Heterolithic bedding is a

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

structure made up of

interbedded In geology, interbedding occurs when beds (layers of rock) of a particular lithology lie between or alternate with beds of a different lithology. For example, sedimentary rocks may be interbedded if there were sea level variations in their sedimen ...

deposits of

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is usually defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural ...

and

mud Mud (, or Middle Dutch) is loam, silt or clay mixed with water. Mud is usually formed after rainfall or near water sources. Ancient mud deposits hardened over geological time to form sedimentary rock such as shale or mudstone (generally cal ...

. It is formed mainly in

tidal flats Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal ...

but can also be formed in glacial environments. Examples from

fluvial A river is a natural stream of fresh water that flows on land or inside caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of its course if it ru ...

environments have been documented but are rare.Martin, A.J. (2000). Flaser and wavy bedding in ephemeral streams: A modern and an ancient example. ''Sedimentary Geology'', 136, 1–5. Heterolithic bedding forms in response to alternations in

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

supply and

tidal Tidal is the adjectival form of tide. Tidal may also refer to: * ''Tidal'' (album), a 1996 album by Fiona Apple * Tidal (king), a king involved in the Battle of the Vale of Siddim * TidalCycles, a live coding environment for music * Tidal (servic ...

velocity. The fluctuations result in the interbedded layers of sand and mud. The rippled sand layer is formed during high

tidal currents Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables c ...

, while the mud is deposited during

slack tide Slack tide or slack water is the short period in a body of tidal water when the water is completely unstressed, and there is no movement either way in the tidal stream. It occurs before the direction of the tidal stream reverses. Slack water ca ...

periods.

Heterolithic bedding types

The three main types of heterolithic bedding are flaser, wavy, and

lenticular Lenticular is an adjective often relating to Lens (optics), lenses. It may refer to: *A term used with two meanings in botany: see *Lenticular cloud, a lens-shaped cloud *Lenticular galaxy, a lens-shaped galaxy *Lenticular (geology), adjective de ...

. Starved ripples and

cross bedding In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The original ...

with flasers can also be considered forms of heterolithic bedding. Differentiating of these various types of heterolithic bedding is based on the relative volume of mud and sand. This key determining factor is controlled by the timing, and duration of both the

high tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...

, and

depositional periods.Reineck, H., Wunderlich, F. (1968)
Classification and Origin of Flaser and Lenticular Bedding
''Sedimentology'', 11, 99–104.Davis, Richard A. (1985). ''Coastal Sedimentary Environments''. Second Revised Expanded Edition, Springer-Verlag, New York.

Flaser bed

If the amount of deposited sand exceeds mud deposits, we get ripples with isolated mud drapes in ripple troughs and crests. This type of sedimentary structure is called flaser bedding. Flaser bedding can be subdivided according to how the mud is deposited over the rippled sand. The various types of flaser bedding are simple flaser, bifurcated flaser, wavy flaser, and bifurcated wavy flaser. Simple flaser bedding is when the flasers do not contact one another and they are concave when the bed is upright. Mud in simple flaser deposits is limited to ripple troughs. Bifurcated flaser bedding occurs when a second flaser is deposited over a pre-existing flaser. The bifurcated flasers resemble a sideways “y” shape. Wavy flaser bedding occurs when a flaser drapes both the ripple trough and crest but forms no continuous layer.

Wavy bedding

If mud and sand deposits are equal, wavy bedding is produced. Wavy bedding occurs when mud is deposited over the whole area of a bed of rippled and/or cross stratified sand. It usually loosely follows the alternating concave-convex nature of the ripples creating a wavy appearance. In wavy bedding the ripples are laterally discontinuous. Wavy bedding marks the boundary between flaser and lenticular bedding.

Lenticular bedding

Lenticular bedding occurs when sand ripples are deposited in mud in an isolated distribution pattern. Ripples in lenticular bedding are both laterally and horizontally discontinuous. Lenticular bedding can be subdivided according to whether or not they have connected lenses and the type of lenses they feature. Lenticular bedding with connected lenses can have up to 75% of discontinuous lenses. Lenticular bedding with single lenses has to have more than 75% discontinuous lenses. Lens thickness is based on a length to height ratio. Thick lenses have a length/height ratio that is < 20 and flat lens ratio is > 20. So the possible types of lenticular bedding are lenticular bedding with connected flat or thick lenses and lenticular bedding with single flat or thick lenses.

References

{{Reflist Sedimentary structures