Calcareous dinoflagellate cysts from the Tithonian - Valanginian Vaca Muerta Formation in the southern Mendoza area of the Neuquén Basin, Argentina

Abstract

The Late Jurassic - Early Cretaceous marine sediments of the Andean region show an excellent record of different calcareous microfossils (e.g., ostracoda,foraminifera, nanoplankton, saccocomids, calpionellids and calcareous dinoflagellate cysts). Detailed micropaleontological studies of Vaca Muerta Formation (Early Tithonian - Early Valanginian) in the southern Mendoza Neuquén Basin from three sections are conducted with the aim of establishing a major presence of microfossil representatives from different microfossil groups. The analysis of thin sections from the outcrops: Arroyo Loncoche, Río Seco delAltar and Tres Esquinas section reveals a relatively rich micropaleontologicalassemblage of calcareous dinoflagellate cysts, as well as levels with poor preserved calpionellids and benthic foraminifera. Particularly, calcareous dinoflagellate cysts include 25 known species (with 4 subspecies): Cadosina parvula, C. minuta, C. fusca fusca, C. fusca cieszynica, Crustocadosina semiradiata semiradiata, C. semiradiata olzae, Colomisphaeralapidosa, C. nagyi, C. carpathica, C. tenuis, C. fortis, C. lucida, C.heliosphaera, C. conferta, C. radiata, C. vogleri, Stomiosphaera moluccana, S. wanneri,S. acuelata, Committosphaera pulla, C. sublapidosa, Parastomiosphaera malmica, Carpistomiosphaera tithonica, C. valanginiana, Stomiosphaerina proxima. The calcareous dinoflagellate cysts constitute one ofthe most common calcareous microfossil groups widely distributed in the pelagicsediments in the Tethyan realm during Late Jurassic - Early Cretaceous time,and have significant stratigraphic importance. Some specieswith biostratigraphic value of the Tethyanregion have been identified also in the Andean region: 1) Parastomios phaeramalmica, Colomisphaera nagyi and Committosphaerapulla are known only from lower Tithonian.2) Colomisphaera tenuis, Colomisphaera fortis and Stomiosphaerina proxima are important markers for uppermost Tithonian - Berriasian (although their FO should bereviewed in detail in Argentina). 3) Colomisphaeraconferta appears in the uppermost part of upper Berriasian. 4) Carpistomiosphaera valanginiana and Colomisphaera vogleri are markers forthe lower Valanginian. More detailed studies of these groups will allow their correlation with Tethyan biozones, and contribute to improve biostratigraphic schemes in the Neuquén Basin.