4.3: Structures Formed by Bidirectional, Oscillatory, and/or Fluctuating Flows
Wave Motion and Wave Ripples
In deep bodies of water (where depth>½ wavelength) energy is transferred through water causing particles to move in a circular pattern.
When water depth is less than ½ wavelength, the wave “feels” bottom and friction causes the circular motion to change into back-and-forth motion which in turn causes symmetric wave ripples to form. Unlike asymmetric ripples, wave ripples are most commonly preserved in three dimensions on bedding planes. The crests of symmetric ripples form parallel to the local shoreline direction.
Hummocky Cross-Stratification
Hummocky cross-stratification (sedimentary structure) forms when storms cause chaotic wave motion and irregular “sloshing” of water at depth. The resulting hummocky cross-stratification consists of convex-up ( hummocks ; bedform) and concave-up ( swales ; bedform) cross-laminae that cross-cut one another.
In areas with muddy bottoms, storms may transport sand from shallow water and form them into isolated hummocks.
Heterolithic Bedding
As the name implies, heterolithic bedding contains alternations of sand and mud. This feature indicates fluctuations in flow velocity and is most commonly formed in tidal regimes where water is moving rapidly during rising and falling tide and calm at high and low tide. In addition to fluctuations in flow velocity, many examples have ripple cross-laminae that indicate reversing flow - a trait that, if present, supports the tidal interpretation.
Four main types of heterolithic bedding are recognized:
- Flaser bedding is dominantly sand with isolated lenses of mud
- Wavy bedding has subequal amounts of sand and mud
- Lenticular bedding is mostly mud with elongate lenticular sandy zones
-
Starved ripples
are isolated ripple forms preserved within mud.
Herringbone Cross-Stratification
Herringbone cross-stratification is a name applied to successions where cross-beds overlying one another record paleoflow in opposite directions. Because this requires fast flow that rapidly reverses direction, most occurrences record tidal conditions.
Readings and Resources
- Reineck, H.-E., and Wunderlich, F., 1968, Classification and origin of flaser and lenticular bedding, Sedimentology, v. 11, p. 99-104.