5.5: Depositional Environments

Depositional Environments

The ultimate goal of many stratigraphy studies is to understand the original depositional environment. Knowing where and how a particular sedimentary rock was formed can help geologists paint a picture of past environments—such as a mountain glacier, gentle floodplain, dry desert, or deep-sea ocean floor. The setting in which sediments are accumulated is called a depositional environment. In the case of clastic rocks, the key environmental conditions are related to the amount of energy available to transport sediments, and how far the sediments get from their source before being deposited.

Broadly, depositional environments can be said to be terrestrial, marine, or to reflect a transitional zone between the two. Terrestrial refers to depositional environments on land. These may be depositional environments such as deserts found on dry land, but they could also be environments such as freshwater lakes or rivers. Marine refers to environments associated with saltwater seas and oceans. Transitional depositional environments include environments such as deltas, where freshwater rivers empty into saltwater seas or oceans.

Figure \(\PageIndex{1}\): Some of the important depositional environments for sediments and sedimentary rocks. Source: Karla Panchuk (2021) CC BY-SA 4.0. Modified after Mike Norton (2018) CC BY-SA 3.0. View source.

Tables \(\PageIndex{1}\) and \(\PageIndex{2}\) provide a summary of the processes and sediment types that pertain to the various depositional environments illustrated in the figure above.

Table \(\PageIndex{1}\): Terrestrial Depositional Environments.

Environment	Key Transport Processes	Depositional Settings	Typical Sediments
Glacial	Gravity, moving ice, moving water	Valleys, plains, streams, lakes	Glacial till, gravel, sand, silt, clay
Alluvial	Gravity, moving water	Where steep-sided valleys meet plains	Coarse angular fragments
Fluvial	Moving water	Streams	Gravel, sand, silt, organic matter
Aeolian	Wind	Deserts and coastal regions	Sand, silt
Lacustrine	Moving Water	Lakes	Sand, silt, clay, organic matter
Evaporite	Still water	Lakes in arid regions	Salts, clay
Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. View source.

Table \(\PageIndex{2}\): Marine and Transitional Depositional Environments.

Environment	Key Transport Processes	Depositional Settings	Typical Sediments
Deltaic	Moving water	Deltas	Sand, silt, clay, organic matter
Beach	Waves, long-shore currents	Beaches, spits, sand bars	Gravel, sand
Tidal	Tidal currents	Tidal flats	Fine-grained sand, silt, clay
Reef	Waves, tidal currents	Reefs and adjacent basins	Carbonates
Shallow marine	Waves, tidal currents	Shelves, slopes, lagoons	Carbonates in tropical climates; sand/silt/clay elsewhere
Lagoonal	Little transportation	Lagoon bottom	Carbonates in tropical climates; silt, clay elsewhere
Submarine fan	Underwater gravity flows	Continental slopes, abyssal plains	Gravel, sand, silt, clay
Deep water	Ocean currents	Deep-ocean abyssal plains	Clay, carbonate mud, silica mud
Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. View source.

Marine and transitional depositional environments are described in the chapter on Coastlines. Terrestrial depositional environments are described next.

Terrestrial Depositional Environments

Terrestrial depositional environments are diverse. Water is a major factor in these environments, in liquid or frozen states, or even when it is lacking (arid conditions). The table shows the sediment, rocks, fossils and sedimentary structures typically found in terrestrial depositional environments.

Table \(\PageIndex{3}\): Rock record and terrestrial depositional environments

Location	Sediment	Common Rock Types	Typical Fossils	Sedimentary Structures
Fluvial (river)	sand and mud, can have larger sediments	sandstone, conglomerate	bone beds (rare)	cross beds, channels, asymmetric ripples
Alluvial	mud to boulders, poorly sorted	clastic rocks	rare	channels, mud cracks
Lacustrine (lake)	fine-grained laminations	shale	invertebrates, rare (deep) bone beds	laminations
Paludal (swamp)	plant material	coal	plant debris	rare
Aeolian (dunes)	very well-sorted sand and silt	sandstone	rare	cross beds (large)
Glacial	mud to boulders, poorly sorted	conglomerate (tillite)		striations, drop stones

Fluvial

Fluvial (river) systems are formed by water flowing in channels over the land. They generally come in two main varieties: meandering or braided. In meandering streams, the flow carries sediment grains via a single channel that wanders back and forth across the floodplain. The sediment deposited farther away from the channel is mostly fine-grained material that only gets deposited during floods.

Figure \(\PageIndex{2}\): The Cauto River in Cuba. Note the sinuosity in the river, which is meandering.

Braided fluvial systems generally contain coarser sediment grains, and form a complicated series of intertwined channels that flow around gravel and sand bars [49].

