June 8, 2009 | David F. Coppedge

Mudstones Make Ripples

Most of the sediments in the world are mudstones – including shales and clays.  Until recently these were thought to form only in calm, placid seas.  Now, two geologists are continuing to show that they can form in flowing or turbulent water.
    Two years ago, Schieber and Southard burst a paradigm by explaining how mudstones could form in flowing water (see 12/14/2007).  They’ve been experimenting ever since.  In flume experiments, they have found new ways to image what is going on in turbulent muddy water.  Their latest paper in Geology shows that mudstone particles can form ripples, just like sand.1  The particles clump into floccules several millimeters in size.  Even though smaller and lighter than sand, they behave like sand particles – climbing up slopes and avalanching down the lee sides, forming the familiar ripples kids see on the beach as the waves recede.  This happens even though floccules have slight attractions to each other via van der Waals forces.  They behave as if independent particles – just like sand grains.  The authors were also surprised to find that ripple formation occurs even when the mud is highly dilute: “this is remarkable when one considers that floccule ripples consist of as much as 90 vol% water.
    Why is this interesting?  After all, the authors acknowledged that geologists have been studying ripple formation for as long as they have been studying sediments.  “We might therefore think that the topic has been sufficiently exhausted to be of no further interest.”  Consider first how economically important mudstones are:

Fine-grained sedimentary rocks (grain size <62.5>shales or mudstones, are the most abundant sedimentary rock type.  They contain the bulk of geologic history recorded in sedimentary rocks (Schieber, 1998), and are a key element in organic-matter burial, the global carbon cycle, and the hydraulic isolation of groundwater resources and waste materials.  Economically, they are an important source of hydrocarbons, minerals, and metals (Sethi and Schieber, 1998).  They are susceptible to weathering due to their clay content, and so often appear quite homogeneous to the casual observer.  Because of this, they are much more poorly understood than other types of sedimentary rocks, in spite of their importance.
    An enduring notion about deposition of muds has been that they are deposited mainly in quiet environments that are only intermittently disturbed by weak current activity (e.g., Potter et al., 2005).  Flume experiments have shown, however, that muds can be transported and deposited at current velocities that would also transport and deposit sand (Schieber et al., 2007).  Deposition-prone floccules form over a wide range of experimental conditions, regardless of the exact parameters that drive flocculation in a given experimental run.  Floccule ripples, ranging in height from 2 to 20 mm, and spaced from centimeters to decimeters apart, migrate over the flume bottom and accrete into continuous mud beds at streamwise velocities from 0.1 to 0.26 m/s.

The picture of tiny particles slowly settling to the bottom, producing uniform, homogeneous sediment layers, therefore, can no longer be defended.  Compaction after deposition can mask the turbulent and flowing conditions under which the beds formed.  This means that finely-laminated sediments may not represent cyclic deposition, but could form more quickly under turbulent or flowing conditions.  The authors discussed a paradox about the behavior of mudstone particles and floccules:

There is an apparent paradox in mud sedimentation.  Whereas mud constituents are cohesive and flocculate, floccules made from cohesive particles appear to act noncohesively in transport.  Observation of floccule-ripple migration shows that erosion removes not simply single floccules, but also larger chunks of material.  Once moving, these chunks break up into smaller subunits that presumably reflect the maximum equilibrium floccule diameter for a given level of turbulence (Parthenaides, 1965).  Floccule-ripples migrate significantly slower than sand ripples under comparable conditions.  Thus, cohesive forces between floccules assert themselves once the floccules come to rest next to each other, but they are ineffective as long as the floccules move in turbulent suspension.

OK, maybe you still couldn’t care less how mud particles settle on the bottoms of flumes, the ocean, or your bathtub.  Consider their ending statement: “Because mudstones were long thought to record low-energy conditions of offshore and deeper-water environments, our results suggest that published interpretations of ancient mudstone successions and derived paleoceanographic conditions are in need of reevaluation.”  I.e., here’s another example of “everything you know is wrong.”

1.  Juergen Schieber and John B. Southard, “Bedload transport of mud by floccule ripples—Direct observation of ripple migration processes and their implications,” Geology, June 2009, v. 37, no. 6, p. 483-486, doi:10.1130/G25319A.1.

Go back and read the earlier entry on this topic (see 12/14/2007).  Considering the vast quantities of sedimentary rocks around the world of this type (think major parts of the Grand Canyon), this really is big news.  A lot of geologic dating, fossil interpretation, and economic geology (e.g., oil shale interpretation) could be in for upsets.  The impact of reevaluating most of the geologic record in light of these findings cannot be ignored.

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