24. JURASSIC OSTRACODES OF DEEP SEA DRILLING PROJECT LEG 76, HOLE 534A, BLAKE-BAHAMA BASIN 1

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1 . JURASSIC OSTRACODES OF DEEP SEA DRILLING PROJECT LEG 76, HOLE A, BLAKE-BAHAMA BASIN Henri J. Oertli, Elf Aquitaine, Centre Micoulau, Pau, France ABSTRACT The ostracode assemblage found in Upper Jurassic cores of Hole A, Leg 76 of the Deep Sea Drilling Project (DSDP), bears some resemblance to that of Sites 00 and 0, DSDP Leg, all in the Blake-Bahama Basin. The Hole A assemblage, containing species represented by one or more specimens only, is much less diversified than that of Sites 00 and 0. Both are deemed typical of a deep-sea environment. Even if the Late Jurassic assignment is not in doubt, precise dating using the ostracodes is impossible. Kimmeridgian strata may extend as deep as Core 0. INTRODUCTION I was delighted when Felix Gradstein invited me to study the Jurassic ostracodes of Leg 76. The aim of this leg was similar to that of Leg (in which Sites 00 and 0 had yielded quite diversified ostracode assemblages; Oertli, 97); it was hoped that the same species would be found on Leg 76. Sites 90 and 9 of Leg are situated in the same region, but no Jurassic sediments were obtained (. Swain, 978, who studied the Cretaceous ostracodes of these sites) (Fig. ). However, a study of nearly 00 core samples of Hole A (between Cores and ) put an end to this hope: little more than 0% yielded ostracodes, and half of these were undeterminable, being mostly fragments. Fortunately, Dr. Gradstein later provided me with a collection containing some more material from 9 cores, between 9 and 6, corresponding to the Jurassic sequence of this hole. The few fossiliferous Cretaceous samples yielded only Cytherella sp. and Asciocy there brevis Cornuel (in Samples A-8-, cm and A-60-, 8 cm) among the determinable forms. This explains why the present study is limited to the scarce Jurassic material. FAUNAL RECORD As can be seen in Figure, the total number of distinguishable species is very modest; the "?" is a common sign. Adult or at least "normal-sized" individuals are rare (see magnification scale on the plates), and the "richest column" is that marked "Indet." (indeterminate); nevertheless, a few interesting findings are listed. Bairdia "striate" (Plate, Figs. -) A peculiar form, with its concentric striae, sometimes combined with reticulation, most developed along the anterior, dorsal, and posterior margins. It is the most widely distributed species, having been found in 0 or of samples. The same species has been observed in Leg, Site 0, Cores and 8. Sheridan, R. E., Gradstein, F. M., et al., Init. Repts. DSDP, 76: Washington (U.S. Govt. Printing Office). Procytheropteron aff. prolongatum (Sharapova, 99) (Plate, Figs. 6-9) The second most frequent species, it was found in 6 samples, between Cores 9 and 0. It strongly resembles P. prolongatum (upper Kimmeridgian to middle olgian), olga region and Ukraine (see Fuller, 980), but is less elongate and has a more concentric reticulation. Its size (adults, mm) is little more than half that of the Russian species. Bairdia umbra Oertli, 967 (Plate, Fig. ) A single valve was found in a sample of which the exact depth is undetermined, but which is situated below Core. The species was originally described from the bathyal Upper Jurassic of the Apennines, and a related species (B. italica Oertli, 967) could be determined in Leg, Site 0. After comparison with type material, the specific identification appears "reasonable." This specimen is, however, smaller 0.9 mm. Paranotacytherel sp. (Plate, Fig. ) One right valve was found in Core. The same species was observed in Core 9 of Site 00, Leg (length = 0.8 mm.) Indeterminate Genera and Species ) One form is apparently the same as that found on Leg at Site 00, Core 7 (Oertli, 97, plate, fig. 0). It is located in our material in Core 0; see Plate, Figure (length = 0.9 mm). Other specimens also resemble forms encountered in Leg material, but probably without being co-specific. These are: ) Indet. gen. et sp. aff. Oertli, 97, (plate, figure 7). This elongate, flat form (a single specimen from Core 0, Plate, Fig. ) with large caudal processus toward the anterior has some similarity with the species shown in 97; but those are more blunt, the posterior is turned upward, and the costulation is more pronounced (length = 0.8 mm). ) Indet. gen. et sp. {Argilloecial sp.). This is an incomplete specimen of approximately 0.8 mm, vaguely 8

