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

AGUIRRE-URRETA, M.B.; LOPEZ-MARTINEZ, R.; VENNARI, V.; LESCANO, M.; LENA, L.; CONCHEYRO, A.; NAIPAUER, M.; RAMOS, V.

San Luis, Potosi

Congreso; 10 International Congress on the Jurassic System; 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 other 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 biostratigraphy allowed the fine-tuning of the age of the Vaca Muerta Formation and also consolidated the established correlation with the Mediterranean Province of the Tethys for the Tithonian.The ammonite biozones of Virgatosphinctes andesensis, Pseudolissoceras zitteli and Aulacosphinctes proximus which correspond to the early Tithonian have been recognized on many surface sections as well as in cores. The Virgatosphinctes andesensis zone (formerly V. mendozanus zone) has been tentatively subdivided into two interval subzones: Pseudinvoluticeras primordialis subzone to the base, and Indansites malarguensis subzone to the top (Vennari, 2016). Four bioevents of calcareous nannofossils allowed to characterize the early Tithonian in cores. The first occurrences (FOs) of Polycostella beckmanni, P. senaria, Helicolithus noeliae and Eiffellithus primus allowed us to correlate these levels with the biozones NJ20A, NJ20B, NJKA (Bralower et al., 1989; Bornemann et al., 2003) and NJT 16 (Casellato, 2010) defined for the Tethyan region. Recently, a radiolarian fauna dominated by nasellarian genera has been described from beds of the Virgatosphinctes andesensis zone and represents the first lower Tithonian described from the Neuquén Basin. This fauna can be partially correlated with the base of radiolarian Zone 4 of Pessagno et al. (2009), interpreted as upper lower Tithonian in Mexico (Vennari and Pujana, 2017).The ammonite biozones of Windhauseniceras internispinosum, Corongoceras alternans and Substeueroceras koeneni are widely recognized across the basin, both with surface and subsurface records. They span from the lower upper Tithonian, and the youngest reaches the lower Berriasian. Regarding the nannofossils, several bioevents were identified even including the J/K boundary. The FOs of Umbria granulosa, Rhagodiscus asper and Nannoconus wintereri were recorded allowing the definition of subzones NJKB and NJKC of the late Tithonian (Aguirre-Urreta et al., 2014). All of them have been correlated with the S. koeneni zone.Regarding the radiolarians, a fauna linked to the Substeueroceras koeneni zone yielded abundant representatives of the Family Pantanellidae. The presence of Complexapora kozuri (Kiessling and Zeiss) and Loopus primitivus (Matsuoka and Yao), two important radiolarian primary markers of the Late Jurassic in North America, supports a late Tithonian age for at least part of the S. koeneni zone (Vennari and Pujana, 2017).Recently, Ivanova and Kietzmann (2017) described several species of calcareous dinoflagellate cysts in the Vaca Muerta Formation, some of which have biostratigraphic value in the Tethyan region. Kietzmann (2017) recognized the Chitinoidella and Crassicollaria standard zones and claimed that, unlike in the Tethys where the chitinoidellids abound during the late early Tithonian until the late Tithonian, in the Neuquén Basin they persist in the Berriasian Argentiniceras noduliferum ammonite zone.López-Martínez et al. (2017) studied the distribution of hyaline calpionellids in the welldocumented Las Loicas section that enabled them to correlate their record with ammonitesand nannofossils. They recognized the upper part of the Crassicollaria zone and the lower part of Calpionella zone across the Jurassic/Cretaceous boundary in the basin and were able to expand the calpionellid paleogeographic distribution to the southern Hemisphere, far from their typical Tethyan occurrence.Besides all these recent advances in the biostratigraphy, ash-fall tuffs interbedded within the sediments of the Vaca Muerta Formation have been dated with high-precision UPb zircons techniques which have given, for the first time, a trustworthy absolute agec onstraint (Vennari et al., 2014; Lena et al., 2017). The CA-ID TIMS U-Pb dates are two from the Berriasian and the other one with an age of 140.1 Ma is late Tithonian, located 3 meters 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).