SEDIMENTOLOGY OFFLUVIALGRA VELS OF COLORADO RIVER, ARGENTINA.

Abstract

This paperdeals with the sedimentology of the psephitic deposits of the Colorado River (Argentina), a long stream flowing through the arid areas from the Cordillera de Los Andes (Argentina) to the Atlantic Ocean. The study begins with the description of the essential characteristics of the river basin at headwaters. Grain size analyses of gravels were made by counting and then converting into ponderal frecuency. Mean size of gravels decreses downstream. The different lithological groups also show grain size sorting. Lithologically, the gravels are made up oftwo type ofvu1canites (basaltic an acid-intermediate ones) (more than 71 %), granites (21 to 12%) and sedimentary rock fragments (15 to 2%). Granites, being more resistant to abrasion, tend to concentrate downstream an also in the large size grade. Sedimentary gravels tend to disminish with transport increaase while vulcanites remain fairly stable. The morphological study of the Colorado river gravels comprises determination of geometricity, flatness, sphericity and roundness. It is shown that the shape properties vary with distance oftransportation and that they are affected both by grain size and morphological sorting. The significance of these variations and the evolution of morphological properties are discused either with grain size or in the lithological groups, downstream. Relationships among size, lithological and morphological characteristics are also analysed.