Current and Wave Ripples
Ripple marks are present as undulations on a non- cohesive surface, though they may also be found infrequently in muddy sediments as well. They are produced as a result of the interaction of waves or currents on a sediment surface. Ripple marks are one of the commonest features of sedimentary rocks, both in recent and ancient sediments. The shape and size of ripples vary considerably. The crests usually run parallel to each other or anastomose partially. In transverse section they may be symmetrical or asymmetrical in shape. The crest may be sharp, rounded, or flattened. Only through later modification by change in water depth, etc., they may become rounded or flattened. Most of the classifications of ripple marks are based on their mode of origin, and their shape and size. A general classification is listed below.
|Current Ripples||Wave Ripples|
Small Current Ripples
Relationship of unidirectional current flow to bedforms:
Sediment movement is accompanied by the organization of grains into morphologic elements called bedforms. Experiment has shown that a number of bedforms exist between certain values of flow strength, thus defining various bedform states.
The three main parameters that determine the stable bedform in unidirectional flow conditions are:
1) grain size
2) flow velocity
3) flow depth
In addition, several other parameters are equally important, though for most pure fluid flows on Earth, these parameters can be assumed to be constant. They include:
m = fluid viscosity
rf = fluid density
rs = grain density
g = gravitational constant
Unidirectional (Current) Ripples:
How are unidirectional ripples formed? Above threshold of movement on artificially smoothed bed unidirectional flow ripples are formed at relatively low flow strengths. They may also form from initial bed irregularities on bed surface. Unidirectional flow ripples are sometimes also called "current ripples".
The inititial formation of ripples is not well understood, but probably has something to do with burst and sweep processes which disrupt the smooth surface (sweeps) and deposit a small pile of grains as a result of decelleration (bursts).
Small piles of grains a few grain diameters high can begin to create flow separation and form a small back eddy downstream.
Increased erosion associated with flow re-attachment tends to entrain grains which then move up the stoss side of the downstream pile of grains until they reach the next point of flow separation. Grains accumulate high on the steep lee ripple face.
Periodically, grains become unstable and exceed angle of repose (angle of initial yield) and a grain avalanche occurs down the lee face. Thus ripples form which eventually shift or move downstream.
Structure of ripples: Grain Avalanches down lee slopes result in small scale cross lamination. Sections normal to flow may be horizontal, defining planar cross lamination (2-D ripples) or may be trough-shaped, defining trough cross lamination (3-D ripples).
|Sketch of two dimensional (straight-crested) ripples in a unidirectional flow. Note the planar (also called tabular) cross-bedding.||Sketch of three dimensional (sinuous-crested) ripples in a unidirectional flow. Note the trough cross-bedding on the cut that is perpendicular to flow (i.e. the v-w cut)||This figure shows flow along the bed across a set of sinuous-crested bedforms. Note that this bottom-hugging flow converges into a scour point (i.e., forms an attachment point). Note also the geometry of the cross beds along different cuts relative to the flow direction and that these sinuous-crested bedforms result in trough cross-beds in the cut perpendicular to flow.|
Net deposition during ripple formation produces an element of vertical motion of ripple crests as well as an element of horizontal motion. Sets of cross lamination may be formed, bounded by erosive surfaces. Climbing ripples are formed as a result; require net deposition, as in decelerating flows associated with river floods or turbidity currents.
Unidirectional Flow Bedforms (G432: M. Hendrix, Univ. of Montana)
SMALL CURRENT RIPPLES
In small ripples there is a tendency for crests to become more discontinuous or lobate with an increase in the energy of the environment.
Straight-Crested Small Ripples
Migrating small straight crested ripples, flatter than asymmetric wave ripples
Migrating small straight crested ripples, flatter than asymmetric wave ripples
These migrating small flat straight crested ripples, are probably current ripples. (From Kendall, U. South Carolina)
Asymmetric Current Ripples, Upper Mississippian – Pennington Formation,Pound Gap. (From Kendall, U. South Carolina)
Undulatory Small Ripples
Undulatory small ripples represent a transition form between low-energy straight-crested ripples and higher energy lingoid ripples.
Block diagram showing cross-bedding produced by migration of undulatory small ripples. The cross-bedded units are weakly festoon-shaped. Lower units are strongly trough-cross-bedded transverse to current direction.
