Clastic Detritus

I am putting together a post about the general geology of the region of southern Chile we work in, what we are working on exactly, etc. but is taking a lot longer than I’d like.
As most of you out there who do relatively long stints of field work you know how it is getting your non-geology life back in order when you get home.

In the meantime, here a some photos and superficial information about them, and anecdotes from this year’s session. (Click on photos for hi-res versions).

The above photo illustrates one reason why we travel so far to look at sedimentary rocks — the exposure. Much of the stratigraphic section we are investigating is shale/siltstone, which does not typically create great outcrops. In this area, however, the recent glaciation has ‘cleaned up’ the mountainsides giving us a glimpse of these strata. You’ll notice in the bottom portion of this exposure an odd lens-shaped area….this section is riddled with deep-sea mass wasting deposits (slumps, slides, debris flow deposits, etc.). The interior of that lens-shaped area is chaotically folded and deformed (syn-sed, soft deformation). There are very few places in the world where this kind of syn-sedimentary deformation is this well-exposed. The photo below is zoomed in a bit. Note the discordant strata in the lower half.

For this particular study area we were lucky to be able to stay in a nice little puesto…a glorified shack where the gauchos stay when they are in the area tending to sheep (see below). Not that I dislike camping…but when working, it is nice to come back to a roof and wood-burning stove in the evening. Plus, being able to drive to the puesto allows us to bring additional supplies (i.e., wine).

Up until this year, all of the work i’ve done in this area has been near, but outside of the national park boundaries (Torres del Paine). A new student is starting a project looking at the structure and thermal/exhumation history of the fold-thrust belt. This requires going further west into the more deformed part of the belt and thus towards the Patagonia ice cap (the 3rd largest continental ice sheet after Antarctica and Greenland, I believe). As you can imagine, accessibility is a major issue….the roads/trails within the park remain the best way to access a lot of these rocks efficiently, safely, and cheaply. So…the bonus is the spectacular scenery. The photo below is from our campsite early one morning…if you’ve ever read anything about this area you’ve no doubt seen this image. It is, by far, the most iconic image of this massif…called Los Cuernos (or ‘horns’).

The light rocks are the granitoid laccolith rocks intruding into the dark shales of the sedimentary sequence we are studying. The sed sequence is Upper Cretaceous…the laccolith is Miocene…I promise I’ll get a post going putting all this random geo-info into some context. By the way, the water in the foreground is essentially sea level and the top of those peaks are nearly 3,000 m (10,000 ft). A little bit of relief.

Another day we were sampling along a trail that led to Glacier Grey. We, of course, had the proper permission and paperwork for sampling in the park, but still got many curious (and some dirty) looks from tourists along the trail. I suppose we may have been ruining their solitude by hammering away….but, hey, tough break. But most people were simply curious and once they find out we are scientists they will ask questions about the area. This photo below is a view of the glacier during our lunch break.

The geologic map of this area that we have is from the late 1970s. Note the two rock ‘islands’ at the glacier front…the bigger one to the right and the much smaller to the left in the photo above. The smaller one is not on the 30 year old map…the glacier has retreated that much.

After this work, me and one other guy set out for an excursion to a rather remote area in an area northeast of the park (again…I will post about the geography/geology more properly sometime soon). I’ve been working in this area for 4 years, but this was the first time to this particular area. It required being horse-packed in and camping for 10 days. Essentially, a gaucho guide takes us in, drops us off, and then comes back on a specific day to retrieve us. I shot this photo (see below) of the gaucho leaving the canyon we were in after dropping us off. He was much more efficient in traveling once he got rid of the two gringos (not necessarily master horseman) and all the supplies.

This was one of the harder areas i’ve worked physically. We couldn’t really get too close to the outcrop we wanted to work on with the horses due the the ruggedness of the canyon we were in. So, we had a 5 km cross-country hike every day to the rocks (about 4 hours). By the end of the excursion we were not only in better shape but had found a good network of game trails that made the commute not so hard on our bodies.
Now that I’m sitting comfortably at home and not cursing my blistered feet and shaking a fist angrily at the weather gods, I can say it was worth it. The sedimentary sequence we were investigating in this area is important to the overall understanding of the basin fill because it is the record of delta progradation that eventually constructs the shelf and fills in the deep-water foreland during the latest Cretaceous. So, in a short sequence we saw turbidites intermingling with hummocky cross-stratification (wave-base) and deltaic deposits. This is another student’s research, so I’m not gonna steal his thunder by posting too much about it….stay tuned.

On a nice day, we got this view from the top of the outcrop (photo above). The tops of those mountains in the distance (to the west) is the Patagonia ice cap…it’s a few km thick in that area…or something like that.

Stay tuned for a post with more geology and context for all of this.

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