Hallazgo de gasterópodos en la formación Sijes (Mioceno superior) Salta, República Argentina

Abstract

The Upper Miocene to Pliocene Sijes Formation consists of a thick sequence of continental sediments and is of major economic importance because it contains the beds of borate minerals that are being exploited in the Puna. Radiometrically dated as between 7 and 4 Ma, it contains only a few fossils, principally diatoms, the remains of birds, and plant fragments, none of which have stratigraphic significance. External molds of small gastropods discovered recently in the Esperanza (upper) Member of the Sijes Formation near the Sol de Mañana borate mine in the Salar of Pastos Grandes represent the first identifiable remains of this kind of organism in these deposits. The Esperanza Member, about 780 m thick, is composed of beds of clastic alluvial fan sediments that are overlain by fine grained clastics, then gray tuffs and pumicites that have been dated at 5.9 Ma by the fission-track method. A sequence of rythmically bedded lacustrine muds and evaporites overlies the volcaniclastic beds. The gastropod-bearing sediment, an altered volcanic ash, is near the base of this lacustrine section. A fission-track date from a volcanic ash near the top of the section is 4.0 Ma. A disconformity separates the top of the Esperanza Member from a 2,000 m thick Pliocene conglomerate. The gastropods recovered are a new species of Littoridina, a lacustrine genus that is widely distributed in lakes that range from nearly fresh water to very salty water. Generally, living species of Littoridina are small, fragile, and unornamented; they live on a firm substrate of fine-grained sediments in a permanent body of water that does not become highly turbid. Their presence in lacustrine sediments of the Sijes Formation indicates that the lake, though undoubtedly salty at the time, probably was deep enough that the snails generally were below wave base or lived in an environment, perhaps on rooted plants, that did not become highly turbid. Littoridina probably was transported to this lake on the feet or feathers of birds. The genus is dioecious, but a fertilized female of another genus in the family (Amnicola) is capable of depositing viable eggs throughout the year, as long as the water remains at an appropriate temperature.