Dish structures

Dish structures are sedimentary structures found in thick sand (or sandstone) that have concave-up, bowl-like shapes. They form when water is trying to escape from rapidly deposited sand but encounters horizontal barriers of somewhat lower permeability (usually zones with smaller grain size and/or dispersed mud). These force the water to flow laterally until it finds a place where it can go upward again. In the meantime, the subtle permeability differences get enhanced as muddy particles are washed away from the cleaner parts of the sand and concentrated in zones of lower permeability. The sides of these lower perm zones bend upward as the water finds its way up. Eventually pillar structures, vertical zones of cleaner sands can form on the sides of the dishes.

Initially dish structures were thought to be related to the (still somewhat fuzzy) mechanics of sediment transport and deposition in high-concentration gravity flows. However, clear examples that showed primary sedimentary structures (like cross lamination) being cross cut by dish structures proved that the latter are secondary structures, formed soon after deposition.

Probably because rapid deposition of sand is a requirement for the formation of dishes, these sedimentary structures are largely restricted to deep-water sands. Here are some examples that I think are blogworthy:

This one is from the northern California coast. Note the pillar structures between the dishes. [Apologies for the lack of scale – I think this bed is about 4 feet thick].

This is a zoom-in of dish structures in the Cerro Toro Formation of Southern Chile. Lighter-colored areas probably contain less mud than the darker zones.

No scale on this one either (there was no way I could climb up there), but trust me, these are probably among the largest dish structures in the known universe. They were photographed in northern Peru, near the town of Talara.

And to prove that they are really big, here is a photo that gives an idea of their scale:

Geologic misconceptions: 2D vs. 3D

[This is a contribution to Accretionary Wedge #5]

One of the problems that a geologist is often faced with is the difficulty of reconstructing a complex three-dimensional geometry and history from limited information that is often one-dimensional (e.g., well data, cores) or two-dimensional (outcrops, 2D seismic sections). Humans in general, and geologists in particular tend to look for evidence where the light is better, and we are tempted to think that the beautiful core we have described, the one good outcrop face we have, the one textbook-quality seismic line on our wall is a good representation of the geology and stratigraphy of a much broader area, and that one can build a coherent story without knowing much about the third dimension.

That, of course, may well be true of ‘layercake’ stratigraphy: after all, a single thickness value can be used to fully characterize the geometry of a layer that has the same thickness over a large area. But, as Brian points out, ‘layercake stratigraphy’ should be considered an oxymoron: every sedimentary layer shows some thickness variations if traced for a long enough distance, even if some layers change their thickness more slowly than others. Stratigraphy is only layercake-like for human observers; subtle but persistent variations in thickness and relief can become striking geometries with some vertical exaggeration. Again, if this variation only occurred in one direction, a two-dimensional section along the same direction would summarize very well the whole story.

However, complex three-dimensionality is the rule rather than the exception in geology. Take for example a meandering river: its geometry is complex enough as it is, a single snapshot of a snaky morphology in time. But try imagining what happens as point bars and levees are deposited and cutbanks are cut; the channel changes its position over time and, over thousands and hundreds of thousands of years, it leaves behind an extremely complicated stack of deposits that would probably be difficult to fully understand even if you somehow could see and describe everything at the greatest detail in 3D. Obviously, a nice outcrop or a number of cores through such a deposit can provide a wealth of information, but we would be fooling ourselves if we thought that a single fining-upward sequence with some cross-bedding (that is, the classic point-bar facies model) was enough to understand a fluvial system.

But strong three-dimensionality is not restricted to fluvial deposits; look at any present-day depositional system in Google Earth and you will find that alluvial fans, deltas, barrier islands and tidal inlets, wind-blown dune fields are all intricate patterns, usually with lines running in more than one direction. Yet many of the classic facies and stratigraphic models are either one- or two-dimensional. Maybe, probably, these are necessary and useful simplifications and conceptual models, but they can only be useful if one is also aware how far they are from capturing the full 3D complexity of nature.

