Sequence stratigraphy and grade-dating by planktonic foraminifera of the Mio-Pliocene series from the Achotal area (State of Veracruz, Mexico)

Cadena, B.-Compan; Peybernes, B.; Gourinard, Y.

Bulletin de la Societe d'Histoire Naturelle de Toulouse 132: 49-64


ISSN/ISBN: 0758-4113
Accession: 009400046

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The Mio-Pliocene mchlt basin mchgt of Achotal (SE Mexico) is a terrigenous turbiditic structure, lightly folded, situated in the oil province of Veracruz, open towards the Gulf of Mexico offshore. The study (realized in collaboration with PEMEX) is specially founded on two recent methods : sequence stratigraphy and grade-dating by planktonic Foraminifera. The stratigraphic series includes four types of deposits:olistostromes with carbonate blocks ; slump sandstones ; BOUMA's turbidites sometimes with basal microbrecciated intervals reworking unknown inner shelf clasts ; pelitic hemipelagites. It consists of five depositional sequences including coarse LST deposits, such as olistostromes, and thinner HST deposits (almost exclusively pelitic) bearing planktonic assemblages. From a micropaleontologic point of view, Foraminiferal assemblages are dated by comparison with the Neogene zonal standard (zones N4 to N22) ; so the use of the grade-dating method is regarded as possible in Mexico. The employed method is based on the biometric evolution of the apparently anagenetic Globigerinoides trilobus lineage, the successive stages of which being dated on a referential curve established thanks to 40K-40 Ar absolute datations and datum planes in Europe and Africa and elsewhere found again (Indonesia). In the Achotal basin, 128 points were numerically grade dated in million years, from Burdigalian to Zanclean. This network of absolute values, associated to the classic field data, allows us to draw isochronic curves wich reveal the cartographic repartition of depositional sequences and sub-systems and give new ideas on the previously identified tectonic structures.