THE PAPAGAYOS CREEK SEDIMENTARY ENVIROMENTS, AN EXAMPLE OF A MODERN FLUVIAL DISTRIBUTARY SYSTEM, EASTERN PIEDMONT OF LA HUERTA ? LAS IMANAS RANGES, SAN JUAN, ARGENTINA

SANTI MALNIS, PAULA; COLOMBI, CARINA ESTER; LIMARINO, OSCAR; RODRIGUEZ POSATINI, NATALIA; ROTHIS, LUIS MARTÍN

La Pampa

Congreso; XV Reunion Argentina de Sedimentología; 2016

Asociación Argentina de Sedimentología

Classical alluvial fans and large fluvial system have been defined as the most common sedimentary environments in intermontane basins. However, recent studies demonstrate that Distributary Fluvial Systems (DFS) occupy a great proportion in these areas. To such extent, that it has been questioned whether most of the models of fluvial and alluvial fan systems interpreted in the geological record, are not really DFS. From this point, the importance of studying in detail the present DFS appears, with the aim of obtaining facial models to study in more detail the ancient deposits. In this contribution, we analyze and discuss the sedimentary environments included in the modern deposits of the Papagayos creek, Western Pampean Ranges in San Juan province, interpreted as a Fluvial Distributary System (DFS). We perform the study of the Papagayos creek using the texture and structure of the accumulated sediments that form the different architectural elements, which in turn characterize the different subenvironments along the creek, from the mountain front to the distal playa lake, known as Salinas de Mascasín. After the Papagayos creek leaves the mountain front it is confined for 12 km until it develops a cone shaped morphology, that limit with the playa lake. Along this course, four zones were recognized, from the proximal areas close to the mountain front to distal areas close to the playa lake in the center of the basin. The proximal zone is characterized by a gravelly (sandy) - bed braided system, where there are longitudinal gravelly alternate bars, hollows, proximal splays and gravelly and sandy bedforms. The medial zone is characterized by a gravelly and sandy wandering fluvial system composed of longitudinal bars, gravelly and sandy lateral accreted bars, sandy and gravelly bedforms, hollows, gravelly-sandy levees, crevasse channels, proximal and distal crevasse splays and secondary channels passing through these floodplain deposits. The distal zone is defined as a distal flashy ephemeral meandering sandy-muddy fluvial system with eolian interaction, where the main channel disappears by infiltration being replaced by many minor channels. This fluvial system is defined by the presence of sandy and muddy pointbars, laminated sand sheets, hollows, levees, crevasse splays, muddy terminal splays, ponds and small dunes. Finally, the fourth zone is defined as a transitional area between the DFS and the coastal shoreline of the saline playa lake, where there are higher dunes, dry interdune, temporally used by the secondary channels and saline ponds. A particular feature observed along the four zones is the presence of mudflows in the floodplain areas, probably as the result of the arid climate, where strong seasonal storms collect once or twice a year lot of loose material. The sedimentary process and environments recognized along the Papagayos creek allow a better understanding of arid DFS depositional dynamics, as well as identifying parameters to recognize ancient distributary fluvial systems. This DFS was developed as a response to the morphometry of the drainage basin of the Papagayos creek, the climate interaction between the upland catchment area (seasonal semiarid) and the valley (seasonal arid), smooth piedmont gradient, and the high tectonic subsidence, which allows high rate of aggradation.