Upper Jurassic-Lower Cretaceous Ammonoids of the Vaca Muerta Formation, Neuquén Basin, Argentina

VERÓNICA VENNARI

Göttingen, Alemania

Congreso; 21º International Colloquium on Latin American Earth Sciences; 2009

Geowissenschafliches Zentrum der Universität Göttingen

Vaca Muerta Formation (Weaver, 1931) consist of an Upper Jurassic-Lower Cretaceous marine sedimentary succession of dark bituminous shales, marls and several types of bioclastic carbonates, reaching hundreds of meters at some localities. It is widely distributed over all the Neuquén Basin, which is located at the eastern side of the Andes of Argentina and central Chile, between 32º and 40ºS latitude. It has a more or less triangular shape and it has been nearly always limited to the NE by the Sierra Pintada Massif, to the SE by the Nordpatagonian Massif and to its western side by the Andean magmatic arc.Vaca Muerta Formation is a very important unit, not only by its rich fosiliferous content but also by its petroleum source potential. It is also well known by its abundant ammonoid fauna which allowed the construction of a reasonable biozonation for the early Tithonian-late Valanginian interval (Leanza, A., 1945; Leanza, H. 1980, among other previous authors). Nevertheless, this scheme needs to be updated to achieve in the future a greater level of detail and accuracy, especially to improve the biostratigraphic resolution around the Jurassic-Cretaceous boundary. To accomplish that aim the ammonoid fauna of some localities of the Vaca Muerta Formation are studied together with the sedimentary context and their stratigraphic framework (Vennari et al., 2007; Vennari 2008; Kietzmann and Vennari, 2008).One of the sections, 522 meters thick, was measured at the locality of Cerro Domuyo (36º41´ S; 70º26´ W), a region with difficult access in northern Neuquén Province in western central Argentina. At this place the sedimentites of the Vaca Muerta Formation conformably lie over the greenish sandstones of the Tordillo Formation (Kimmeridgian), while miocene volcaniclastic and piroclastic rocks cover them in turn. The section is characterized by the rhythmic alternation of dark shales and marls with microbial or concretional mudstones, laminated mudstones and/or wackestones, bioclastic wackestones, packstones and floatstones. Some event beds have been recognized as well, including sandy wackestones with HCS and sandstones facies associated with storm and turbidity currents deposits. The whole sedimentary succession has been interpreted as a carbonate-siliciclastic ramp dominated by outer ramp and basin facies (Kietzmann and Vennari, 2008).In association with the abundant ammonite shells and aptychus spread all over the section, there were found too some belemnite rostra, bivalves (including pectinids, oysters and lucinids), disarticulated large bones and scales of undetermined vertebrates and also some wood fragments.The traditional ammonoid biozonation proposed by Leanza, H. (1980, 1981a, b) was followed to analyze particularly the ammonoid fauna found at this locality.0-7 m: Virgatosphinctes mendozanus biozone, defined by Burckhardt (1900), is restricted, until now, to the lower Tithonian. Here includes specimens of Choicensisphinctes choicensis sutilis Leanza, H., Choicensisphinctes choicensis (?) (Burckhardt), Pseudoinvoluticeras douvillei Spath, Pseudoinvoluticeras windhauseni (?) (Weaver) and Virgatosphinctes dorsoplanus (Vischniakoff). This biozone extends over outer ramp facies where microbial mudstones and hardgrounds are quite frequent.7-60 m: Pseudolissoceras zitteli biozone, also established by Burckhardt (1900), is considered equal to the upper portion of lower Tithonian and the lower part of middle Tithonian. It is represented here by many shells assigned to Pseudolissoceras zitteli (Burckhardt) and its related aptychus (Laevilamellaptychus). It is worth noting that on the same beds there were find some specimens of Virgatosphinctes denseplicatus rotundus Spath and, despite their bad preservation, probably also of Virgatosphinctes andesensis (Douville), both of them are traditionally considered to belong to the previous biozone. The Pseudolissoceras zitteli biozone is associated with anoxic to disoxic basin facies and more oxygenated outer ramp facies.60-67 m: Aulacosphinctes proximus biozone, although originally established by Burckhardt (1900) as Aulacosphinctes colubrinoides zone, was then redefined by other authors (e.g. Groeber, 1946). It is typically associated with the middle Tithonian. At Cerro Domuyo section, it includes specimens of Aulacosphinctes proximus (Steuer), Aspidoceras andinum Steuer, Laevaptychus (commonly found in association with Aspidoceratids) and Odontoceras ellipsostomum Steuer. This briefly extended biozone is found also related to outer ramp facies.67-158 m: Windhauseniceras internispinosum biozone, differentiated by Weaver (1931), is mostly extended over the upper Tithonian and here is represented by laterally compacted specimens of Windhauseniceras internispinosum (Krantz) and some of Corongoceras lotenoense Spath. This biozone extends over outer ramp facies with and without microbial and concretional mudstones, and open basin facies.158-220 m: Corongoceras alternans biozone, was by created by Leanza (1945) and it is considered equivalent to the lower part of the upper Tithonian. At this locality is represented by some fragmentary and bad preserved specimens of Berriasella sp. The Corongoceras alternans biozone is related at this section with outer and middle ramp facies.220-281 m: Substeueroceras koeneni biozone, was defined by Gerth (1921) and was traditionally associated with the upper part of the upper Tithonian. The section studied includes a sandy wackestones/sandstones interval corresponding to the Huncal Member of the Vaca Muerta Formation (Leanza, H. et al. 2002). At this interval some specimens of Pectinatites striolatus (Steuer) and Subteueroceras koeneni (Steuer) were found; hence this subunit should be considered of an earlier age as previously proposed by its authors (Leanza, H. et al., 2002; 2003).281-312 m: Argentiniceras noduliferum biozone established by Leanza, A. (1945) and equivalent to the lower Berriasian, has not yet been documented here. This interval is represented by open basin and slope facies.312-522 m: Spiticeras (Kilianiceras) damesi biozone, differentiated by Gerth (1921), it is considered equivalent to the late Berriasian. Although not spiticeratids had been found in situ here, there were identified some specimens of Thurmanniceras duraznense (Gerth), Cuyaniceras transgrediens (Steuer), Neocomites regularis (Leanza) and Neocosmoceras sp. This last biozone recognized here corresponds to open basin and outer ramp facies, without microbial mudstones and with several concretional beds.Therefore, at Cerro Domuyo locality the sedimentites of the Vaca Muerta Formation span from an early Tithonian to a late Berriasian age.