Friday Field Foto #26: Birds on a corral fence

I snapped this photo of some ibis this past March after a long horse ride out of the woods. Happy Friday!

Friday Field Foto #25: Ancient organic detritus

Many modern river delta environments are chock full of organic matter (think Louisiana swamp). Much of it is plant and other woody material. In some cases, this material is buried and remnants or evidence of it are preserved when the sediment lithifies.


The photos above show the bedding plane view (looking down on top of the surface of sedimentary layer) of a deposit rich with organic detritus (i.e., pieces of stuff). A lot of it is preserved woody fragments. Also note the dark black flecks...this material is mostly carbonaceous shale, which is kind of like coal. Click on these to see the high-res versions.

The organic detritus is abundant in thin (~1 cm) layers. The photo below is a cross-sectional view where you can see the layering of the sedimentary rock. The darkest layers (the pencil is pointing to one) are dark in color because they are full of this organic detritus and coaly bits.


But, wait, these are turbidites? It says so in the post label. How can you have all this terrestrial organic matter on the deep sea floor? In this case, this turbidite system is being fed by a delta. In the upper photographs, notice how broken up all the organic matter is. It has been ripped up and flushed off the delta environment and into the deep sea, where it was likely buried rather quickly. We find abundant organic detritus on the very tops of turbidite beds....this material is relatively light compared to the medium-grained sand and, therefore, is deposited after the sand as the flow slows down (ending up on top).

The organic detritus-rich nature is, in fact, one of many lines of evidence telling us that these Cretaceous strata are delta-fed turbidite deposits.

Conglomeratic intrusions

Okay....this is another long post....maybe too long, but oh well.

A former fellow student (now professor), me, and our adviser have a paper out this month characterizing some very unique sedimentary features in Cretaceous strata of southern Chile: conglomeratic injectites.
Clastic intrusions, now termed injectites, have been recognized for a long time. In some cases, they resemble igneous intrusions (dikes, sills, etc.) with respect to their geometric relationship to the surrounding rock.

What are injectites?
They represent the violent remobilization of typically coarse sediment like sand due to over-pressurization. The best visual I can give you is imagine that you could press down on a jelly donut that was somehow sealed along the sides. That is, the jelly does not squeeze out of the side when you put pressure on it from above (overburden). Now imagine some plane of weakness in the fabric of the donut above the jelly layer. The overburden pressure will force the jelly into that plane of weakness and upward. If the movement of the jelly is of sufficient magnitude it will bust its way through the donut, creating its own path, and perhaps erupting at the surface of the donut.

Coarse sediment (usually sand) that is deposited in deep-marine settings is a mixture of sediment and water and it takes a while for the deposit to slowly de-water. If the sediment accumulation rate is high enough -- that is, if these water-laden deposits get buried relatively quickly by more sediment this layer becomes the "jelly". The increasing overburden pressure combined with some sort of trigger (seismicity? a threshold?) creates a situation for this large-scale injection to occur.

What's cool about our study (at least we think its cool) is that we have (1) a great outcrop example of this phenomenon where the features are explicitly exposed and (2) the injected material is very coarse -- cobble-sized material.

Now, we'll be the first to admit that this paper is not high science. We literally came across these features and spent a few days mapping them out and characterizing them. There's only so much time in a field season and we had other tasks to accomplish. So, this paper is very descriptive in that way...that is, we did not take it to the level of really figuring out how these things work. The mechanics of clastic injection is not well understood.

Because injectites are associated with thick piles of sediment they are being found by oil companies exploring for hydrocarbons in the subsurface. In fact, one hypothesized mechanism for contributing to over-pressurization is gas charging. Believe it or not, there is an entire book about the occurrence of clastic injectites and their relationship to petroleum systems. This paper is in that book. We were happy that the editors really wanted this stuff in there since we weren't sure where else it would ever get published. Take it when you can get it!


Okay...enough set up....here we go.

The figure above is the location information. I've said in the past that I would put together a post about the general geology of the area i've been working in Patagonia....I still mean to do that....someday. As always, click on the image to get a higher resolution view. The study area, Cerro Benitez, is in the center of the satellite image at right. These Cretaceous strata are part of the Magallanes foreland basin and now nicely exposed in the foothills of the southernmost Andes.

The photos above show some of the close-up characteristics of these conglomeratic deposits. Notable are the near vertical groove-like features in F and G. By themselves, however, these small-scale features are not very telling.


This next set of photographs (above) takes a step back and shows the conglomerate bodies in their full glory. The one shown in A and then enlarged in B is particularly nice (note circled person [me] for scale in each). When we first came across these features we thought the juxtaposition of the conglomerate against the shale had to be a fault contact. It's not...we mapped the entire area. Then we thought these were some kind of channelized deposits, but the geometries were just not working out. When we spent a few days mapping out the multiple bodies and their relationship, an interesting picture emerged.


