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.

Figure \(\PageIndex{1}\): Processes, bedforms and structures formed by waves, oscillatory or sloshing flows (Page Quinton via Wikimedia Commons; CC BY-SA 4.0).

 

 

Figure \(\PageIndex{2}\): Modern and ancient symmetrical wave ripples (all from Michael C. Rygel via Wikimedia Commons; CC BY-SA 3.0 or CC BY-SA 4.0). A) Modern wave ripples exposed at low tide on the Bay of Fundy are similar in size and shape to those shown in B) from the Permian Snapper Point Formation, New South Wales. C) A more typical bedding plane exposure of symmetrical wave ripples from the Cambrian Potsdam Sandstone. D) Although much less common, wave ripples are also occasionally preserved in cross section as with this example from the Permian Wasp Head Formation, New South Wales.

 

 

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.

Figure \(\PageIndex{3}\): Examples of hummocky cross-stratification (all from Michael C. Rygel via Wikimedia Commons; CC BY-SA 3.0 or CC BY-SA 4.0). A & B) Hummocky cross-stratified sandstone from the Permian Wasp Head Formation, New South Wales. C) Isolated sandstone hummock in mudrock of the Silurian Arisaig Formation, Nova Scotia. D) Isolated sandstone swale in offshore transition mudrocks of the Permian Pebbley Beach Formation, New South Wales.

 

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.
  
   

Figure \(\PageIndex{4}\): Structures formed in response to changes in flow direction and/or strength (Page Quinton via Wikimedia Commons; CC BY-SA 4.0). Diagram after Reineck and Wunderlich (1968).

 

Figure \(\PageIndex{5}\): Examples of heterolithic bedding (all from Michael C. Rygel via Wikimedia Commons; CC BY-SA 3.0 or CC BY-SA 4.0). A) Wavy bedding with ripple cross-laminae that record reversal of flow direction B) Starved ripples atop a sandy zone interval that could be described either as flaser bedding or ripple cross-laminae with mud drapes. C) Wavy bedding. A-C all from the from the upper Pebbley Beach Formation (Permian). D) Mud drapes on foreset laminae in the Indian Cave Sandstone (Pennsylvanian), Nebraska.

 

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.

Figure \(\PageIndex{6}\): Herringbone cross-bedding in a tidal channel, Eocene Delmar Formation, Torrey Pines State Park, California (Geozz86 via Wikimedia Commons; CC BY-SA 4.0).

 

Readings and Resources

• Reineck, H.-E., and Wunderlich, F., 1968, Classification and origin of flaser and lenticular bedding, Sedimentology, v. 11, p. 99-104.