Estudio sedimentológico y geomorfológico del mega abanico del río Huaco, cuenca del Bermejo, San Juan, Argentina

Abstract

En los últimos años varios autores han considerado que los mega-abanicos están incluidos dentro de los Sistemas Fluviales Distributivos (SFD), los que, por su abundancia en los ambientes sedimentarios continentales, sus características geomorfológicas y sedimentológicas son considerados potenciales fuentes de recursos hídricos y energéticos. En este estudio se propuso obtener un modelo geomorfológico y sedimentológico del mega-abanico del río Huaco utilizando imágenes satelitales, análisis fotogeológico y relevamientos de campo. Con estos datos, evaluar si el mega-abanico corresponde con el modelo de un SFD y, en caso de que lo sea, determinar de qué tipo de acuerdo con las clasificaciones propuestas. El mega-abanico del río Huaco se localiza al Este de la provincia de San Juan en el valle del Bermejo, que constituye la cuenca de antepaís fragmentada de la faja plegada y corrida andina entre los 29°LS y 31°LS. A través del procesamiento de imágenes multiespectrales se reconoció la distribución de materiales a escala regional. El estudio geomorfológico permitió distinguir siete unidades geomorfológicas, determinar la sinuosidad de los canales fluviales y cambios de pendiente. A partir del análisis sedimentológico se calcularon las características texturales de las unidades geomorfológicas identificadas. Con estos criterios se definieron cuatro zonas principales: a) zona 1 o proximal formada por un sistema fluvial multicanalizado de baja sinuosidad b) zona 2 o media formada por un sistema fluvial monocanalizado de alta sinuosidad, c) zona 3 o distal formada por un sistema fluvial monocanalizado de baja sinuosidad, d) zona 4 que es la región de interacción entre el mega-abanico del río Huaco, el río Bermejo y el río Jáchal. De acuerdo con las características estudiadas, el mega-abanico del río Huaco corresponde a un SFD, de tipo entrelazado dominante y anabranching multicanalizado (según las clasificaciones de SFD propuestas). Es importante destacar que la tasa de aporte de sedimentos estudiados en el megaabanico no sólo se produce asociada a la acción fluvial, sino también a la eólica la que es muy importante, especialmente cuando las geoformas son abandonadas.

