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

Abstract

Un corte de la ruta nacional 60, 500 m al oeste de Villa Mervil (Sierras de Mazán, La Rioja) muestra el contacto entre las Formaciones Salicas y Las Cumbres, con excelente exposición de la arquitectura fluvial que pudo ser extendida en tres dimensiones y por más de 700 m en sentido N-S, a ambos lados de la ruta. El corte muestra el contacto entre las Formaciones Salicas (Plioceno) y Las Cumbres (Plio-Pleistoceno). Pueden ser reconocidos cinco estadios de sedimentación: A-S, el inferior, constituido por areniscas friables fluviales y eólicas; F, de capas tabulares consistentes en limos de barreal y areniscas de planicie de distributarios (escorrenteras y barras efímeras) con abundantes calcretos y rizoconcreciones; B, el mejor preservado y que ha permitido el análisis 3D, de conglomerados arenosos, que corta profundamente los dos estadios inferiores. El Estadio B está organizado en una serie de mesoformas de canal, con barrancas izquierdas (al Oeste) bien definidas. En su etapa final de abandono (Estadio L), fue rellenado por capas tabulares de arenas finas limos pardos, que contienen una arcilita verde y una capa de micrita blanca, que pueden asignarse a un depósito lacustre somero. Finalmente yace sobre todo el conjunto anterior, el Estadio C, de conglomerados finos arenosos que forman un extenso sistema de canales y planicie aluvial pero mal representado en estos afloramientos. Las particularidades de estos depósitos pueden ser comparadas con expresiones locales del ambiente actual: el río Pituil (al oeste de la Sierra de Velasco) la llanura arenosa del Bolsón de Pipanaco y el barreal del Señor de la Peña, donde impera un clima árido con una escasas lluvias estivales, indicado condiciones climáticas similares.

A cut along the National Road 60, 300 m west of the Mervil Village, locate at the southern end of the "Bolsón de Pipanaco" (Pampean Ranges of NW-Argentina) unveils a 3-D fluvial architecture in beds located 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 3 contacts define stages that could be divided in storeys limited, in turn, by order 2 contacts. 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 11 storeys (Fig. 3) composed of moderate sorted sandstones massive or cross-bedded, with few small pebbles (schists and granites in composition) near their concave bases. Most of the bases of events “A”, 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 10 storeys formed by well-sorted fine to medium size sandstones with occasional mud intraclasts (pebble to block sizes). The meter thick silty “F1”storey Fig 4 b and e) is composed of 10 to 11 tabular decimeter slightly endurated beds, interpreted as playa lake ("barreal") deposits by comparation with very similar sediments that fill the nearby barreal “El Señor de la Peña” a typical endorheic depression. This “F1” horizon is the source of most irregular blocks found at the foot of the left margin bank cuts of the “B” storeys. The F2 stage (Figs. 3 and 4b) is composed of 0,90 m tabular to lenticular coarse sandstone horizon crowded of calcretes and rhyzoconcretions defining a paleosol. There are six irregular and lenticular storeys in the “B” stage, defined by concave and convex stratification designs. These storeys could be considered mesoforms in the sense of Bridge (1985, 1993) of the macroform “B”. A close correlation of the contacts shown in the two walls of the road cut associated with contact orientation measurements and paleocurrents, allowed to built models of the 3D situation in several evolution steps starting with storey B2 ending up in storey B6 (Figs. 6 a-c). The last block (Fig. 6d) shows the abandon “L” stage of lacustrine fillings ( see Figs.3 for stratigraphy relations and Fig. 4 for details). The B channel stage is deeply incised on the F and A-S stages indicating a change of the equilibrium profile. Several stages of bank cut are shown in the road cut walls (Fig. 8) indicating an active displacement of the channel toward the West. The B stage itself, represented by storeys B5 and B6, were mapped beyond the road cut for an extension of 700 m (in N-S sense) yielding a map that shows a NE-trending low sinuosity river with cut banks, lateral bar deposits, overbank deposits and chute channel deposits with variable width. Most mapped features yielded also paleocurrents measurements on cross-bedding and pebble imbrications that were integrated with the textural and structural evidences (Fig. 7). The model of Fig. 7b, is a cartoon of a slightly ondulate landscape crossed from SSW to NNE by the B stage river channel. Calcretes and rhizoconcretions over convex or tabular gravels storeys, mapped as inside features of channel “B”, indicate an ephemeral regime with riparian vegetation (Fig. 7). The Pituil River (Fig. 9) draining the northwest end of the Velasco Range and the sandy plains located north of the Mazán Range and near the center of the “Bolsón de Pipanaco” were used as modern analogs of the ancient Mervil channel system (stages B and A-S respectively). The structural details shown by the Salicas Formation at the Mervil area around the road cut, are also present in the modern analogs, indicating a similar arid climate. However, this Pliocene paleoenvironment was developed in a low land geography dominated by plains and surrounded by mountains of low to moderate relief that acted as source areas.