The Tithonian chrono-biostratigraphy of the Neuquén Basin, Argentine Andes: a review and update

AGUIRRE-URRETA BEATRIZ; LOPEZ MARTINEZ RAFAEL; VENNARI VERÓNICA; LESCANO MARINA; LENA LUIS; CONCHEYRO ANDREA; NAIPAUER MAXIMILIANO,; RAMOS VÍCTOR

San Luis Potosí

Simposio; 10th. International Symposium on the Jurassic System (San Luis Potosí); 2018

The Tithonian of the Neuquén Basin of western Argentina is mostly represented by the marine rocks of the Vaca Muerta Formation, deposited after a period dominated by the accumulation of non-marine clastics (Tordillo Formation and equivalents). In the initial stages of this widespread transgression from the Paleo-Pacific across an active volcanic arc, the waters remained shallow and slightly hypersaline and the Neuquén embayment was covered by a thin stromatolitic zone. Upwards the common rocks are dark bituminous shales and marls reaching several hundred meters thick. Meanwhile the eastern and southeastern sides of the Neuquén Embayment record mixed carbonate siliciclastics assemblages represented by the Picún Leufú and ther time-equivalent formations (Legarreta and Uliana, 1991, 1996). As Vaca Muerta is today one of the biggest non-conventional reservoirs in the world, its knowledge is based not only in studies on its extensive outcrops but also as result of wells exploration and development activity performed by oil companies.U?Pb detrital zircon ages have been presented for the underlying continental clastics of the Tordillo Formation. The source region of sediment supply was the Andean arc and the maximum deposition age for the Tordillo Formation is around 144 Ma (Naipauer et al., 2015). It is worth noting here that this age has a discrepancy of at least 7-8 Ma from the absolute age of the Kimmeridgian-Tithonian boundary (152.1± 0.9, in the International Commission Stratigraphy chart v. 2017/2) which should be in the upper part of the Tordillo Formation.The timing of the depositional sequence of the Vaca Muerta Formation was traditionally based on age-diagnostic fossils and for nearly a century ammonoids allowed the most consistent local zonation (Riccardi, 2008, 2015; Vennari, 2016). This biostratigrahic dating has recently been supported by studies on other groups as nannofossils, radiolarians and calpionellids (Ballent et al., 2011; Vennari et al. 2014; Vennari and Pujana, 2017; López-Martínez et al., 2017). This integrated biostratigraphymeters below the J/K boundary based on the first occurrences (FOs) of the nannofossils Nannoconus kamptneri minor and N. steinmanni minor which correlate with the NJK-D subzone (Bralower et al., 1989) and NKT (Casellato, 2010) (Vennari et al., 2014; López Martínez et al., 2017).