Ripples

Introduction

Ripples are the smallest bedform produced by fluid flow, and they are among the most widespread sedimentary structures in both modern environments and the ancient rock record. They form in siliciclastic and carbonate sediments alike, under both water and wind transport, and they record the direction and character of the flow that created them with unusual fidelity. Despite their ubiquity in modern settings, however, they are less common as preserved features in ancient rocks than the cross-lamination they produce - because the ripple itself is easily eroded before burial.

Types of Ripples

Ripples are common sedimentary structures in modern environments, occurring in both siliciclastic and carbonate sediments. They can form by both water and wind transport. Ripples can develop under either unidirectional current flow or oscillatory flow (wave action). ^[1]

This difference in driving mechanism - steady current versus oscillating waves - produces two fundamentally different ripple types with different shapes and different environmental significance.

Current Ripples

Ripples that develop in response to unidirectional flow are asymmetrical in shape, with the steep (lee) side facing downstream in the direction of current flow. Asymmetrical ripples of this type are called current ripples. Under natural conditions they form by river and stream flow, by backwash on beaches, and by longshore currents, tidal currents, and deep-ocean bottom currents. ^[1]

The asymmetry of current ripples is their most diagnostic feature. The gently inclined stoss (upstream) face contrasts with the steeper lee face, and that asymmetry immediately tells a geologist which direction the current was flowing when the ripple formed. This makes current ripples one of the most reliable paleocurrent indicators available from sedimentary rocks.

In plan view (looking down from above), the crests of current ripples take a variety of shapes: straight, sinuous, catenary, linguoid, and lunate. The plan-view shape of ripples is apparently related to water depth and velocity, though the controls on shape are not well understood. More complex forms tend to develop in shallower water and at higher velocities than the less complex forms. The progression from lower to higher complexity - and from greater to lesser water depth and velocity - follows the order: straight → sinuous → symmetric linguoid → asymmetric linguoid for ripples, and straight → sinuous → catenary → lunate for dunes. ^[1]

Oscillation Ripples

Ripples that form by wave action under oscillatory flow are called oscillation ripples. Oscillation ripples tend to be nearly symmetrical in shape and have fairly straight crests. ^[1]

The symmetry arises directly from the back-and-forth motion of waves. Unlike a unidirectional current, which pushes sediment consistently in one direction and builds an asymmetric lee face, oscillating wave motion pushes sediment first one way and then the other. The result is a ripple whose two flanks are roughly mirror images of each other. Finding symmetric ripples with straight crests in an ancient sedimentary rock is therefore evidence of wave action - a nearshore or shallow-water setting - rather than a directional current.

Preservation Potential

The preservation potential of ripples is relatively low, so they are not extremely common features on the bedding planes of ancient sedimentary rocks. Cross-beds, by contrast, are exceedingly common in many ancient sandstone successions. ^[1]

This contrast explains a fundamental asymmetry in how bedforms are recorded in the rock record. The ripple itself is a surface feature that tends to be eroded or reworked before the next depositional event buries it. The internal cross-lamination that the ripple produces as it migrates, however, is preserved within the body of the bed - protected from later erosion. A geologist studying an ancient sandstone is therefore far more likely to read the record of ripples through the cross-lamination they left behind than through preserved ripple surfaces.

References

1. Boggs, S. Jr. (2012). Principles of Sedimentology and Stratigraphy, 5th ed. Pearson Prentice Hall.

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