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Bedding and lamination

bedding and lamination sketch

Sketch illustrating the first-order alternations of different lithologies (bedding) in a stratigraphic sequence and the small scale laminae that occur within beds. Graphics: Samuele Papeschi/GW.

The most common feature of sedimentary rocks is that they are organized in layers of different composition piled on top of each other. Any layer of rock in a sedimentary sequence that can be distinguished from the layers above and below is a bedding plane. Bedding planes, also called beds or strata (singular: stratum) are the simplest sedimentary structures and the smaller lithostratigraphic unit used by geologists to describe sedimentary formations. What allows the differentiations of a bed from the surrounding ones are differences in lithology (e.g. alternations of limestone and shale) or grain size (e.g. alternations of sandstone and conglomerate). In geology each bed represents an event, i.e. a period of time in which the physical processes active in a sedimentary basin allowed the deposition of a specific sediment. An ‘event’ in geology can be a long period of time, for example the settling of clay particles on the seabed over thousands of years, but also a fast process, like the rapid deposition of a submarine turbidity current. Bedding planes deposit by gravity and are influenced by other physical processes like currents that distribute the sediments within the basin. The combined effect of these phenomena spreads the sediments laterally and horizontally, in some cases for hundreds of square kilometers, towards the margins of the sedimentary basin, producing horizontal with very limited thickness and wide lateral extension. 

The order of beds in an undisturbed sedimentary sequence (i.e. when the sequence was not tilted/folded by tectonic processes) follows the order of deposition, with the younger strata on top and the older at the bottom (Principle of Superimposition). Each bed is unique and reflects an instant of the history of a sedimentary basin when its sedimentary input, energy, and eventual life forms (fossils) remained constant. A sequence of beds can, hence, be seen as the sequence of events that occurred over geologic time scales in a basin.

Laminations
In a sequence of sedimentary rocks, bedding represent the first-order lithological alternations that occurs at the scale of more than a cm (centimeters to meters). Subtler and less pronounced planar lithological alternations within beds, by convention at a scale smaller than the centimeter, are laminations or laminae (singular: lamina). Laminations are the smaller units in which a sedimentary sequence can be divided. Laminations are important, because their geometry provides information on the processes (e.g. currents) that were active during the deposition of beds.

sedimentary layers in the Grand canyon

No place on Earth highlights the horizontal character of bedding planes like the Grand Canyon! The deeply incised canyon shows (mostly) horizontal strata of sandstone, limestone, and shale extending in all directions. Photo Samuele Papeschi/GW.

bedding planes in the Grand Canyon

Detail of bedding along the Bright Angel Trail (Grand Canyon). The reddish layers are sandstone and shale (Supai Group and Hermit Shale; Carboniferous – Permian). The white layers on top are sandstone (Coconino Sandstone – Permian) and Kaibab limestone (the very top, of Permian age). Photo Samuele Papeschi/GW.

bedding planes in the Kaibab Limestone

Beds defined by alternations of carbonatic and siliciclastic rocks in the Kaibab Limestone (Permian). Grand Canyon, Arizona. Photo Samuele Papeschi/GW.

Layers of turbidites

Beds of turbidites (sandstone, prominent) and shales (eroded) from the Macigno Sandstone (Oligocene). Note the presence of subtle, planar laminae in the layer behind the hammer. Cala del Leone, Quercianella, Italy. Photo Samuele Papeschi/GW.

Lamination in sandstone

Laminations are subtle structures within beds. In this example they are defined by tiny laminae of sand with slightly different grain size in a turbidite layer. Cala del Leone, Quercianella, Italy. Photo Samuele Papeschi/GW.

convolute lamination in sandstone

Laminations, like stratifications, may show complex geometries. Here, plane parallel laminae (bottom) transition to curved, convolute laminae (top) within a sandstone layer. Cala del Leone, Quercianella, Italy. Photo Samuele Papeschi/GW.

Related pages

plane bedding

  • Plane bedding Plane bedding (or parallel bedding) is the simplest sedimentary structure. It occurs when bedding planes are parallel to each other. In undisturbed (non deformed) sedimentary sequences, plane bedding continues laterally as horizontal beds at the scale of kilometers to hundreds of kilometers. Beds end against the margins of the sedimentary basin or gradually fades into progressively thinner beds, moving away… Read More »Plane bedding

 

cross bedded sandstone

  • Cross-bedding Cross-bedding (or cross-stratification) is a primary sedimentary feature characterized by layers that intersect at an angle with each other. In general, cross-bedding is characterized by planar erosional surfaces that separate beds with inclined strata or laminae. This architecture is the result of the migration of bedforms, such as dunes, ripples, and megaripples, produced by wind or water currents in sand-rich… Read More »Cross-bedding

amalgamated bed

  • Amalgamated beds In sedimentary sequences, a bed generally represents a single deposition event: a period of time in which the conditions of sedimentation in a basin remained constant, resulting in a layer with characteristic composition and structures that can be distinguished from the underlying and the overlying beds. Amalgamated beds are an exception. Amalgamation occurs in high-energy environments characterized by alternating phases… Read More »Amalgamated beds

References
Campbell, C. V. (1967). Lamina, laminaset, bed and bedset. Sedimentology8(1), 7-26.
Ingram, R. L. (1954). Terminology for the thickness of stratification and parting units in sedimentary rocks. Geological Society of America Bulletin65(9), 937-938.
Lyell, C., & Deshayes, G. P. (1830). Principles of geology: being an attempt to explain the former changes of the earth’s surface, by reference to causes now in operation (Vol. 1). J. Murray.
Mackenzie, F. T., & Garrels, R. M. (1971). Evolution of sedimentary rocks. New York: Norton.
McKee, E. D., & Weir, G. W. (1953). Terminology for stratification and cross-stratification in sedimentary rocks. Geological Society of America Bulletin64(4), 381-390.
Pettijohn, F. J. (1975). Sedimentary rocks (Vol. 3). New York: Harper & Row.
Schlager, W. (2004). Fractal nature of stratigraphic sequences. Geology32(3), 185-188.
Steno, N. (1669). De Solido Intra Solidium Naturaliter Contento Dissertationis Prodromus: Florence, Italy, Library of Grand Duke Ferdinand II, – V. iv, 131 p. English version: Stensen, Niels 1671, The prodromus to a dissertation concerning solids naturally contained within solids. J. Winter, London, 112 p.

        

See also
Features from the Field – bedding/stratification – EGU Tectonics & Structural Geology Blog.
SEPM Strata – Bed.
SEPM Strata – Bedding plane

it_IT Italiano
Detrital and Authigenic Minerals
Textures
Sedimentary Structures
Fossils
Sedimentary Rocks

 

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