Evidences of an unknown debris avalanche event (<0.58 Ma), in the active azufral volcano (Nariño, Colombia)

Azufral volcano is an active composite volcano located in SW Colombia, whose edifice is mostly formed by pyroclastic deposits and lava flows. Recent road works in the region, NE of the volcano, revealed unknown deposits, one of them related to a debris avalanche event. This deposit, herein named San Roque debris avalanche deposit, outcrops along a 13 km section in the Túquerres - Samaniego road. Using this long exposure, the deposit was studied through sedimentological, grain size, microtextural and mineralogical analyses. The deposit is massive, varicolored, poorly sorted, with a predominance of gravel and sand-sized clasts; it contains angular megaclasts some of them with jigsaw cracks, and exhibits injections, faults, and horsts-and-graben structures. Surface microtextures in some clasts evidence collisions and shearing interactions among them, while mineralogy evidences the collapsed material involved in the avalanche. Based on stratigraphic relationships, the collapse occurred between 0.58 Ma and ~18 ka. The San Roque debris avalanche deposit covers an area of 56.01 km2, has an estimated volume of 0.51 km3, an approximated runout of 11 km, and an apparent friction coefficient H/L of 0.109; these parameters are indicative of a large and very mobile flow. Its main transport mechanism responded to a dynamic disintegration of the larger clasts and a subsequent interaction by contact or collision (inertial-granular flow model), although some zones moved in a cushioned way remaining coherent over long distances (multi-shear model). •A new debris avalanche deposit in Azufral volcano is defined.•Jjigsaw cracks and surface microtextures evidence collisions and shearing interactions among fragments.•The transport mechanism responded to both an inertial-granular flow model and a multi-shear model.•The collapse occurred between 0.58 Ma and ~18 ka.