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.

Friday Field Foto #23: Flame structures

Today, we have another sedimentary structure commonly seen in turbidite successions. The features above that look like flames are called.......drum roll........ flame structures (brilliant!). Right in the center of the photograph notice the slightly darker and curved lines in the rock. They meet upwards at a tip and are pointing to the right.

Flame structures form as the upper part sinks into the underlying sediment, often referred to as 'loading'. This loading commonly occurs in localized pods and then the sediment in between gets squeezed up a bit. The directional component of this (the tip of the 'flame') is produced because there is still some current strength which shears the material just slightly before everything comes to rest as a deposit. So, in the case above the local current direction was left to right.

How does loading occur in the first place?
Firstly, the underlying bed cannot be completely dewatered. That is, it is still a mixture of sediment and water is likely in a 'soupy' state. Secondly, there needs to be a density contrast. The overlying material has to sink into the soupy bed to be able to displace the material like this. You may have seen flame structures before where coarser material (usually sand) sinks into much finer-grained material. But, in this case all the material is sand of similar grain size. The density contrast is set up because the overlying sediment has much more water in it still. Remember, sediment gravity flows are a mixture of sediment in suspension and water....this is very different than fluid flows where the sand is transported as bedload.

So, what does this tell us?
In this case, if you were to examine this deposit laterally away from where this photo was taken the loading and flame structures disappear and the bed is completely homogeneous. This is just a localized structure within a bed and not a boundary between discrete events. Turbidity currents commonly exhibit 'surging' behavior. As the flow moves down-slope it will begin to separate into multiple sub-flows. Or so we think....our knowledge of how these things actually work is incredibly limited.

Friday Field Foto #22: Rip-up clasts

More turbidites!

I'm actually going to visit these rocks early next week for a couple days. This is north of the Bay Area near the charming little coastal town of Gualala (which is fun to say out loud).

In this photo (note coin for scale) we see a complex mixture of pebbly sandstone (near bottom), coarse sandstone, and siltstone and mudstone (darker-colored rock). Note how the coarse sandstone seems to surround the large area of dark mudstone. The mudstone areas are actually clasts within a thick (>5 m) sandy turbidite bed. We call these "rip-up clasts" as they were deposits on the sea floor that were ripped up by a subsequent turbidity current, incorporated into the flow as a clast, and eventually deposited. Muddy material can be quite cohesive and can stay together as a coherent clast like this. Think about making a mudball vs. a sandball with your hands. The sand disaggregates much easier.

As mudstone rip-up clasts go, these are probably slightly above average in size. We commonly see smaller (centimeter scale) and once in a while you'll see a deposit with larger rip-up clasts.

Real Earth vs. GoogleEarth

I took some photos the other day as we were landing in San Francisco. The picture below is looking to the northeast, that is the town of Fremont and the East Bay hills.

Now compare with the GoogleEarth view...this was as close as I could get it to match.

That's it...I don't have anything interesting to say about that.
Happy Fourth of July!

Friday Field Foto #21: Climbing ripples

Today we are gonna look at some ripple cross-laminated sandstone. More specifically these are called 'climbing ripples', or sometimes you may see this sedimentary structure referred to as 'ripple drift'. These show up particularly nice in this photo because of the light-dark contrast of the sandy vs. muddy laminae.

Climbing ripples record both migration (lateral) and aggradation (vertical) of the bedform. In this example note the very prominent ripple set in the middle section of the photo nicely showing the 'climb' from left to right. Different angles of climb represent different ratios of migration and aggradation. Climbing ripples are commonly interbedded with or grade upwards into wavy laminae.


This image at left is a classification of types of climbing ripple-laminated structures based on the angle of climb (click on image for larger view; click here for source of image).

Climbing ripples are most commonly seen in river and turbidity current deposits, typically in sub-environments of high rates of deposition from decelerating flows.









On a side note, Dr. Lemming has pointed out a new geology-related blog out there. It's called All of My Faults Are Stress Related and looks to be a nice addition to the geoblogosphere. I have a few more new links over there on the sidebar that i'll post about soon.

I also have an updated album of Patagonia photos you can check out here. The above image is from that collection.

Friday Field Foto #20: Volcanic hills

Last weekend our research group along with some others went on a short field trip that was a cross-section of the California convergent margin. We were principally looking at the Mesozoic system -- the continental arc plutons of the Sierra Nevada, the sedimentary rocks of the adjacent forearc basin (Great Valley Group), and then the deformed cherts, pillow lavas, and greenstones of the Franciscan subduction complex. Normally, the forearc and accretionary prism do not get well preserved, but thanks to the transition from a subduction margin to a strike-slip margin (San Andreas Fault system), these major elements of the Mesozoic system are preserved in their general relative positions for us to examine.

Although we focused on the Mesozoic system, we looked at many of the younger features along this transect as well. This week's photo is of Sutter Buttes, or sometimes known as Marysville Buttes. This hills are sticking out of the middle of the fantastically flat Central Valley like a giant zit on a teenager's forehead.


