Bedforms: ripples and dunes
Ripples, dunes, antidunes are all bedforms, structures that form in sand when it is moved by water or wind. Bedforms are ubiquitous on our planet. It is very common to see ripples, undulatory structures in sand, under shallow waters close to seashores or along riverbanks. And deserts, they are commonly covered with large sand dunes, in turn sprinkled by smaller sand waves, all structures produced by wind. All these structures have a strong preservation potential and can survive in the rock record, either as fossil ripple marks, visible on the surface of strata, or, more commonly, as various types of cross-bedding, inclined laminae and layers that represent cross-sections through past migrating bedforms.
One of the fundamental principles of geology is that the present is the key to the past. In the case of bedforms, this means that we can study the bedforms that currently form on our planet to understand how past sedimentary environments looked like, thanks to the structures preserved in sedimentary rocks. In particular, bedforms are accurate storytellers of the direction and strength of currents in past environments.
What are ripples?
Ripples, commonly known as sand waves, are undulatory structures produced by a current (water or wind) on the surface of a sandy sediment. In geology there are several types of ripples: centimeter-scale ripples or ripple marks, megaripples, which are higher than 5 cm and several meters wide, and sand dunes, which are large, meters to tens of meters high sand mounds. In general, these structures are known as bedforms, because they form at the bottom of a basin at the contact between the sediment and air/water. When ripples (or other bedforms) are produced by a unidirectional current, they show an asymmetric shape with a gentler and a steeper side. On the other hand, oscillatory currents like waves often produce symmetric ripples.
Bedforms form only in sand or sand-rich sediment because fine-grained muddy sediments are transported in suspension by water/wind, whereas sand is mostly transported by dragging or saltation/jumping at the sediment/water (or air) interface. This creates an interaction (drag) between the current and the underlying sediment that molds the surface of the sediment, producing forms that are controlled by the speed of the current, depth, grain size, and direction of flow. Different types of bedforms form when these parameters change.
Formation of asymmetric ripples
Asymmetric ripples form at the contact between water/air and the sediment drag by a unidirectional current. The initial stages of formation of ripples are not well understood. In general, erosion of the sediment forms an uneven surface with small highs and depression, disturbing the flow of the current. The current accelerates where highs form, pushing the sand ahead: these structures soon start to migrate as ripples and rapidly reach a stable shape with a well-defined wavelength (distance between the crests of two neighboring ripples) that depends from the velocity of the current. It may sound simple, in principle, but the mathematical description of ripples is not easy! Ripples are structures that self-organize and, when they start to form in a sediment, they will all attain the same geometry, shape, and height, in function of the physical conditions at which they formed.
The stable shape of ripples produced by a unidirectional current is characterized by a gentler stoss side, leaning against the flow, and a steeper lee side, leaning towards the direction of the flow. The lee side usually lies at the critical angle of repose of the sediment. The sediment is pushed forward by eroding the stoss side of the ripple and depositing it on its lee side, where the flow suddenly expands allowing the deposition of sediment. The lee side accretes new sediment that either falls from the crest of the ripple or is accreted by the reverse flow of the current that forms in the troughs between neighboring ripple crests. Here is a video from Michael Calzi that shows the migration of ripples:
Here another example with wind ripples:
3 minutes in 2.5 seconds. pic.twitter.com/FqSbCnoDyI
— Wolfgang Schwanghart 🚴♂️🌍💻 (@WSchwanghart) March 22, 2022
The internal structure of ripples is characterized by foreset laminae that lean in the direction of the flow and are hence useful paleocurrent indicators when observing fossil ripple marks or cross-bedding preserved in rocks. Minor laminae may form parallel to the stoss side but their preservation is difficult because they are subject to erosion. Ripples may also show bottom set laminae or a tangential termination of foreset laminae occurring at the base of the ripple. These structures form because the reverse flow can deposit a flat lamina of sediment in front of the ripple or because the sand falling off the crest of the ripple can roll down further in the trough. In any case, these structures occur only at the base of the ripple and are, hence, useful indicators of polarity of sedimentary rocks, in case of deformed or overturned sequences.
From ripples to megaripples and plane bed
The main parameter that controls bedforms is the velocity of the current. The current must initially reach a critical velocity, beyond which it has enough energy to start transporting the sediment. When this critical velocity is reached, sediments begin to move by traction on the bed and ripples begin to form.
