Tsunami deposits of September 21st 1985 in Barra de Potosí: comparison with other studies and evaluation of some geological proxies for southwestern Mexico

Abstract

Residents of Barra de Potosí village in southwestern Mexico witnessed inundation by waves up to a distance of ~500 m from the shore after the Mw 7.5 earthquake on September 21st, 1985. Sediments deposited by the tsunami wave were identified near El Potosí estuary and their geological characteristics (sedimentology, mineralogy and chemical composition) were compared with pre-tsunami sediments and deposits from the nearby-unaffected area. Tsunami sediments were characterized by well and moderately well sorted (standard deviation: 0.4-0.7 Φ) fine sand (mean size: 2.13-2.47 Φ) and contain higher amounts of both finer and coarser fractions (negative to positive skewed) and had leptokurtic to extremely leptokurtic distribution. Sedimentological characteristics of tsunami and pre-tsunami deposits were similar. Abundance and association of heavy minerals were also comparable both in tsunami and pre-tsunami deposits. However, lower amounts of Br and Fe2O3 and higher SiO2 and TiO2 differentiate tsunami deposits from the pre-tsunami sediments. Comparison with sediments deposited during the tsunamis of March 14th, 1979, September 21st, 1985, and March 11th, 2011, in the region did not yield any characteristic signature. Except for stratigraphy (i.e., erosive base), no other geological characteristic was useful for identifying paleo-tsunami in the region.