2 H. J. OERTLI Figure. Location map of the sites discussed c 0 c related to what was called in Oertli, 97, Fabalicypris sp. (found in Core, Plate, Fig. ). ) Indet. gen. et sp. (aff. Pseudoperissocytheridea parahieroglyphica Whatley, 970). Of this form too, a single but badly preserved specimen was found in Core (Plate, Fig. ). It is not dissimilar to the cited species from the Scottish Oxfordian, though the latter is approximately twice its size (length of our specimen = 0. mm). Other problematical identifications at the genus level include: ) Pontocyprellal sp. from Core 9 (the determination is not sure because the posteroventral margin may be incomplete). (Length = 0. mm; Plate, Fig. 7.) ) Macrocyprisl sp. or Paracyprisl sp. (Core 9). A left valve, broken posteriorly. (Length = more than 0.9 mm. Plate, Fig. 8.) ) Bythocyprisl sp. A single carapace from Core 97 (Plate, Fig. 9) one of the rare specimens of normal size (0.7 mm). Our specimen bears a slight resemblance to Bythocypris ambitruncata Peterson, 9 (Rierdon Formation, Montana). ) Trachycytherel sp. esticytherural sp. Found in Core 0 (Plate, Fig. 6), this form is most probably an internal mold and therefore of little interest. (Length = 0.8 mm.) ) Patellacytherei sp. Found in Core 0, this is a very badly preserved specimen for which the given determination is most uncertain. (Length = 0.8 mm; Plate, Fig. 0.) Finally, in Plate, Figures and, two specimens of very limited size are shown that do not deserve special comment, but may possibly be familiar to colleagues. DATING CORRELATION AND PALEOECOLOGY The lack of similarity to western European Callovian and Upper Jurassic ostracode assemblages prevents precise dating of the Leg 76 association. Of the species already observed in Leg, Bairdia "striate" is not stratigraphically interesting, obviously having a long range. Better information (though to be used with caution, because it is based on single specimens of rare species) includes: For core 0: Indet. gen. et sp. (figured on plate, fig. 0, in Oertli, 97); its occurrence (in DSDP Section 00-7-) is attributed to the Tithonian to Kimmeridgian (?) by Luterbacher (97). For core : Paranotacytherel sp. (figured as Orthonotacythere sp. on plate, figs. 6-7, in Oertli, 97, and found in DSDP Sample 00-9,CC); Luterbacher (97) attributes this sample to the Kimmeridgian^) to Oxfordian^). Bairdia umbra Oertli, 967 was found in the Apennines in Upper Jurassic deposits, but below the Tithonian Ṫhere is a close resemblance to Upper Jurassic ostracode assemblages from the Scotian Shelf (Ascoli, 976, 98; Jansa et al., 980). According to P. Ascoli (personal communication, May and September, 98), Oxfordian ostracode assemblages are very scarce on the Scotian Shelf, except in a conventional bottom core, where he tentatively identifed over 0 species. Among these, three species described in 97 by this author in the Jurassic sediments of DSDP Leg {Acrocytherel sp., Bairdia italica Oertli and Indet. gen. et sp. of plate, fig. 8) seem to be present. 8

3 Appe aring i n Plate CSI CSI CSI Figure Collection nos. of photographed specimens ill ,.7,6,7 III 7 - ^^ Core Species ~~ - ^ Section Interval cm Pontocyprella? sp. (instar) Indet. (fragments, deformed and/or badly preserved specimens, molds) Bairda striate (mostly instars and fragments) ( Bairda? sp. in Leg ) Macrocypris? sp. or Paracypris? sp. (instar) Procytheropteron aff. prolongatum (Sharapova, 99) Bythocypris? sp. Trachycythere? or esticytherura? sp. (mold) CC Jurassic ostracodes of Leg 76, Hole A cc Below A B 0 S Patellacythere? sp. CSI CSI 9 I I I I Indet. gen. et sp. aff. Oertli, 97 (pl., fig. 7) Indet. gen. et sp. of Oertli, 97 (pl., fig. 0) Paranotacythere? sp. Orthonotacythere sp. of Oertli, 97 (pl., figs. 6-7) Indet. gen. et sp. [Argilloecia? sp.; cf. Fabalicypris? sp. of Oertli, 97 [pl., fig. 7]) Indet. gen. et sp. (aff. Pseudoperissocytheridea parahieroglyphica Whatley, 970) Bairda umbra Oertli, 967 specimens / 6 specimens» Determination i Figure. Distribution of ostracodes. (! " means: SEM stub, third row, third specimen from the left, and so on.)

