Ostracods (Crustacea, Ostracoda) and charophytes (Chlorophyta, Charales) from the Desemcuentro Formation (Upper Miocene), La Rioja Argentina

GABRIELA CUSMINSKY, ADRIANA GARCÍA AND RAFAEL HERBST

Robertson, Australia

Simposio; 4 th Symposium of the International Research Group on Charophytes; 2004

School of Earth and Environmental Sciences University of Wollongong

The Desencuentro Formation is part of the San Nicolás Group, Campo de Talampaya, La Rioja, Argentina. The Desencuentro Formation is divided in 3 Membert (<d1 to D3). The ostracods and charophytes were found in the Member D3, which are fine grain sandstones, limolites, with some carbonates and tuffs intercalated. The tuff has been dated in 7.4 ±0,9 Ma and 7.54 ±1.65 Ma. which indicates an Upper Miocene age. The ostracods and charophytes from the Desencuentro Formation (Late Miocene) are described and illustrated using the scanning electron microscope. The association has low diversity with six taxa of ostracods: Eucypris sp., Darwinula sp., Cyamocytheridea sp., Cyprideis salebrosa hartmanni Ramírez, Limnocythere aff. L. reticulata Sharpe, Limnocythere sp. The charophytes identified are Chara zeylanica Klein ex Willdenow and Chara musacchioi García sp. nov.. This new species is characterized by a large gyrogonite., subtruncate at the apical parte, more protruding in the basal contour. The spiral cells are wide and thick showing a low number of circumvolutions in lateral wiew, the apex shows al “apical pore” formed by thespiral cells dipping into  it. The discovery of a large number of well preserved gyrogonites of Chara musacchioi, showed the morphological variability within the taxon allowing the establishment of its relationship with the extant Chara hornemannii Wallman and with the Chara ? sp nov. described from the Upper Miocene of Valles de Santa María and El Cajón Catamarca, Argentina. The assemblage indicates a calm aquatic environment with subsaline to mesosaline water, with the possible input of running fresher water, in a tropical to subtropical area, with median diurnal temperature of 20ºC.