The last day of characterizing the injection complex, we hiked to the other side of the lake and climbed up a mountain to take the photograph above. We felt we needed a view zoomed out even more....it took a day to get this photo....not the first time we spent a day to get a photograph down there. In the corresponding drawing below the photo, the yellow is the "regular" conglomerate bodies (referred to as stratified conglomerate) and the dark brown denotes the injected conglomerate. The cool part about this outcrop is that the old glacial lake created a flat terrace (shown in lighter gray) between two more vertical slices of the stratigraphy (shown in the dark gray). So, it's sort of a "chair" display if you can visualize it. This little bit of three-dimensionality helped immensely in mapping out the different bodies.


The schematic cartoon above (or, as the first author Steve likes to call it, the geophantasmogram) attempts to summarize the outcrop and how we've mapped it. Again, note the "chair" set up. The injectites emanate from the side of the channel-fill deposits, which is a pattern seen in injectites in the subsurface over and over again. Also, note the left complex on the diagram bifurcate and then head up through the strata until they terminate. The photos above with me for scale is the right branch of that bifurcating complex. The very ends of those injectite bodies lack conglomerate and are instead only sand reflecting a "fining-away" trend. That is, the "flow" of material had to come to a stop at some point, and the brief slowing down before stopping was enough to sort the material being carried.


The last thing I want to touch on is the scale of this injectite complex compared to other known and well-studied examples from the subsurface. The figure above (which is the bottom half of the geophantasmogram) shows the cross-sectional outline of the injectite complex we characterized with some that have been mapped using seismic-reflection data in the North Sea. Firstly, notice the "winged" appearance of nearly all of these examples. Secondly, the bottom line is -- our complex, as big as it seemed, is a tiny little sucker. For example, note the small rectangle on one of the little nubs in D. That rectangle represents the size of the complex we characterized. These North Sea injectite complexes are friggin' HUGE! It kind of blows my mind.
Anyway, i'm gonna cut myself off....this post is already ridiculously long. If you'd like to know more about the nitty gritty, please feel free to comment below or e-mail me at romansbrian AT gmail.

Friday Field Foto #24: Conglomerate injectites

I don't have a lot of time right now...still traveling...but wanted to get this Friday Field Foto posted while I wait for my flight (hopefully a better experience than the trip here). I'm not traveling at all for the next 5 weeks, so I should be blogging more frequently (and finishing my dissertation).

A paper I'm a co-author on just came out, which I will post about in more detail next week. The photo above (that's me for scale) is a great view of some conglomerate injectites in Cretaceous sedimentary rocks down in Patagonia. I showed a photo of some smaller injectites previously on this blog.

Note the flat lying shale and thin-bedded sandstone that I am standing on juxstaposed against a more resistant conglomeratic body that is nearly vertical.

That must be a fault, right? Wrong. I know this may sound crazy, but this feature represents the upward injection of coarse-grained material through the overlying deposits. Yeah....right. Give me a few days and I'll put together a better explanation of all this. It's pretty wild stuff.

Friday Field Foto #16: Sand injectites

Okay, back to Patagonia for this week's photo.


This isn't in my specific field area, but very close to it and in the same formation. What's really cool about this area is the presence of clastic dikes....or, what have now been termed "injectites", as in the injection of sand. In this photo you'll notice the light-colored streaks cutting up and to the right across the flat-lying strata. These features are sandstone.

Injectites have been recognized for over a hundred years, but have been more appreciated in recent years. Oil companies searching for petroleum in the North Sea have seen huge networks and complexes of injectites with seismic-reflection data. And I mean huge...some of the individual sandstone "dikes" can be kilometers long cutting sub-vertically through the strata.

In the case above, the orientation of this swarm of injectites are parallel to a growth fault (i.e., syn-depositional) lower in the section.

There is still much debate about the mechanics of these things...what kinds of overpressures are required, how fast is the unlithified sand injected, and so on.

Photographic summary of recent Patagonia field work

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.

Back from field work safe and sound

Yet another field season in Patagonia...and i've made it out alive and relatively unscathed. Although there's little to no jet lag involved with the journey from Patagonia to California, it is 24 hours of traveling and if you're like me and can't sleep on planes (what a drag!)....it feels about as bad as jet lag.

I will blog more about it and post some photos soon...this weekend I hope. There are a lot of great posts out there in the geoblogosphere that I'd like to catch up on too...and Highly Allochthonous has moved to Scienceblogs...congrats.

