Upcoming conference on seismic geomorphology

Ever since geoscientists and engineers started using seismic waves to figure out what lies under our feet, seismic reflection technology kept improving and today most oil-rich sedimentary basins have a wide coverage of high-quality three-dimensional datasets. In addition to finding structures and locations in the subsurface that are likely to be filled with hydrocarbons, these huge data volumes can also be used to reconstruct landscapes that are long gone from the Earth's surface. After all, stratigraphy is what is left behind from an ever-changing topography, and it is a mistake to think that stratigraphy can be understood without knowing a few things about geomorphology and landscape evolution.

Seismic data confirms that the past, indeed, is not that different from the present: if you peek (that is, listen) into these volumes of rock, you see ancient meandering rivers and submarine channels, deltas, barrier islands, mouth bars, and estuaries. However, in addition to and beyond the excitement of seeing another beautiful example of a sinuous channel or other depositional and erosional features, a lot remains to be learned from the true and large-scale three-dimensionality of these datasets. For a geologist, there is no other data type that offers such a degree of three-dimensionality. Even the largest outcrops offer only random two-dimensional sections through a system; the temptation is strong to convince ourselves that we can extrapolate to get an idea about the third dimension, but more often than not we are probably wrong, at least in the details of our extrapolation. This is actually worse than the case of "The Blind Men and the Elephant"; it is more like the blind men and a random cut through the elephant (but I will stop this chain of analogies right there).

Long story short, I (re)started to blog about this subject because there is going to be an SEPM research conference in Houston, a conference that focuses on using three-dimensional seismic data to better understand how sediment moves or gets deposited on continental slopes. It should be an interesting collection of talks and papers.



The image above is from the conference website and announcement; it happens to come from a paper that I am going to present, on a shelf-edge delta and its related slope channels in the Gulf of Mexico (the higher-resolution version is coming soon...). Colors represent thickness (red is thick, blue means thin). There are two leveed channels taking sediment from the shelf-edge delta into the deep ocean.

Two gigapans from Cliffs of Moher, Ireland

I shot these gigapans recently, while we were visiting some deep-water rocks in County Clare, Ireland (see more detail on these rocks and a few photos from the trip). One afternoon we took some time off from the turbidites to do a bit of geo-tourism at the Cliffs of Moher, a series of spectacular escarpments along an 8 km long stretch of the western coast of Ireland. They are 702 feet (214 meters) high at the highest point and expose Late Carboniferous (Namurian) sandstones and shales that were mostly deposited as deltaic and fluvial sediments of the Tullig and Kilkee cyclothems.

This place is one of the most visited tourist attractions in Ireland, and for a good reason: the combination of the cliffs, the landscape, and abundant wildlife is, indeed, spectacular.

This is a view to the south (launch full screen viewer):


And this is a view to the north (from O'Brian's Tower; launch full-screen viewer):


Unfortunately, these stamp-sized windows do not do justice to the panoramas; it is a good idea to click on the "Launch full-screen viewer" links.

More reasons to conclude that coastal 'chevrons' are not related to mega-tsunamis

If there was any doubt left that coastal sand accumulations called 'chevrons' are *not* related to gigantic tsunamis (see previous thoughts on the subject here and here; Ole also has a recent blog post, and see a news release here), the May issue of Geology provides additional arguments to show that this is the case. Joanne Bourgeois of University of Washington and Robert Weiss of Texas A&M University, both experts in the sedimentology of tsunami deposits, present two lines of arguments. First they show that the orientation of the Madagascar chevrons is significantly different from what is predicted through modeling the tsunami. While the tsunami wave tends to hit the coast with an overall perpendicular orientation, due to wave refraction, the 'chevrons' are oriented at various angles to the coast, angles that are more consistent with predominant wind directions. Second, they look at the sediment transport conditions and suggest that even coarse sand must have been in suspension in flows deep enough to create the chevrons. However, dune-like bedforms cannot develop without sediment being transported as bedload; therefore, the bedforms must have a different origin than mega-tsunamis. The obvious alternative is parabolic dunes; these well-known bedforms show up when vegetation partially covers the dune's tails and slows down sediment transport. The authors don't hesitate to draw the conclusion that

The extraordinary claim of "chevron” genesis by mega-tsunamis cannot withstand simple but rigorous testing.

I am far from being a tsunami expert, but I find this subject fascinating. The issue of suspended load vs. bedload and stratified or laminated vs. graded bedding is equally important for deposition from tsunami waves and turbidity currents. It is worth spending a bit of time and blogspace to explore the kind of analysis of sediment transport conditions that this paper presents.

