Posted by: paulgarner | June 11, 2009

Overturning conventional thinking about mudstones

Mudstones, which are sedimentary rocks dominated by silt- or clay-sized particles, are very abundant, perhaps comprising up to three-quarters of the entire sedimentary record. They are usually considered to have been deposited by the very slow settling of fine dispersed grains in quiet water conditions. For this reason, the assumption has been that thick mudstones must have taken extremely long periods of time to be laid down.

However, conventional thinking about mudstones is being challenged by recent research. Silt and clay particles are susceptible to flocculation, a form of coagulation that causes fine particles to clump together into larger aggregates. This process causes silt- and clay-sized particles to settle out of suspension rapidly as ‘flocs’ or ‘flakes’ which are about the size of sand grains.

In 2007, Schieber et al described flume experiments demonstrating the formation of muddy floccules that travel in bedload in the form of ripples. This allows muds to be transported and deposited at current velocities that are sufficient to transport and deposit sand. The Indiana University team even posted an educational video on YouTube — part 1, part 2 and part 3 — showing the experiments and resulting sediments. 

Juergen Schieber inspecting . Photo by Chris Meyer. Courtesy of Indiana University

Geologist Juergen Schieber inspecting the "racetrack flume" in which his experiments were conducted. Photo by Chris Meyer. Courtesy of Indiana University.

In the June edition of Geology, Schieber and Southard (2009) have reported additional experiments on the rapid sedimentation of muds. Observations of the sediment transport in floccule ripples is usually hampered by the turbid nature of the muddy suspensions. However, Schieber and Southard have now succeeded in imaging the sediment movement associated with these floccule ripples.

Although the ripples contain as much as 90% water (by volume), they have a shape that is very similar to the ripples formed in sandy sediments. Furthermore, even though the particles that make up the floccule ripples are fragile and much less dense than sand grains, the way in which the mud is transported across these ripples is essentially the same as in sandy ripples. The investigators say that their new experiments provide the first direct observations of the processes that shape and propagate mud ripples.

I think it’s really exciting that the deposition of mudstones, so long considered problematic for catastrophist scenarios, is beginning to be viewed in a different light. Macquaker and Bohacs (2007 p.1735) even refer to “a paradigm shift” in mudstone science. There is now a need to re-evaluate conventional interpretations of mudstone formations in the geological record and the assumed conditions under which they were deposited.

References

Macquaker J. H. S. and Bohacs K. M. 2007. On the accumulation of mud. Science, 318:1734-1735.

Schieber J. and Southard J B. 2009. Bedload transport of mud by floccule ripples — direct observation of ripple migration processes and their implications. Geology, 37:483-486.

Schieber J., Southard J.B. and Thaisen K. 2007. Accretion of mudstone beds from migrating floccule ripples. Science, 318:1760-1763.

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Responses

  1. I just read the Schieber and Southard article in the June 2009 issue of Geology that you referenced, and would like to make a few points:

    1. The authors call for a reevaluation of the interpretations of mudstone depositional environments and associated paleoceanographic reconstructions. They are not saying that all mudstone interpretations are wrong, nor do they seem to be calling for a revolution in mudstone sedimentary geology.

    2. The amount of flocculation of clay in water is a function of many factors, such as clay mineralogy, pH, salinity, sediment concentration, and amount of organic matter in solution. Because of this, flocculation will occur at a lower rate in some settings than what is described in this study. It wasn’t clear in the report, but I would guess that they set up the experiment for maximum flocculation.

    3. Flocculation is certainly not a new concept, and I don’t see how this study supports flood geology in some new way. The fluid environment in the Schieber and Southard experiments is nothing like what is needed for catastrophism. In their setup, the water was 5 cm deep, fluid flow was less than 0.26 m/s, and the mud ripples moved along the rate of 24 cm/h. To go from this to catastrophic sedimentation of mudstones is quite a leap.

    4. Reconstruction of mudstone depositional environments is based on more than just “mud settles slowly.” A number of factors are considered in determining the sedimentary setting, such as fossils (usually benthic), trace fossils (there shouldn’t be a whole lot of time for worms to be making sophisticated burrows during catastrophic deposition), and lateral and vertical relationships to adjoining sedimentary units.

    With respect,
    Kevin N

    • Kevin, thank you once again for taking the opportunity to comment. Time pressures necessitate a brief response on this occasion.

      Schieber et al. (2007 p.1762) concluded: “Many ancient shale units, once examined carefully, may thus reveal that they accumulated in the manner illustrated here, rather than having largely settled from slow-moving or still suspensions. This, in turn, will most likely necessitate the reevaluation of the sedimentary history of large portions of the geologic record.” Likewise, Macquaker and Bohacs (2007) say that the mechanism documented by Schieber et al. is “at odds with perceived wisdom” (p.1734), that “mudstone science is poised for a paradigm shift” (p.1735) and that the results “call for critical reappraisal of all mudstones previously interpreted as having been continuously deposited under still waters” (p.1734). It seems to me that those working in the field consider that something of far-reaching significance is going on.

      Nevertheless, there’s clearly much more work to be done in this area and you’re right to point out that these pioneering experiments have some limitations. Schieber et al. (2007 p.1760) have themselves said that mudstones, despite their abundance, are arguably the most poorly understood type of sedimentary rocks. However, you ought to be aware that one sedimentologist has told me that not just ripple cross-laminated mudstone but parallel-laminated mudstone may form by the rapid deposition of flocs at even higher velocities in the upper-flow regime plane-bed stability field at Froude numbers >1. There’s a lot of rethinking to be done!

  2. Kevin N said: “It wasn’t clear in the report, but I would guess that they set up the experiment for maximum flocculation.”

    I am highly experienced in the area of flocculation and coagulation; from watching the video (Youtube link #1) they did not seem to indicate whether they added dissolved salts (particularly nulti-valent cations) but it did not seem that they did.

    It did not seem to me that they used maximum coagulation/flocculation potential – probably not even close (the floccules weren’t really that big….you can get much bigger without too much trying…)

    My 2c


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