The first structures that form are small undulatory ripples with a height of a few centimeters. At increasing velocity of the current, megaripples start to form. Megaripples are meter-scale structures with height > 5 cm and large wavelength and that can be covered by smaller ripples. The internal structure of megaripples is identical to ripples.
At higher velocities, the current is so fast that it does not allow the formation of ripples, producing a plane bed, where sand grains move by traction. In sedimentology this marks the transition between the so-called lower flow regime and the upper flow regime. The sedimentary structure that forms under these conditions is plane lamination. Another structure that forms in the upper flow regime are antidunes, very small bedforms that, contrarily to ripples, migrate in the direction opposite with that of the flow. There are no known fossil antidunes in the geological record because the preservation potential of these structures is extremely low.
Formation of dunes
Dunes are significantly larger than ripples but their internal structure, and mode of migration is not different. Very large dunes are common in deserts, where they can basically grow undisturbed in height, contrarily to water ripples, whose height is controlled also by water depth. Another environment where dunes are common are beaches, in particular the back side of beaches, where the sand accumulated by coastal processes is reworked by winds. The initial stages of formation of dunes are linked with the presence of obstacles, like boulders or plants. When the wind transporting the sand encounters an obstacle, its flow vectors break and separate, producing local turbulent flow with areas where sand is eroded and areas where it is deposited. Eventually, the process produces a little mound of sand around the obstacle which will become the nucleus of the dune. Once this mound has formed, like ripples, the dune starts to migrate as sand is eroded from the stoss side and deposited on the lee side, pushing the dune forward.
Geometry of ripples and dunes
The geometry of the crests of ripples largely depends on the speed of the current and the depth of the water column under which they form.
In general, ripples with straight crests are stable at low flow velocity or at relatively large depth. They are also commonly produced by wind. Straight ripples (or, at large scale, dunes) are linked to the formation of planar cross-bedding. At higher flow velocity or lower water depth, ripple crests become progressively more sinuous, then catenary (the shape of a flexible hanging chain fixed to poles, like the Greek letter ω), and finally highly irregular (linguoid, tongue shaped, or lunate, crescent shaped). The difference between the linguoid and lunate ripples is the orientation of the concavity, opposite to the direction of the current in linguoid ripples, in favor of it in lunate ripples. Irregular ripples produce deposits characterized by trough cross-bedding. At higher flow velocity or even lower depth, ripples cannot form and the dominant bedform is a plane bed that produces a plane lamination.
The geometry and shape of ripples is highly influenced not only by the velocity, but also by the depth of the water that forms them. Rhomboid ripples are very small ripples with limited height (a few mm) that develop in very shallow waters (a few mm to some cm). They are common in beaches where they form by wave backwash and washover on sand.
The bedforms described above are all produced by unidirectional currents, wind or water flowing in the same direction. On Earth, there are also oscillatory or bidirectional currents, waves that move water alternatively in two opposite directions. This oscillatory movement produces symmetric ripples or wave ripples, as the sediment is dragged in a direction and then in the opposite one. The crests of wave ripples can be pointed (chevron) or rounded and they are oriented parallel to the wave front (another useful paleocurrent indicator). In cross section, wave ripples, form wave-ripple cross-lamination with foreset laminae pointing alternatively in opposite directions, and often with undulatory bases.
Here is a video from jctkao showing how wave ripples form:
The symmetric shape allows to distinguish wave ripples from unidirectional current ripples easily. However, wave ripples can also attain an asymmetric shape in situations where one of the two currents generating them is stronger (generally the onshore current). In this case, wave ripples can be recognized thanks to the presence of well-developed bifurcation of crests visible in map view. The video above shows many, bifurcating wave ripples.
In present environments, currents change speed and direction, wave fronts can change orientation with, and winds may sweep sand on different directions. Ripples (and the fossil structures they leave) are sensitive to these variations. In the geological record, it is possible to find traces of these changes as complex cross-bedded structures. Care if needed when trying to reconstruct the orientation of paleocurrents from the analysis of fossil ripples and cross-bedding!
- Cross-bedding – Cross-bedding (or cross-stratification) is a primary sedimentary feature characterized by layers that intersect at an angle with each other. In general, cross-bedding is characterized by planar erosional surfaces that separate beds with inclined strata or laminae. This architecture is the result of the migration of bedforms, such as dunes, ripples, and megaripples, produced by wind or water currents in sand-rich… Read More »Cross-bedding
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