SANDBIAN (LATE ORDOVICIAN) CONODONTS AT QUEBRADA LA POLA, SIERRA DE VILLICUM, SAN JUAN PRECORDILLERA (ARGENTINA).

SANDBIAN (LATE ORDOVICIAN) CONODONTS AT QUEBRADA LA POLA, SIERRA DE VILLICUM, SAN JUAN PRECORDILLERA (ARGENTINA). In this contribution we better define the age and discuss the stratigraphic significance of a Late Ordovician brownish siliciclastic unit at the top of the La Cantera Formation at Quebrada La Pola (Sierra de Villicum), in the light of new biostratigraphic evidences here provided. The La Polo Formation includes siltstones and interbedded sandstones with brachiopod shells deposited in a proximal to intermediate shelf environment. A conodont association contained in this unit allows defining the Amorphognathus tvaerensis Zone, in the lower Late Ordovician. Within this zone, the unit can be assigned to the Baltoniodus variabilis Subzone, defining quite accurately the time of deposition. The conodont association suggests some paleobiogeographical affinities with the Midcontinent Province but some North Atlantic Province influence is also possible.