Evidencias estratigráfi cas y geoquímicas de la variación temporal de sedimentos naturales y antropogénicos en la planicie aluvial del río Guanajuato

Abstract

In this paper we analyze the stratigraphy of fl oodplain sediments from the Guanajuato river, based on sedimentology, 14C ages and geochemistry of trace elements of anthropogenic and natural sequences. The natural sediments were deposited in a fl oodplain environment between 7460 ± 40 BC (bottom) and 500 ± 50 BC (middle). Anthropogenic fl oodplain sediments, related to mining activities in the Guanajuato mining district, were deposited at least since 1660 ± 40 AD over the natural fl oodplain sediments. The stratigraphic interpretation is supported by trace elements analysis, which reveal that the anthropogenic fl oodplain sediments contain high levels of Pb, Zn and Cu as well as relatively low values of Cr and Ni, while the natural fl oodplain sediments, contain low values of Pb, Zn, Cu and high values of Cr and Ni. The combination of stratigraphy and geochemistry proves a useful tool in the detection and discrimination of natural and anthropogenic sediments related to the historic mining activity in the Guanajuato mining district.