Clast orientations of the 1985 lahars of the Nevado del Ruiz, Colombia and implications for depositional processes

Abstract

The eruption of the Nevado del Ruiz (Colombia) on 13 November 1985 produced lahar flows, resulting in extensive deposits. The imbrications of flat clasts within the deposits as well as the orientations of large boulders resting on the surface of these deposits were measured. The imbrications of the long (A) axes of the small clasts were found to dip 25° upstream against the lahar-flow direction (with a dispersion of 30°) indicating transport of such clasts by normal stream flow. Contrary to this, the boulders (mostly on the surface of the deposits) are aligned with their long (A) axes normally to the lahar flow, indicating rolling on the surface. Both orienting mechanisms occurred in runoff pulses subsequent to the main event which produced only unordered deposits “en masse”. Thus, the genesis of the emplacement of the lahar deposits in question has been mechanically explained.