Geomorfología y facies sedimentarias 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 y 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, encaso de que lo sea, definir de qué tipo según 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 es la cuenca de antepaís fragmentada de la faja plegada y corrida andina entre los 29° 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 obtuvieron 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; y d) zona 4, correspondiente a la región de interacción entre el mega-abanico del río Huaco, el río Bermejo del río Jáchal. De acuerdo con las características estudiadas y las clasificaciones de SFD propuestas, el mega-abanico del río Huaco corresponde a un SFD de tipo entrelazado dominante y anabranching multicanalizado. Es importante destacar que la tasa de aporte de sedimentos en el mega-abanico no sólo se produce asociada a la acción fluvial, sino también a la acción eólica la que es muy importante, especialmente cuando las geoformas son abandonadas.

In the past 15 years, studies in modern intermontane valleys and the geological record proposed the model for distributive fluvial systems (DFS) as a way to explain fluvial deposits from piedmont areas (e.g., Nichols and Fisher, 2007; Hartley et al., 2010; Weissman et al., 2010). The DFS model includes several previously defined sedimentary proposals, such as terminal fans or fluvial fans (Kelly and Olsen, 1993; Blair and McPherson, 1994; Nichols and Fisher, 2007; Colombo, 2010). DFS are generally located away from the mountain front, developed on lowslope piedmonts, dominated by fluid sedimentary processes, and have a fan-shaped convex and lobate topography (Friend, 1978; Nichols and Fisher, 2007; Hartley et al., 2010). Since DFS are widely located in modern continental basins, understanding their facies distribution is crucial for natural resource exploration. This study aims 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 (S30°14’34.98”; W68°15’50.41”; Jordan, 2001; Fig. 1) and to test if it corresponds to the DFS model. The Huaco River watershed includes Central and Eastern Precordillera sedimentary rocks developed under a seasonal snowy high-mountain climate and a seasonal desert climate. Unlike the Huaco River watershed climate, the mega-fan depositional area develops under a seasonal arid to hyper-arid climate. 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 landforms were recognized in the GIS platform. Multispectral satellite image processing helps in obtaining proxies for sediment textures that were checked in the field. Fieldwork included facies analysis in natural and artificial trenches. The Huaco River flows through the fold and thrust belts of the Central and Eastern Precordillera for 18 km until it loses confinement in Punta del Agua, where it deposits a mega-fan. 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): 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 is 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 Huaco mega-fan is divided into proximal, medial, distal, and interaction zones. The proximal zone displays a multichannel braided fluvial system with a low sinuosity index (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 a higher sinuosity index (2.32) and a lower slope (0.20°) than the proximal zone. A singlechannel, high-sinuosity fluvial system dominates the medial zone with BM and CP geomorphological units and a well-developed 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 sinuosity index (1.21), changes the river planform to a single-thread low-sinuosity fluvial system and defines the distal zone. The latter includes the geomorphological units BM, CP, CPA, LTI, LTIA, and PT, which resemble abandoned and active branches of the Huaco megafan. Based on these criteria, the distal zone of the Huaco mega-fan was split up into five regions from West to East. From them, the third region shows more active geomorphological units because it is currently the functional branch of the Huaco mega-fan. In contrast, the second region exhibits the most significant degree of abandonment where aeolian deposits cover a large portion of the zone. Although aeolian aggradation occurs during the abandonment stage of the dry season, geomorphological units are subjected to intense aeolian aggradation and deflation according to the position in the mega-fan. Finally, the interaction zone occurs downstream, where the Huaco mega-fan and the Jáchal and Bermejo rivers deposits join together. Natural and artificial trenches from proximal and distal zones of the mega-fan allowed additional observations of geomorphological units. CP and AI geomorphological units exhibit stream flow processes (facies Gci, Gcm, Sh, Sl, Sr, Fl). PE covers channelized facies characterized by Slg, Shm, Sre, and Fl facies. Terminal splay lobes are typically sandy and silty facies deposited when channelized facies lose capacity and competence. Terminal plain channels are shallow (10 cm deep) and connect different terminal splay lobes. Aeolian 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 Huaco mega-fan fits to the DFS model. Considering the proposed types, it corresponds to a type III distributive fluvial system of Hartley 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 the Douglas Creek and El Ebro basin (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 DFSs of Argentina under different climates, the aeolian activity could significantly contribute to the sediment supply of the depositional systems, especially during the river branches abandonment stage.