Berriasian to early Aptian calcareous nannofossils from the Vocontian Trough (SE France) and Deep Sea Drilling Site 534: new nannofossil taxa and a summary of low-latitude biostratigraphic events.

Recent investigation of 17 sections in the Mesozoic Vocontian Trough (southeastern France) spanning the Callovian to Cenomanian has shown great potential for improving Tethyan nannofossil biostratigraphic resolution. Both predefined zonal taxa and newly-recognized biostratigraphic events were calibrated to the Lower Cretaceous ammonite biostratigraphy. Coeval strata drilled at Deep Sea Drilling Program Site . 534 offered the chance to test the viabil ity of these new events and evaluate the calibration of the deep~ nannofossil biozonation. Reported here is a summary for the Berriasian to lower Aptian, with the most significant improvements being for the V alanginian and Hauterivian. Ten new species and two new combinations are introduced. Introduction The thick basinal sequence of the Mesozoic Vocontian Trough in southeastern France contains a fairly complete record of nannofossil evolution and now provides an excellent opportunity to calibrate the nannofossil record to the Tethyan ammonite biostratigraphy. The Vocontian Trough also contains several of the stage-stratotypes (Berriasian, Biuremian, and Aptian ; Valanginian hypostratotype) and has been the subject of recent sequence stratigraphic work. The first published Lower Cretaceous nannofossil biozonation by Thierstein ( 1971 , 1973) also was founded on sections in this basin. Bralower, Monechi, and Thierstein ( 1989) improved biostratigraphic resolution for the Tithonian to Berriasian and were able to tie the nannofossil biostratigraphy to both the Tethyan ammonite biostratigraphy and palaeomagnetic record. Deep Sea Drilling Project Site 534 is an important reference section for M-sequence palaeomagnetic record. Roth (1983) published the original nannofossil biostratigraphy of this site, for which he assigned numbers his Lower Cretaceous zones (NCl-10). Sissingh (1977, 1978) pu~ lished a numbered, zonal scheme for the Cretaceous (CC Zones); Applegate and Bergen ( 1988) further refined this zonal scheme based on Lower Cretaceous sections drilled on the Galicia Margin during O.D.P Leg 103. This paper describes the new species found during study of the Lower Cretaceous sections in the Vocontian Trough and D.S. D.P Site 534. An outline of the sequence of nannofossil events in common between these two localities is provided and correlated to the ammonite biostratigraphy of the French sections. Comparisons also are made to the aforementioned nannofossil biozonations. The taxonomic problems encountered are beyond the scope of the current paper and, hopefully, will be addressed in a future publication on the entire Callovian-Cenomanian interval. Methods Research on the Vocontian Trough sections was completed during 1991 at Amoco Production Research in Tulsa, Oklahoma. Two factors had a significant impact on the results obtained during this study: centrifugation of samples and analysis time. These factors improved the continuity of stratigraphic ranges, especially for many of the rare taxa. Abbreviations used in this paper include: LM for light microscope; SEM for scanning electron microscope; LO for lowest occurrence; HO for highest occurrence; TR for total range; Sa for samples; T for top; B for bottom; and SZ for Sub zone . Two rough estimates for the biostratigraphy reliability of an individual species are cited: ( 1) percentage of samples in which the species was found throughout its entire documented stratigraphic range; or (2) the number of samples in which the species was found in either the top or bottom ten samples of its documented stratigraphic range. Acknowledgments Special thanks to Luc Bulot for his interpretations of the ammonite biostratigraphy. I also would like to thank Amoco Production Company for the opportunity to work on this research project, Eric de Kaenel for his help with the specimen transfer (LM/SEM) documentation of specimens illustrated herein, and Tim Bralower for reference samples from the BGS Speeton Borehole and DSDP Site 534. Thanks once again to Jeremy Young, as well as Paul Bown and David Rutledge for their helpful suggestions and reviews of this article. Sections Considered TheprimarysectionsforthecurrentpaperincludeD.S. D.P. Site 534 (Biake-Bahama Plateau, NW Atlantic) and four outcrops in SE France (see Text-Figure 1): (1) Berriasian to Valanginian at Angles; (2) Hauterivian at Vergons; (3) Barremian stratotype at Angles; and ( 4) the lower Aptian stratotype at La Bedoule. Also referenced are the Speeton section in Great Britain, the BGS Speeton Borehole, as well as other sections in SE France (see Text-Figure 1): (A) Albian near Blieux; (B) upper Aptian at Angles; (C) the Berriasian stratotype; (D) Tithonian to lower Berriasian sections at Broyon and Le Pouzin (Route de Grads & Carriere de Anges); and (E) Upper Jurassic sections in SE France at La Melle and Lavilledieu. Relevant nannofossil publications include: Thierstein ( 1971 , 1973) on the Barremian stratotype, Berrias, and Broyon; Bralower et al. (l989)onBerrias, Broyon, andD.S. D.P. Site534; Roth (1983) on D.S. D.P. Site 534; and the most recent paper on the Speeton section (Crux, 1989). The; BerriasianBarremian ammonite biostratigraphy is based on Bulot (1993 & 1994, pers. comm.) and the published framework of Hoedemaeker & Bulot (reporters) (1990) and Hoedemaeker & Company (1993). The lower Aptian ===========================59=========================== Journal ofNannoplanldon Research , 16, 2, 1994. owns • Sections • J. A. Bergen: &m.ssian to Early Aptian Calcareous Nannofossils. , p. 59-69.