That being said, I have to add that 3D is not always better than 2D. Nowadays, some of the best three-dimensional geological datasets are 3D seismic surveys, and, with the increasing availability of such gold-mines of stratigraphic beauty (there are other uses as well, but let’s focus on one thing for now 🙂 ) it is easy to fall victim to the temptations of colorful three-dimensional displays. Despite claims like ‘3D interpretation and visualization are the future’, the truth is that a good set of old-fashioned maps and cross sections are more valuable in the long term than some glossy presentation slides with no exact spatial location.

Unless, of course, you can visualize and share your data relying on an easy-to-use and truly three-dimensional viewer. Like Google Earth. Even William Smith would be excited about that.


Detail from “Geological view and section through Dorsetshire and Somersetshire to Taunton, on the road through Yeovil toWimborn[e] Minster, &c.”, by William Smith, 1819. Source: Oxford Digital Library

Geo-highlights from Hindered Settling 2007

It is kind of late to do this 2007 retrospective, but what the heck. As pointed out by Ron, 2007 has been the year when a real geology blogger community started to develop. The evolution of Hindered Settling from an eclectic mix of notes about science, geology, skepticisim, atheism, technology, etc., written in Hungarian and in English (or Hunglish?), to a much more geoscience-oriented, English-only site is in part the result of this trend.

So here are a few posts from 2007 that I think should be on this list:

Photos from Brazos Bend State Park – if you live in Houston, Brazos Bend State Park is one of the best places to get away from the city and see some wildlife & nature. No mountains, of course, but at least you can look at oxbow lakes and learn about photography. For some reason, the photos I have taken there over the years have become fairly popular.

On the Great Unconformity, James Hutton, and Geologic Time

Photos and impressions from a stunning glacial lake and delta in the Canadian Rockies, with some sedimentology mixed in

On flame structures

Sedimentology on Mars
– wet or dry gravity flows?

Thoughts about the Black Sea flood and its potential link to the spread of agriculture in Europe

Absurd catastrophism

If there was an icon for ‘blogging on non-peer-reviewed non-research’, in the style of ‘blogging on peer-reviewed research’, this post would qualify for it. Although it is advertised as publishing “cutting-edge, peer-reviewed, creationist research papers”, Answers Research Journal (published by Answers in Genesis) is definitely not cutting-edge, not peer-reviewed, and is clearly not research. The evidence: the first few materials that are available online. There is a paper on “catastrophic granite formation”; here is a passage that gives you a flavor:

“Thus the formation of granite intrusions in the middle to upper crust involves four discrete processes — partial melting, melt segregation, magma ascent, and magma emplacement. According to conventional geologists (Petford et al. 2000), the rate-limiting step in this series of processes in granite magmatism is the timescale of partial melting (Harris, Vance, and Ayres 2000; Petford, Clemens, and Vigneresse 1997), but “the follow-on stages of segregation, ascent, and emplacement can be geologically extremely rapid – perhaps even catastrophic.” However, as suggested by Woodmorappe (2001), the required timescale for partial melting is not incompatible with the 6,000–7,000 year biblical framework for earth history because a very large reservoir of granitic melts could have been generated in the lower crust in the 1,650 years between Creation and the Flood, particularly due to residual heat from an episode of accelerated nuclear decay during the first three days of the Creation Week (Humphreys 2000; Vardiman, Snelling, and Chaffin 2005). This very large reservoir of granitic melts would then have been mobilized and progressively intruded into the upper crust during the global, year-long Flood when the rates of these granite magmatism processes would have been greatly accelerated with so many other geologic processes due to another episode of accelerated nuclear decay (Humphreys, 2000; Vardiman, Snelling, and Chaffin 2005) and catastrophic plate tectonics (Austin et al.1994), the likely driving mechanism of the Flood event.”

