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.
UPDATE: Check out Hindered Settling’s post about flame structures here.
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Where on (Google)Earth #27?
The Where on (Google)Earth? quiz started on this blog back in January of this year has grown up and transformed into a community-based series. Many more geo-nerds are getting involved and the installments are featuring oblique views from time to time. I finally had a chance to solve one in time so now it comes back to me. The last one I did was #14 and this is #27!!
I feel like a proud parent. My kid went off to bigger and better things and has now come back for a short visit.
I can never tell anymore what is difficult and what is easy in WoGE. People seem to solve ones ridiculously fast in some cases. I didn’t include a scale on this screenshot…it’s a few miles across…or something.
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HiRISE images of Mars
The image above is from the Marte Vallis region of Mars and nicely shows some mass wasting on the hillslopes.
The High Resolution Imaging Science Experiment (HiRISE) is a camera aboard the Mars Reconnaissance Orbiter. The project is part of the University of Arizona and the website is a great portal into high resolution of Mars’ surface.
They have a lot of the images tagged by categories such as mass wasting, tectonic processes, fluvial processes, and others. See all the science themes here.
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A two-hour and twenty-three minute GM commercial
I really didn’t expect a movie from Michael Bay to be anything but the cheesiest cheese that Hollywood has to offer but I didn’t expect Transformers to be a commercial.
These days movies have a lot of product placement and the like, but this one really takes the cake. All the vehicles, especially the good ones (Autobots), were GM….Chevy and Pontiac cars and GMC trucks. Any chance they got….I mean any….they had a close-up of the logo on the grill of the car/truck. It wasn’t even subtle.
I was a kid in the ’80s, had the Transformers toys, and watched the cartoon. One of the favorite characters was an old yellow Volkswagen bug, named “Bumblebee”. This movie of course couldn’t have a German car in it…so, they made Bumblebee a damn Chevy Camaro!! What!? He’s supposed to be a cute little VW bug!! A#@holes!
The commercial….er, I mean movie….takes any chance it gets to have the leading girl lean over the car in her skimpy outfits. It looked like the cover of one of those hot rod magazines you see in the gas station on the road during a geology field trip.
Does GM really think this is gonna help their sales? Perhaps they oughtta work on designing and building cars Americans actually want to buy, instead of spending money on pushing the same junk. Anyone who goes out to buy a new Camaro simply because it was featured in this movie is the only customer GM has left.
We have crossed the line from movies that have a lot of ads in them to an advertisement that is disguised as a movie. The end is coming.
image above from Movieweb
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Spectacular visualization of flight patterns
If you haven’t seen by now…i’m a sucker for images showing complex patterns. I found a fantastic website that is part data visualization and part art. The author/artist of this project, Aaron Koblin, compiled flight pattern data from the FAA for the United States (and most of North America) and created stunning images and animations. The animations are really the coolest part of this…to see our flight transportation system like this is jaw-dropping. It looks like the flurry of activity you see on an ant hill…or something.
Here are a few of the images….check out the site for more.
The look and feel of the work reminds of the Cabspotting project that I posted about several months ago.
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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.
Diabolical propaganda
Depending on your mood, this will make you laugh or it might make your skin crawl.
On Rush Limbaugh’s radio show Tuesday, a 13 year old caller named Patrick complained that he was forced to read “liberal magazines like Time and Newsweek” in school which explained the globe was getting warmer. The caller said he was skeptical of the science because “my parents have always been skeptical of it.” Limbaugh then offered the following encouragement to the caller:
RUSH: Patrick, this will be a good lesson. There are liberals everywhere. You may think that just because your town is conservative — there are liberals. They’re hiding in the shadows, and they are lurking there, and they’re around and the odds are that many of them are in the school system. You’ll probably at some point probably have to watch [Al Gore’s movie], unless your parents and other parents find out about it and demand, “If you’re going to show this movie, you better show the Great Global Warming Swindle and put the other side to our kids out there.” Well, congratulations. I’m glad you called and told us this. This is the kind of thing that gives us all encouragement for the future. Here you are at 13, already aware of when you’re watching propaganda. That’s great.
How Orwellian is that!? Denounce propaganda as you spew it. Hmmm….the “other side”…. why are people so goddamn preoccupied with distilling every issue to “sides”? If an asteroid doesn’t destroy our civilization, then this dangerously oversimplified dualistic worldview will.
Okay….back to work. I need to do some evil scientific research. I wonder if Rush considers Sedimentology a liberal journal?
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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!
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Global socio-economic cartograms
As an Earth scientist I live and breath maps.
