The Geology of the Upper Juncal Valley

Upper Juncal Valley

After an elevation gain and about an hour of walking the breadth of the upper Juncal Valley is made apparent from a rocky overlook. Below is the Vega Nacimientos and beyond the imposing flanking ridge of Alto de Los Leones and the tip of the Nevado Juncal. At 6110 meters, the summit of Nevado Juncal rises 3500 meters from the meadow at its base. Although somewhat lower, Alto de Los Leones is equally imposing due to the extreme relief of its northeast face.

Both these peaks, as well as the slopes of the valley leading up to them, are formed of volcanic rocks belonging to the Abanico formation which is about 25 million years old. There are two broad categories of volcanic rock present here, pyroclastics and lavas. The pyroclastics are formed by explosive eruptions where volcanic ash clouds are deposited as flows or airfall sheets. These rocks are generally softer than lavas and many eroded spires and fins can be seen along the main valley walls. Lava flows generally tend to form harder, more layered rock such as can be seen high up on the walls of Alto de Los Leones.

Striking rock textures can be seen especially in the boulder field at the upper end of Vega Nacimientos. Here there are some fine examples of porphyritic textures where long white crystals of plagioclase feldspar up to 3 cm. long are surrounded by a dark fine-grained matrix (photo 2).

Porphyritic textures result when molten rock starts cooling slowly in an underground reservoir known as a magma chamber. Plagioclase crystals begin to form, but before the magma can solidify completely, it is erupted to the surface where rapid cooling crystalizes the much smaller mineral grains of the matrix. This porphyritic texture occurs with other mineral types as well.

The volcanic rocks of Nevado Juncal, Alto de Los Leones, and the main valley walls are deformed from the relatively horizontal position in which they were originally laid down. Eastward compression from the subducting Nazca plate has created north-south trending folds in the rocks (see geologic map, Appendix. 1). A broad, upward-arched fold known as an anticline runs along the crest of the peaks on the west side of the valley and through Nevado Juncal. Because the valley is eroded almost parallel to the anticline, it is hard to see the true shape. But one can see a glimpse of this phenomenon where the weakly layered rocks near the top of Alto de Los Leones are bent downward closer to Nevado Juncal. There are some areas high on the east side of the main valley where long narrow ridges and fins indicate the original beds have been tilted nearly vertical.

The most impressive feature of Nevado Juncal´s north face is its glacier (photo 3). Huge icefields near the top feed into mid-slope icefalls, and lower down it flattens into a valley type glacier. As is the case with most glaciers worldwide, this one is melting back. The end of the glacier has retreated approximately 170 meters from 1955-1997. Evidence of the glacier’s last maximum extent can be seen by the barren “bath tub ring” scarp that wraps around the lower slopes on both sides of the entrance to Monos de Agua canyon and also along the west side of the upper valley above Vega Nacimientos (photo 4).

 

Conspicuously absent, however, is the presence of a terminal moraine (the crescent-shaped pile of rubble and boulders that a glacier pushes ahead of it). Instead, a spread-out field of boulders overlies the upper end of Vega Nacimientos. One possible explanation for this is that Vega Nacimientos was a lake at the time. Instead of pushing up a pile of moraine, the rock-strewn front of the glacier would have been calving icebergs off into the lake where the rocks within would have sunk to the bottom randomly as the ice melted.

A couple features support this idea. In the lower end of the meadow, a clay bench outcrops. This is a lake bed deposit. What formed the lake at that time and why is it gone now? The likely answer lies just downstream. Just north of Vega Nacimientos there is a massive fan-shaped landslide deposit at the base of the valley slope on the west side. A broad concave depression on the hillside above marks the source of the slide material. This landslide could have dammed the Juncal River creating a temporary lake. Eventually the river would have eroded through the slide (as it is actively doing today) and the lake would have drained, leaving a muddy lake bottom littered with boulders. The clay rich sediment forms a relatively impermeable suface over which water emerging from springs at the base of the surrounding talus slopes meanders and supports the lush meadow plant life.