Here is what I honestly do not understand. Let’s accept for a moment the idea that granites can form relatively fast, and pretend that radioactive dating has some major issues, as these people claim (it doesn’t, of course), so that all the granites on Earth fit the 6000-year timeframe. But what about the stuff that the granites were generated from? That must be older, right? And if the whole crust is less than a few thousand years old, what about the mantle? And, if one can speculate about “accelerated nuclear decay during the first three days of the Creation Week” or “catastrophic plate tectonics”, why not just say that granites were created on the second day, after the mantle was ready to start convection by the end of the first day? Or, even better and simpler (Occam’s razor!), why not just come up with something like:

And God said, Let there be granite: and there was granite. And God saw the granite, that it was good: and God divided the crust from the mantle.

I am looking forward to the time when somebody realizes that the Universe was created yesterday, and it is only an illusion that we have been around for a bit longer than that. Imagine all the wonderful research opportunities that such a revolutionary working hypothesis would generate. I can already see papers and headlines like:

Updated relativity theory shows that time is shorter than you think

Plate tectonic hit-and-run: after hitting North America yesterday with several microcontinents, the Pacific Plate continued to subduct as if nothing happened

Fossil record from 4:15 pm yesterday shows that lightning-fast giant snails were abundant on Earth for more than 7 minutes

Accelerated ice flow during the last few minutes of Creation Hour is likely responsible for death of Ötzi the iceman

Scuba diver killed by massive rain of pelagic forams

Catastrophic hair growth in early humans

Additional research ideas are welcome; ‘Answers Research Journal’ is calling for papers now.

UPDATE: In my rush to publish the above results, I forgot to mention some previous work on similar subjects: Afarensis – an expert in points to the Precambrian archaeology –ist, who suggested long ago that the Cambrian explosion was caused by a bacteria trying to form a synthesis with a mitochondria and that peanut butter is a leftover from this explosion. I would only add that there is new evidence suggesting that the Precambrian started at 1 am yesterday, after Creation Hour ended, and it probably lasted for several hours, until the bacteria committed adultery.

Catastrophic flooding of the Black Sea and the expansion of agriculture in Europe

ResearchBlogging.orgBoth Ole and Chris have blogged about a new paper, published in Quaternary Science Reviews, that discusses the link between the catastrophic inundation of the Black Sea and the expansion of agriculture in Europe during the Neolithic. I am not going to repeat what they already summarized well; I only want to expand a bit on what I see as the weak points of the Black Sea flood story. [Disclaimer: I am not an expert in the geology and stratigraphy of the Black Sea, and even less of an expert in archaeological matters].

While the recently published Turney & Brown paper presents some nice data and argues convincingly that the start of Neolithic expansion in Europe roughly coincides with the ~8300 yr BP age estimate for the catastrophic flooding of the Black Sea, this correlation does not necessarily suggest a cause-and-effect relationship. The question is still open: yes, the flooding might have caused the migration, but it is also possible that the two events are independently related to the same climatic changes. Very few of the radiocarbon dates, representing the earliest Neolithic sites in Europe, come from the territory of present-day Romania, yet one would expect that the low-lying areas along the lower Danube River would be the first places to be colonized by the population forced out from the inundated shelves of the Black Sea that are the widest east of the Danube Delta. Why are early Neolithic settlements so scarce in this area, which is good for agriculture? In their 1998 book, Ryan and Pitman suggest that Vinca farmers showed up abruptly along the Danube valley, soon after the flooding occurred. The earliest Vinca settlements however are dated at 7500 yr BP, so there is a gap of a few hundred years between the flood and these first settlements. That seems too long; in addition, even if the initial displacement of people living near the Black Sea resulted in the expansion of agriculture into Southeastern Europe, it is questionable how much effect this flooding had on the spread of Neolithic people into Northwestern Europe. It is unlikely that the main motivation to cross the English Channel for people living in today’s Northern France was that their ancestors were scared off the shores of the Black Sea many hundreds of years before.

The image below is a screenshot from Google Earth, showing the data published by Turney and Brown. The Neolithic locations are color-coded according to their age, red being the oldest, and dark blue being the youngest. The KMZ file (made with GPS Visualizer and some help from Matlab) is available here.


