Facies y geometrías de los depósitos aluviales cuaternarios en la quebrada del Portezuelo, Sierra de Mojotoro, provincia de Salta, Argentina

salta city is located at the northern end of the lerma valley, which is a quaternary tectonic depression bounded along the east by the mojotoro range, lying on the eastern border of cordillera oriental geological province (baldis et al., 1976). the mojotoro range, with peaks at about 1,500 m a.s.l., is underlain by proterozoic and lower paleozoic sedimentary rocks in an east-vergent anticline. the portezuelo canyon is an east-west wind gap across the mojotoro range (fig. 1). the portezuelo canyon is a narrow valley, 500 to 1,000 m in width at a mean altitude of 1,200 m a.s.l. quaternary deposits resting unconformably on paleozoic and older strata are locally preserved in the portezuelo canyon. recent road works on national highway 9 have produced excellent exposures of the quaternary deposits; these exposures are studied in the present paper. studied quaternary deposits in the portezuelo canyon occur as inclined beds (strike/slip: n350°/3°w) exposed almost continuously along approximately 350 m. these deposits are characterized by two main facies, fig. 2: (1) conglomerate with a coarse-grained sandstone matrix, in light yellow (5y 6/2 and 5y 8/2; rock-color chart committee, 1991) beds showing imbricated pebble clusters, trough and low-angle planar cross-stratification, with convex-upward bases and concave-upward to flat tops; bed thickness ranges from 1 to 2 m and exposed widths are in the order of several tens of meters; and (2) siltstone to very fine-grained, light yellow red (10yr 6/4 y 10yr 4/4; rock-color chart committee, 1991), horizontally stratified sandstone beds 1 to 3 m thick, showing parallel lamination and scarce pebbles. more detailed facies subdivision is given in table 1. significant differences with established quaternary formations in the lerma valley lead us to separate these deposits as the portezuelo formation. clast source probably was mostly local from the sides of the ancient portezuelo canyon, fed by small alluvial fans or rock-avalanche dep