Sedimentology of a rock avalanche older than 5.4 ka in the Sierra San Bernardo, Chubut, Argentina

Abstract

This study characterizes a mass movement deposit exposed in the Matasiete Canyon (Sierra San Bernardo, Chubut). Lithofacies analysis and structural survey allow the deposit to be interpreted as the result of a rock avalanche (RA) emplaced prior to 5401 cal yr BP (Middle Holocene). The deposit is composed of breccias of claystones, tuffs, and conglomeradic sandstones of the Matasiete Formation, organized into facies consistent with the classical carapace-body-base model, displaying diagnostic features of cataclastic laminar flows (high fragmentation and differential shearing), such a jigsaw and survivor clasts, boudin-like structures, R and R’ shears, folds, and normal faults. Two main shear zones control the architecture of the deposit (PCPA and ZCPB) and indicate a transport direction toward the WSW. The absence of surface morphological features associated with the deposit hinders direct interpretation of the landslide (emplacement type, volume, and extent), although climatic conditions (intense precipitation) are considered the main triggering factor.