Paleoenvironmental reconstruction in the western lacustrine plain of Llancanelo Lake, Mendoza, Argentina

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Abstract

Lakes are key sites for studying paleoclimates. Llancanelo Lake (southern Mendoza Province, western Argentina) is an endoreic, highly saline water body located in the southern extreme of a tectonic basin, the Central or Huarpes Depression. The lake is located between the Andean Cordillera, San Rafael Block and Payenia Volcanic Field. The lake evolved as a major regional depocenter during the Pliocene-Quaternary, hence it contains important thicknesses of intra and extra basinal clastic and evaporitic sediments mainly dominated by volcaniclastic products. The main conditioning factors in the lake evolution were arc and back-arc volcanism as well as climatic changes. Geomorphological and sedimentary evidence supports the hypothesis that the lake was in past times larger than in present days. This paper estimates the lake’s former extension on the western lacustrine plain using electromagnetic induction (EMI) and geoelectricity (Multielectrode Resistivity Meter) surveys, as well as shallow wells, along an 8 km long transect perpendicular to the lake’s western shoreline. The geophysical and sedimentological information, as well as microfaunal studies, lab analysis and petrographic/EDAX determinations, support the presence, in the subsoil, of a lacustrine sequence at least 30 m thick composed mainly of volcaniclastic sediments. Volcanic eruptions and climatic changes influenced the evolution of the lake, producing intercalations in the lacustrine sedimentary sequences of ash layers, evaporites, soils, and eolian and swamp deposits.

Introduction

Lakes contain in their sedimentary sequences some of the most complete worldwide paleoclimatic archives at different resolutions and temporal scales (e.g.: Cohen, 2003, Juggins, 2008). In the particular case of Patagonian lakes, they represent key sites for inter-hemispheric linkages for two reasons (Zolitschka et al., 2009): (a) they are located in a very narrow continent dominated by an “oceanic” character, particularly important because the Southern Ocean plays a key role in the global climate system; and (b), the region is located near the Andean cordillera where volcanism has been very active throughout its evolutionary history, and therefore it represents one of the most important source regions for southern hemispheric dust.

In this context, Llancanelo Lake, in southern Mendoza Province, western Argentina (Fig. 1), is the northernmost Patagonian Lake, which is surrounded by both the Andean volcanic arc and the Payenia volcanic back-arc, and therefore it represents a suitable place for paleoclimatic reconstructions.

On the other hand, Llancanelo Lake is a wetland of high environmental sensitivity which is considered, together with the neighboring region of Payenia, as a Natural and Faunistic Reserve. In 1995 the lake was declared a Ramsar Site (Ramsar Convention on Wetlands, Matthews, 1993, http://www.ramsar.org) because of its role in the support of biodiversity, and in 2001 was included in the Montreux Record (a register of endangered and threatened sites as a result of human interference, Ramsar Convention Secretariat, 2004) to ensure its conservation.

Because of these considerations, the region deserves special attention and efforts in carrying out scientific research aimed at studying environmental aspects, which must undoubtedly include paleoenvironmental and evolutionary studies. In this sense, constitutes a significant site for this kind of research activities because of its geotectonic and environmental context as well as the condition given by the complex interplay between extra and intra-basinal sedimentation strongly dependant of external and internal volcanic sources.

The region is located at the foot of the Andes cordillera in a transitional zone between the Patagonian (to the south) and extra-Patagonian (to the north) regions. Major conditioning factors in the regional evolution were: (a) the influence of a strong volcanic activity characterized by the ash-dominated Andean arc volcanism and basaltic flows-dominated back-arc volcanism and (b) climatic changes. The lake is an endoreic basin with predominance of depositional processes and lack of strong erosive processes, hence it allowed a good and eventually complete preservation of the sedimentary sequences deposited through geological times. Therefore, it represents an appropriate setting for the study of geological, environmental and climatic changes occurred in the region during the Quaternary.

