CHARACTERISTICS OF THE DEBRIS AVALANCHE AT SIERRA LAS NAVAJAS, HIDALGO, MEXICO

Sierra Las Navajas is an extinct peralkaline rhyolite volcano in the eastern Mexican Volcanic Belt. Sector collapse of the northeastern quadrant of the volcano about 2 million years ago produced a debris avalanche deposit that extends 48 km from the source, covers an area of 510 km² with a volume of about 30 km³, and is well exposed in the canyon walls of the Rio Grande de Tulancingo to the north. The deposit consists of 4 distinct units that maintain their stratigraphic relations throughout the area. The basal unit, termed the basalt mixed unit, contains large sub-rounded clasts of basalt supported in a matrix of rhyolitic rock flour. The basalt clasts were derived from the lower flanks of the volcano and also incorporated as the avalanche traveled over the basaltic plain north of the volcano. In this unit large flame structures occur, suggesting some turbulence during emplacement. The rhyolite block unit lies above, and is sometimes intruded by, the basalt mixed unit. This unit is clast supported with rhyolite clasts ranging from centimeter to meter size, and also contains megablocks of stratified rhyolitic lavas and pyroclastics that measure up to 1500 m and preserve intact sections of the pre-collapse volcano. Irregularly above this unit is a ~1 m thick layer of white devitrified rhyolitic ash. The presence of this unit suggests a hiatus during emplacement. The uppermost unit, called the rhyolite mixed unit, is matrix-supported and contains various pumice, obsidian and rhyolite clasts. This unit is conformably overlain by airfall and pyroclastic flow deposits, which in some places show gradational contacts with the underlying debris avalanche deposit, suggesting contemporaneous eruption. The distinct nature of each unit implies that each was emplaced separately. Preservation of blocks of unconsolidated pyroclastic deposits in each unit suggests little turbulent flow during emplacement. Emplacement mechanisms include mechanical fluidization of smaller clasts and incorporation of water into the basal layer as the avalanche traveled across the paleovalley. Prior investigation of Sierra Las Navajas showed that a distinct magma composition was being erupted both pre and post collapse. Movement of this magma apparently triggered the collapse event and concurrent Bezimianny style eruption.