4 H. J. OERTLI Ostracodes from the Kimmeridgian and younger Jurassic strata of the Scotian Shelf resemble European Shelf assemblages of that time, which contrast with the associations of Legs and 76. The specific composition of the Leg and Leg 76 associations and their scarcity make it probable that the depositional environment was situated beyond the continental shelf. The reduced size may be interpreted as a sign of precarious nutritional conditions. ACKNOWLEDGMENTS My sincerest thanks go to Piero Ascoli, Richard Benson, and John Neale for their critical reading of the paper and helpful comments. The management of Elf Aquitaine provided facilities for working on the material and granted permission for publication. REFERENCES Ascoli, P., 976. Foraminiferal and ostracod biostratigraphy of the Mesozoic-cenozoic, Scotian Shelf, Atlantic Canada. Marit. Sediments, Spec. Pubi, :6-77., 98. Foraminiferal-ostracod Late Jurassic biozonation of the Scotian Shelf. Geol. Surv. Ottawa, Open File 7. Fuller, N., 980. On Procytheropteron prolongatum (Sharapova). A Stereo-Atlas of Ostracod Shells, 7:7-. Jansa, L. F., Remane, J., and Ascoli, P., 980. Calpionellid and foraminiferal ostracod biostratigraphy at the Jurassic-Cretaceous boundary, offshore eastern Canada. Riv. Itai. Paleontol., 86: Luterbacher, H., 97. Foraminifera from the Lower Cretaceous and Upper Jurassic of the northwestern Atlantic. In Hollister, CD., Ewing, J. I., et al., Init. Repts. DSDP, : Washington (U.S. Govt. Printing Office), 6-9. Oertli, H. J., 97. Jurassic ostracodes of DSDP Leg (Sites 00 and 0) preliminary account. In Hollister, C. D., Ewing, J. I., et al., Init. Repts. DSDP, : Washington (U.S. Govt. Printing Office), Swain, F. M., 978. Notes on Cretaceous ostracoda from DSDP Leg, Sites 90 and 9. In Benson, W. E., Sheridan, R. E., et al., Init. Repts. DSDP, : Washington (U.S. Govt. Printing Office), Date of Initial Receipt: February, 98 8

5 JURASSIC OSTRACODES Plate. Jurassic ostracodes. (All specimens with the exception of Figs. and are magnified approximately 60; Figs. and are magnified 80. All specimens are single valves.) -. Bairdia "striate," () Sample A-0.CC. () Sample A-9-, 0- cm. () Sample A- 96-, 6-8 cm. () Sample A-0-, 0- cm.. Bairdia umbra Oertli, 967. Sample A, below core Procytheropteron aff. prolongatum (Sharapova, 99). 6. Sample A-99-, cm. 7. Sample A-99-, -6 cm. 8. Sample A-0-, 0- cm. 9. Sample A-9,CC. 8

6 H. J. OERTLI v Plate. Jurassic ostracodes. (All specimens with the exception of Fig. 9 are magnified approximately 60; Fig. 9 is magnified 80. Figs.,,, 8, 9,, and represent single valves, the others are carapaces.). Paranotacytherei sp. Sample A--, 6-8 cm.. Indet. gen. et sp.: Argilloecial sp. Sample A--, 6-8 cm.. Indet. gen. et sp.: aff. Pseudoperissocytherideaparahieroglyphica Whatley, 970. Sample A--, 6-8 cm.. Indet. gen. et sp.: aff. Oertli, 97, pl., fig. 7. Sample A-0,CC.. Indet. gen. et sp.: Oertli, 97, pl., fig. 0. Sample A-0.CC. 6. Trachycytherei sp., esticytherural sp. (probably mold). Sample A-0-, 60-6 cm. 7. Pontocyprellal sp. Sample A-9-, -6 cm. 8. Macrocyprisi sp. or Paracyprisl sp. Sample A-9-, 0- cm. 9. Bythocyprisi sp. Sample A-97-, 8-9 cm. 0. Patellacytherei sp. Sample A-0-, - cm.. Indet. Sample A-6-, 0- cm.. Indet. Sample A--, 6-8 cm. 86