Mudstones make up the majority of the geological record. The picture of tiny particles slowly settling to the bottom, producing uniform, homogeneous sediment layers, can no longer be defended. Latest observations do not support the notion that muds can only be deposited in quiet environments with only intermittent weak currents. Instead, bedload transport of clumped together material occur at current velocities that would also transport and deposit sand. Ripples form under swiftly moving currents in the 10 cm/s to 26 cm/s velocity range, just like sand.
The mud particles adhere into clumps 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. The results suggest that published interpretations of ancient mudstone successions and derived paleoceanographic conditions are in need of reevaluation now!
Most of the sediments in the world are mudstones including shale 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). Theyve 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 µm), commonly known as 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 couldnt 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., heres another example of everything you know is wrong.
Mudstones make up the majority of the geological record. However, it is difficult to reconstruct the complex processes of mud deposition in the laboratory, such as the clumping of particles into floccules. Using flume experiments, we have investigated the bedload transport and deposition of clay floccules and find that this occurs at flow velocities that transport and deposit sand. Deposition-prone floccules form over a wide range of experimental conditions, which suggests an underlying universal process. Floccule ripples develop into low-angle foresets and mud beds that appear laminated after postdepositional compaction, but the layers retain signs of floccule ripple bedding that would be detectable in the rock record. Because mudstones were long thought to record low-energy conditions of offshore and deeper water environments our results call for reevaluation of published interpretations of ancient mudstone successions and derived paleoceanographic conditions.Considering the vast quantities of sedimentary rocks around the world of this type (think major parts of the Grand Canyon), this really is shocking news. A lot of geologic dating, fossil interpretation, and economic geology (e.g., oil shale interpretation) has been based on faulty science and will be in for dramatic new theories. The impact of reevaluating most of the geologic record in light of these findings could be earth shattering. Guy Berthault a french sedimentologist has been saying for a number of years that the laminations, striations, and other phenomena has been the result of higher energy flowing water processes (Julien & Berthault 1993) (Berthault 2002) (Berthault 2004).
A century ago, Henry Clifton Sorby, one of the pioneers of geology, pointed to the study of muds as one of the most challenging topics in sedimentary geology (1). Today, with our knowledge clearly expanded, muddy sediments are still considered highly complex systems that may require as many as 32 variables and parameters for a satisfactory physicochemical characterization (2). More research may clarify interdependencies between a number of these parameters and may allow us to consider a smaller number of variables, but the fundamental complexity of muddy sediments is likely to remain. A key issue in mudstone sedimentation is flocculation, a phenomenon in which a number of these parameters, such as settling velocity, floccule size, grain-size distribution, ion exchange behavior, and organic content "come together." A joining of smaller particles to form larger aggregates, flocculation enhances the deposition rate of fine-grained sediments, and its understanding is critical for modeling the behavior of mud in sedimentary environments.
Flocculation is affected by particle concentration within the fluid and intensity of turbulence (3, 4). Over time, floccules enlarge to a maximum equilibrium diameter that is related to the intensity of turbulence (5). Floccule deposition is influenced by turbulence, bed shear stress, sediment concentration, and settling velocity. We currently still miss critical data on floccule formation and on the influence of floccule structure and turbulence on the formation of muddy sediments (6). We will collect data concerning these issues with new instrumentation in the near future, although the importance of our observations will not be affected. The notion is widely held that slow-moving currents or still water are a prerequisite for substantial mud deposition (7, 8) because shear stress in swift-moving currents disrupts previously formed fragile floccules and prevents their deposition, but our observations suggest an alternative mode of mud deposition that apparently left its imprint in the rock record.
Mudstones constitute up to two-thirds of the sedimentary record and are arguably the most poorly understood type of sedimentary rocks (9). Mudstone successions contain a wealth of sedimentary features that provide information about depositional conditions and sedimentary history (1013), but presently we lack the information that would allow us to link features observed in the rock record to measurable sets of physical variables in modern environments.
Although various small-scale sedimentary structures have been described from modern muds, these have not been observed in the making. This forces us to infer controlling parameters (e.g., current velocity and density of suspension) from temporally and spatially very limited measurements in the overlying water column (1417). Such measurements (e.g., flow velocity, sediment concentration) in modern environments are commonly considered representative of depositional conditions for the uppermost millimeters to decimeters of the accumulating deposits. However, upon close examination, modern sediments show considerable heterogeneity at the millimeter to centimeter scale (16), an indication that what we observe in surficial sediments is not a direct response to measured conditions in the overlying water column. To improve on this situation, it is essential to conduct experiments that replicate natural conditions and to compare the experimental sediments to the rock record.
Here, we report experimental insights into the sedimentology of mudstones. In past experimental studies, centrifugal pumps were used to recirculate mud suspensions (1820), destroying the clay floccules that are of such key importance in mud transport and deposition. Therefore, to minimize the risk of shredding clay floccules once formed, we built a racetrack flume that uses a paddle belt (21) for moving the mud suspension.
Our observations do not support the notion that muds can only be deposited in quiet environments with only intermittent weak currents . Instead, bedload transport of flocculated mud and deposition occurs at current velocities that would also transport and deposit sand . Clay beds can accrete from migrating floccule ripples under swiftly moving currents in the 10 cm/s to 26 cm/s velocity range, a range likely to expand as flows with larger sediment concentrations are explored. Whereas the clay beds formed in our experiments consist of downcurrent-inclined laminae, they appear to be parallel-laminated once fully compacted (Fig. 4A). Because floccule ripples are spaced 30 to 40 cm apart, ancient sediments of this origin are likely to appear parallel-laminated (Fig. 4C) as well. Detection of ripple-accreted muds in the rock record will require carefully defined, and yet to be developed, criteria. Things to look for might be subtle nonparallel lamina geometry, as well as basal downlap and top truncation of laminae. Examination of ancient shale units may, for example, yield low-amplitude bedforms (Fig. 4, D and E) as indicators of lateral accretion and ripple migration. Bedding-oblique orientation of larger flat particles, such as spores, microfossils, plant debris, and mica flakes, could be another indicator. If such particles are deposited on the inclined foresets of floccule ripples, they may record the gentle inclination of the latter even when compaction has rendered the depositional fabric of inclined laminae (Fig. 4B) unrecognizable.
In the course of two decades of detailed studies of shales and mudstones, one of us has seen comparable low-amplitude bedforms in shale units that were deposited in a wide variety of environments. Examples can be found in the Mid-Proterozoic Belt Supergroup, the Devonian of the eastern United States, the Jurassic Posidonia Shale, the Cretaceous Mancos Shale, and the Eocene Green River Formation. This suggests that mud accretion from migrating floccule ripples probably occurred throughout geologic history. 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.
Elucidating the mechanisms of mudstone deposition not only helps to better understand the rock record but also benefits hydrocarbon exploration, hydrogeology, and coastal and shelf engineering. Managing mud is important for the maintenance of harbors, shipping lanes, and water reservoirs, especially given the impact of climate change. How mudstones act as barriers to fluid migration (oil and water) is probably linked to depositional processes that affect mud microfabrics. For example, if a mud accumulated from current-transported floccules, one might expect a network of larger pores, poorer sealing capacity, and easier release of liquid and gaseous hydrocarbons. Conversely, accumulation in still water from dispersed clays and low-density floccules should lower permeability and may produce an oil shale that clings tightly to its generated hydrocarbons. These qualities are also critical for the ability of a mudstone unit to protect aquifers from contamination and to compartmentalize groundwater reservoirs.