The “Mera” lahar deposit in the upper Amazon basin: Transformation of a late Pleistocene collapse at Huisla volcano, central Ecuador

Abstract

The Sub-Andean zone east of the Cordillera Real, Ecuador and out to the Amazon basin's western margin has been the depository of voluminous lahars related to volcanic activity in the Andean highlands. These lahars arrived to the Sub-Andean zone via gravitational transport through narrow river canyons and emplaced volumes surpassing several cubic kilometers over robust inundation zones. This paper discusses the origin, flow route, depositional zone, terrace formation and geomorphic significance of the most important lahar deposit yet mapped in this region, that of the Mera lahar, which likely formed from a late Pleistocene collapse of Huisla volcano, followed by impounding by temporary dams behind drainage-blocking debris avalanche deposits (DAD), then subsequent rupture of the blockage. The actual deposit of the Mera lahar has a thickness between 30 and 70 m, is mainly comprised of DAD breccia, is matrix-supported (c 70%), of reddish gray color and is well-consolidated. Based on geochemical and petrographic similarities, Huisla volcano's DAD is the best candidate for the lahar's source. Huisla volcano is located some 90 km up valley of the bulk of Mera lahar's mapped deposit. Clasts of Mera lahar rocks and Huisla's DAD breccias have 57–61 wt% SiO2, corresponding to andesites of the calc-alkaline series with mean values of 1–1.5 wt% for K2O. The mapped Mera lahar deposit has an actual volume estimated in 1.2 km3 compared to its original estimated volume of 5.4 km3. Cross-sectional widths of 1.5 to 4.5 km span and extend laterally beyond the actual Pastaza river channel where the lahar's deposition produced high-standing isolated surfaces that are notable local geomorphic features of the upper Sub-Andean zone. The flow modeling program LaharFlow, employing the modern landscape as the topographic base, adequately simulates the flow route and inundation zones of the Mera lahar.