. Linking mid-scale Distributive Fluvial Systems with drainage areas: geomorphic evidences from the Gastre Basin, Argentina.

ANDRÉS BILMES; DANIEL ARIZTEGUI

Fribourg

Congreso; Twenty-second Meeting of Swiss Sedimentologists; 2014

Distributive Fluvial Systems (DFS) have been recentlyidentified as dominant Quaternary geomorphic features in many modern, aggradational continental sedimentary basins (Moscariello, 2005; Weissmannet al., 2011).For all the complete range in scale of DFS ? represented by alluvial fans,fluvial fans and megafans?previous studies have demonstrated relationships between the DFS area and the catchment basin (Al-Farraj & Harvey, 2005; Davidson & Hartley, 2013). Thus the idea of that DFS area can be used to predict the magnitude of fluvially- transported sediment deposited in a sedimentary basin was considered. Mid-scale DFS are well-preserved in the endorheic Gastre Basin, central Patagonia, Argentina. They are part of the Quaternary record of this basintogether with small-scale distributive fluvial systems, lake shorelines and lacustrine deposits that interplay with a volcanic field 0.3-1 Ma old. The drainage network of the feeding DFS is transitory and extends over an area which has arid climate conditions. Hence, the study of these mid-scale DFS provides an excellent opportunity to analyse fluvial depositional patterns in sedimentary basinsincluding relationships between DFS properties and feeder catchment properties. Detailed geological, geomorphological and hydrological mapping, together with trial pits and sedimentary cores were performed in order to analyze the sedimentological and geomorphological characterization of the Quaternary record from the Gastre Basin with focus in the mid-scale DFS. The piedemont zone of the basin is characterized by remnant fans and bajadas (lower to middle Pleistocene) which are incised bymodern drainage systems, connected to younger small?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 0.23° to 0.82° and cover areas of 49 to 151km2.Catchment areas of the feeding rivers range from 400 km2to 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 sedimentological characterization exhibits pebble to cobble-grade conglomerates with fine-to coarse-grained sandy matrix. The four examples have the same source and thus the same bedrock lithology. 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 evidences of streamflow processes typical of fluvial fans. The present work indicates that the relationships between DFS area and catchment basin only agree in one case of the four DFS with the existing regression equations models (Al-Farraj & Harvey, 2005; Davidson & Hartley, 2013). Thus, others variables such as available horizontal accommodation space and drainage basin slope need to be included to predict mid-scale DFS length.