ABSTRACT: In the last 15 years, studies in modern intermontane valleys and the geological record proposed the distributive fluvial system model (DFS) as a way to explain fluvial deposits from piedmont areas (e.g., Nichols y Fisher, 2007; Hartley et al., 2010; Weissman et al., 2010). The DFS model includes several sedimentary proposals previously defined (terminal fans, fluvial fans; Kelly y Olsen, 1993; Blair y McPherson, 1994; Nichols y Fisher, 2007; Colombo, 2010). DFS are generally located away from the mountain front, developed on low-slope piedmonts, dominated by fluid sedimentary processes, and have a fan-shaped convex and lobate topography (Friend, 1978; Nichols y Fisher, 2007; Hartley et al., 2010). This study aimed to contribute to the geomorphological and sedimentological characterization and comprehension of the Huaco River mega-fan, located in the central-northern part of the Bermejo Broken Foreland Basin (30°14'34.98" S-68°15'50.41" W, Jordan, 2001; Fig. 1). The Huaco River watershed includes Central and Eastern Precordillera sedimentary rocks developed under a seasonal snowy highmountain climate and a seasonal desert climate. Unlike the Huaco River watershed climate, the mega-fan depositional area displays under seasonal arid to hyper-arid climate. Since DFS are widely located in modern continental basins, understanding its facies distribution is crucial for natural resources exploration. This study aims to analyze the geomorphological units comprising the Huaco mega-fan and its sedimentologic features to test if it corresponds to the DFS model. A detailed geomorphological survey was performed on a GIS platform (QGIS 3.22 and Google Earth). The geomorphological survey included multispectral satellite image processing (Aster and Sentinel 2), high-resolution natural color satellite images (Google Earth, Bing) slope determination through an ALOS PALSAR DEM and Google Earth DEMs, and sinuosity index calculation. A total of eight geoforms were recognized via the GIS platform. Multispectral satellite image processing was helpful in obtaining proxies of sediment textures that were checked in the field. Fieldwork included facies analysis in natural and artificial trenches. The Huaco River flows through the fault and thrust belts of the Central and Eastern Precordillera for 18 km until it loses confinement in Punta del Agua, where it deposits a megafan. The Huaco mega-fan exhibits a maximum width of 39 km and a maximum length of 35 km, covering a total area of 504 km2 in the Bermejo Valley. Seven geomorphological units were recognized in the Huaco mega-fan, most of them comprising several subunits (Fig. 3). The units are Eolian Plain (PE), Main Channels (CP), Interchannel Area (AI), Abandoned Interchannel Area (AIA), Floodplain (LI), Undiscriminated Terminal Splays (LTI), Terminal Plain (PT) and Huaco, Bermejo and Jáchal Interaction zone (InterHBJ). The CP unit includes gravel bars (BG), abandoned main channel (CPA), and scroll bars (BM) subunits. Sandy gravel bars (BAG) and minor channels (CM) are part of the AI unit. The abandoned stage of AI are included in the AIA geomorphological subunits. Crevasse splays are part of the LI unit and terminal splays of the LTI unit. Finally, terminal plain channels (CPT) and their abandoned stage (CPTA) are located in the PT unit. Based on the geomorphologic analysis, the mega-fan of the Huaco River is divided into proximal, medial, distal, and interaction zones. The proximal zone displays a multichannel braided fluvial system with a low IS (1,27) and low slope (0.39°). The geomorphological units that characterize the proximal zone are CP (BG), AI, and PE. The medial zone exhibits higher IS (2.32) and lower slope (0.20°) than the proximal zone. A single-channel high sinuosity fluvial system dominates the medial zone with BM and CP geomorphological units and a welldeveloped floodplain area. Floodplain (LI) includes crevasse splays (LD), abandoned terminal splays (LTA), and the PE. Also, some abandoned branches of the Huaco River are observed (CPA). A decrease in slope (0.17°), though an increase in IS (1.21), changes the river planform to a single thread low sinuosity fluvial system and defines the distal zone. The distal zone includes many geomorphological units since the Huaco mega-fan is or has been split into five regions with active and abandoned geomorphological units (BM, CP, CPA, LTI, LTIA, PT, among others). The third region is the active branch of the Huaco mega-fan, and the second region shows the most significant grade of abandonment. Although eolian aggradation occurs during the dry season, in the abandonment stage, geomorphological units are subjected to intense eolian aggradation and deflation according to the position in the mega-fan. Finally, the interaction zone occurs downstream, where the mega-fan of the Huaco River and the deposits of the Jáchal and Bermejo Rivers gather. Natural and artificial trenches were studied in geomorphological units observed in proximal and distal zones of the mega-fan. CP and AI geomorphological units exhibit stream flow processes (Gci, Gcm, Sh, Sl, Sr, Fl). PE covers channelized facies and is characterized by Slg, Shm, Sre, and Fl facies. Terminal splay lobes are typically sandy and silty facies deposited when channelized streams lose capacity and competence. Terminal plain channels are shallow (10- cm deep) and connect different terminal splay lobes. Eolian aggradation occurs in channelized and terminal plain facies, reworking deposited material and creating zibars in the forested terminal lobes area. According to the results, the domain of stream flow processes, downstream loss of channel capacity and competence, and distributary drainage pattern, the mega-fan of the Huaco River fits the DFS model. Considering the proposed types, it corresponds to a type III distributive fluvial system of Hartyel et al. (2010), single braided to high sinuosity channel, and a type II multi-thread anabranching according to Davidson et al. (2013). Terminal splays of the Huaco River show similar facies arrangement than those described in Douglas Creek and El Ebro basin (Nichols y Fisher, 2007; Fisher et al., 2008). Terminal splays of the Huaco River are characterized by laminated sands and silty, faintly convex-shaped bodies up to 0.70 m thick, 1-km wide, and 2-km long. They also present higher vegetation concentrations than floodplain areas. Old trees and shrubs occupy terminal splay complexes, serving as natural traps for transported vegetation during unconfined flow occurrence. As it is observed in other DFS of Argentina of different climatic zones, the aeolian activity could significantly contribute to the sediment supply of the depositional systems, especially during the river branches abandonment stage