Consideraciones petrográficas y paleoambientales de la Formación Tambolar (Silúrico) del río Sasso, Precordillera Central sanjuanina.

LEÓN, L. I.; PERALTA, S H; VALLECILLO, G.

Salta

Congreso; XIV Congreso Geológico Argentino; 1999

Asociación Geológica Argentina

ABSTRACT: The Silurian shallow-water silici-clastic deposits of the Tambolar Formation, San Juan River area, Central Precordillera, outcropping in the Sasso river section, is considered. Here, the Silurian sequence, 70 m thick, includes two members: The lower Member,composed mainly of greenish pelitic deposits, and scarce siltstone and very fine-grained sandstone (subgrawacke) interbedded, overlay the fossiliferous limestone of the San Juan Formation, Early Ordovician in age. The upper Member composed of greenish and redish very fine to fine grained sandstone (protoquarzite),which shows normal gradation, overlay transitionally the lower Member,and underlay in paraconformity bioturbated mudsatones of the Talacasto Formation, lower Devonian in age. At the top of this unit a ferriferous-phophatic layer, related to an omission surface, occurs. In this section, the basal cherty conglomerate and mudstone levels related, which display on the other Silurian sections (Sassito river, Tambolar Pass and Pachaco creek), are absent. The thin beds of grawacke show tabular and lenticular stratification,which include sme small-size hummock cross stratification (HCS) and trace fossils of the Cruziana Ichnofacies. The upper Member shows thickenning-upward arrangement, and is composed chiefly by protoquarzites interbedded with thin bedded siltstones.In the protoquarzite beds internal normal gradation is common, as well as hummocky cross stratification and bioturbation characterized by the Cruziana Ichnofacies. The set of physical (sedimentological and petrological) and paleobiological (trace fossils) features, support the previous shallow-water interpretation to this sequence, and a paleoenvironment between the fair weather wave base and the storm wave base is suggested.