Sedimentological Study Of Distal Rain-Triggered Lahars: The Case Of West Coast Of Ecuador

The coastal sector of Ecuador, on the area comprised between Salango and Jama (Manabí Province), is characterized by the presence of whitish to grey, centimeters to meters thick ash deposits associated to secondary rain-triggered lahars. The ash deposits, outcropping far to 160 km from the nearest volcanic vents, varied from parallel stratified fine rich ash to a massive lithic-rich fine-rich ash to a massive lithic breccia tuff and a faintly stratified lapilli-rich tuff debris flows. Sharp contacts, very fine ash laminated layers and sand and silt layers interbedded with volcanic ash beds confirmed that multiple lahar events affected the area related with secondary remobilization of the fall ash deposits. Rain events generated the multiple pulses both in short time and after large periods. The presence of human remains confirmed also that this kind of events strongly affected the ancient populations (Manteña) present on the coastal sector of Ecuador. This study confirms that the coastal sector was affected by rain triggered lahars events that nowadays can affect towns sited on distal areas (>160km) after VEI 4 to 6 eruptions, on short time and within more years.