An interesting finding of Chittinoidellids (Calpionellidea Bonet) in the Jurassic-Cretaceous boundary of the Neuquén Basin

Abstract

Calpionellids are a useful biostratigraphic group of planktonic protozoa widely distributed in the Tethys realm during the Late Tithonian up to Berriasian times (e.g., Blau and Grün, 1997), but they are really rare in mid and high latitudes. Presence of calpionellids in the Neuquén Basin was mentioned by Fernández Carmona et al. (1996) and Fernández Carmona and Riccardi (1998, 1999), however these reports were considered doubtful by some authors (Aguirre-Urreta. pers. comm.). The results of this study are part of the microfacies analysis conducted at the outcrops of the Vaca Muerta Formation (Early Tithonian - Early Valanginian) in the southern Mendoza Neuquén Basin. This contribution analyze the Arroyo Loncoche (284 m) and Río Seco de La Tosca (320 m) sections, where the Vaca Muerta Formation is characterized by a rhythmic alternation of marls, shales and bioclastic wackestones, packstones and floatstones (Kietzmann et al., 2008). Following the traditional ammonite zones (e.g., Leanza, 1980) the Jurassic-Cretaceous boundary lies in the top of the TST-3 of the third depositional sequence (DS-3) (see Spalletti et al. 2000 and Kietzmann et al. 2008). Calpionellid record is poor, hyaline calcitic loricas are often recrystallized due to the high neomorphism making difficult the systematic determination. However, the presence of a horizont with microgranular calcite wall calpionellids (microgranular calpionellid level) results of particular interest. Calpionellids are related to other incertae sedis grups, such as calcisphaeres. Lower Tithonian interval (Virgatoshinctes mendozanus to Aulacosphinctes proximus Ammonite Zones) contains only calcisphaeres. However, the late Lower Tithonian (Windhauseniceras internispinosum Ammonite Zone) contain Chitinoidella sp. (Chitinoidella Zone), Cadosina sp. and Colomisphaera sp., while Upper Tithonian levels (Corongoceras alternans and Substeueroceras koeneni Ammonite Zones) contains high recrystallized hyaline calpionellid forms, that for the moment have not been able to determine. The Jurassic-Cretaceous transition encloses the calpionellid-level, which contains Calpionellidea of the genus Chitinoidella. Finally, Berriasian beds (Argentiniceras noduliferum and Spiticeras damesi Ammonite Zones) contains abundant Colomisphaera sp., and some poorly preserved hyaline calpionellids. In the Loncoche section the microgranular calpionellid level is located 3.2 m above the last bed with Substeueroceras koeneni (Steuer) (Late Tithonian) and 1.8 m below of the first occurrence of ammonites of the Spiticeras damesi Zone (Late Berriasian). The Argentiniceras noduliferum Zone was not recognized in the Loncoche section, but neither condensation evidences were observed. On the other hand, in the Río Seco de La Tosca section the microgranular calpionellid level is located 5 m above the last occurrence of ammonites of the Substeueroceras koeneni Zone and 4 m above the first occurrence of ammonites of Berriasian age. Therefore, the microgranular calpionellid level should be assigned to the latermost Late Tithonian to Early Berriasian time, even if any of the both biostratigraphic schemes (Leanza, 1980 or Riccardi, 2008) are used. Presence of calpionellids in the Neuquén Basin is not unexpected, since other typical Tethyan faunas have been recently recognized, such as ammonites (e.g., Riccardi, 1991), bivalves (e.g., Damborenea, 2002), and saccocomid microcrinoids (Kietzmann and Palma, 2009). However, the importance of this presence lies in the fact that it represents the first record outside the Tethys area. Additionally, Tethyan chitinoidellids are restricted to the Tithonian, with its highest expression in the late Lower Tithonian Chitinoidella Zone, and calpionellids with -microgranular- loricas appeared again during middle Aptian (for a review see Raháková and Michalík, 1997). The presence of large forms of Calpionella alpina Lorenz, Crassicollaria sp. and Tintinnopsella sp. in association with ammonites of the Late Tithonian (Corongoceras alternans Zone and lowermost part of the Substeueroceras koeneni Zones; Fernández-Carmona et al., 1996) rule out the possibility of correlating this level with the Chitinoidella Zone.