Materiales parentales de los suelos de la Llanura Pampeana: El aporte volcánico andino

Abstract

La mayor parte de los suelos pampeanos se desarrollan a partir de una extensa cubierta sedimentaria de depósitos loéssicos hacia el este y mantos de arena -campos de dunas hacia eloeste, acumulados principalmente en los últimos ~30.000 años. El objetivo de este capítulo esevaluar el papel del vulcanismo en la generación de los materiales parentales caracterizadospor la dominancia de partículas de naturaleza volcánica (trizas vítreas, líticos volcánicos, mineralesvolcánicos) procedentes de los Andes. El comienzo de la acumulación de sedimentosvolcánicos andinos en la llanura pampeana se vincula con el levantamiento andino duranteel Mioceno (~12 Ma), motivo por el cual se los clasifica como materiales sedimentarios sinorogénicos. Los aportes volcánicos andinos directos (caída de cenizas a partir de erupcionesen la cordillera) proceden de la zona volcánica del sur (33°-38°S), la Puna (zona volcánicacentral, sensu Stern, 2004) y la Patagonia austral (e.g. volcán Hudson, Puyehue) al sur de los38°S. La señal geoquímica (elementos traza y elementos de tierras raras) es importante pararastrear las fuentes de emisión de las cenizas. La re-suspensión de las partículas en los mesesposteriores es un mecanismo de importancia a tener en cuenta en las consideraciones e interpretaciones generales sobre la adición de partículas volcánicas en la superficie de los suelos.

The Pampean plain, east of the Andes (~32-38°S, Fig. 1 ), is the economic and agricultural center of Argentina. The origin and evolution of the Pampean soils of central Argentina is the subject of numerous contributions including several studies on the composition of the parent materials. The oustanding feature of the sediments is the dominance of Andean volcanic particles (volcanic glass, volcanic lithics, volcanic minerals). The goal of this chapter is to discuss the role played by the volcanism in the generation of the parent material of the Pampean plain soils. In this respect, several questions are posed ¿Why was there Andean supply to the Pampean plain?, What process is responsible for the abundance of volcanic material? ¿When did the Andean supply begin?, was it chronological restricted?¿Was there a continuous supply of Andean sediments, since when?. Why the Pampean plain was the depositional setting of the Andean volcanic sediments?. The Pampean plain is a vast geomorphological environment (~600.000 km2 ) of transitional boundaries with low relative relief and heights gradually decreasing to the Atlantic Ocean. The wide latitudinal and longitudinal range of the plain is dominated by temperate climate with precipitations gradually decreasing from NE to SW. The parent material of the Pampean soils is an extensive sedimentary cover eastwards and sand mantles-sand dunes westwards, resulting from the aeolian dynamic during the last ~30,000 yr. The accumulation occurred under more arid and colder conditions during most of the last Glacial Maximum and the deglaciation, until the early Holocene/mid Holocene (~11, 700- 8000 yr BP), and the mid Holocene (~5000 yr BP), according to the sector of the Pampean plain considered (Zárate, 2015 and references therein). The still meager geochemical analysis of the parent material, reveals a compositional variation during the last 12 Ma (i.e. since the Late Miocene to the present) showing a more silicic trend which indicates a compositional change in the Andean magmatic arc (Schultz et al., 2006). A remarkable characteristic of the Pampean soil parent material is the occurrence of interbedded tephras with variable thicknesses and ages ranging from the late Miocene -Pliocene to the Holocene (e.g. Frenguelli, 1928; Kraglievich, 1952; Bigazzi et al., 1995, 1996; Gentile & Ribot, 2001). The tephras represent ash falls episodes (sand grain size) from Andean eruptions, basically the Southern volcanic zone (Ramos & Almeida, 2000; Stern, 2004). The eruption of the Quizapú volcano (April 10, 1932) located in the Andes of southern Mendoza, was the focus of numerous contributions. It was the greatest XXth century volcanic event, a plinian eruption that generated ~9.5 km3 of pyroclastic material (Hildreth & Drake, 1992). The grain size zonation and the mineralogical-geochemical composition indicate that most of the soil parent material come from outcrops of volcanic and pyroclastic sedimentary rocks exposed in the Andes Cordillera. Also, contributions from different secondary sources are present according to the area considered (Pampa Ondulada, Pampa Arenosa, Pampa interserrana, Pampa arenosa). The occurrence of volcanic episodes in the Cordillera de los Andes and the resulting generation of pyroclastic sediment and volcanic rocks since the late Miocene, are related to the general geotectonic setting. Accordingly, the evolution of the Pampean plain, located in the distal Andean foreland, is associated with the tectonic dynamic of the Andes, and the expansion of the South Atlantic Ocean. Hence, the volcanic activity is related to the development and evolution of the magmatic arc resulting from the collision of the South American plate with the Pacific plate (Aguilera, this volume). Thus, the volcanic activity generated by the Andes uplift, determined the abundance of volcanic and pyroclastic rocks of the Andes Cordillera. The Andean volcanic sediments were accumulated in the Cordillera piedmont environments represented by ample alluvial fans of the drainage system located in the fringe of the Pampean plain (Colorado river system, and its major tributary Desaguadero-Salado-Chadileuvú-Curacó). The fluvial system drains the Andes Cordillera and the mountain blocks of the fragmented foreland along an extensive latitudinal segment (~29°-41°S). A complex mechanism of erosion, transport and sedimentation in the alluvial fans has been continuous since the Andean uplift up to the present. The alluvial fans are geomorphological settings of high sediment availability from which the volcanic particles were deflated and transported to the Pampean plain. Also, extensive outcrops of volcaniclastic sediments are present south of 38°S, at the headwaters of the Colorado and Negro rivers (Andreis, 1965), another source area of the volcaniclastic sediment of the southern Pampas (Zárate & Blasi, 1993). The deposits of the Mendoza piedmont and the Colorado-Negro rivers have been subjected to repeated cycles of erosion, transport, and reworking, explaining the occurrence of rounded grains of heavy mineral in the loess deposits. The sedimentary material was deflated, and accumulated in the Pampean plain (e.g. pampa interserrana, Buenos Aires province, Zárate & Blasi, 1993). The dynamic of the tectonic setting in southern South America gave way to the Andean uplift and the beginning of the volcanic sediments accumulation around 12 Ma (late Miocene) (Folguera & Zárate, 2009). It is the beginning of the low relief environment of the Pampean plain. The history of its development is recorded by an average thickness of 200 meters of Miocene-Pliocene and Quaternary sandy silts made up of sedimentary material related to the uplift of the Andes Cordillera. As a result, the sedimentary sequence including the parent material of the Pampean soils at the surface, is composed of sinorogenic deposits (i.e. related to the uplift of the Andes, Folguera & Zárate, 2009). Due to the latitudinal range of the Pampean plain, direct volcanic supplies come from eruptions in the Southern Volcanic zone (33°-38°S), the Puna (central volcanic zone, sensu Stern, 2004) and southern Patagonia, south of 38°S (e.g. Hudson, Puyehue volcanoes). Supplies of volcanic ashes from Puna eruptions are present in the northern Pampas. Consequently, the heterogeneity of the Andean volcanic sediments must be considered when the soils profiles are analyzed. The geochemical signature (trace elements, REE elements) is essential to track the source of the volcanic ashes. Besides, the volcanic eruptions of the last decades permitted to understand the dynamic of the direct supply of pyroclastic material (e.g. volcanic ash) to the soil profile. The process is controlled not only by the type of volcanic eruption itself, but the weather conditions during and after the event. The re-suspension of particles already accumulated after an eruption can be added to the soil surface.