Characterization of alluvial bajada facies distribution using TM imagery

Automatic mapping techniques using multiband satellite image information have been used to study sediment grain‐size variations on an alluvial bajada. A previous study of sparsely vegetated alluvial surfaces in central‐western Argentina showed that the reflectivity recorded in seven‐band Landsat TM images is controlled by sediment composition, age and grain size. At diameters >4 mm, clast composition influences image information, while at grain sizes smaller than sand, clay mineralogy begins to influence spectral characteristics. The progressive increase in desert varnish and the loss of fine grain‐size fractions as a result of deflation cause the age influence. The bajada sedimentary environment is well suited for testing the influence of grain‐size variability on image recovery, as most drainage basins are small and sourced by a single geological unit, which produces compositionally homogeneous alluvial surfaces. Additionally, most drainage lines are active within intervals of 10–50 years, reducing the effect of surface ageing. Larger than average drainage basins produce oversized drainage lines that disrupt the bajada, generate individual alluvial fans and have slightly different compositions. Two typical bajadas were selected to map grain‐size characteristics using automatic classification techniques. The obtained classes were checked in the field, and grain‐size and compositional counts were completed. Although both bajadas are very different in composition, most grain‐size curves showed similar shapes, suggesting that deposition took place by the same process (hyperconcentrated flash floods). The values of the median and mean were consistent across the same class between both bajadas. Thus, unsupervised classification techniques are useful for mapping sediment grain size, although minor field control is needed. Image classes represent areas of similar grain size, which are elongated parallel to the mountain front in an active alluvial bajada, indicating a homogeneous distribution of sedimentary processes along strike. Changes in the width of image classes indicate different downstream fining rates closely related to the topographic gap and the slope change rate. In contrast to bajadas, alluvial fans have semi‐circular belts and a pie‐piece‐shaped area in which most active streams are located. Thus, unlike fans, bajadas lack autocyclic mechanisms for producing heterogeneous sedimentary sequences. The sedimentary log of an ancient bajada was measured in order to show the influence of allocyclic factors in the absence of autocyclicity.