Dynamics of the slope at Bastos Archaeological Site - Dourado/SP/Brazil

Buried soils and paleosols may be important paleoenvironmental indicators, providing information about the processes that act in the relief elaboration. The Bastos archaeological site is currently considered the oldest in the State of São Paulo. Archaeological records were preserved due to erosive/depositional events that led to the burial of the site. The present work aims to identify the depositional events associated with the burial. The methodological procedures consisted of a morphological description of a vertical section, physical-chemical analysis, in addition to the collection of undisturbed samples for micromorphological description. The area is located in a low-slope terrace condition, close to the Silvestre stream with small irregular levels, lowered by an average of 2.9 m in relation to the colluvium ramps. Its contact occurs through ruptures of concave-converging slopes. The vertical section investigated presents 229 cm depth, in which 10 horizons were identified, composing 7 distinct soil profiles. The highest values ​​of silt and clay occur in the A, Bi, 5Bt, 7Cg1 and 7Cg2 horizons, with mean values ​​of 25.3% and 34.4% respectively. The 2C, 3C1, 3C2, 4Cg and 6C horizons present high levels of sand, between 53.2% and 84.8%, with predominance of the coarse fraction. The terraces show evidence of alternations in erosion and local deposition cycles, where a sequence of truncated and buried soil profiles occur. Two types of events may be described, the colluvial processes, interspersed by fluvial depositions. On the colluvium ramps and slopes, fine materials (silt and clay) predominate, due to basalt alteration. Along the valley, we see conical shapes modeled by the action of concentrated surface runoff through narrow channels. When unconfined, the detrital flows are dispersed, forming fan-shaped deposits. The deposits truncate soils and deposit poorly selected materials, especially basalt fragments, in the midst of a clayey matrix. The process supplies material to form the profiles with clayey texture in the studied section, evidenced by the similar textural and chemical characteristics. However, in cases such as the discontinuity that marks the abrupt transition between the 7Cg1 and 6C profiles, the presence of silcrete clasts and the relative proportion of sand in the fine land fraction (+87%) highlight the existence of an alluvial deposit. In the higher areas, close to the channel's drainage head, the slopes are built on the sandstones of the Vale do Rio do Peixe Formation. Thus, the materials that make up the archaeological site present both alluvial and colluvial origin, indicating the complex geomorphological dynamics of the valley. Currently, the channel continues to rework the slope deposits that fill the valley and are distributed throughout the study area. The channel thalweg is approximately 115 cm below its most recent depositional event (2C), and 15 cm above the oldest (6C), evidencing the dynamics of the deposition and erosion processes in the valley.