Abstract
The Las Chacritas Fm in the type section in the Sierra de La Trampa, Central Precordillera (San Juan Province) was deposited in an outer carbonate ramp setting that evolved from relatively deep to shallow water. Samples from this well-exposed Middle Ordovician section yielded collections of low-diversity conodont faunas stratigraphically significant. The top of the underlying San Juan Fm and the lower and middle parts of the Las Chacritas Fm contain conodonts representative of the Lenodus variabilis Zone, and the upper part of the Las Chacritas Fm yields conodonts that correlate with the Paroistodus horridus subzone of the upper part of the Lenodus variabilis Zone. The occurrence of Dzikodus tablepointensis and Eoplacognathus pseudoplanus in the upper part of the Paroistodus horridus subzone is especially significant. The occurrence of Histiodella kristinae in the highest levels indicates that the top of the Las Chacritas Fm correlates with the Eoplacognathus suecicus Zone. Conodont biofacies are analyzed trough the entire section, supporting an outer platform or open sea conditions. Baltic and Sino-Pacific affinities are stated.