Sedimentary Record of the Zacapu Basin, Michoacán, México, and Implications for P’urhépecha Culture During the Preclassic and Postclassic Periods

The analysis of syn-sedimentary deformations is a helpful tool for paleoenvironmental reconstructions; these structures are better preserved in fluvio-lacustrine sequences. In the Zacapu basin, lacustrine sedimentation has been active since late Pliocene until modern times. During the Holocene, the lake evolved into a wetland that was the home of the first P’urhepecha settlements in Michoacan, Mexico. Since the beginning, the Zacapu basin was affected by volcanic activity from the Michoacan-Guanajuato Volcanic Field and seismicity from the Morelia-Acambay Fault System. The effects of these geologic events were recorded in the basin sediments and in the pictorial records of the P’urhepecha people. According to archeological studies, P’urhepechas settled in the Zacapu basin around 150 BC (2100 y BP) (Loma Alta phase) occupying the site for at least 1700 years until the Milpillas phase (also known as Late Uricho phase). In this time span, the population may have witnessed the volcanic activity of the Capaxtiro volcano (between 193 BC (2143 y BP) and 84 BC (2034 y BP) and the Malpais Prieto volcano (AD 826 - 962 or 1124 - 988 y BP), but also the tectonic activity of nearby faults (i.e., Villa Jimenez fault). In this work, we analyze the syn-sedimentary deformations exposed in the sedimentary record of trenches at the uplifted area of Las Lomas, and identify the presence of liquefaction structures, folding, microfaulting, and slumps, which have characteristics that suggest that they are related to seismic activity. We propose the occurrence of at least four earthquakes with moderate to high magnitude (MI ≥ 5) in the basin based in the deformations. The oldest syn-sedimentary deformations recorded in the lacustrine sequence are liquefied dikes and flames dated around 40 ka. Meanwhile, the youngest syn-sedimentary deformation are microfaulted layers which contain fragments of P’urhepecha pottery dated in the Milpillas/Late Uricho phase (1350 AD). Because of the time span when the volcanic and seismic events occurred, we discuss their relation with the human activities of the early settlements in the Zacapu basin.