Alluvial deposits and tectonic terraces, Pleistocene, Medeiro Hills, NW-Argentina

GEORGIEFF, S.M., GONZÁLEZ BONORINO, G.

Delft, Holanda

Conferencia; 8th International Conference on Fluvial Sedimentology; 2005

Universidad de Delft

Medeiro Hills are located in an intermountain valley in Salta Province, northwestern Argentina. These hills are the end south of  the Vaqueros Range anticline and they are elongated in north-south direction. In the northeast corner of hills there are six terraces, and a high ravine in the west flank, sedimentology of these deposits and the origin of terraces are the main goals of this work. Geologic map from aerial photographs, satellite images and field control, sedimentological descriptions on vertical walls of terraces and ravines, descriptions of the sedimentary conglomerates (size and lithology), paleocurrents directions measured on imbricated clasts and two topographic cross sections, each one of 2 km long, measured normal to terraces were recorded in different places of Medeiro Hills. Angular unconformities were also observed between Tertiary and Quaternary deposits in three terraces. Tertiary deposits correspond to Piquete Formation (Pliocene-lower Pleistocene; a radiometric age of 1.3 Ma was obtained near the top of formation), formed by sandy gravel in tabular strata alternating with beds of yellow red siltstone. Strike and dip of strata is N 30º/25º W in the west flank of the anticline, the dip in the east flank is about 10º. Quaternary deposits belong to Tajamar Formation (Pleistocene), and they are formed by pebbles, cobbles and blocks, from 2 to 50 cm, in tabular to lenticular strata of 2 to 10 m thick. The terraces were formed on the right margin of Vaqueros River and they were not preserved on the other margin. The cross sections show the changes of both transverse slope and ravine highs on each terraces. The angles of transverse slope decrease systematically from 6.2º (1st, oldest, terrace) to 0,5º(6th, youngest, terrace), these changes can be grouped in two series: 1st and 2nd terraces show angles above 4.2º and from 3rd to 6th terraces the angle is lower than 1.5º. The ravine highs of each terraces vary from 22 to 30 m and they also show systematic changes. The analysis of  paleocurrents indicate a gentle rotation from SSE (paleocurrents of 1st terrace) to SE (paleocurrents of 6th terrace)  direction; at present, the Vaqueros River flows toward the ESE. Other relevant data is that the Tajamar Formation grain size is coarser than Piquete Formation. The Tajamar Formation deposits are interpreted as fluvial gravel channel-bars probably developed onto braided plain. The evidences point out that the uplift and folding of Vaqueros Range and its south end, Medeiro Hills, started on Pleistocene. The decrease in the transverse slope from the older to the younger terrace and the sedimentary deposits over the terraces are related with this tectonic activity. This work was granted financial support from CONICET (PIP 400/98) and CIUNSa (Nº 1230).