Biostratigraphy of Jurassic and Triassic successions in the Balazuc-1 borehole (GPF Programme). Stratigraphic setting inferred from dinoflagellate cysts, pollen, spores and calcareous nannofossils

Abstract

Biostratigraphic studies - palynology (dinoflagellate cysts, spores, pollen) and calcareous nan-nofossils—of core from the borehole Balazuc-1 (BAl) allowed the various formations intersected from the Lower Kimmeridgian-Upper Oxfordian to the Toarcian to be dated. Within the studied interval in Balazuc-1 borehole, 34–1652 m, only ditch cuttings samples were available through the Upper and Middle Jurassic section (34–554 m), whereas conventional cores were studied from the LowerJurassic and Triassic sections (556–1652 m). The beds were dated on dinoflagellate cysts and calcareous nannofossils for the Lower Kimmeridgian (34 m) to Lower Bajocian-Aalenian (554 m). A hiatus was identified between the Lower Oxfordian and the Upper Callovian. The upper levels of the Liassic contain only a poor microflora, providing a Toarcian-Aalenian dating. The Rhaetian-Hettangian boundary is characterized at 1347.50 m and the Triassic was identified in the 'Grès du Roubreau' (Anisian-Ladinian) and in the 'Argilites sulfatées supérieures anhydritiques' (Carnian). The identification of major sedimentary cycles in the borehole [Razin et al. (1996) Mar. Petrol. Geol. 13, 625–652] has enabled correlations between variations in sea level and the estimated abundance of both dinoflagellate cysts (in the Middle Jurassic) and organic matter (in the Upper Jurassic). The distribution of calcareous nannofossils in the Balazuc-1 borehole was compared with that of sections in the Ardèche region and has enabled the establishment of bioevents, in particular for the Mid-Oxfordian and Upper Bathonian periods.