Micromorphology and OSL dating of the basal part of the loess–paleosol sequence at La Mesada in Tucumán province, Northwest Argentina

The basal part of the 42-m loess–paleosol sequence at La Mesada near Tucumán in NW Argentina reported by Zinck and Sayago (J. S. Am. Earth Sci. 12 (1999) 293; Quat. Int. 78 (2001) 11) is re-examined here using a combination of micromorphology and optically stimulated luminescence (OSL) dating. Depth distributions of microstructure, clay coatings and secondary carbonate features enable the development of this part of the sequence to be reconstructed in terms of a series of pedosedimentary stages. Stage one involved accretionary pedogenesis with loess being modified by bioturbation (and possibly leaching) processes as it accumulated. Stage two was marked by a reduction in loess deposition and establishment of a relatively stable surface at which bioturbation, leaching and clay translocation processes were dominant. Stage three represented a return to accretionary pedogenesis followed by establishment of a new relatively stable surface and vertical leaching and clay translocation processes with some welding of secondary carbonate features on the underlying older soil. The three OSL dates, ranging from ca. 150 to Full-size image (<1 K) BP, suggest that the whole sequence is significantly older than indicated by the radiocarbon chronology (Full-size image (