FIRST RECORD OF BOCHIANITES NEOCOMIENSIS (AMMONOIDEA) FROM ARGENTINA AND ASSOCIATED CALCAREOUS NANNOFOSSIL BIOEVENTS: STRENGTHENING THE EARLY VALANGINIAN CORRELATION OF THE ANDES WITH THE MEDITERRANEAN TETHYS

MARIN, L; VENNARI, V.; LESCANO, M.; AGUIRRE-URRETA, B.

Varsovia

Congreso; 11th International Cretaceous Symposium; 2022

Bochianites neocomiensis, a long-ranging and widely distributed ammonoid species from the Tithonian to the Lower Barremian, is recorded for the first time from the Neuquén Basin, from the upper levels of the Vaca Muerta Formation at Cañada de Leiva, southern Andes of Mendoza, Argentina. A 40 m section encompassing the Lissonia riveroi and Olcostephanus atherstoni zones was sampled bed-by-bed for ammonoids and calcareous nannofossils. A total of 125 specimens of B. neocomiensis, either flattened or preserved as imprints, were retrieved from two levels of dark-brownish mudstones near the base of the section. The same beds yielded L. riveroi, an endemic Andean ammonite restricted to the Lower Valanginian (Aguirre-Urreta and Rawson 1999). The occurrence of Eiffellithus windii from the base of the studied section allowed the recognition of nannofossil subzone CC3-B (Applegate and Bergen 1988), which together with the co-ocurrence of B. neocomiensis and L. riveroi, enable the correlation of those levels with the Neocomites neocomiensiformis ammonite zone in the Western Mediterranean Province of the Tethyan Realm (Reboulet et al. 2014). In addition, in the last level yielding O. atherstoni, the FO of Eiffellithus striatus was noted, a calcareous nannofossil bioevent that indicates the base of nannofossil subzone CC4-A and correlates with the base of the Western Mediterranean Saynoceras verrucosum ammonite zone. This finding reinforces the correlation of the O. atherstoni Subzone with the Karakaschiceras inostranzewi ammonoid zone. The mix of a cosmopolitan ammonoid-like B. neocomiensis in association with endemic Andean ammonites and their calibration with new calcareous nannofossil data helps to refine the long-distance correlation between the west-central Argentina biozonal scheme and the ammonite standard zonation for the Valanginian.