Abstract

SURIANO, Julieta  and  LIMARINO, C. Oscar. Piedmont sedimentation in the headwaters of theRío Jachal and Pampa de Gualilán, Precordillera of San Juan. Rev. Asoc. Geol. Argent. [online]. 2009, vol.65, n.3, pp.516-532. ISSN 0004-4822.

This paper deals with the sedimentological characteristics ofpiedmont deposits exposed in intramontane basins of the Precordillera. Detailedstudies carried out along the upper Jáchal river and in the Gualilán areaallowed identifying and characterizing six major types of piedmontaccumulations: 1) talus, 2) colluvial fans dominated by gravity flows, 3)colluvial fans dominated by fluid flows, 4) collector-conoid fluvial system, 5)piedmonts dominated by channalized flows and 6) alluvial fans. The talusdeposits are characterized by the dominance of clast-supported breccias(bearing minimum matrix amounts) that commonly exhibit massive bedding, planaror high angle imbricated clasts fabric. Colluvial fans dominated by gravityflows are formed by massive matrix-supported breccias with minor percentages ofclast-supported breccias. On the contrary, colluvial fans dominated by fluidflows are composed of amalgamated channels infilled by both clast- andmatrix-supported breccias together with intercalation of graded breccias.Accumulations of the collector-conoid fluvial systems are the most diverse incomposition, including from massive matrix-supported breccias to cross-beddedconglomerates and scarce mudstones. In relation to the piedmont dominated bychannalized flows, they comprise massive clast-supported breccias and lessamounts of imbricated or cross-bedded ones. The alluvial fan deposits aremostly composed of cross-bedded clast-supported conglomerates, thoughmatrix-supported conglomerates and breccias also appear as proximal deposits.Different from the previously mentioned deposits, the clast of alluvial fanaccumulations shows more varied lithologic composition. The comparativeanalysis of the piedmonts found in the studied valleys permits to propone aconceptual model for classification of piedmont, on the basis of the extensionof the basins and vertical space of accommodation.

Keywords : Intramontane basins; Piedmonts; Sedimentary models.

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