Lingoid Small Ripples
In lingoid small ripples, the crest of the ripples is discontinuous and small curved crests extend forward in a tounge-like lobe.
|Block diagram showing cross-bedding produced by migration of lingoid small ripples. The cross-bedded units are strongly festoon-shaped and units are strongly trough-cross-bedded transverse to current direction.||
Rhomboid Small Ripples
Crests of rhomboid small ripples show a scale-like pattern in the form of rhomboids. These ripples develop in very low water depths, usually on the seaward slopes of beaches by backwash and may be common on the landward slopes of beach bars produced by washovers.
|Well-developed, small rhomboid ripples on North Sea tidal flats. Flow is from right to left.|
Net deposition during ripple formation produces an element of vertical motion of ripple crests as well as an element of horizontal motion. Climbing ripples are formed as a result; require net deposition, as in decelerating flows associated with river floods or turbidity currents. Depending on the relative magnitude of the climb angle vs. the stoss angle, climbing ripples can be classified as subcritically-climbing, critically-climbing, or supercritically-climbing.
Some examples of current ripples:
|The angle of climb of cross-stratified deposits increases with deposition rate, resulting in ‘climbing ripple cross lamination'. Climbing ripples require net deposition, as in decelerating flows associated with river floods or turbidity currents.||Climbing ripples in sandstone. Note coin for scale. Flow was right to left. Some of these ripple sets are critically-climbing, whereas others are supercriticallly-climbing.||Critically(?)-climbing ripples from an core that Exxon recovered from one of its oil fields in Texas. Flow was left to right.|
|Subcritically-climbing ripples in a modern river sand bar. Note the translatent surfaces that truncate the ripple foresets. Flow was left to right.||a series of critically-climbing to
supercritically-climbing ripples from a core Exxon recovered. Flow was right to
||Some subcritically-climbing ripples recovered by Exxon from an oil-soaked reservoir. Note the translatent bounding surfaces. Flow was left to right.||A good example of a translatent surface in a set of ripple drift strata from a core recovered by Exxon. Flow was right to left.|
Megaripples have similar forms to small ripples but are formed, rather abruptly at increased stream velocities. Their length ranges from 60 cm to several metres but they are less easily identified in outcrop and core due to their large size and the reworking of the upper surface which means they leave a record only through large-scale cross-bedding.
The most common form is the lunate megaripple in which the crest is broken producing a crescent-shaped lobe with the horns facing down-flow.
|Block diagram showing cross-bedding produced by migration of lunate megaripples. The cross-bedded units are strongly festoon-shaped and units are strongly trough-cross-bedded transverse to current direction.|
Antidunes are produced by in-phase, shallow flow at Fr > 0.8 or so.
Spacing is roughly dependent upon the square of the mean flow velocity
L = U2g/2P
Antidunes migrate upstream, giving rise to low angle cross
lamination that is dipping upstream but is faint (no grain sorting by
avalanching.). Significantly, antidunes to not migrate due to grain avalanching
(unlike ripples and dunes), but due to grain accretion on the upstream side of
|Some antidunes forming in a small tidal channel on the California coast. The antidunes are forming under the train of prominent waves in the middle of the photograph. Flow in the tidal channel is from lower right to upper left (i.e. towards the surf).||A standing wave train with associated antidunes in another tidal channel on the California coast. The bottom of the photograph is about 4 meters across. Note that the wave train is only about 1 meter in width; on either side of it other bedforms (mostly upper plane bed flow) is the stable structure.|
If flow strength is increased beyond the ripple field for medium and coarse sand, dunes result. Dunes are similar to ripples, but dynamically distinct. Dune wavelengths commonly range from 0.6 m to hundreds of meters; heights range from 0.05 -10.0 m.
Dunes commonly show correlation of spacing and height with flow depth y, whereas current ripples do not. From experiment and field measurements,
L = 1.16y1.55 and H = 0.086y1.19
A word about Bedform Hierarchies
Commonly, ripples are preserved on the stoss side of dunes. Uncertainty exists as to whether this is a result of equilibrium flow or changing flow with time.
With large-scale trough cross stratification, there is commonly a tangential contact between the ‘toe’ of the foreset and bounding surface below, due to weak development of lee-side eddy and high fallout rate of sediment on the lee side of a dune. If back eddy is well developed, it is possible to develop counterflow ripples as grains are swept back up the lee side by near bed flow in the separation bubble.
