Cajon del Estero Monos de Agua

Cajon del Estero Monos de Agua

Photo 5, Syncline

Estero Monos de Agua is a major tributary to the Juncal River, joining it at Vega Nacimientos (see map, Appendix 2). Most of the Monos de Agua canyon is cut into the same volcanic rocks that make up the main Juncal Valley. From about halfway up the canyon, one can look downstream and see a good example of a syncline in the northeast canyon wall. A syncline is a down-bowed fold in the rocks, the opposite of  an anticline. The structure is much more apparent here than the anticline in the main Juncal Valley because erosion has cut through it perpendicular to the syncline axis.

Farther upstream the canyon splits, with one arm continuing south up a rock glacier, passing in front of a finger-like glacial cascade falling from the main Juncal glacier and the other turning eastward. In the middle of this bifurcation rises the Cerro Mono Verde.
This juncture marks a major change in rock type. The eastward-trending canyon starts with an abrupt bench of sedimentary rock some 200 meters high. These sandstones, shales, and limestones butt up against the volcanic rocks of the main Monos de Agua canyon along a major thrust fault that is exposed in the canyon wall immediately north of the bench.

Photo 6, Thrust Fault

Photo 8, tightly folded anticline

This fault is where the younger rocks of the Abanico formation from the west were driven up and over the older sedimentary rocks of the San Jose formation to the east. It is easiest to see this juxtaposition of rock types high on the canyon wall where the light-grey banded limestone abruptly terminates against the red-brown volcanics. Blocks of the limestone sitting on top of the sedimentary bench contain scattered shell fossils in thin bands (photo 7).

Photo 7, fossils in thin bands

The bench also has a tightly folded anticline

visible on its southern end (photo 8). Nearby is Cerro Marisco, where Ulrich Lorber has so named because of the shell fossils scattered at the peak.

Another interesting feature here is the presence of gypsum diapirs. A diapir is a geologic structure in which certain types of sedimentary rock like salt or gypsum have slowly flowed under pressure, like toothpaste, from one place to another.
Gypsum originally formed as a flat-lying bed that was deposited from solution in a shallow, evaporating sea or lake. Compression from the subduction zone squeezed it out of its original location to where it lies today. Gypsum also dissolves in water, which has produced a couple of fancifully carved canyons on top of and beside the bench (photo 9).

Rising above the sedimentary bench to the southeast is the bulk of Mono Verde (photo 10).

This rounded green mountain is an igneous intrusive body. Intrusions occur when molten rock at depth rises into overlying rocks but does not reach the surface. The Mono Verde intrusion apparently intruded the gypsum because a band of gypsum can be seen high up on the mountain. The intrusive rock is highly altered, or was subjected to great heating, which accounts for the greenish color.

The green is mainly due to a mineral called chlorite which forms when hot groundwater around the intrusion reacts with the original rock minerals and chemically changes them. Another common alteration mineral found here is epidote which is a distinctive apple green color (photo 11).

Photo 11, an apple-green epidote

Photo 12, end of the Estero Mono de Agua

The upper Monos de Agua valley, past the high sedimentary bench, continues east through sedimentary rocks of the San Jose and Rio Damas formations. The head of the valley is occupied by an arm of the Plomo glacier (photo 12, end of the Estero Mono de Agua).

by John Rygh, of Cordillera Consultants