Heterolithic bedding is a

sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentati ...

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 defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...

and

mud A MUD (; originally multi-user dungeon, with later variants multi-user dimension and multi-user domain) is a Multiplayer video game, multiplayer Time-keeping systems in games#Real-time, real-time virtual world, usually Text-based game, text-bas ...

. 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 fl ...

but can also be formed in glacial environments. Examples from

fluvial In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluviog ...

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 naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...

supply and

tidal Tidal is the adjectival form of tide. Tidal may also refer to: * Tidal (album), ''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 * ...

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 can ...

, while the mud is deposited during

slack tide Slack water is a 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, and which occurs before the direction of the tidal stream reverses. Slack water can be estim ...

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

lenticular Lenticular is an adjective often relating to 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 describing a ...

. 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 can ...

, and

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

Heterolithic bedding types

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

{{Main, 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

Sedimentary structures