Regarding the link between the Black Sea flood and Noah’s story — I think it is an interesting idea, but not much more. There were and there will be numerous large floods that affect human lives and human history, and the one featured in the Bible is so generic that it will be difficult to unequivocally link it to any specific event. In addition, the catastrophic flooding of the Black Sea might have been catastrophic only in a geological sense; calculations suggest that it took 34 years to erase the 155 m difference in water level between the Black Sea and the Mediterranean. The slow and relentless rise of the sea must have been disquieting and annoying to people living close to the shore, but, unless they built their huts in the middle of the Bosporus, it probably was not as traumatic for most of them as Hurricane Katrina was for lots of Gulf Coast residents.

The recent (I mean geologically recent) history of the Black Sea region is certainly fascinating and the controversy surrounding the exact sequence and nature of the events can only result in more top-notch oceanographic and archeologic research. And that is always exciting.

The importance of numbers in sedimentary geology

A few years ago, Chris Paola published a paper in Sedimentology on “Quantitative models of sedimentary basin filling”. I was skimming through it today, and found these thoughts about the role and status of quantitative reasoning in sedimentary geology:

…what is needed is researchers who are skilled in the field but at the same time understand what quantitative modelling is about: why and how people make approximations, why approaches to modelling can and must differ, and, above all, what the mathematics in the models mean physically. Just as there is no substitute for experience in learning to work in the field, there is no substitute for experience in developing physical insight. And there is no shortcut: we need researchers who are good at at both traditional, descriptive geology and quantitative geology. For the ‘modal’ sedimentary-geology student, it is not sophisticated computational skills or training in advanced calculus that is lacking, but rather the routine application of basic quantitative reasoning. This means things like estimating scales and rates for key processes, knowing the magnitudes of basic physical properties, and being able to estimate the relative importance of various processes in a particular setting. Understanding scales, rates and relative magnitudes is to quantitative science what recognizing quartz and feldspar is to field geology. Neither requires years of sophisticated training, but both require repetition until they become habitual.

And:

Some 30 years after the initial ‘physics scare’ associated with bedforms and sedimentary structures, a set of basic principles from fluid and sediment mechanics now appears routinely in introductory sedimentology textbooks. Popular items include settling velocity and Stoke’s Law, the Reynolds and Froude numbers, and the basic force balance for steady, uniform channel flow. This material is typically presented somewhere near the beginning of the book and then is largely ignored. (…) There remains a striking contrast between the role of fluid and sediment physics in sedimentary geology and that of thermodynamics in igneous and metamorphic geology. In ‘ig-met’ texts the underlying thermodynamic principles are introduced and then applied repeatedly. Whereas in hard-rock petrology, thermodynamics permeates the discipline, in sedimentary geology, sediment mechanics still seems a little like taking vitamins: it is surely good for you, but most people cannot say exactly why. There are several reasons for this. In current practice, process-based interpretation is often applied in a piecemeal, descriptive way, to no apparent end beyond providing the interpreter with one more adjective. In addition, the quantitative material that is traditionally taught more often not the most important. For instance, a real appreciation of the implications of the sediment-continuity equation as the governing relation for physical sedimentation is far more useful than the details of sediment-transport formulae or even the definition of the Reynolds number.

Although I still have a lot to learn myself, I couldn’t agree more.

ps. Check out what Lord Kelvin had to say about the importance of numbers in science.

Changing one’s mind is not a sign of weakness

Seed Magazine’s second annual science writing contest is over now, and the essays of the first and second prize winners are available online. Here is something worth noting in the piece by Thomas W. Martin:

The goal of science is to find those ideas that can withstand the long and hard barrage of evidence-based argument. That lesson must be experienced anew by the members of each generation, irrespective of their careers. Mastery of scientific concepts and theories is a necessary starting point, but it serves only as a prerequisite to joining the never-ending dialogue. Students must learn first-hand how to both imaginatively create new hypotheses and to dispassionately critique them. Many commentators have rightly implored us to make certain that young people encounter the “thrill” of discovery. While this is undeniably desirable, it is arguably even more crucial that they experience the agony (if only on a modest scale) of having a pet hypothesis demolished by facts.