Although I see no reasons to disagree with the paper's conclusions (as it could be predicted from my previous posts on the subject), at first reading I didn't fully understand the line of reasoning about suspended load vs. bedload. So here goes my attempt to understand it.

The argument goes as follows. The Rouse number is the ratio between the settling velocity of a certain grain size and the shear velocity of the flow, multiplied by von Karman's constant (which is ~0.4): Ro = ws/k*u_shear. For a grain of a given size, if the Rouse number is larger than 2.5, the grain's settling velocity is much larger than the upward-directed component of the turbulence, and the grain tends to stay close to the bottom, in the bedload. [This is equivalent to saying that the settling velocity has to be larger or equal to the shear velocity, a condition also known as the suspension criterion]. If the Rouse number is less than 0.8, the flow is turbulent enough to keep the grain fully suspended. In between these values, there is a zone of transitional behavior. For the flows that might have deposited the chevrons, the Rouse number is always less than 2.5, regardless of how the other parameters like the Froude number, grain diameter, and roughness length are varied. Although the authors state that the flows must have been deeper than 8 m (because most chevrons are higher than 4 m, and the flow must be at least twice as high as the bedform), there seems to be no other constraint on tsunami behavior [note that I did not have access - yet - to the supplementary web material].

So the question is: doesn't this reasoning apply to other types of flows as well? For example, the Mississippi River is certainly deeper than 8 m in many places -- does this mean that it is able to suspend very coarse (2 mm diameter) sand? In other words, what is the difference between flow in a tsunami run-up and the Mississippi River? The answers might be obvious to many, but they are certainly not obvious to me.

One thing we can do is to create a different kind of plot: instead of plotting the Rouse number against flow depth, let's plot velocity vs. depth. I have a better feeling for what are reasonable velocities for different kind of flows than I do for Rouse numbers. The Rouse number would form the third dimension of the plot; one can visualize that as a contour map of Rouse numbers as a function of flow depth and velocity:


The Rouse numbers shown in this plot are valid for a grain diameter of 2 mm and roughness length of 1 m (using the same equations for settling velocity and shear velocity as in Bourgeois & Weiss 2009). Anything coarser than this cannot be called sand any more. So if this grain size doesn't stay in the bedload, there is no chance for finer sediment either. It is obvious from the plot that, for flows deeper than 8-10 m, very coarse sand will be part of the bedload unless flow velocity is larger than ~5 m/s. The Mississippi River at New Orleans has velocities on the scale 1.5 m/s, so 1-2 mm sand should definitely stay close to the bottom, and in fact it does.

We know however that tsunamis are not exactly tranquil flows like the big old Mississippi at New Orleans. The larger ones are fast and furious and Google Earth might need massive updates after they rearrange entire coastal landscapes. [Don't get me wrong, I am not trying to diminish the power and destructive force of the Mississippi.] In other words, the Froude number of a tsunami run-up is larger than the Froude number of the Mississippi River. The Mississippi is relatively slow and deep; the tsunami is fast and relatively shallow. The Froude number is the ratio between velocity and the square root of gravity multiplied by flow depth:


Fr = u/√(g*d)
This number for the Mississippi is much less than one (these flows are called subcritical flows). On the other hand, tsunamis are waves of very large wavelengths, and they behave even in the open ocean as shallow water waves (wavelength 20 times larger than water depth). For these kinds of waves, the velocity is solely a function of water depth:


u = √(g*d)
If we assume that the tsunami run-up has a comparable velocity to that of the tsunami wave in the nearshore zone, we find that the Froude number of the run-up must be around 1. This is obviously a very back-of-the-envelope argument, but the point is that these flows must have in general relatively large Froude numbers. If we plot the lines for Fr = 1 and Fr =1.5 on the depth-velocity diagram (see above), we can see how different likely tsunami behavior is from that of large rivers. It also becomes evident that even coarse sand would not be part of the bedload in these flows, especially not in flows deep enough to build the 'chevrons'. Which means that sandy tsunami deposits are likely to be largely unstructured or poorly structured sand sheets rather than several m thick accumulations of cross-bedded sand.

And that ends my Saturday exercise in Fluid Mechanics 101.

Reference
Bourgeois, J., & Weiss, R. (2009). "Chevrons" are not mega-tsunami deposits--A sedimentologic assessment Geology, 37 (5), 403-406 DOI: 10.1130/G25246A.1

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Scientia Pro Publica #4

Normal grading

In sedimentology, the word 'grading' has nothing to do with exams and assignments. Instead, it refers to a regularly decreasing or increasing grain size within one sedimentary layer. Because it is much more common than the other alternative, upward decreasing grain size is called 'normal grading'. Grains that consistently increase in size toward the top of the bed are responsible for 'inverse grading'. Upward fining and coarsening are related terms that are often used to describe grain-size trends in not one, but multiple beds.

