Whispers of rock

If climbers are straining to conquer the scream of raw stone, geologists are straining to hear the whispers of rock that often yields its secrets reluctantly. Learning to “read the rocks”—to decipher Earth’s history—is like reading a story where most of the pages are missing, and the pages that remain are shuffled into a random order. So learning to interpret Earth’s history involves trying to put the pages into their proper order and using a lot of imagination to fill in the pages that are missing entirely.

Two PhD students who are working with Steve Graham, the same professor who was my graduate research advisor at Stanford University in the 1980s, are doing field research in the area around El Calafate and El Chaltén to decipher the history of the sedimentary rocks in this area. As luck would have it, our time in El Chaltén has overlapped with one of the student’s period of field investigation here. So we have spent parts of several days in the field with Matthew and his field assistant Corey. Here we are (photo from Jay), with Matt explaining the large size of the boulders in one of the sedimentary deposits.


Steve’s students have been working in the area south of here, around Torres del Paine national park in Chile, for a decade or more. They have read the clues to establish a complete chronology (sequence of events through time) that continues to be refined. Matt’s job is to extend that work to the north—to the El Chaltén area—where the details remain obscure. Here I am with Matt and Corey and the geologic map of the region, which at this point is not very detailed (photo from Jay).


Here is the basic story—as Matt can tell you, the devil is in the details! In the Jurassic Period (Mesozoic Era) the continents that were united as Gondwana begin to separate and the Atlantic Ocean began to open. In Patagonia, rifting created a large ocean basin that continued to subside and fill with sediments that were mostly mud. Here is the field team looking for clues in the mudstone, with Cerro FitzRoy as backdrop (photo from Jay).


Fossils are, of course, one important clue. This locality is famous for ammonites, a marine organism that evolved rapidly throughout the Mesozoic and that went extinct when the dinosaurs did, at the end of the Mesozoic. So these fossils are important for putting the layers of sedimentary rock into their proper order. Here is a photo (from Jay) of an ammonite from the site shown in the photo above. There can be no collecting, however. Much of the geologic mapping here has been done in the context of paleontology, and in Argentina fossils are a serious matter. You would not want to be caught leaving the country with samples in your luggage.


During the Cretaceous Period (latter part of the Mesozoic) this entire region was compressed and, although the marine basin continued to exist, more sand began to be supplied from areas uplifted above the water. Here is an example of a sand bed that is graded—that is, there are larger sand grains at the base and smaller grains toward the top. We interpret these beds as deposits of turbidity currents that are basically underwater debris flows. As the flow moves seaward from the land, the slope becomes more gentle and the sand falls out of the water to make a graded bed.


Other evidence of the compression is the abundant folds that are visible throughout the area. Imagine that you take a flat piece of paper (the original layer of sediment) and squeeze it until it’s crumpled. That would make a fold. Here are some beautifully exposed folds on the hill slope along the road between El Calafate and El Chaltén.


Many complex structures were formed during this period of contraction. Our hike to the top of Loma del Pliegue (slope of the fold) left us puzzled when we didn’t see the fold. But on the following day, on a hike in the valley below, the fold was clearly revealed on the side of the slope. These layers were folded so much that the top layer is completely upside down (take that piece of paper and squeeze it so the one side of the paper faces downward). It’s hard to get the scale of this fold, but a person would be only a tiny speck on the top of the peak.


Finally, a note about the culture of geology. Geologists like to drink beer, so it’s certainly a bonus when the local town has a microbrewery! This is a popular spot for everyone in El Chaltén, not just for geologists.


Geologists, also like many others, are attracted to mountainous landscapes in exotic locations. Here is the field team with Cerro Torre as backdrop. Field sites don’t get much better than this! (Person on the left is a German traveler who accompanied us for awhile to learn more about what geologists do.) Matt and Corey are now on a several-day excursion to a more remote location east of here. We wish them success in their continuing investigations.


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