Las Cornisas and the Valle de la Muerte
The morning of November 24th saw Kevin and I hiking on the Las Cornisas trail. This trail follows the ridge above and on the west side of the Valle de la Muerte (the Valley of Death). The trail itself is rather unremarkable as it is fairly level with only two short steep sections at the beginning and the end of the trail. The descent from the ridge is done by slipping, sliding or jumping (your choice) down a large sand dune. While the trail itself was nothing special, the view from the trail was exceptional, especially the views that it gave of the Valle de la Muerte.
The ridge that the trail follows is composed of three rock formations. Two of these formations are volcanic in origin, while the third is sedimentary. The oldest rock unit in this area is the San Pedro Formation. This sequence of rock layers is found throughout the Cordillera de la Sal (the Salt Range). The deposition of this formation occurred in the Oligocene and Miocene Epoch, which is between 36 to 5 million years old. The formation is composed of sedimentary layers of shale, sandstones, conglomerates, rock salt and gypsum, which were deposited in a salt lake similar to the present day Salar de Atacama or the Great Salt Lake found in the United States. Erosion of the surrounding terrain, other rock formations and the Andes Mountains supplied the material which eventually became the San Pedro Formation. The San Pedro Formation makes up the rocks found in the Valle de la Muerte. Erosion of this material since its deposition has resulted in the formation of some very spectacular badlands.
Most of the ridge is made up of the Pelón Ignimbrite and the Sifón Ignimbrite. Both are volcanic tuffs deposited as the result of large eruptions of volcanic ash. Both are also part of a larger formation called the San Bartolo Group. The Sifón Ignimbrite has a dacitic composition, and is generally brownish or gray in color. Radiometric dating shows that the deposits are between 8 and 9 million years old. The Pelón Ignimbrite is a gray to pink colored, dacitic tuff. Radiometric dating indicates an age of 7 million years old.
Two more recent deposits of sediment have partially buried the San Pedro Formation, Pelón and Sifón Ignimbrites. On top of the Pelón Ignimbrite and San Pedro Formation is the Vilama Formation. This formation is alluvium - silt, sand and gravel deposited by streams. Erosion of the Andes Mountains appears to have been the source of the sediment as indicated by the large number of volcanic and intrusive igneous rock fragments that are found within the deposits. The Vilama Formation forms an angular unconformity with the San Pedro Formation. The rocks of the San Pedro Formation have been deformed by tectonic activity. As a result the layers are no longer in their original horizontal position. After these layers were deformed, erosion removed some of this material. The Vilama Formation sediments were then deposited on this new surface, creating the angular unconformity.
The other major and most recent deposition in the area is the current wind-blown sand dune activity. These loose deposits of sand are derived from erosion of the surrounding terrain and dry stream bottoms.
Parts of the information above have been summarized from the following source: Marinovic, N. and A. Lahsen, 1984, Hoja Calama, Region de Antofagasta, Carta geologica de Chile No. 58, 1:250,000. Servicio Nacional de Geologia y Mineria, ISSN 0716-0194.