Resumo

SANCHEZ-NUNEZ, Juan M. et al. Granulometric characterization of alluvial fan deposits at the Motozintla Basin, Chiapas, Mexico: a dormant geologic hazard due to flood events. Bol. Soc. Geol. Mex [online]. 2017, vol.69, n.3, pp.529-554. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2017v69n3a2.

Motozintla basin is situated in the southern mountainous region of Chiapas State, Mexico, where debris flows have frequently occurred. These natural phenomena are latent geohazards for the population of Motozintla. Their sudden nature, their high energy, related unstable slopes (> 30°), rugged topography and anthropogenic footprint contribute to make them more dangerous. This basin is located in the western end of the left lateral Polochic fault - the structural limit of North America and the Caribbean plates. Tectonically, the basin encompasses alluvial fans and terraces that cut both sides of the plate boundary. This study deals with the alluvial fans that are composed of successive deposits, some separated by underdeveloped paleosoils. Granulometric characteristics of 57 deposits sampled in 30 stratigraphic columns were determined. The results revealed that most alluvial fans were sourced from immature flows (lithological, heterometric and various other sources) transported on steep slopes (> 30°) with less than 10 km of transport from the source. The repeated occurrence of these events in the past indicates that the events of debris flow are influenced by hydrometerological and seismological processes. This research is the first of its kind in an active tectonic setting of Mexico.

Palavras-chave : Motozintla; alluvial fans; debris flows; geohazard.