The region has not been suitably studied in detail up to now from an integrated paleoclimatic–paleoenvironmental perspective. Few but very significant references exist on those subjects. Groeber (1939) mentioned for the first time the existence in the past (post-glacial times or “Platense”) of a larger-than-today lacustrine body formed during climatic periods wetter than the present. Delpino, 1987, Bermudez and Delpino, 1987, Bermúdez et al., 1993 and Delpino (1993) explored deeper into the evidences of a former larger lake. Particularly Delpino (1993) stated that the ancient lake extended from present Llancanelo to El Nihuil lakes, probably covering an area of 5000 km2 with a water level about 50 m above present lake. According to that author, the evidences that support this preliminary conclusions are: (a) the presence of lacustrine sediments in topographic levels higher than the present lake shoreline; on this matter, recent works by Dieguez et al., 2004, Gil et al., 2005, Guerci et al., 2006 and De Francesco and Dieguez (2006) carried out in outcropping sequences in the Atuel river cliffs northwest of Llancanelo Lake (Fig. 2), indicate the presence of sediments and faunas typical of shallow and vegetated lacustrine environments evolved in periods of larger water availability; (b) the subterranean hydrological system that reveals a connection between Llancanelo and El Nihuil lakes (already mentioned by Ruiz Huidobro and Serrano, 1987, and also by Prieto and Abraham, 1994); and (c) the presence of hydroclastic volcanic cones around Llancanelo Lake, which indicate that past eruptions occurred through a substratum a lot more humid than today with higher subterranean water levels; this last statement is supported by the works of Risso and Drosina (2003) and Risso et al., 2005, Risso et al., 2008 who studied in detail some of the volcanic morphologies which were interpreted as hydroclastic cones and maars. Additionally, the works by Neme et al. (2005) and Gil and Neme (2006), based on archeological evidences, considered paleoclimatic aspects – particularly for Holocene times – of great importance in human occupation.

If it is considered the hypothesis that the lake was larger in the past, the lowlands formerly covered by lacustrine waters that remained exposed to subaerial conditions as the lake level progressively decreased, should be considered as “lacustrine plains”.

As a result of the tectonic, morphological, hydric and climatic aspects, Llancanelo Lake and the ancient lake regions are considered to having had the largest accumulation of sediments in the southern region of Mendoza during the Cenozoic. Because of the depressed and endoreic nature of the environment, the lake contains important thicknesses of sedimentary and volcanic products which have remained there, preserving continuous sequences of the environmental changes.

With the aim of exploring the complexity of paleoenvironmental aspects of the lake useful for demonstrating its larger extension in the past as well as the importance of volcanism and climatic changes as the major conditioning factors that participated in its evolution, an integral, multidisciplinary Project was recently initiated. The project is aimed at studying paleoenvironmental, paleoclimatic and paleovolcanic evidences preserved in the lacustrine plains by applying geophysical and sedimentological methodologies. The present contribution deals with the conclusions arising from the first two years of development of the Project, mainly focused on the evidences of evolution of the western lacustrine plain of Llancanelo Lake.

Section snippets

Geotectonic setting

Llancanelo Lake is located in the southern extreme of a tectonic basin known as Central or Huarpes Depression, which extends along more than 100 km between the Andes cordillera to the west and San Rafael Block to the east, and goes further to the north through the so-called Tunuyán half-graben (Fig. 1). The southern extreme of the basin is the Payenia Volcanic Field.

The basin evolved under tectonic control of extensional character and acquired its definitive configuration in the Quaternary (

Methods

Field works were carried out in April and November 2007, which consisted in: (a) geological observations at a regional scale; (b) geophysical surveys; and (c) realization of short drilling (6 m long) and sampling of subsurface sediments for stratigraphic studies on the western coast of the lake (site known as “La Playa”, Fig. 3a), at the latitude of 35°38, 89′S.

Regional geology and geomorphology

The landscape is dominated by two main geomorphic features: (a) the lake and its surrounding lacustrine plains – mainly composed by volcaniclastic sediments – and (b) a volcanic plain composed of volcanic cones and basaltic lava flows, which undoubtedly represent outstanding elements as geomorphological modelling factors. The boundary between the lacustrine and the volcanic plains is well defined by the sharp termination of the outer border of the basaltic lava flows as they disappear below the

Regional geology

Regional geological and geomorphological aspects are very important as evidences of paleoclimatic, paleoenvironmental and paleovolcanic factors. Large volumes of extra-basinal ash deposits from the active volcanic Andean front as well as from intra-basinal monogenetic volcanoes are present in the sedimentary sequences and soils. The large extension of basalt flows surrounding Llancanelo Lake indicates effusive processes of very fluid lavas during long periods which undoubtedly conditioned the

