BLACK CLAYEY SOIL (VERTISOL) IN THE LERMA VALLEY, MICHOACÁN, NORTH-CENTRAL MEXICO: A HINT TO THE ENVIRONMENTAL AND CULTURAL CHANGES DURING THE LATE CLASSIC PERIOD? (Invited Presentation)

A conspicuous feature of cultural history in Mesoamerica is its cyclic trajectory: extensive periods of ascendant development alternated with rapid processes of deterioration. Recent studies put an increasing emphasis on the hypotheses involving ecological factors. Catastrophic events (climatic, tectonic, volcanic, etc.) frequently provoke geomorphological instability which interrupts normal pedogenesis and switches the processes of surface erosion and/or sedimentation. In the Lerma Valley, particularly in the Tres Mezquites area, extensive excavations have been conducted to estimate the nature and density of the pre-hispanic settlements in the alluvial plain. Most of the archaeological structures are covered by dark clayey soils (Vertisols). Besides the dark Vertisol burying the archaeological structures, an accumulation of black earth has been detected on the floors and walls of a house-like structure. We have studied in detail the Vertisols and the black earth around the sites in order to establish to which extent this dark earth is an in situ product of the soil forming factors; or if its accumulation is due to erosion/sedimentation processes, which could affect the occupation of the site. The Vertisols and the dark earth are characterized by different means (physical, chemical, micromorphology, clay mineralogy and radiocarbon dating). The results show that both the Vertisols and the black earth are very clayey, with angular blocky structures and slickensides (properties which are typical of vertic soils), and smectite as the main clay mineral. Besides smectite, under the microscope a high amount of fresh volcanic glass is identified, suggesting that volcanism can be the responsible of the abandonment. However, there are no volcanic events related to the age of abandonment. In consequence, we consider that landslides produced by seismic activity are the catastrophic events that strongly impacted the settlements.