Bed

Bed, layer of sediment or volcanic material that can be identified as a single layer separate from its neighbouring layers. Multiple beds are also referred to as strata. Beds can vary in thickness, but most are in the range of 1 cm (– in) to 1 m (3 ft) thick. Beds thinner than 1 cm are called laminae, and an individual thin bed is called a lamination. Most beds are flat and planar or lens-shaped, but some can be highly irregular. Beds are the smallest formal rock unit designation. An accumulation of many similar beds can define a larger formal rock unit called a formation if: (i) the beds are distinct from underlying and overlying rocks; (ii) the beds attain a collective thickness that is great enough to show on a geological map; and (iii) the beds cover enough area to show on a geological map.

Individual beds are easily identified because they exhibit slight differences in grain size, resistance to weathering, or colouration from the adjacent beds. Each bed is separated by a planar surface called a bedding plane that accentuates these differences. Additionally, bedding planes are typically surfaces of weakness that guide fracturing and cause individual beds to stand out even more.

A single bed forms as material is deposited on a surface in a relatively continuous way. New beds form when the conditions of deposition change, which in turn causes noticeable changes in the material’s grain size, texture, or composition. These changes can be very slight, as displayed by strata in many fine-grained sedimentary rocks, or they can be extreme, as displayed by alternating coarse- and fine-grained deposits or where volcanic ash beds overlie sedimentary beds. In many cases, the underlying bed is mildly eroded before new material is deposited on top of it.

Beds may display features, called sedimentary structures, that give clues to their origins. Examples of sedimentary structures include desiccation cracks (cracks that form when sediments dry), ripples, cross-bedding, and graded bedding. Desiccation cracks indicate an alternately wet and dry environment of deposition. Ripples in sedimentary rock indicate that the sediment was deposited by running water or blowing wind.

Cross-bedding consists of multiple layers of sediment that occur within, and at an angle to, a single sedimentary bed. Cross-bedding can form when sediment is deposited from moving water or wind. In these circumstances, ripples or dunes form and create an irregular surface on which sediment is deposited. The ripples or dunes obstruct the wind or current, creating a sheltered area on their back sides, or down-current slopes, where the wind or current deposits sediment. Hence, the sediments are deposited in downward sloping layers, while the bed as a whole remains fairly horizontal. Thicknesses of cross-bedding depend on the size of the surface irregularities. They range from less than a centimetre in the case of ripples to greater than several metres in the case of dunes. Cross-bedding forms mostly within sandstone, but can also form within other sedimentary rocks such as siltstone, conglomerate, clastic limestone, and dolomite, and even in volcanic ash flows.

Graded bedding forms when particles of varying sizes settle at differing rates. Graded bedding usually has the coarsest and heaviest material near its bottom because this material settles first and the finest and lightest material near its top because this material settles last. One example of graded bedding is the kind of bed that forms in a submarine fan. A submarine fan is a huge fan-shaped deposit of material that settles on the ocean floor at the end of submarine canyons. Occasionally, masses of sediment mixed with water hurtle down the submarine canyons. These fast-moving masses of sediment and water are known as turbidity currents. When the turbidity currents exit the bottom of the canyon, they slow down. As they slow down, the water drops its load of suspended sediments, heavy particles first, forming a graded bed. Through time, repeated flows can result in a thick sequence of beds called turbidites, each of which grades from coarse- to fine-grained. Sequences of graded beds can also form in floodplain environments, where repeated flooding events deposit sediment as the flood wanes.

A sequence of beds, one on top of the other, can also reveal information about how a particular environment changes through time. A sequence of beds where each bed is generally coarser than the bed below it is said to become coarser grained upwards. In general, bedding sequences that become coarser grained upwards indicate that an environment’s energy has increased through time, whereas those that become finer grained upwards indicate that the energy decreased through time. For example, if a river builds a delta that gradually fills in a lake, its sedimentary record would become coarser grained upwards. At the bottom, fine-grained sediments, deposited in the low-energy lake environment would dominate, while at the top, coarser sediments, reflecting the higher energy of the river, would dominate.

The term bioturbation describes the effects of organisms on the sediment before the sediment hardens into rock. Because organisms live within the sediment as well as on its surface, the sedimentary structures they create can appear within beds as well as on their surfaces. Well-known features include crawling or feeding tracks and burrows. Bioturbation can result in the wholesale mixing of the sediment. In most cases, strong mixing of the sediment results in the destruction of any other sedimentary structures.