Resumen

SPALLETI, Luis Antonio. Evolución de los ambientes fluviales en el Triásico de la Sierra Pintada (Mendoza, Argentina): análisis sobre la influencia de controles intrínsecos y extrínsecos al sistema depositacional. Rev. Asoc. Argent. Sedimentol. [online]. 1994, vol.1, n.2, pp.125-142. ISSN 1853-6360.

The Puesto Viejo Formation (Triassic) outcrops at the Sierra Pintada in San Rafael (Mendoza) and constitutes a continental volcaniclastic succession of approximately 300 m thick, in which pyroclastic flow and fall deposits as well as basic volcanic rocks are intercalated. From the survey of three detail sections (scale 1:100), facies and sedimentary facies associations were defined and nine types of sedimentary bodies were determined. Among the coarse-grained deposits, paraconglomerates (Gms), resedimented-type conglomerates (Gmhy) and orthoconglomerates (Gm, Gmi. Gh, Gl, Gt and Gp) were found. Sandy facies (S), sandy-gravelly facies (SG) and tuffaceous sandstones facies (ST) are very common, each of them showing different features according to their primary structures. Among claystones Fm facies predominate. Primary pyroclastic deposits generated by fall (tuffs and chonites, T and FT facies, respectively) as well as flow (facies I) processes are frequently found. From the hierarchization of unconformities, the vertical relationships between facies and the elementary sequences, different types of sedimentary bodies have been defined. These bodies have allowed to assign the clastic deposits to various processes and agents, such as unchannelized proximal floods, hyperconcentrated flows, subaerial debris flows, fluvial currents (channels, longitudinal bars. Cross bars) and flood plains (aqueous and aerial suspensions, crevasse and crevasse-splay). This multiple variety of bodies shows the complexity of the accumulation processes that participated during the generation of the Triassic succession. For the Puesto Viejo Formation three main depositional systems are proposed. The first one belongs to an alluvial fan (medium-distal) characterized by coarse materials in channelized bodies and sandy bars, sheet floods, hyperconcentrated flows and debris flows (lahars). This environment was affected by remarkable variations in the discharge, instability of slopes in the source area, poor incision and braided drainage network. The second system is a low sinuosity meandering fluvial system. In this system, which is characterized by a coarse member made up of conglomerates and conglomeratic sandstones in amalgamated bodies formed by migration and aggradation of tridimensional channel bars. In the fine member, fine fall out deposits with thin lenticular conglomerates assigned to crevasse channel fills are seen. The third system was formed in lower gradient conditions and corresponds to a high sinuosity fluvial environment. Its sandy coarse member is arranged on important unconformities and shows a marked finning upward arrangement; facies and primary structures suggest that most sandstones accumulated in processes such as point bars. The fine member, of the high sinuosity system is formed by thick fine-grained flood plain deposits produced by aqueous and eolian (loessites) decantation, with frequent paleosoil levels. According to sedimentary-body geometry, intercalations of coarser deposits are assigned to crevasses, crevasse splays and levees. Sedimentation of the Puesto Viejo Formation was strongly controlled by external or allocyclic factors. On one side, the acidic explosive volcanism which was increased towards the final stages of the accumulation process was a fundamental factor in the surrounding relief modelization and in the supply of clasts of quite different grain-size to the depositional systems. It has also had influence on the generation of some epiclastic deposits, such as those of viscous flows (lahars and hyperconcentrated flows), the mixed-load fluvial deposits and the loessic deposits. Based on the attributes of the flood plain and interfan deposits and also due to the existence of loessites and paleosoils, it is inferred that the climatic conditions were subhumid to humid; however, data suggest a greater desiccation for the uppermost terms of the sedimentary succession. Finally, the tectonics has had an important influence on the development of the unit. Transcurrent movements with vertical component along marginal faults (Huarpic diastrophic phase) created the space for the generation of the depocenter, which in its early filling stages was characterized by proximal fluvial systems developed during a period in which an important fall in the base-level occurred. The ulterior development of fining-upward megasequences reflects a reduction in the coarse sediment supply due to a decrease in the current power, which is here to the combined effect of denudation of the surrounding positive areas, a base-level rise and a fast quick basin subsidence. The posthumous evolution stage in sedimentary fill shows a tectonic reactivation with intensified explosive volcanism, probably related to the generation of calderas in areas which are near the Puesto Viejo basin.

Palabras clave : Triassic; Facies; Fluvial systems; Tectosedimentation.

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