CONICET | Buscador de Institutos y Recursos Humanos

The Gastre Basin is an intermontane topographic depression located in central Patagonia, Argentina. It has a NW-SE main trend and extends over an area of 4,200 km2 which has arid climate conditions. The basin formed during the Middle Miocene and suffered minor structural reconfigurations during the Pleistocene. The heights of the NE bounding mountains (1800 m) are higher than the ones in the SW (1200 m), both composed of pre-Miocene basament. The basin floor has a minimum height of 830 m. The drainage network is transitory and endorheic, defining a bolson. The record of this basin is 400 m maximum, with a maximum of 150 m of Quaternary deposits that include alluvial, fluvial and lacustrine deposits that interplay with a volcanic field 0.3-1 Ma old. Detailed geological, geomorphological and hydrological mapping, together with trial pits and sedimentary cores were performed in order to analyze the sedimentological characterization of the quaternary geomorphological units. Two distinctive stages of accumulation were recognized for the Quaternary fill of the Gastre Basin. Stage I (lower to middle Pleistocene) is characterized by remnant fans and bajadas of an area around 50 km2, which are incised by the modern drainage systems. The surface of these units is characterized by the development of mature soils that include argilic and calcic horizons, and varnished gravels. The slopes of the remnant fans and bajadas range from 1° to 3°. These units are mainly composed of pebble to cobble-grade conglomerates with abundant fine- to coarse-grained sandy matrix. In some cases, these deposits are clast-supported. Internally, they are mainly massive although a faint horizontal stratification and clast imbrications can be registered. These facies can be associated with fine- to coarse-grained sandtone lenses with planar cross-bedding. Subordinated facies of matrix-supported, cobble to pebble-grade conglomerates with a clay matrix are also registered. The younger stage (Stage II) is related to the upper Pleistocene-Holocene and is composed of three large alluvial fan systems, around 170 km2 in area, and by many inset fans with less areal extension (45 km2). The record of Stage II incises deposits of Stage I, causing topographic ridges up to 50 m high near the mountain front. The fans end in playa lakes and are connected to the recent drainage systems. The catchment area of the large fans ranges from 400 km2 to 1,300 km2, and from 27 km2 to 240 km2 for the inset fans. The slopes of the alluvial fans and inset fans of Stage II range from 0,1° to 1°. They consist of pebble to cobble-grade conglomerates with fine-to coarse-grained sandy matrix. Clast and matrix-supported textures are registered with planar cross-bedding, horizontal stratification and clast imbrication as the most common sedimentary structures. Lenses of coarse-grained and pebbly sandstones with planar cross-bedding are also recorded. The overlapping of alluvial fan lobes over playa lake deposits and the presence of remnant elevated coast lines are indicate episodes of base level fall during the development of Stage II. All the geomorphic units exhibit evidences of streamflow processes. Minor debris flow facies are observed in Stage I. The marked differences in slopes between the two identified stages and the presence of debris flow deposits only associated to Stage I indicate a change in depositional conditions between these two evolutionary stages. Despite neotectonic activity inside the basin (indicated by aligned volcanic cones), there is no evidence of tectonic control over the quaternary sedimentary record. The development of large, streamflow-dominated alluvial fan systems is most likely associated with more humid conditions recorded towards the end of the Pleistocene. Therefore the actual configuration of the Gastre Basin reflects processes that took place in the Pleistocene-Holocene boundary and little modifications have occurred since then.

enviar mensaje