Differences in flow conditions throughout the history of dune (e.g. rising and falling stage flows) may result in reactivation surfaces. A reactivation surface is produced when the lee side of a dune is partially eroded by low stage flow. When normal flow resumes, avalanching process begins again and dune migrates.
|A series of large, exposed subaqueous dunes from a river in southern Louisiana. Note the group of people standing in the upper right hand part of the photo. Flow was from lower left to upper right. Most of these dunes have well-developed sinuous-crested current ripples on their stoss sides.||A cross-sectional cut, parallel to flow direction, through the tops of one of the dunes. The object in the upper left corner is the tip of our trenching shovel. Note the small ripple drift sets in the bottom half of the exposure, the prominent dune foresets in the top half of the exposure, and the one set of current ripples at the very top of the exposure.||Tabular dune cross-beds in a Sinian sandstone from northwestern China. Flow was left to right.|
|Multiple sets of dune foresets forming tabular cross-stratification. The outcrop is about 4 meters high. Flow was right to left. Note that all sets are subcritically-climbing as is typical for dunes, and that some of the dune foresets are overturned.|
Wave ripples are symmetrical or slightly asymmetrical undulations produced by the action of waves on a noncohesive surface. They are usually straight-crested, frequently showing bifurcation. Such bifurcations are never observed in the case of current- formed, small ripples and megaripples.
Symmetrical Wave Ripples
Symmetrical wave ripples are marked by the sym metrical shape of their crests (Figs. 9, 10). The crests are usually pointed and the troughs rounded. Occasionally, crests may be rounded in shape. However rounding of crests is a result of reworking of ripples during the process of emergence. Sometimes a secondary ridge of small magnitude may be present along the axis of the trough. Symmetrical wave ripples are essentially straight-crested, partly showing bifurcation. A typical symmetrical wave ripple shows a distinctive internal structure characterized by super imposed chevron-like laminations.
Fig. 9 Wave ripples (slightly asymmetric) with wave rippled cross bedding. Fig. 10 Symmetric wave ripples with wave rippled cross bedding. Note irregular bounding surface.
However, as shown by the investigations of Newton (1968), most of the symmetrical wave ripples in the nearshore zone show an internal structure in which foreset laminae are only in one direction (Fig. 11). Such ripples may be regarded as forms transitional between symmetrical and asymmetrical wave ripples.In this case the forward motion of a wave is somewhat stronger than the backward motion, and produces foreset laminae. On the other hand, the backward motion of a wave is only strong enough to maintain the symmetry of the ripple, but too weak to produce foreset laminae. The net result is that, although the symmetry of the ripple is maintained, laminae are produced only in one direction , i..e direction of wave propagation.
Fig. 11. Symmetrical wave ripple with foreset laminae dipping in one direction (shoreward). Relief cast. North Sea. (After Newton, 1968)
Fig. 12 Asymmetric wave ripples with a form-discordant internal structure. A younger set of foreset laminae dipping to the left has truncated the older set of foreset laminae and caused a reversal in ripple sym metry. (After newton, 1968)
Asymmetrical Wave Ripples
Asymmetrical wave ripples show much similarity to straight-crested current ripples, in possessing a steep lee side and a gentle stoss side (Fig. 12).
The internal structure of normal asymmetrical wave ripples is almost identical to the internal structure of current ripples, composed of a single bottomset lamina and a few stoss side laminae. The main body is composed of foreset laminae dipping in a single direction. This is the form-concordant internal structure of asymmetrical wave ripples. However, they may also possess internal structure that is form-discordant. In such cases ripples show a composite structure, made up of laminae of earlier formed ripples. In other words, the outer form of the ripples is genetically not related to the internal structure.
Asymmetrical wave ripples may also develop as climbing ripples or ripple laminae in-phase, if sufficient sediment suply is available. It is often very difficult to distinguish asymetric wave ripples from current ripples in core. However, ripples smaller than 4.5 mm are known only as asymmetrical wave ripples.
|Photos are of generally asymmetric wave ripples. Note bifurcation of crests in both pictures.|
COMBINED CURRENT/WAVE RIPPLES
In shallow-water reas, where both current and waves are present, ripple patterns under a combined influence are produced.
Longitudinal Combined Current/Wave Ripples
Straight crested and mainly due to wave action, often found on muddy substrates. The current seems to cause erosion in the ripple troughs and helps to maintain the form of the crests. The internal structure always seems to be form discordant.
Transverse Combined Current/Wave Ripples
The direction of wave motion is parallel to the axis of current flow and these are quite often found near the shore-lineIsolated Ripples
Isolated ripples occur when there is insufficient sand to cover the whole sub-strate. when isolated ripples are found embeded in muddy sediments, they are designated lenticular bedding.
• Lenticular beds are small lenses of sand in a predominant matrix of muddy beds
• Wavy beds are subequal mixtures of small lenses of sand and muddy beds
• Flaser beds are predominantly stacks of small lenses of sand in less than 50% muddy matrix