Arquitectura y paleoambientes de los depósitos fluviales gravosos de la Formación Las Cumbres (Neógeno), en Villa Mervil, La Rioja, Argentina

a cut along the national road 60, 400 m west of the mervil village, a small town located at the southern end of the "bolsón de pipanaco" (pampean ranges of nw-argentina) unveils a 3-d fluvial and eolian architecture in beds that belong to the stratigraphic passage between the salicas (pliocene) and las cumbres (plio- pleistocene) formations (fig. 1 and 2). fossil mammals found in the salicas formation belong to the huayquerian fauna that could be dated in the pliocene (bossi et al., 1996). a four-fold classification of contacts hierarchy (allen, 1983) was used to define individual sedimentary bodies and their associations: order 0, are the contacts between laminas within a laminaset; order 1 are the contacts between laminasets (represented by parallel or crossbedded cogenetic packages); order 2 contacts, cut order 1 contacts and embrace bedded packages or storeys, limited themselves by order 1 contacts, that show a similar genetic emplacement (similar features in outcrops); finally the order 3 contacts, cut and embrace sedimentary bodies limited by order 2 contacts, defining major groups or stages. five distinct stages were recognized (fig. 3): a lower a-s stage, covered by an extended floodplain f stage, cut deeply by the gravelly stage b, partially filled by l stage, and covered by the fluvial gravelly c stage at top (fig. 4 and 5). the sandy a-s stage consists of channel and sandflat deposits formed by alternating fluvial (a) and eolian (s) storeys. facies assamblage "a" is an association of 9 storeys (fig. 3) composed of moderate sorted massive or cross-bedded sandstones, with few subrounded pebbles (schists and granites) in or near their concave bases. most of the a storeys bottoms, are covered by a 1-5 cm thick silty shale drapes slightly disturbed or forming clay galls partially curled and/or removed (fig. 4b and c). the facies assemblage s, is an association of 6 storeys (painted white in fig. 5b and c) formed by well-sorted fine to medium size sandstones wi