Abstract

LAZO, Darío G. et al. Integrated biostratigraphy of the AgrioFormation in its type locality, Early Cretaceous of the Neuquén Basin. Rev. Asoc. Geol. Argent. [online]. 2009, vol.65, n.2, pp.322-341. ISSN 0004-4822.

The Lower Cretaceous of the Neuquén Basinrepresents one of the most complete series in the Andes of South America.Highly fossiliferous exposures make correlation easy among different localitiesand allow to perform detailed studies of the fossil content. This paperpresents, for the first time, an integrated biostratigraphic zonation for theAgrio Formation in its type section, based on ammonoids, bivalves, nannofossilsand palynomorphs. A detailed section from base to top of the Agrio Formationwas measured in the Bajada del Agrio region near the junction of Salado andAgrio Rivers in Central Neuquén. Our focus centred on the two marine members.In the field 39 ammonoid levels were identified and 74 samples of black andgrey shales were collected to search calcareous nannofossils and palynomorphs.In addition, 41 Steinmanella Crickmay bearing levels and four levelscontaining Neocomiceramus curacoensis (Weaver) were identified. Fourammonoids zones, eight ammonoids subzones and two bivalve zones were recognizedin the Pilmatué Member encompassing the late Valanginian to the earlyHauterivian. Four ammonoids zones and two bivalve zones were recognized in theAgua de la Mula Member encompassing the late Hauterivian and reaching the basalBarremian. Another bivalve zone was identified in the Spitidiscus riccardiiZone, but left undefined due to unsolved taxonomy. The analyzed fossil groupshave mainly tethyan affinities. Correlations to the European standard zonationsare possible due to the presence of some index ammonoids, but also due to somenannofossil bioevents. Palynomorphs resulted not very useful inbiostratigraphy. Continental palynomorphs have affinities to Gondwana whilemarine palynomorphs have clear tethyan affinities. Bivalves have proved locallyuseful in biostratigraphy, but index species are endemic to the basin and thusdifficult to correlate to other regions or continents.

Keywords : Biostratigraphy; Ammonoids; Bivalves; Nannofossils; Palynomorphs; Valanginian; Hauterivian.

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