Alfredo J. Cuerda

GRAPTOLITES FROM THE UPPER LLANDOVERY OF THE TALACASTO AREA, CENTRAL PRECORDILLERA, SAN JUAN, ARGENTINA

Graptolites are the most important biostratigraphic tool in the study of the Silurian of the Argentine Precordillera. Since, however, the graptolites are concentrated primarily in the Lower Llandovery and Ludlow of the region, the ages of parts of some stratigraphic units are poorly known. The recovery of a small Upper Llandovery graptolite assemblage in the lower one-third of the La Chilca Formation of the Talacasto area helps in resolving this problem, and that assemblage is the focus of this study. Brachiopods, trilobites, bivalves, and ostracodes are present in the same stratigraphic intervals, but are less common and less studied and, thus, of less biostratigraphic value. Silurian sedimentary rocks are widely distributed in the Central Precordillera of San Juan Province (Fig. 1) (Baldis and Chebli, 1969). They are composed of siliciclastic marine deposits, including those crossing the Ordovician-Silurian boundary in the lowermost part of the sequence (Cuerda et al., 1983, 1988; Melendi and Volkheimer, 1982). Figure 1 —Location map and partial geology of the Central Precordillera of San Juan in the Talacasto region (after Baldis et al., 1984). The key graptolite collection locality of the Quebrada Ancha is shown by star number 2. The Los Banos and Salto Macho collection localities are shown by (smaller) stars 1 and 3, respectively The Central Precordillera includes the classical, widespread, and well-exposed Silurian deposits of the Tucunuco Group (Cuerda, 1965, 1969) containing assemblages of graptolites, brachiopods, and trilobites. Outcrops of the Tucunuco Group range geographically from the tributaries of the Jachal River in the north to the La Dehesa Range in the south, a belt 140 km long. The structure is a system of blocks tilted to the west, bounded between north-south oriented reverse faults. The Tucunuco Group includes the La Chilca Formation (up to 200 m thick; Late Asghill …