Below is a very general geologic map of this part of California pointing out the location of Sutter Buttes.

So, what are these hills all about? They consist of rhyolites and andesites that were erupted about 1.5 million years ago. This young and small volcanic complex appears to be part of the Cascade system. Mt. Lassen, which is ~100 miles to the north of Sutter Buttes, is the southernmost volcano of the Cascade arc to have erupted in historical times. There is some debate about the position of Sutter Buttes with respect to the northward migrating triple junction and associated timing of cessation of volcanism. Some of these questions remain unanswered.

If you ever find yourself in the Great Valley and are feeling topographically-deficient, go check out the Buttes.

Friday Field Foto #19: Trough cross-bedding

I'm puttin' these up a day early because we are actually going on a 4-day field trip starting tomorrow. Yay!This photo is a shot looking down on the bedding plane of cross-bedded sandstone. The pencil is in the area between two migrating arcuate dunes. If you're lucky enough to get a bedding plane exposure of cross-stratified deposits you can use them as paleocurrent indicators. In this case, the pencil is pointing downstream.
This is in a shallow-marine unit of the Cretaceous in central Utah.


All Friday Field Fotos are taken by me unless otherwise noted.

Friday Field Foto #15: Pillow basalts

One of the best parts about living in the San Francisco Bay Area is that there is great geology very close by. I snapped this photo just a couple weeks ago when we showing some out-of-town visitors around the area.

These beautiful pillow basalts are exposed out at Point Bonita, which is the most seaward promontory adjacent to the Golden Gate. Barely visible is a seagull in the upper left corner of the photo for scale. This area is part of the Marin Headlands, which is composed of rocks of the Franciscan accretionary complex. Very near to this site are spectacularly folded radiolarian cherts.

In addition to the pillow basalts, which are obviously the main attraction, there is a cool old lighthouse.

Friday Field Foto #14: Landslide deposit

In the spring of 2006 I was lucky enough to attend a geology conference in Mendoza, Argentina, which is in the foreland of the central Andes east of, and just over the continental divide from Santiago, Chile. Part of the program was a day trip up to the Andes to look at the fold-thrust belt structure. This is very close to Aconcagua, which is the highest peak in the western hemisphere (almost 7,000 meters).

I snapped this photo on the last stop of the day (near the divide/border) of a landslide that occured about 100 years ago. Apparently, the timing of this is known from notes and observations from European explorers/settlers. Note the building in the lower right foreground for scale.

Friday Field Foto #12: Combined-flow ripples

I realized today that I haven't done a Friday Field Foto since early February...go here to see all the previous FFF posts.

Today, I decided to show a close-up instead of the usual landscape-scale views that I usually post. This is from the Cretaceous of central Utah (associated w/ these rocks) and shows some very nice ripple cross-laminated sandstone. In this case you can very nicely see some slightly climbing ripple-laminated sandstone toward the bottom overlain by some ripples heading the other way (apparently), and then some wavy laminae and smaller-scale ripples on top. Above and below this bed are abundant hummocky cross-stratified sandstones indicating that these ripple-laminated deposits are likely the product of combined flow (oscillatory + unidirectional) associated with storm waves (i.e., between storm- and fair weather-wave base).

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.

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 #8: Cretaceous shoreline deposits

I've missed the last few Fridays...

The Book Cliffs are in central Utah and western Colorado and are famous for their spectacular outcrops of Cretaceous shoreline deposits. The seaway, which once stretched across the whole of North America from the Gulf of Mexico to the Arctic Ocean, left a fantastic record of relative sea level changes in these sedimentary rocks. The landward or basinward movement of the shoreline can be mapped along the Book Cliffs by identifying the various sedimentary environments (coastal plain, beach, shoreface, shelf, etc.) in the record.

The map above illustrates this paleogeography nicely. Note how much of Colorado is part of this seaway at this time. Eventually, the rivers and shorelines (as recorded by the stratigraphy) march across Colorado too. This image taken from a fantastic website by geologist Ron Blakey at the University of Northern Arizona. Check it out.

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.

Friday Field Foto #6: Sand-rich turbidite deposits

This is in the desert of west Texas...in the Delaware Mountains. I spent many weeks here when I was working on my master's (2000-2003). Because of the school calendar, we had to go in the summer mostly. It was frickin' hot! But a beautiful place, a lot of rock. The cool thing about this place is that each of those cliff-faces has the same layers of rock...so, we were able to map out how they changed in some detail.

Check out more photos of the Delawares 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 #4: Patterns in the beach

A day late ...
Here's a photo I took this October while walking on a beach up near Eureka, CA. These little erosional rills formed in this one area of the beach. You can see how the sand was re-sedimented into some neat patterns just a few feet downhill (towards the top). The width of the frame is about 3 or 4 feet across.

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.