Richard Harwood

Chile - 2001

Lago Sarmiento

Sarmiento Lake

On the morning of November 20th I took the half-day hike to Lago Sarmiento. This hike proved to be very interesting geologically for two main reasons.

First there is Lago Sarmiento itself. The lake and Lago Sarmiento Chico are both lined with calcium deposits. The lakes literally look as if they have a ring around them, kind of like a bath tub ring. A brief examination of the deposits and distribution suggests two possible explanations. The first hypothesis is at some time in the recent past, Lago Sarmiento, and to a lesser extent Lago Sarmiento Chico, have experienced hydrothermal activity. The calcium deposits are similar to deposits that are found in other hot spring areas, like Yellowstone in Wyoming. Hot waters rising up through the underlying rocks will dissolve some of the minerals, calcite in particular. These dissolved minerals are then deposited at the surface due to evaporation. Because this hydrothermal activity took place in the lake, the deposits are found along the shore, where evaporation would leave these minerals coating the rocks. There appears to have been multiple pulses of activity, based on there being at least three terraces of deposits in some locations along the shore. The different levels also suggest that the periods of hydrothermal activity may have caused the lake level to rise and fall. Another explanation for the lake level changes is climatic changes - periods of increased or decreased precipitation. This is something that needs to be investigated further. This area is part of the Andes Mountains, which is a long chain of volcanoes, and hydrothermal activity is common in areas of current or recent magmatic activity. The numerous basaltic dikes and the Torres del Paine granite are both direct evidence of the past magmatic activity in the area.

The second hypothesis that has also been advanced by other geologists suggests that the deposits are the result of biologic activity. Stromatolites are sedimentary deposits that are formed by sediment being trapped in mats of cyanobacteria (blue-green algae). Some of the deposits at Lago Sarmiento do resemble known stromatolite deposits. There are a lot of unanswered questions concerning the deposits surrounding Lago Sarmiento - hopefully some day we'll have some answers instead of hypotheses.

The other interesting aspect of the hike was the structural deformation of the Cretaceous aged sandstone and shale layers. These rock layer are present throughout the park and surrounding regions, but this hike helped to highlight some of the intense deformation that has occurred. Large blocks of this material have been faulted, inclined and folded, with all three occurring within short distances of each other. In many cases the layers in one hill are in a different orientation than the layer in an adjacent hill.

In addition to the structural deformation of the rock layers, volcanic activity has intruded numerous basaltic dikes into these sedimentary rocks. Basalt is a type of igneous rock that is relatively rich in iron and magnesium, but relatively poor in silicon and aluminum. It is typically black in color and can contain crystals of the minerals olivine, pyroxene and plagioclase. It is commonly found as lava flows, but is also found in thin, sheet-like bodies which intrude as a magma into other rocks - an igneous dike. This differs considerably from granite which is typically white or pink in color, rich in silicon, aluminum and potassium, but poor in iron and magnesium, and contains the minerals quartz, orthoclase and plagioclase, and often hornblende or biotite. Granite is also found within the park.