The Tithonian-Berriasian ammonite fauna and stratigraphy of Arroyo Cieneguita, Neuquén-Mendoza Basin, Argentina

In this paper we present the results of the revision of the ammonite fauna and the stratigraphy of the Tithonian- Berriasian succession of Arroyo Cieneguita (Mendoza, Argentina), based on new material recently collected and the collections studied by Steuer, Gerth and Krantz. The studied succession belongs to the Vaca Muerta Formation which overlies the Tordillo Formation in concordance. The lower part of the succession consists of marls and shales, being rich in ammonites, bivalves and gastropods; the middle (shales) and upper (limestones and marls) parts contain almost exclusively ammonites. The majority of the species described by Steuer and a part of those studied by Gerth and Krantz have been revised and several type specimens designated and refigured. Ammonites of the Haploceratoidea have been only recorded in the interval Picunleufuense - lower Internispinosum zones including the genera Pseudolissoceras, Cieneguiticeras, Pasottia, ?Parastreblites, ?Uhligites and ?Semiformiceras. The Catutosphinctes and Choicensisphinctes genera are recorded from the Picunleufuense Zone (lowermost Tithonian) up to the Koeneni Zone (uppermost Tithonian). Two new genera of the Lithacoceratinae (family Ataxiocertidae) are introduced: Krantziceras n. gen. and Platydiscus n. gen. Remaining Ataxioceratid genera recorded are: Lithacoceras, Mazatepites and probably Malagasites (a doubtful specimen). The Berriasellinae are represented by Parodontoceras, Substeueroceras, Blanfordiceras and Chigaroceras (a single specimen), and the Spiticeratinae by Spiticeras and Groebericeras. The Himalayitidae include species of Steueria n. gen., Micracanthoceras, Windhauseniceras, Corongoceras and Himalayites (a single specimen). The Aspidoceratinae are well represented by Aspidoceras, Toulisphinctes, Pseudhimalayites and Physodoceras, although as typical in the basin, the material is scarce. The chronostratigraphic classification and time-correlation of the studied succession are based, as usual, on the biostratigraphy of its ammonites. In some levels occur associations which enable the definition of ammonite (bio-)horizons, useful biostratigraphic units consisting of beds with a characteristic fauna. Eight new ammonite horizons are introduced, from below: cf. erinoides hz. (Zitteli Z.), falculatum hz. (Proximus Z.), vetustum hz., bardense hz. (Alternans Z.), striolatus hz. (Koeneni Z.), compressum hz., noduliferum hz. (Noduliferum Z.) and transgrediens hz. (Damesi Z.). The succession of these horizons and other ones already defined in previous papers, are associated to the Andean chronostratigraphic scale which is then used for fine time-correlation between sections of several localities of the Neuquén-Mendoza Basin where a number of them could be recognized by means of their characteristic ammonite assemblages. The vetustum hz., Alternans Zone, upper Tithonian is recognized in Madagascar and probably in parts of Antarctica.