Several current presidential candidates have insisted that they oppose the scientific account of earth’s natural history as a matter of principle. In the present cultural climate, altering one’s beliefs in response to anything (facts included) is considered a sign of weakness. Students must be convinced that changing one’s mind in light of the evidence is not weakness: Changing one’s mind is the essence of intellectual growth.

New images from Mars: the idea of very recent watery flows is evaporating

A few months ago I commented on the fact that, despite numerous scientific and media reports, the existence of recent watery flows on Mars is far from being obvious or proven. While there are many rock formations exposed at the planet’s surface that clearly suggest flowing water some time in the ancient past – for example, the delta near Holden Crater -, many of the young gullies and debris fans have no unequivocal signatures of recent watery flows.

The high-resolution images with the relatively recent gullies were released in 2000, and a paper was published in Science about how these features suggest the presence of liquid water on the Martian surface. Last year, this idea seemed to get new support, in the form of some images taken in 2005 were showing sedimentary activity on crater walls, when compared to images shot a few years earlier.

(image from Malin Space Science Systems)
The problem is, as I said, that nothing in these images suggest unequivocally the presence of water. Geologist Allan Treiman published a paper in 2003 stating this, but at that time his views were representing the minority viewpoint. Needless to say, the news reports got rid of the last remaining uncertainties and doubts in the story, and presented it as if it was 100% sure that liquid water exists today on Mars.

Now there is new evidence that the recent watery flows are not so watery after all. Rather, they are probably dust avalanches, dry flows similar to the ones that occur on windblown dunes here on Earth. Such flows can only form on steep slopes, that are close to the angle of repose. The problem, of course, is complicated – as many problems in science are – and there is no simple answer. For example, in the image shown above, you can see a fan that has been reincised after its deposition by its own feeding channel, so that the latest active deposition occurs further downdip. Such erosional valleys are probably associated with turbulent flow, suggesting that these fans were probably deposited by watery flows. More recent images (see below) also show details of erosional channels that are suggestive of watery flows. Unless the dust avalanches were highly turbulent density flows, similar to some snow avalanches, and they were even able to cut channels. Again, I think there is no easy and obvious answer.

photo from NASA’s Planetary Photojournal
In any case, there are two new papers in Science on this subject, check them out if you have online access (I don’t 😦 ).

Flame structures


Flame structures are sedimentary structures that usually consist of upward-pointing flame-shaped finer-grained sediment tongues that protrude into coarser sediment (like sand). Almost invariably, the ‘flames’ are inclined in a downslope direction (in a paleogeographic sense, of course) — like in these two images from the Precambrian Windermere Group in the Canadian Caribou Mountains.


Flame structures are often interpreted as load structures: the overall higher-density sand sinks into the lower-density underlying shale. That would put flame structures into the category of Rayleigh-Taylor instabilities, which result from density inversion. In geology, one of the most important types of Rayleigh-Taylor instability is related to salt: if buried deep enough, the density of the compacting overlying sediment exceeds the density of salt, and the latter starts flowing upward, giving rise to salt diapirs. Salt diapirs often have mushroom shapes, typical of Rayleigh-Taylor instabilities.

The shapes of the flame structures above actually remind me more of the Kelvin-Helmholtz instability, which is related to shear (that is, different velocities) across a fluid interface, and can occur even if the densities are not inverted. K-H instabilities in the atmosphere can result in elegant clouds. K-H billows are common at the tops of turbidity currents, due to the shear between the static water column above and the moving sediment-laden current below. There is no reason why the instability could not occur at the base of the current as well, if the underlying sediment is still fluid enough, and the current itself is not too erosive.

Here is the classic picture of K-H billows at the top of a density current, from Van Dyke’s Album of Fluid Motion.


Clastic Detritus has more on flame structures.