Cyclic steps are rhythmic

bedform A bedform is a geological 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 ro ...

s associated with Froude super-critical flow instability. They are a type of sediment wave, and are created when supercritical sediment-laden water (

turbidity current A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...

s) travels downslope through sediment beds. Each ‘step’ has a steep drop, and together they tend to migrate upstream. On the ocean floor, this phenomenon was first shown to be possible in 2006, although it was observed in

open-channel flow In fluid mechanics and hydraulics, open-channel flow is a type of liquid flow within a conduit with a free surface, known as a channel. The other type of flow within a conduit is pipe flow. These two types of flow are similar in many ways but di ...

s over a decade earlier. Geological features appearing to be submarine cyclic steps have been detected in the

northern lowlands of Mars (Latin 'northern waste') is the largest lowland region of Mars. It is in the northerly latitudes of the planet and encircles the northern polar region. Vastitas Borealis is often simply referred to as the northern plains, northern lowlands or ...

in the Aeolis Mensae region, providing evidence of an ancient Martian ocean.

Formation

There are many parameters which govern the formation of cyclic steps; bed slope, bed porosity, erosion resistance, sediment concentration, and flow rate all play a role. Tilting

flume A flume is a human-made channel for water, in the form of an open declined gravity chute whose walls are raised above the surrounding terrain, in contrast to a trench or ditch. Flumes are not to be confused with aqueducts, which are built to t ...

s can be used to create cyclic steps in subaerial laboratory conditions, provided the

Froude number In continuum mechanics, the Froude number (, after William Froude, ) is a dimensionless number defined as the ratio of the flow inertia to the external field (the latter in many applications simply due to gravity). The Froude number is based on t ...

is high enough. If the Froude number is lower than required,

antidune An antidune is a bedform found in fluvial and other channeled environments. Antidunes occur in supercritical flow, meaning that the Froude number is greater than 1.0 or the flow velocity exceeds the wave velocity; this is also known as upper flo ...

s will form instead. Additionally, if the sediment is too fine then chute-and-pool features will form. In subaqueous conditions, most of the work has traditionally been in building mathematical, rather than physical, models of cyclic step formation. However, cyclic steps have attracted increasing scientific attention in the past decade, and numerous real world examples of cyclic steps have now been found. Cyclic steps can be categorized by the rate at which sediment is deposited (the aggradation rate) on different parts of the steps. The categorization concerns the difference in rate on the stoss (flow-facing) and lee (flow-opposing) sides of the feature. Type-1 cyclic steps have more lee erosion than there is stoss aggradation, Type-2 have a roughly equal amount of lee erosion and stoss aggradation, and Type-3 has aggradation on both sides. Type-1 cyclic steps play an important role in canyon formation. Type-2 cyclic steps have been created in the laboratory, in contrast to Type-3 which is common on the sea floor but is harder to create in laboratory conditions - it was first made experimentally in 2013. Types 1, 2, and 3 are also called 'falling', 'transportational', and 'climbing', respectively. Laboratory work has successfully created all three types of cyclic steps in

Relation to other bedforms

In density flows, antidunes can turn into cyclic steps by wave breaking. Fluid flow is Froude-supercritical over the entirety of antidunes, whereas the flow alternates between the sub- and super-criticality over cyclic steps (with hydraulic jumps between cycles). Additionally, cyclic steps tend to have a much larger wavelength-to-flow-thickness ratio and a higher suspension index (ratio of shear velocity to sediment settling velocity). Antidunes are typically unstable (although they can be made stable in laboratory conditions), in contrast to cyclic steps. Despite these differences, it is not uncommon for researchers to incorrectly label a cyclic step as an antidune. Cyclic steps also have similarities to chute-and-pool features. Like cyclic steps, chute-and-pool flows undergo hydraulic jumps, although the flow does not undergo repeated transitions from sub- to super-critical. When the flow remains subcritical over the whole feature, ripples and dunes form instead.

Examples

Attention on real world cyclic steps has mostly been focused on the ocean floor and at river deltas. Several submarine cyclic steps have been discovered off the coast of California, such as those in the underwater canyons

Monterey Canyon Monterey Canyon, or Monterey Submarine Canyon, is a submarine canyon in Monterey Bay, California with steep canyon walls measuring a full 1 mile in height from bottom to top, which height/depth rivals the depth of the Grand Canyon itself. It is ...

and Eel Canyon. They have also been discovered in the South China Sea, at the South Taiwan shoal and the West Penghu submarine canyons. The cyclic step structure at the South Taiwan shoal is the longest ever observed (as of 2015), consisting of 19 steps and ranging over . They have also been discovered in the Japan Sea at the Toyama deep-sea channel. On Mars, they have been observed at Aeolis Mensae. At prodeltas (the portion of a river delta furthest from shore), cyclic steps have been observed in the Mediterranean. The wavelength of prodelta cyclic steps tends to be an order of magnitude smaller than their seafloor counterparts; the Mediterranean cyclic steps have a wavelength ranging from whereas submarine cyclic steps are typically measured in kilometers. While no modern examples have been found, cyclic steps can also form within rivers. Geologic evidence from the Cambrian-Ordovician Potsdam Group strata indicates that the Quebec Basin once possessed this type of cyclic step.

Glaciolacustrine Sediments deposited into lakes that have come from glaciers are called glaciolacustrine deposits. These lakes include ice margin lakes or other types formed from glacial erosion or deposition. Sediments in the bedload and suspended load are carr ...

cyclic steps have also been found in modern Quebec. Cyclic steps can also form along underwater volcanos, such as those in the Punta del Rosario fan, as well as along carbonate slopes and under bedrock streams. Cyclic steps do not need to form underwater - wind can cause them too.

Katabatic wind A katabatic wind (named from the Greek word κατάβασις ''katabasis'', meaning "descending") is a drainage wind, a wind that carries high-density air from a higher elevation down a slope under the force of gravity. Such winds are sometim ...

s may have caused cyclic steps to form on the ice sheet of Antarctica, and are actively forming cyclic steps at Mars’ poles.{{Cite journal, last1=Smith, first1=Isaac B., last2=Holt, first2=John W., last3=Spiga, first3=Aymeric, last4=Howard, first4=Alan D., last5=Parker, first5=Gary, date=2013, title=The spiral troughs of Mars as cyclic steps, url=https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/jgre.20142, journal=Journal of Geophysical Research: Planets, language=en, volume=118, issue=9, pages=1835–1857, doi=10.1002/jgre.20142, bibcode=2013JGRE..118.1835S, issn=2169-9100

References

Sedimentology Coastal and oceanic landforms Submarine topography Oceanographical terminology