Bedding and lamination
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.
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.
- 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-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 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
Campbell, C. V. (1967). Lamina, laminaset, bed and bedset. Sedimentology, 8(1), 7-26.
Ingram, R. L. (1954). Terminology for the thickness of stratification and parting units in sedimentary rocks. Geological Society of America Bulletin, 65(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 Bulletin, 64(4), 381-390.
Pettijohn, F. J. (1975). Sedimentary rocks (Vol. 3). New York: Harper & Row.
Schlager, W. (2004). Fractal nature of stratigraphic sequences. Geology, 32(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.