FOSSIL ASSEMBLAGES AND K-BENTONITE BEDS FROM THE UPPER MEMBER OF THE SAN JUAN FORMACION (EARLY ORDOVICIAN), VILLICUM RANGE, PRECORDILLERA, ARGENTINA.

PERALTA, S.H; BERESI, M.S.

Praga, República Checa

Simposio; 8th International Symposium on the Ordovician System; 1999

Charles University, Prague

In the eastern slope of the Villicum Range, Eastern Precordillera of San Juan Province, Argentina, outcrops an early Ordovician thick carbonate sequence ranging from Arenig to Lower Llanvirn in age.These limestones of the San Juan Formation, correspond to a warm carbonate platform, that developed in the Eastern and Central belts of the Cuyo Precordillera sensu González Bonorino and González Bonorino 1991). At the top of the San Juan Limestones, occurs a red ocher unit found in the here named Quebrada Gustavo (see Martínez 1987). This unit, classically named as Upper Member of the San Juan Formation (see Baldis and Beresi 1981, Sarmiento 1985, among others authors) is 9 meters thick. This unit is composed mainly of marls and calcarenites, and interbeded K-bentonite beds (fossil volcanic ash layers). The K-bentonite layers are predominant in the upper part of the unit. At the top, the limestone bears conodonts of E. suecicus Zone (Sarmiento 1985, 1991), which represents and age slightly above the beginning of the Llanvirnian. This Quebrada Gustavo unit, containing an interval of numerous K-bentonite layers, is a new locality for the occurrence of volcanic ash layers in the eastern thrust belt of the Precordillera. This unit is similar to others K-bentonite bearing intervals discovered in others localities of the Precordillera by Huff (1995), Bergström et al. (1998).This new K-bentonite interval in the Villicum range adds to the growing information on time in the Eastern Precordillera. Because of the relatively instantaneous deposition over an extremely wide area of the K-bentonite beds, they are an excellent source of isochrons or time lines for the area. They can be used as stratigraphic tools for biostratigraphic correlation within the Precordillera and for global correlation particularly with coeval K-bentonites from the Ordovician of Laurentia, such as was suggested by Huff et al. 1995.