Discussion

The deployment of geophysical methods like geoelectric and electromagnetism made possible to determine, in the surroundings of Llancanelo Lake, the extension of the sedimentary sequences in the subsoil as well as their regional variability. Although conductivity shows a regional homogeneity, indicating similar sedimentological characteristics for all the surveyed area, slight variations permit to discriminate sectors with differentiated aspects, which can be later interpreted in terms of

Conclusions

The integration of geophysical and geological information permitted the characterization of the uppermost sedimentary sequences in the surroundings of Llancanelo Lake. As a first conclusion it can be stated that in studied area in the western lacustrine plain there are no basalt layers in the upper 30 m of the sequences, even though they crop out in the surroundings.

The superficial very soft and most conductive sediment layers can be associated to the presence of the phreatic level, whereas the

Acknowledgements

This work is part of Project ANPCyT-PICT 2006–07/01311 (Geology and Geophysics of Laguna Llancanelo, Payenia region, Mendoza), period 2008–2011. Field work activities were carried out within the framework and logistical support of the geological surveys performed for the mapping of the Geological Chart N° 3769-03 “Santa María Volcano” in charge of Dr. E.I. Rovere (Plan of Geological Charts of SEGEMAR – Argentine Geological and Mining Survey –). Geophysical activities were partially funded by

References (56)

  • Cátedra de Obras Hidráulicas, 2005. Estudios Hidrológicos, Hidráulicos y Ambientales – Laguna Llancanelo – Mendoza....
  • A.S. Cohen

    Paleolimnology

    (2003)
  • Cortés, J.M., Vinciguerra, P., Yamín, M., Pasini, M.A., 1999. Tectónica Cuaternaria de la región Andina del Nuevo Cuyo...
  • De Francesco, C.G., Dieguez, S., 2006. Paleoambientes del Cuaternario tardío del sur de Mendoza: estado del...
  • D.H. Delpino

    Erupciones basálticas a través de fracturas en el retroarco andino (35-36° lat. S), Mendoza, Argentina

    10° Congreso Geológico Argentino Actas

    (1987)
  • Delpino, D.H., 1993. Fue el sur Mendocino similar a Hawaii? Evidencias del Pasado para entender el Presente. Primeras...
  • Dieguez, S., De Francesco, C., Páez, M., Navarro, D., Quintana, F., Guerci, A., Zárate, M., Giardina, M., Neme, G. and...
  • C.G. Farquharson et al.

    Simultaneous 1D inversion of 685 loop–loop electromagnetic data for magnetic susceptibility and electrical conductivity

    Geophysics

    (2000)
  • Gil, A., Neme, G., 2006. Distribuciones arqueológicas superficiales en Payunia-Llancanelo. Anales de Arqueología y...
  • Gonzalez Diaz, E.F., Fauqué, L. 1993. Geomorfología de Mendoza. Relatorio XII’ Congreso Geológico Argentino y 11’...
  • González Ferrán, O., 1993. Principales erupciones volcánicas en los Andes meridionales. Fuentes potenciales de peligros...
  • Groeber, P., 1939. Informe Geológico sobre la Zona de Embalse del Proyectado Dique en Nihuil (Provincia de Mendoza)....
  • Guerci, A., Paez, M., Dieguez, S., De Francesco, C., Gil, A., Neme, G., olimeni, M., 2006. Estudios paleoambientales al...
  • B.M. Gutiérrez Tellez

    Diatomeas del Pleistoceno de Pehuen-Có (provincia de Buenos Aires): inferencias paleoambientales

    Ameghiniana

    (1996)
  • Hernández, J., Martinis, N., 2006. Particularidades de las cuencas hidrogeológicas explotadas con fines de riego en la...
  • Juggins, S., 2008. Lake sediments as climate archives: strategies for quantifying and reducing uncertainty. In: Proxy...
  • S.M. Kay

    Tertiary to Recent transient shallow subduction zones in the Central and Southern Andes

    15° Congreso Geológico Argentino (Calafate), Actas

    (2002)
  • S.M. Kay et al.

    Magmatic sources, setting and causes of Eocene to Recent Patagonian Plateau magmatism (36–52°S latitude)

    Rev. Asociacion Geologica Argentina

    (2004)
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