Black clayey earth in archaeological contexts in the Lerma Valley, Michoacán, North-Central Mexico: a hint to apprehend changes during the late Classic period.

International audienceCatastrophic events (climatic, tectonic, volcanic, etc.) frequently provoke geomorphological instability which interrupts normal pedogenesis and switches the processes of surface erosion and/or sedimentation. These processes have an effect in the distribution of prehispanic settlements and can be the cause of the abandonment of cities. In the Lerma Valley, particularly in the Tres Mezquites area, surveys, test-pits and excavations revealed a dense pre-Hispanic occupation during the first millennium in the alluvial plain and piedmonts. Most of the archaeological structures are covered by dark clayey soils (Vertisols). In particular an accumulation of black earth with Vertisol properties was detected at the Classic PA 3_El Pitayo site, on the floor of collapsed buildings and adjacent exterior spaces. The archaeological data shows that the collapse was caused by a sudden and uncontrolled event and that the destroyed structures were quickly rebuilt. We have studied in detail the Vertisols and the black earth in and around the archaeological site in order to establish if the accumulation was due to an intentional levelling, or to which extent this dark earth was an in situ product of the soil forming factors, or if it was due to erosion/sedimentation processes, which could have affected the occupation of the site. In this paper, we present the archaeological context and its stratigraphy, and the main results of the Vertisols and black earth analysis. The sediments were 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 an angular blocky structure and slickensides (properties typical of vertic soils), and smectite as the main clay mineral. The characteristics of the black earth and how it was deposited within the archaeological context lead us to abandon the hypothesis of an intentional filling. Besides smectite, we identified under the microscope a high amount of fresh volcanic glass, suggesting that volcanic activity could be related with the deposition of the black earth. However, there are no known dated volcanic events in the region for the time of destruction of the site. Consequently, and until additional volcanic data becomes available, we propose that the deposition of the black earth is the result of a mudslide triggered by seismic activity/climatic events which impacted the settlements in the Lerma Valley