Here's a photo from this year of us on one of the more beautiful days I've experienced down there. That's the torres of Torres del Paine in the background. A lot of tourists travel a long way and never see them because they'll be shrouded in clouds for days at a time.

still in the field

We are in town right now (Puerto Natales, Chile) to resupply, get a shower, and sleep in a bed for a night. There are a bunch of internet cafes in this town because it is a jumping off point for backpackers and such (mostly Europeans) traveling around Patagonia.

The weather has been the usual...either really nice (60s F and sunny) or completely miserable (30s F with snow or rain). I either feel ridiculous carrying so many articles of clothing in my day pack when its nice or wish I had more to wear when its cruddy.

Probably the most exciting thing to happen so far this year...is that I finally saw a puma! They are essentially the same species as a mountain lion in North America but a bit bigger I think. They are around and probably see us all the time, but are timid and difficult to actually see. Today, we saw one cross the dirt road we were driving. I was happy to be in the car...the thing was big and muscular. It could kill and eat me without much trouble if it wanted to. Luckily, they are fairly well fed in this region...plenty of sheep and guanacos (and turistas).

¿Dónde está ...Or Something?

This blog will be dormant for the next month or so while traveling and doing field work in Patagonia. We have sporadic web access in public internet cafes when we are in town re-supplying, but I can't promise any interesting posts.

In the meantime, check out some of these other geo-related blogs:
Apparent Dip
Highly Allochthonous
Lab Lemming
Southern Exposure
Green Gabbro

¡...Or Something volverá pronto!

Friday Field Foto #11: Guanaco

I am headed down south again very soon (next Tues) for my last field season in Patagonia...as a student anyway. I've had one other post about wildlife we encounter down there.

The above animal is a guanaco, which is sort of like a cross between a llama and a camel. These things are all over the place. We were able to get close to this particular one because we were in Parque Nacional Torres del Paine, where they are tamer. When we see them in other areas outside of the park, they keep their distance. Typically one of them will stand between us and the rest of the herd...keeping the herd informed on what we are doing. Guanacos make a noise that sounds like a horse whinney combined with a laughing hyena.

Whenever we see them, we wish we could ride them up the mountain...they can get up the hills a lot faster than us.

Friday Field Foto #9: Igneous intrusion cutting across folded strata

This week...another photo from Chile.

Here we got a good ol' geologic puzzle...the kind you'd learn in an intro class. Here we can use the intuitive law of cross-cutting relationships to determine the relative timing of events. First the sediments were deposited, then the lithified sediments (rocks) were uplifted and folded, then an igneous intrusion cuts across the fold. Finally, erosion reveals it all for us to see.

To see all the Friday Field Fotos, go here.
To see more photos of Patagonia, go here.

Friday Field Foto #7: Stack of deep-water deposits

(a day late)

Here's another shot of my field area in southern Chile. The more resistant cliff-forming rocks are mostly sandstone and the gray-brown slopes are shale. These sedimentary rocks were deposited on the sea floor about 65-70 million years ago (kind of around the time the dinosaurs became extinct). Good stuff.

Augusto Pinochet and the other 9/11

Because I've been traveling to Chile for research for the last few years, I've naturally taken an interest in their history, culture, and politics.

Yesterday Augusto Pinochet, 91, died. Pinochet led a military coup that ousted democratically-elected president Salvador Allende on September 11, 1973. The U.S. secretly backed this coup as part of their 'anti-communism' program of that era.

Pinochet ruled Chile as a right-wing military dictator until 1990. Thousands of people (there's debate about how many) were rounded up, tortured, and killed for their known or inferred dissent of his politics (if you weren't with him, you were against him). There were detention and torture centers, concentration camps essentially...all that good stuff military dictatorships are made of.

He has a long list of human rights violations against him and has evaded justice over the last decade with the excuse of failing health.

Many Chileans view him as a brutal fascist dictator that ended democracy, while others view him as someone who brought economic growth to Chile with fierce free market policies (which is why the U.S. gov't backed him).

You can learn more about this subject here, here, or here.

Friday Field Foto #5: Pygmy owl

Although we are usually looking at rocks, it's impossible to not have encounters with wildlife while in the field. One morning while we were having breakfast at our campsite in southern Chile, this pygmy owl hung out on a tree branch near us for about 20 minutes. This little guy is about the size of your fist.

See more photos of mine from Patagonia here.

Friday Field Foto #2: Shallow-marine strata

This week, another photo from southern Chile. This is a ridge called Sierra Dorotea very close to the town of Puerto Natales. We climbed up this on the very last day of our field season last March. These rocks are shallow marine and deltaic deposits of the Dorotea Formation (~60-65 million years old). More photos of Chile are here.

Friday Field Foto #1: Snow-covered outcrop

Here's a picture of my main field area in southern Chile, called Cerro Divisadero. This photo was shot about an hour after a snow storm came out of nowhere and chased us back to camp. If you want to see more photos from this area...check here.