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Bedform

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Bedform

Current ripples preserved in sandstone of the Moenkopi Formation, Capitol Reef National Park, Utah, USA.

A bedform is a feature that develops at the interface of fluid and a moveable bed, the result of bed material being moved by fluid flow. Examples include ripples and dunes on the bed of a river. Bedforms are often preserved in the rock record as a result of being present in a depositional setting. Bedforms are often characteristic to the flow parameters,[1] and may be used to infer flow depth and velocity, and therefore the Froude number.

Bedforms vs. flow

Typical unidirectional bedforms represent a specific flow velocity, assuming typical sediments (sands and silts) and water depths, and a chart such as below can be used for interpreting depositional environments,[2] with increasing water velocity going down the chart.

Flow Regime Bedform Preservation Potential Identification Tips
Lower
Lower plane bed High Flat laminae, almost lack of current
Ripple marks High Small, cm-scale undulations
Sand waves Medium to low Rare, longer wavelength than ripples
Dunes/Megaripples High Large, meter-scale ripples
Upper
Upper plane bed High Flat laminae, +/- aligned grains (parting lineations)
Antidunes Low Water in phase with bedform, low angle, subtle laminae
Pool and chute Very low Mostly erosional features

This chart is for general use, because changes in grain size and flow depth can change the bedform present and skip bedforms in certain scenarios. Bidirectional environments (e.g. tidal flats) produce similar bedforms, but the reworking the sediments and opposite directions of flow complicates the structures.

This bed form sequence can also be illustrated diagrammatically:

Bedforms formed in sand in channels under unidirectional flow. Numbers correspond broadly to increasing flow regime, i.e., increasing water flow velocity. Blue arrows show schematically flow lines in the water above the bed. Flow is always from left to right.

Types of Bedforms

Lower Plane Bed

"Lower plane bed" refers to the flat configuration the bed of a river that is produced in via low rates of sediment transport.[3]

Upper Plane Bed

Parting lineation, from lower left to upper right; Kayenta Formation, Canyonlands National Park.

"Upper plane bed" features are flat and characterized by a unidirectional flow with high rates of sediment transport as both bed load and suspended load. Upper plane bed conditions can produce parting current lineations, which are typically subtle streaks on the bed surface due to the high energy flow.[3]

See also

Megaripple from Utah

References

  1. ^ Southard, J B (1991). "Experimental Determination of Bed-Form Stability". Annual Review of Earth and Planetary Sciences 19: 423.  
  2. ^ Prothero, D. R. and Schwab, F., 1996, Sedimentary Geology, pg. 45-49, ISBN 0-7167-2726-9
  3. ^ a b editors, Klaus K.E. Neuendorf, James P. Mehl, Jr., Julia A. Jackson. (2005). Glossary of geology. Alexandria: American Geological Institute. p. 382.  



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