Figure \(\PageIndex{3}\): The braided Waimakariri river in New Zealand.

Alluvial

A distinctive characteristic of alluvial systems is the intermittent flow of water. Alluvial deposits are common in arid places with little soil development. Lithified alluvial beds are the primary basin-filling rock found throughout the Basin and Range region of the western United States. The most distinctive alluvial sedimentary deposit is the alluvial fan, a large cone of sediment formed by streams flowing out of dry mountain valleys into a wider and more open dry area. Alluvial sediments are typically poorly sorted and coarse-grained, and often found near playa lakes or aeolian deposits [50].

Figure \(\PageIndex{4}\): An alluvial fan spreads out into a broad alluvial plain. From Red Rock Canyon State Park, California.

Lacustrine

Lake systems and deposits, called lacustrine, often form as sediment from rivers is dumped into a lake and slowly settles to the bottom. Lacustrine deposits are found in lakes in a wide variety of locations. Lake Baikal in southeast Siberia (Russia) is in a tectonic basin. Crater Lake (Oregon) sits in a volcanic caldera. The Great Lakes (northern United States) came from glacially carved and deposited sediment. Ancient Lake Bonneville (Utah) formed in a pluvial (from rainfall) setting during a climate that was relatively wetter and cooler than that of modern Utah. Oxbow lakes, named for their curved shape, originated in fluvial floodplains.

Figure \(\PageIndex{5}\): Oregon’s Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama.

Lacustrine sediment tends to be very fine-grained and thinly laminated, with only minor contributions from wind-blown, current, and tidal deposits [51]. When lakes dry out or evaporation outpaces precipitation, playas form. Playa deposits resemble those of normal lake deposits but contain more evaporite minerals. Certain tidal flats can have playa-type deposits as well.

Paludal

Paludal systems include bogs, marshes, swamps, or other wetlands, and usually contain lots of organic matter. Paludal systems typically develop in coastal environments but are common in humid, low-lying, low-latitude, warm zones with large volumes of flowing water. A characteristic paludal deposit is a peat bog, a deposit rich in organic matter that can be converted into coal when lithified. Paludal environments may be associated with tidal, deltaic, lacustrine, and/or fluvial deposition.

Aeolian

Aeolian, sometimes spelled eolian or œolian, are deposits of windblown sediments. Since wind has a much lower carrying capacity than water, aeolian deposits typically consist of clast sizes from fine dust to sand [52]. Fine silt and clay can cross very long distances, even entire oceans suspended in the air.

With sufficient sediment influx, aeolian systems can potentially form large dunes in dry or wet conditions. The figure shows dune features and various types. Parabolic and linear dunes grow from sand anchored by plants and are common in coastal areas.

Figure \(\PageIndex{6}\): Formation and types of dunes. (By NPS; public domain via Flickr.)

Compacted layers of wind-blown sediment are known as loess. Loess commonly starts as finely ground-up rock flour created by glaciers. Such deposits cover thousands of square miles in the Midwestern United States. Loess may also form in desert regions. Silt for the Loess Plateau in China came from the Gobi Desert in China and Mongolia.

Figure \(\PageIndex{7}\): Loess Plateau in China. The loess is so highly compacted that buildings and homes have been carved in it.

Glacial

Glacial sedimentation is very diverse and generally consists of the most poorly-sorted sediment deposits found in nature. The main rock type is called diamictite, which literally means two sizes, referring to the unsorted mix of large and small rock fragments found in glacial deposits [53]. Many glacial tills, glacially derived sediments, include very finely-pulverized rock flour along with giant erratic boulders. The surfaces of larger clasts typically have striations from the rubbing, scraping, and polishing of surfaces by abrasion during the movement of glacial ice. Glacial systems are so large and produce so much sediment, they frequently create multiple, individualized depositional environments, such as fluvial, deltaic, lacustrine, pluvial, alluvial, and/or aeolian.

Figure \(\PageIndex{8}\): Wide range of sediments near Athabaska Glacier, Jasper National Park, Alberta, Canada.

References

• 49. Cant, D. J. Fluvial facies models and their application. (1982).
• 50. Stanistreet, I. G. & McCarthy, T. S. The Okavango Fan and the classification of subaerial fan systems. Sediment. Geol. 85, 115–133 (1993).
• 551. Sly, P. G. Sedimentary processes in lakes. in Lakes (ed. Lerman, A.) 65–89 (Springer New York, 1978). doi:10.1007/978-1-4757-1152-3_3
• 52. Bagnold, R. A. The physics of blown sand and desert dunes. Methum, London, UK 265 (1941).
• 53. Eyles, N., Eyles, C. & Miall, A. D. Glacial facies models. Geology 16, 374–375 (1988).