Volcaniclastic sequences at the foot of Tacaná Volcano, southern México: implications for hazard assessment

Abstract

Extensive volcaniclastic deposits have originated from the Tacana Volcanic Complex (TVC) throughout its evolution. The valleys of the Coatan and Cahuacan rivers have been the main routes for descending flows which have occasionally reached the Pacific Ocean. As a result, three fans have been deposited or increased their volume. The most recent, Tapachula, is 13 km long and covers an area of ~48 km2 that has been largely urbanized by the city of Tapachula, which is located 30 km SW of the TVC. The ~0.7-km3 fan is bounded by the Coatan River to the west and the Cahuacan River to the east and is composed of a complex succession of volcaniclastic deposits accumulated between ~23,000 and ~1,300 years ago in response to remobilization of debris along the Coatan River basin. Collectively, the three fans consist of 34 stacked units grouped into five stratigraphic sequences. From the oldest to the youngest, these sequences are as follows: (1) Pre-Tacana (Chanjale Fan), comprising five units associated with the remobilization of material produced during the formation of the Chanjale Caldera ~1 Myr ago; (2) Mal Paso (Mal Paso Fan), comprising four units emplaced between ~100,000 and >>23,000 years ago; (3) Lower Tapachula (Tapachula Fan), consisting of two units deposited around 23,000 years ago; (4) Upper Tapachula (Tapachula Fan), consisting of 14 units emplaced between ~14,300 and ~1,300 years ago; and (5) the topmost sequence, Coatan, is made up of 11 units deposited during the past ~1,300 years inside the valley of the Coatan River. Today, ~200,000 people live on top of the Tapachula Fan, which makes the city vulnerable to future lahars from the TVC.