The Tithonian-Lower Valanginian stratigraphy and ammonite fauna of the Vaca Muerta Formation in Pampa Tril, Neuquén Basin, Argentina

The sedimentary succession corresponding to the Vaca Muerta Fm (Tithonian-Lower Valanginian) at Pampa Tril (Neuquén Basin, Argentina) is described in detail. In this framework, the ammonite succession, sampled bed-by-bed, is described for first time. The Vaca Muerta Fm reaches 421.6 m in thickness in the study area, consisting of a succession of shales (strongly bituminous in the lower part) with several relatively thin intercalations of calcareous shales and fine-grained sandstones; it overlies the Tordillo Fm (Kimmeridgian continental sandstones) and underlies the Quintuco Fm (Lower Valanginian greenish gray claystones). The scope and definition of the Quintuco Fm are shortly reviewed. Four lithofacies are differentiated in the Vaca Muerta Fm: (1) bituminous shale: gray to black, fine-grained clastic deposits with high organic matter contents, calcareous concretions, and abundant fossils, (2) sandy shale: moderate contents of organic matter, lighter in colour, with tuffaceous sandstone levels intercalated, and few fossils, (3) calcareous shale: carbonate material with varying proportions of fine-grained epiclastics and organic matter, forming thick-massive beds of marly aspect and conchoidal fracture, with moderately abundant fossils (ammonites and marine reptiles), and (4) finegrained sandstone: psamitic components (quartz grains) with total absence of pelitic components and minimal contents of organic matter, scarcely fossiliferous. Volcanic ash evidences the activity of the western magmatic arc. Storm event records are frequent and recurrent all throughout the succession. The sequence reflects an upward-shallowing marine environment with euxinic-anoxic passing to dysoxic conditions, and increasing input of terrigenous epiclastics along an outer ramp. The fauna of the succession is composed mainly of ammonites, followed by bivalves, gastropods, reptiles, fishes, and nautiloids. Ammonites occur in abundance throughout the sequence, mainly in the calcareous and sandstone levels. Nine ammonite families are represented in the fauna, including 35 genera, of which one is new: Lytoceratidae (Lytoceras), Ataxioceratidae (Lithacoceras, Choicensisphinctes, Krantziceras, Catutosphinctes, Mazatepites, Paraboliceras), Neocomitidae (Parodontoceras, Pseudoparodontoceras gen. nov., Substeueroceras, Blanfordiceras, Neocosmoceras, Argentiniceras, Raimondiceras, Cuyaniceras, Subthurmannia, Thurmanniceras, Pseudoblanfordia, Lissonia, Pseudofavrella), Olcostephanidae (Groebericeras, Spiticeras, Aspidostephanus), Himalayitidae (Windhauseniceras, Corongoceras, Himalayites), Aspidoceratidae (Physodoceras, Pseudhimalayites, Toulisphinctes), Haploceratidae (Haploceras), Lissoceratidae (Pseudolissoceras), and Oppeliidae (Metahaploceras, Pasottia, Parastreblites, Cieneguiticeras). The fauna (especially the perisphinctoids) is dominated by adult and juvenile macroconchs whereas adult microconchs are very scarce. Lithacoceras picunleufuense is very well represented by adult macroconchs which show clearly the aspect of the successive transients from the very base of the Andean Tithonian sequence. The genera Choicensisphinctes and Catutosphinctes are also well represented throughout the Tithonian, exhibiting the main evolutionary changes of the lineages. The well controlled stratigraphic distribution of the 55 species described allows, considering the succession of their assemblages, a rather confident chronostratigraphic subdivision of the column at zonal level. Furthermore, the recognition of fourteen ammonite biohorizons (four of them tentatively) increases notably the resolution of the cronostratigraphic subdivision, enabling much more precise time-correlations. Three of these biohorizons are new: internispinosum alfa Hz. (base of a standard Internispinosum Zone), azulense Hz. (Alternans Zone), and koeneni Hz. (Koeneni Zone). The Internispinosum (Tithonian) as well as the Noduliferum and Damesi zones (Berriasian) are standarized by fixation of their bases by means of biohorizons. The rich succession studied allows to time-correlate some few Andean horizons with the Tethyan Primary Standard Scale through the Tithonian and Berriasian. For first time the tipically Tethyan Berriasian Subthurmannia boissieri is documented confidently from horizoned samples (well preserved macroconchs) in the upper Noduliferum-Damesi zones interval.