Mid-scale Quaternary Distributive Fluvial Systems: an example from the Gastre Basin, Argentina

BILMES, A.; VEIGA, G.D.; FRANZESE, J.R.

Congreso; 30th IAS Meeting of Sedimentology; 2013

Distributive Fluvial Systems (DFS) have been recently identified as dominant Quaternary geomorphic features in many modern, aggradational continental basins. Of all the complete range of scales of DFS, midscale DFS ? represented by fluvial fans and braided deltas ? are intermediate between megafans and alluvial fans in size, gradient and catchment areas. Therefore, the analysis of mid-scale DFS provides an excellent opportunity to study fluvial depositional patterns in sedimentary basins in a way similar to the complex megafan distribution but under more homogeneous climatic and tectonic conditions that are related with the development of small?mid scale DFS. 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 with no younger structural reconfigurations. The heights of the NE bounding mountains (1,800 m a.s.l.) are higher than the ones in the SW (1,200 m a.s.l.), both composed of pre-Miocene units. The basin floor has a minimum height of 830 m. The drainage network is transitory and endorheic, defining a bolson. The Quaternary record of this basin is up to 150 m, which include small to mid-scale distributive fluvial systems, lake shorelines 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 sedimentologycal and geomorphological character of the Quaternary deposits, with focus on the mid-scale DFS. The piedmont zone of the basin is characterized by remnant fans and bajadas (lower to middle Pleistocene) which are incised by modern drainage systems, connected to younger small? to mid-scale DFS (upper Pleistocene-Holocene). Four mid-scale DFS are observed, all of them related to mountain-fed transitory rivers developed in the NE bounding mountains. They have a semiconical shape, slopes from apex to toe ranging from 0.10° to 0.52° and cover areas of 90 to 180 km2. Catchment areas of the feeding rivers range from 400 km2 to 1,500 km2. The dominant channel planform of the mid-scale DFS is a single braided channel that bifurcates downstream into braided, or more rarely meandering, channels that terminate in playa lakes or salinas. The four examples have overlapping lobes over playa lake deposits at topographic heights lower than remnant lake shorelines. The main facies of these DFS are 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. All the mid-scale DFS exhibit evidence of streamflow processes typical of fluvial fans. However, the presence of overlapping lobes over playa lake deposits at topographic heights lower than remnant elevated coastlines suggest that much of the fans could have been developed as braided deltas. Thus, the development of mid-scale DFS systems is most likely associated with more humid conditions recorded towards the end of the Pleistocene. Therefore, many patterns of the DFS of the Gastre Basin could reflect processes that took place in previously wetter periods, extraordinarily preserved by the present arid climate conditions.