Hummocky cross-stratification is a type of sedimentary structure found in sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

s. It is a form of cross-bedding

Cross-bedding

In geology, the sedimentary structures known as cross-bedding refer to horizontal units that are internally composed of inclined layers. This is a case in geology in which the original depositional layering is tilted, and the tilting is not a result of post-depositional deformation...

 usually formed by the action of large storms, such as hurricanes. It takes the form of a series of "smile"-like shapes, crosscutting each other. It is only formed at a depth of water below fair-weather wave base

Wave base

The wave base is the maximum depth at which a water wave's passage causes significant water motion. For water depths larger than the wave base, bottom sediments are no longer stirred by the wave motion above....

 and above storm-weather wave base. They are not related to "hummock

Hummock

A hummock is a boss or rounded knoll of ice rising above the general level of an ice-field, making sledge travelling in the Arctic and Antarctic region extremely difficult and unpleasant....

s" except in shape.

History

The name we use today was introduced by Harms et al. in 1975. Before this time, these structures were recognized under many different names. When hummocky cross-stratification was founded, it was originally given the name “truncated wave-ripple laminae,” by Campbell (1966, 1971). The main features were listed by Bourgeois (1980), Harms et al. (1982), and Walker (1983), in order to identify the structure. Dott and Bourgeois launched an idealized hummocky stratification sequence. From bottom to top, these include: first-order scoured base (± sole mark

Sole markings

Sole marks are sedimentary structures found on the bases of certain strata, that indicate small-scale grooves or irregularities. This usually occurs at the interface of two differing lithologies and/or grain sizes. They are commonly preserved as casts of these indents on the bottom of the...

s); characteristic hummocky zone with several second-order truncation surfaces separating individual undulating lamina sets; a zone of flat laminae; a zone with well-oriented ripple cross-laminae and symmetrical ripple

Ripple marks

In geology, ripple marks are sedimentary structures and indicate agitation by water or wind.- Defining ripple cross-laminae and asymmetric ripples :...

 forms; all overlain by a more or less burrowed mudstone

Mudstone

Mudstone is a fine grained sedimentary rock whose original constituents were clays or muds. Grain size is up to 0.0625 mm with individual grains too small to be distinguished without a microscope. With increased pressure over time the platey clay minerals may become aligned, with the...

 or siltstone

Siltstone

Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...

. Walker (1983) wanted to create a second sequence, but it was decided that this sequence offers the best basis for studying hummocky cross-stratification for the future.

Composition

This structure is commonly found in silt to fine sand. It is typically interbedded with bioturbated mudstone. It commonly contains concretion

Concretion

A concretion is a volume of sedimentary rock in which a mineral cement fills the porosity . Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'...

s of abundant mica

Mica

The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

 and plant detritus in the tops of many laminae. This helps indicate a shape sorting. Although hummocky cross-stratification is usually found in shallow marine sedimentary rock

Sedimentary rock

Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....

s, it has also been found in some lacustrine sedimentary rocks.

Common characteristics

In plan view (seen from above), it takes on the form of hummocks and swales that are circular to elliptical, with long wavelengths (1–5 m) but with low height (10s of centimeters). Laminations drape these hummocks; in cross-section view, these laminations have an upward curvature, and low angle, curved intersections. Hummocky cross-stratification can form in sediments up to about 3 cm in diameter, with near-bed water particle velocities between about 40–100 cm/s.

Formation of structure

This structure is formed under a combination of unidirectional and oscillatory flow that is generated by relatively large storm waves in the ocean. Deposition involves fallout from suspension and lateral tractive flow due to wave oscillation. As the large waves drape sand over and irregular scoured surface, this strong storm-wave action erodes the seabed into low hummocks and swales that lack any significant orientation. It is usually formed by redeposition below normal fair weather wave base delivered offshore by flooding rivers and shoals by large waves.

Depositional environments

During ancient times, hummocky cross-stratification was located in shallow marine environments, on the shore face and shelf by waves. It can also form on land during especially large storms when large amounts of water are pushed up onto the tidal flat. These landward deposits feature smaller bed forms due to the attenuation of storm waves as they move onto the land. While it is usually formed in marine settings by the action of storms, it may also be deposited in fluvial strata; a fluvial origin is more likely if the unit solely comprises sand.