Ripple Cross-Lamination
Introduction
Ripple cross-lamination - also known as climbing ripples - is a distinctive structure that forms when sediment accumulates so fast that each successive ripple is buried before it can be fully eroded by the next one. The result is a set of cross-laminae in which successive ripple crests appear to climb upward through the deposit, recording a brief but intense pulse of sedimentation from suspension.
Formation Mechanism
Ripple cross-lamination forms when deposition takes place very rapidly during migration of current or wave ripples. A series of cross-laminae are produced by superimposing migrating ripples. The ripples climb on top of one another such that the crests of vertically succeeding laminae are out of phase and appear to be advancing upslope. In cross-section cut normal to the wave crests, the resulting units have the general appearance of waves. In sections cut at other orientations, the laminae may appear horizontal or trough-shaped, depending on the orientation and shape of the ripples. [1]
The appearance of climbing in the ripple crests is a direct record of sediment flux. When deposition rate is high, each ripple is partially buried as it migrates, and its stoss (upstream) side is not fully eroded. The next ripple therefore climbs onto a surface that is already rising, and its preserved crest sits higher in the section than the previous one. That offset is not random - it equals the net vertical accumulation rate during one ripple-migration cycle.
Two Variants: Out-of-Phase and In-Phase
Two conditions of ripple cross-lamination are recognised, reflecting different balances between sediment supply and traction transport.
The common out-of-phase variant forms when ripples migrate forward while accumulating sediment. Suspended sediment supply must be abundant enough to quickly bury and preserve the original rippled layers, but traction transport must supply just enough energy to produce rippling without completely eroding the laminae from the stoss side of ripples. [1]
Some ripple laminae are in phase - one ripple crest lies directly above the other - which indicates that the ripples did not migrate at all. In-phase laminae form where traction transport and sediment supply are balanced so that ripples remain stationary even as the sediment surface grows upward. [1]
In-phase climbing indicates an even higher sediment flux than the out-of-phase type - the bed is aggrading so fast that there is no net lateral ripple migration at all, only vertical growth.
Environments and Context
Ripple cross-lamination occurs in sediments deposited in environments characterised by rapid sedimentation from suspension: fluvial floodplains, point bars, river deltas subject to periodic flooding, and turbidite environments. [1]
A well-documented setting is the waning-flood stage of a river. At peak flood velocity the bed is in the upper flow regime and planar lamination develops. As velocity drops into the lower flow regime, ripple cross-lamination forms on top of those plane-bed laminae - the two structures together recording the deceleration of the event. [1]
The practical significance is that ripple cross-lamination is a rapid-deposition signal. When a geologist sees it in the rock record, it indicates a brief, high-supply event - a flood pulse, a turbidite lobe, or a delta-front progradation episode - rather than slow, continuous sedimentation.
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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