I recently came across the site Worldmapper, which has nearly 400 global cartograms of various metrics. A cartogram is basically a map that shows area re-sized according to the variable being mapped. Worldmapper metrics include things like immigration, wealth, resources, housing, health, disease, education, and many others.
For example, below is a map showing the global distribution of scientific research for 2001 (defined as the number of published scientific papers by authors living in that nation). Further analysis would obviously require the raw data (which you can get off this site as well), but viewing the results in a map form like this conveys a lot of information immediately. The United States, Europe, and Japan (that dark purple blog on the right) stand out as “big” areas while areas like Africa, Central America, and Southeast Asia are “small”.
Another example shown below is a cartogram of crude petroleum exports. Note the big players: the Middle East, Venezuela, Mexico, Norway, and parts of Africa. Nations where imports exceed exports (e.g., the United States) are not shown.
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A key element to understanding sediment flux is the determination of accumulation rates in areas of net deposition. How fast (or slow) is sediment piling up? How is that rate distributed in space? How does it change over time? What does it tell us about the controls of sediment flux?
Sweet. This is simple. Just measure the thickness of sediment (or sedimentary rock), divide by the duration, and interpret! Well…it turns out it’s not so simple after all.
This post is a little bit about a very important paper that I don’t see cited enough.
Sadler, P.M., 1981, Sediment accumulation rates and the completeness of stratigraphic sections: Journal of Geology, v. 89.
The main gist of the paper is summarized nicely in the first statements of the abstract:
A compilation of nearly 25,000 rates of sediment accumulation shows that they are extremely variable, spanning at least 11 orders of magnitude. Much of this variation results from compiling rates determined for different time spans: there is a systematic trend of falling mean rate with increasing time span.
In other words, the longer the time interval measured the lower the sedimentation rate. The image below is Sadler’s Figure 1 with some of my scribblings on it. It is a log-log plot of sedimentation rate (meters per 1000 years) on the vertical axis and timespan (years) and thickness (meters) on the lower and upper horizontal axes, respectively. For now just focus on the ‘a’ figure … the one on the left. And also don’t worry about the Roman numerals (i, ii, etc.).
The trend is pretty obvious. The data in the upper left have very high sedimentation rates, were measured over very short time intervals (hours to months), and are relatively thin. The area to the lower right represents sedimentation rates several orders of magnitude slower and have measured time intervals in the millions to 10s of millions of years. There’s some spread in the data of course, but in general this is a robust trend. What is underlying this relationship?
The tendency for sediment to compact as it is buried (and thus become thinner) is a contribution but does not account for the bulk of the relationship. What you are seeing here is essentially the nature of sedimentation. It is discontinuous and highly spasmodic. In most depositional environments, the duration of actual deposition of sediment is very small compared to the time the system sits there doing little to nothing. I think Derek Ager said the history of sedimentation is like being in the military … hours of boredom separated by brief moments of terror.
So, we are seeing the effect of including more time of non-deposition (i.e., nothing happening) as we increase the span of the measured time interval. We are also seeing the nature of sediment preservation over geologic time scales. That is, the farther back into time we go, the more time of erosion is included (remember, these systems are net-depositional … there is some erosion occurring). The younger the section, the more “complete” it tends to be. Sadler discusses this issue of preservation and stratigraphic completeness very elegantly with this statement:
Sedimentary sequences…record the passage of geological time as an alternating set of sedimentary increments and gaps. The ratio of these two components is stratigraphic completeness.
Sadler certainly was not the first to recognize this (another famous paper by Barrell, which I will post about someday, is notable for discussing completeness back in 1917) but this paper was the first to quantify it. A sizeable chunk of the paper goes through methods for calculating completeness.
Several studies in recent years, including the popular paper by Molnar & England (1990), discuss the apparent increase in sedimentation rate observed in the last 20-30 million years. This has led to countless discussions about whether this signature is recording changing uplift rates or changing climate or both. Now of course we shan’t attribute this trend solely to Sadler’s measurement interval bias, because there very well may be an actual increase in rate. But, the point of this post is to highlight Sadler’s work and how we shouldn’t forget about it. Many of these papers do not even mention it. Sampling bias as a result for a study is not very exciting and probably won’t result in the cover of Nature or Science, but if we are to adequately deconvolve all the factors at play we need to remember biases like this.
Molnar, P., and England, P., 1990, Late Cenozoic uplift of mountain ranges and global climate change: chicken or egg? Nature, v. 346.
Sadler, P. (1981). Sediment Accumulation Rates and the Completeness of Stratigraphic Sections The Journal of Geology, 89 (5), 569-584 DOI: 10.1086/628623
Clastic Detritus 