Black clayey soil (Vertisol) in the Lerma valley, Michoacán, North‐Central Mexico: A hint of the environmental and cultural changes during the Late Classic period?

Abstract

Catastrophic events (climatic, tectonic, volcanic, etc.) frequently cause geomorphological instability, which interrupts normal pedogenesis and switches the processes of surface erosion and/or sedimentation. These processes have an effect on the distribution of ancient settlements and can be the cause of the abandonment of cities. In the Lerma Valley, particularly in the Tres Mezquites area, surveys and extensive excavations have been conducted to estimate the nature and density of pre‐Hispanic settlements on the alluvial plain and in the piedmonts. Most of the archaeological structures are covered by dark clayey soils (Vertisols). This includes the PA3_El Pitayo site, where an accumulation of black earth has been detected on the floors and walls of a collapsed building. We have studied in detail the Vertisols and the black earth around the site to establish to what 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, or the result of anthropic activity for leveling. 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 that 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 could be responsible for the abandonment of the study site. However, there are no known volcanic events related to the age of abandonment. As a consequence, we consider that landslides triggered by seismic activity/climatic events impacted the settlement.

Impactos de la actividad volcánica en época prehispánica: Estudios edáficos en los sitios arqueológicos de Cerro del Teúl y El Pitayo, Tres Mezquites en el occidente de México

“…En particular, El Pitayo está rodeado por diversas estructuras volcánicas de composición química y edad desconocida. El sitio arqueológico, específicamente la estructura arquitectónica estudiada, se ubica en el piedemonte del Cerro El Arco, asentado sobre sedimentos coluviales, con pendientes suaves cercanas a los 5° (Solleiro-Rebolledo et al, 2021). El Cerro El Arco es un volcán tipo escudo mediano, posiblemente monogenético, de composición andesítico-basáltica, con una altitud que varía entre los 1800-2210 m.s.n.m.…”

THE PETATE AND THE COSMIC ORDER: DISCOVERIES FROM A CLASSIC PERIOD RESIDENTIAL GRAVE IN MICHOACÁN, MEXICO

“…However, the last level of occupation, which was entirely destroyed by recent levelling for agricultural activities, was characterized by three monumental platforms and ceramic materials dating from the Epiclassic period. Excavations of the Classic occupation levels revealed the presence of black clay sediment covering most of the sectors studied and signs of brutal destruction (Darras et al 2017; Solleiro et al 2021).…”

“…Different indicators found on the older occupation surface, including the partial destruction of the north wall and its covering, traces of a fire, large quantities of broken ceramic vessels, and food remains, suggest that the building was destroyed by a violent event (Darras et al 2017). The nature of these indicators and the geoarchaeological approach, which included micromorphological analysis of black clay sediment found over the occupation surface, permit the hypothesis that the destruction of the building was caused by a landslide (Solleiro et al 2021). The seven radiocarbon dates associated with the occupation, destruction, and reconstruction of the edifice show that the successive events occurred within a very short period, some time between cal.…”

“…To address these points, this approach is based on a detailed description of the archaeological context, including the spatial distribution of its features, and a sequential chronology of the cultural and natural deposits. The contextual interpretation and the understanding of how petate remains were formed and preserved are based on an upstream intra- and supra-site geoarchaeological approach, including a micromorphological study (Solleiro et al 2021). Several different methods were used to study the archaeological remains discussed in this article: traditional bioarchaeological tools for bone remains and the application of the Bruzek method for sexual diagnosis (Bruzek 2002); chemical analyses of floors and ceramic residues with the method developed by the Archaeological Prospecting Laboratory of the Universidad Nacional Autónoma de México (Barba and Ortiz 1992; Barba et al 1991).…”

“…Para abordar estos puntos, el enfoque del artículo se basa en una descripción escrupulosa del contexto arqueológico, teniendo en cuenta la distribución espacial de todos sus hallazgos, así como en un acertado análisis cronoestratigráfico para comprender la sucesión de los depósitos culturales y/o naturales. La interpretación contextual y la comprensión de cómo se formaron y conservaron los restos de petate se basaron en un enfoque geoarqueológico intra- y suprasitio, que incluyó análisis micromorfológicos de los sedimentos (Medina et al 2018; Solleiro et al 2021). Los vestigios arqueológicos discutidos en este artículo fueron estudiados siguiendo diferentes enfoques: las herramientas bioarqueológicas tradicionales para los restos óseos y la aplicación del método Bruzek para el diagnóstico sexual (Bruzek 2002); y análisis químicos de pisos y de residuos en cerámicas con el método desarrollado por el Laboratorio de Prospección Arqueológica de la Universidad Nacional Autónoma de México (Barba y Ortiz 1992; Barba et al 1991).…”