La secuencia Terciaria de Pachaco: Sedimentología, edad, correlaciones y significado paleogeográfico

MILANA, J.P., CEVALLOS, M.F., ZAVATTIERI, A.M., PRÁMPARO, M.B. & PAPÚ, OH.

Mendoza

Congreso; XII Congreso Geológico Argentino y II Congreso de Exploración de Hidrocarburos; 1993

Asociación Geológica Argentina

The Pachaco Tertiary section that cropsout in the Central Precordillera is a key-sequence for the understanding of the Neogene paleogeography. Because it shows different lithologies from other Tertiary sections, some workers assignate it to a Triassic age. The finding of recogniseable palynomorphs in the basal levels enabled us to assignate a Tertiary age for the entire secuense, and to prove the existance of a widespread sand-sea during Neogene times. Following early works, three members were recognized. The lower (ca. 150 m thick) outsands due to its light tuffs and carbonaceous levels. It seems to have been deposited in an irregular relief which favoured permanent swamps where carbonaceous remains did not oxidezed. As sedimentation toook place, the relief was filled, and distal ephemeral streams interbedded with aeolian dunes were formed. The middle member (ca. 700 m thick) in entirely aeolian and the upper (ca. 400 m thick) is composed of braided channel belts of mixed-load rivers (gravel and sandy), perhaps with as ephemeral discharge regime, associated minor sandy floodplain deposits. A short interval of normal grading characteriszes the lower part of the Upper Member, but then the upward coarsening tendency of the entire sequence is restablished. The age data for this sequence is not very precise but general evolution suggest its correlation with the Divisadero Largo and Mariño formations of the Northern Mendoza.