A new K-bentonite interval, from Early Llanvirn, Villicúm Range, Precordillera Western Argentina.

BERESI, M. S. AND S. PERALTA.

Czech Republic

Congreso; Acta Carolinae 43(1/2):456-459.; 1999

8th International Symposium on the Ordovician System

    INTRODUCTION    In the eastern slope of the Villicum Range, Eastern Precordillera of San Juan Province, Argentina, outcrops in early Ordovician thick carbonate sequence ranging frem 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 bellS of the Cuyo Precordillera (sensu Gonzalez Bonorino and Gonzalez Bonorino 1991). At the top of the San Juan Limestones, occurs a red ocher unit found in the here named Quebrada Gustavo (see Martinez 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 calcarenites, and interbeded K-bentonite volcanic ash layers). The K-bentonite predominant in the upper part of the unit. At the top, the limestones bear conodonts of E suecicus Zone (Sarmiento 1985, 1991), which represent an age slightly above the beginning of the Llanvirnian This Quebrada Gustavo unit, containing an interval of numerous K-bentonite layers, is a new locality of volcanic ash layers in the eastern thrust belt of the Precordillera. This unit is similar to others K-bentonite bearing intervals discovered in other localities of the Precordillera by Hoff et al. (1995), Bergstrom et at. (1998).