve of Cytherella), ostracode assemblages contain sina, Jugosocythereis, Puriana, Loxoconcha, and species of many specimens and species of genera usually considered ne­ other genera with well-developed eye tubercles. ritic, such as Cytherelloidea, Hulingsina, and Puriana (Hazel, 2. Species of genera exclusively or mainly occurring in the 1970; Valentine, 1971; and others). These genera indicate tem­ modern deep-sea environment (i.e., for some genera this is as perate to subtropical marine climates. However, deep-sea genera shallow as the lower limit of the photic zone). These species in­ (such as Agrenocythere, Bradleya, and Bythoceratina) are ob­ clude Cardobairdia, Henryhowella, Krithe, Trachyleberidea, Agre• served throughout the section. Although not indicated by the nocythere, Poseidonamicus, Bythoceratina, and Pseudocythere. specimens that are well preserved and have asimilar sedimentary 3. Species belonging to common genera that occur at nearly filling, logic suggests that the valves of neritic taxa were trans­ all depths from the infralittoral or mediolittoral range to the ported from the continental shelf. Since the Miocene, a small abyssal or bathyal zone, such as Cytherella, numerous Bair- part of the infraneritic ostracode population could possibly have diidea, Echinocythereis, Pterygocythereis. colonized the upper part of the continental slope. However, the concomitant presence of bathyal with neritic ostracodes may be attributed to the turbidites that occur in lithologic Unit II of Hole 628A (Austin, Schlager, et al., 1986). 1 Austin, J. A., Jr., Schlager, W., et al., 1987. Proc. ODP, Sci. Results, 101: College Station, TX (Ocean Drilling Program). 2 Departement de Stratigraphie et UA 319, CNRS, T15, Universite Pierre et For a discussion of these expressions and concepts see Bruun (1957) and Marie Curie, 75252 Paris Cedex 05, France. Benson (1975a). 139 C. GUERNET, E. FOURCADE Figure 1. Location map showing Hole 628A from which samples containing Cenozoic ostracodes were studied. Therefore, it appears that important events took place be­ during the Pliocene (Cores 101-628A-8H through 101-628A-4H), tween the late Oligocene and the late middle Miocene; these which are well-represented in our samples, are biostratigraphi­ events are responsible for the faunal break and the resedimenta­ cally significant. tion of neritic ostracodes on the continental slope. These ostra­ codes suggest that most of the observed turbidites came from SYSTEMATIC PALEONTOLOGY the infralittoral zone. Only those turbidites associated with large Among the 100 species observed, many occur as having only one or benthic foraminifers (Cores 101-628A-11H and 101-628A-12H) two valves. Their genetic significance (e.g., reworked specimen or down­ probably came from the shallow zones of the ancestral carbon­ hole contamination) is particularly suspect. Consequently, these species ate platform. are neither plotted in Table 1 nor illustrated in Plates 1 through 6. Spe­ The change in specific composition of the ostracode assem­ cies having easily identifiable features and ecologic or stratigraphic im­ blages through time is probably of biostratigraphic importance. portance are discussed next. The systematics used here follow Hart- The possible redeposition of the continental shelf seems to be mann and Puri (1974). penecontemporaneous with specimens indicating off-bank sedi­ Order PODOCOPIDA Muller, 1894 mentation. Note that with regard to the Eocene ostracode fau­ Suborder PODOCOPA Sars, 1866 nas from DSDP Site 390 (Guernet, 1982) and from Barbados Superfamily CYTHERACEA Baird, 1850 (Steineck et al., 1984) species renewal is complete between the Family CYTHERIDAE Baird, 1850 Eocene and the Oligocene, while at the generic level assemblages Genus GANGAMOCYTHERIDEA van den Bold, 1963 bear the same psychrospheric features. In contrast, the Oligo­ Gangamocytheridea cf. dictyon van den Bold, 1963 cene-Miocene transition, despite the early and middle Miocene Pl. 5, Figs. 14 and 15 hiatus at Site 628, is characterized by the appearance of new species, rather than by extinctions. This "enrichment" continued Remarks. The valves assigned to this species are abundant through­ out the Neogene (Table 1) but seem to differ from those described by during the late Miocene and the Pliocene. However, only further van den Bold (1963) by their more complex reticulation. G. dictyon, studies will enable us to determine if some appearances, such as known in the Caribbean in the late Miocene, still lives "on the continen­ Hulingsina sp. 1 (Core 101-628A-13H), Gangamocytheridea cf. tal shelf north and east of Trinidad. The maximum depth at which the dictyon, Puriana sp. 1 and Cytherelloidea sp. during the late species has been observed is 30 fathoms" (van den Bold, 1963b). A Miocene (Cores 101-628A-11H and 101-628A-12H), Semicythe- closely related species, G. reticulata (van den Bold, 1957), occurs in the rura sp. 1, Hulingsina cf. tuberculata, and Ornatoleberis sp. Miocene of Trinidad. 140 CENOZOIC OSTRACODES FROM HOLE 628A Table 1. Range chart for Tertiary ostracodes from Hole 628A. •3 I 2 31 ■g ci a • o a cs a <v o 'V3co o <£? .» 5 •-,> * a, • a- a, si a E co . cy c<v &> CN . * o H -^i f • co • a • en co +? CO ooiuNCjo ■<! OtJ oa- O • O, 3 CJ * a a a <« o • a. « a co o -<> c. -v a co ci) co w «ts ■v IH ui a, a a Wu .^oSdiitj o*? • "? tsO^matiidi*; CVV o • CJHIMSIBBIKW a a •^ o. -a *e cj c-. co -o «< a<u«; ■^<; s o-o S co co ^ei«;«i<;cjcv'Vi« <£•*■?: . ,w d* cl • • o co •-} -v OJ a <s 3 -v a ci -V < u> -c E *) U-i <V tJ rj *; Q *? V SiSi ci +? Sl"T3 W U O Ci B 5»-*? tj tJ ti Si V 3-D O <V Q. 3 W ui Sl'V cj 'V U -Sf Si O 'V <? f^-O^Si-qS-C^'VU « Si C &) Ci> Si O Ci -rfS>o°^+?ouovsi o •<? ci =■? cj O 'V ■H O^SiStf OO SiU Si 0 3S)cnosiOEcis%sstjci)«v^oS)<5noc)o «v si-a o -K cO O Si O -oci'VsuucSEocjcicjEocD-a-vcuosu CJJJ'W u^-vuo-sicj'VcS-c « o cj s 'V o « CJ ci-s cs -v -v -c -v s> -s; 3 "13 orv -c • v 0 N o al C •H 0 en cs -o *? «? -e »e S V =s> ti -O X racS*e*?ES>cy>u«^ * « ■£ o *? OQS<h-<c_)<:Lijft.cS V|-i2Ql-ia.<OUIVCO.UEi!><Oft.CjVlOCOt-l 3 M O- O 141 C.
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