Normal grading in a turbidite from the Talara Basin, Peru

The simplest way to generate normal grading is to put some poorly sorted sand and water in a container, shake it up, and then let it settle. The larger grains will settle faster than the smaller ones (as Stokes' law tells us) and most of the large grains will end up at the bottom of the deposit. [Note that some fine grains will be at the bottom as well - the ones that were already close to the bottom at the beginning of sedimentation.] This kind of static suspension settling is not how most sediment is deposited on a river bed or a beach; even if a grain is part of the suspended load, it usually goes through a phase of bedload transport, that is, a phase of jumping and rolling and bouncing on the bed, before it comes to rest. The resulting deposit usually has lots of thin layers, laminations, and there are no obvious and gradual upward changes in grain size.

What is needed is a sediment-rich flow that suddenly slows down or spreads out and looses its power to carry most of its sediment load. Grains are getting to the bottom so fast that there is not much time for the flow to keep them rolling and bouncing around; instead they quickly get buried by the other grains that are ready to take a geological break. While this is still quite different from static suspension settling (because the flow did not come to a full stop), it can be thought of as a modified version of static settling: all is needed is a horizontal velocity component, in addition to the vertical one. Of course, the segregation of the coarser grains to the bottom of the flow may have started much earlier. Typically, they never made it to the top in the first place.


Conglomerate bed in the Cretaceous Cerro Toro Formation, Torres del Paine National Park, Southern Chile. There is some inverse grading at the base of this bed, before the size of the clasts starts decreasing

Such large, sediment-laden flows are not very common, certainly not on a human timescale. When they do occur, they tend to show up in the news, especially if human artifacts, or humans themselves, become part of the normally graded deposits. Deposits of snow avalanches, volcanic ash-laden pyroclastic flows, subaerial debris flows, tsunamis, submarine turbidity currents can all show normal grading. The images shown here all come from deposits of large submarine gravity flows. Some of them (like the one below) have a muddy matrix, but the grading is still obvious (the two large clasts at the top of the bed have lower densities).

Normally graded conglomerate layer with a muddy matrix, Cerro Toro Formation, Chile

In recent years, some questions have been raised about the common presence of normal grading, especially in turbidites. The fact is that normal grading is often seen in rocks of all ages, and, in a simple view, it is a reflection of larger grains getting quickly to the bottom.

Normal grading is normal, after all.

Description does not suffice for an explanation

On February 3, 1967, J. R. L. Allen gave the fifth "Geologists' Association Special Lecture", entitled "Some Recent Advances in the Physics of Sedimentation". This is from the introduction:

"Two stages can generally be recognised in the historical growth of a reasonably advanced scientific discipline. There is an early, descriptive stage in which with little guide from theory, an attempt is made to collect, define and analyse phenomena. In the later, explanatory stage we see that efforts are concentrated on the production of generalisations and on the explanation of the reduced phenomena in terms of general laws. Of course, there is never a single point in time at which there is change over the entire scope of a discipline from the descriptive to the explanatory stage. The change is, rather, uneven, taking place earlier in some branches than in others, and more gradually in one branch than in another.

Sedimentology stands today in a period of transition. Its subject matter is sedimentary deposits, and its goal the origin and meaning of these in the context of planetary studies in general. But it is apparent, except to adherents of geological phenomenalism, that sedimentary deposits cannot be explained in terms of themselves. Already we are in possession of major generalisations about these deposits, and our chief task for some years should be to explore and ratify them in terms of general laws in order that our understanding of the sedimentary record can be made sharper. In those parts of the field where major generalisations have already been established, the provision of further descriptive data is of little value, except in so far as light is shed on the problems of particular deposits. These are validly a part of the subject, leading to a refinement of certain planetary laws. But the other and no less important laws in terms of which we should seek to frame our understanding are those of general chemistry, physics and biology. In order to achieve this framework in the case of detrital sediments, it will be necessary to set aside for a while the problems of particular deposits. This will, of course, be unacceptable to those who claim that geology, or sedimentology, is only to do with rocks as conceived in a historico-geographical manner. But they will be proved wrong, provided we keep our major goals in mind, for it is a mistake to suppose that a description will suffice for an explanation. Most of our explanations will probably turn out to be no better than qualitative, so complex are most sedimentary systems, but we should nevertheless attempt them and try to frame them as exactly as possible."

Forty years after publication of the paper, this seems as timely as ever.

Reference:
Allen, J. R. L., 1969. Some Recent Advances in the Physics of Sedimentation. Proceedings of the Geologists' Association 80:1-42.