Landslide damming lakes in the Matienzo basin, Mendoza province

- LAURO C., MOREIRAS S.M. Y MASTRANTONIO L.E.

Mendoza

Congreso; 18th International Sedimentologist Congress; 2010

IAS

Along the Mendoza-Las Cuevas river valley (32ºS) in the Central Andes at least four dammed palaeo-lakes have been identified (Fauqué et al., 2000). Besides, four impounded palaeo-lakes were inferred by Suarez (1983) in the upper basin of Las Cuevas river (Matienzo gully) assuming a glacial origin for damming deposits. However, recent studies reinterpreted the genesis of these deposits as landslides (Moreiras, 2007; Moreiras et al., 2008). An integral research to determinate the accurate role of these geodynamic processes and morphometric analyses of associated palaeo-lakes being previously ignored were encouraged in this research.   The origin of Quaternary deposits was established by geomorphological studies and detailed analyses. Four rotational slides denominated respectively Goyete, Negro, Susanita and Matienzo were identified. In addition, the Lagunita rock avalanche generated in the western hillslope of the valley was recognised; as well as three relit Pleistocene lateral moraines and the Holocene moraine of Casa de Piedra.   Landslides are characterized by local deposits with no continuity along the valley.  They are poorly sorted deposits with angular and subangular clasts. Also they show a great variability in the volumetric relation between clasts-blocks/matrix and a homogenous lithology. They usually have low matrix content (30% - 50%) commonly rich in sand fraction (60%). Generally, they have great blocks on the top of landslide deposit. On the other hand, moraines are characterized by heterogeneous lithologies conformed by Jurassic-cretaceous traquites, basalts, vulcanites, granites, shales and sandstones. In these deposits predominates subangular –subrounded blocks/clasts. They have higher sandy matrix content where predominate the 500 - 250 microns fraction.   Accordingly to above, five of the palaeo-lakes were generated by gravitational collapses. These palaeo-lakes had a great areal distribution. The smallest one was generated by Negro landslides turning out 594 m long and covering 55226 m2 with a volume of 0.16 hm3, whereas the biggest lakes associated to the Susanita landslide was 1496 m long with an extension of 32,6140 m2 and 2,83 hm3 in volume.   Concerning to stability of palaeo-lakes, they seem to have a short life as their cost lines could not been determined and lacustrine sediments are absent. Based on the blockage index (BI) (Ermini & Casagli, 2003), all paleo-lakes, except for the stable Goyete lake with a BI= 2.43, result unstable.   The re-interpretation of Quaternary deposits of Matienzo gully improved the knowledge of regional palaeo-climate conditions remarking the hazard of impounded lakes associated to gravitational collapses in the mountains areas.