Abstract: Volcanogenic Sedimentation on Andean Plateau, Northern Ecuador

J. Donald Ryan

The Andes Mountains throughout most of Ecuador consist of two nearly parallel rows of composite volcanoes separated by a high plateau underlain by thick deposits of volcanogenic sediments of Pliocene to Quaternary age. The highest of the volcanic peaks rise to heights of more than 6,000 m above sea level; the plateau rises to a height of about 3,000 m above sea level. Deep canyons cut the plateau, exposing spectacular and nearly complete sections of the volcanogenic sequence.

The plateau appears to be underlain largely by pyroclastic ash, but, in fact, it has been built up by accumulation of several types of volcanogenic sediments, each type representing a different depositional process. The various sedimentary types include: (1) the deposits of airborne pyroclastic ash and pumice, (2) several types of bedded fluviatile gravels and sands, (3) poorly sorted blocky landslide debris, (4) probable ancient mudflows or debris avalanches somewhat similar to the landslide debris but containing a larger proportion of reworked ash and sand, and (5) finely laminated lacustrine deposits of interbedded fine-grained sand and ash. In places, the sediments are associated with ancient lava flows.

The pyroclastic deposits are arranged in layers which follow ancient land surfaces. Dip azimuths of pyroclastic beds are not indicative of direction of sediment transport. Buried soil profiles superposed on successive ashfalls can be recognized in some of the younger ashfalls. High-angle faults, probably related to earthquakes which triggered landslides and debris avalanches, are common.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma