5.4: Depositional Environments

Figure \(\PageIndex{1}\): A representation of common 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 study of depositional environments is a complex endeavor; the table shows a simplified version of what to look for in the rock record. Don't feel that you need to memorize this, this is just to give you a feel for what this looks like. Read a few entries to get an idea of how this works.

Location	Sediment	Common Rock Types	Typical Fossils	Sedimentary Structures
Abyssal	very fine muds and oozes, diatomaceous Earth	chert	diatoms	few
Submarine fan	graded Bouma sequences, alternating sand/mud	clastic rocks	rare	channels, fan shape
Continental slope	mud, possible sand, countourites	shale, siltstone, limestone	rare	swaths
Lower shoreface	laminated sand	sandstone	bioturbation	hummocky cross beds
Upper shoreface	planar sand	sandstone	bioturbation	plane beds, cross beds
Littoral (beach)	very well sorted sand	sandstone	bioturbation	few
Tidal Flat	mud and sand with channels	shale, mudstone,siltstone	bioturbation	mudcracks, symmetric ripples
Reef	lime mud with coral	limestone	many, commonly coral	few
Lagoon	laminated mud	shale	many, bioturbation	laminations
Delta	channelized sand with mud, ±swamp	clastic rocks	many to few	cross beds
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

Marine

Marine depositional environments are completely and constantly submerged in seawater. Their depositional characteristics are largely dependent on the depth of water with two notable exceptions: submarine fans and turbidites.

Abyssal

Figure \(\PageIndex{2}\): Marine sediment thickness. Note the lack of sediment away from the continents.

Abyssal sedimentary rocks form on the abyssal plain which is the vast flat ocean floor. Most abyssal plains do not experience significant fluid movement, so sedimentary rocks formed there are very fine-grained [39]. This includes things like chalk, chert and clay-based rocks like mudstone or claystone. These rocks are discussed earlier in this chapter.

Figure \(\PageIndex{4}\): Turbidites inter-deposited within submarine fans.

A notable exception to the fine-grained nature of abyssal sediment are turbidite deposits [40]. These occur offshore at the base of large river systems. They are initiated during times of low sea level, as strong river currents carve submarine canyons into the continental shelf. When sea levels rise, sediments accumulate on the shelf typically forming large, fan-shaped floodplains called deltas. Periodically, the sediment is disturbed creating dense slurries that flush down the underwater canyons in large gravity-induced events called turbidites. They deposit their sediment loads as the slope decreases. This sudden flushing transports coarser sediment to the ocean floor where they are otherwise uncommon.

Transitional Coastline Environments

Figure \(\PageIndex{8}\): The rising sea levels of transgressions create onlapping sediments, regressions create offlapping.

Transitional environments, more often called shoreline or coastline environments, are zones of complex interactions caused by ocean water hitting land. The sediment preservation potential is very high in these environments because deposition often occurs on the continental shelf (the shallow area underwater just off the coastline).

Sea-level fluctuations come from the daily tides, as well as climate changes and plate tectonics. A steady rise in sea level relative to the shoreline is called transgression. Regression is the opposite, a relative drop in sea level. Some common components of shoreline environments are littoral zones, tidal flats, reefs, lagoons, and deltas. For a more in-depth look at these environments, see the chapter on Coastlines.

Littoral

Figure \(\PageIndex{9}\): Lithified heavy mineral sand (dark layers) from a beach deposit in India.

The littoral zone, better known as the beach, consists of highly weathered, homogeneous, well-sorted sand grains made mostly of quartz. There are black sand and other types of sand beaches, but they tend to be unique exceptions rather than the rule. The beach environment has no sedimentary structures, due to the constant bombardment of wave energy delivered by surf action. Beach sediment is moved around via multiple processes. Some beaches with high sediment supplies develop dunes nearby.

Tidal Flats

Figure \(\PageIndex{10}\): General diagram of a tidal flat and associated features.

Tidal flats, or mudflats, are sedimentary environments that are regularly flooded and drained by ocean tides. Tidal flats have large areas of fine-grained sediment but may also contain coarser sands. Tidal flat deposits typically contain gradational sediments and may include multi-directional ripple marks. Mudcracks are also commonly seen due to the sediment being regularly exposed to air during low tides; the combination of mud cracks and ripple marks is distinctive to tidal flats [45].

Reefs

Figure \(\PageIndex{11}\): Waterpocket fold, Capitol Reef National Park, Utah

Reefs, which most people would immediately associate with tropical coral reefs found in the oceans, are not only made by living things. Natural buildups of sand or rock can also create reefs, similar to barrier islands. Geologically speaking, a reef is any topographically-elevated feature on the continental shelf, located oceanward of and separate from the beach. The term reef can also be applied to terrestrial (atop the continental crust) features. Capitol Reef National Park in Utah contains a topographic barrier, a reef, called the Waterpocket Fold.

Figure \(\PageIndex{12}\): A modern coral reef.

Most reefs, now and in the geologic past, originate from the biological processes of living organisms [46]. The growth habits of coral reefs provide geologists important information about the past. The hard structures in coral reefs are built by soft-bodied marine organisms, which continually add new material and enlarge the reef over time. Under certain conditions, when the land beneath a reef is subsiding, the coral reef may grow around and through existing sediment, holding the sediment in place, and thus preserving the record of environmental and geological conditions around it.

Figure \(\PageIndex{13}\): The light blue reef is fringing the island of Vanatinai. As the island erodes away, only the reef will remain, forming a reef-bound seamount.

Reefs are found around shorelines and islands; coral reefs are particularly common in tropical locations. Reefs are also found around features known as seamounts, which is the base of an ocean island left standing underwater after the upper part is eroded away by waves. Examples include the Emperor Seamounts, formed millions of years ago over the Hawaiian Hotspot. Reefs live and grow along the upper edge of these flat-topped seamounts.

Lagoon

Figure \(\PageIndex{15}\): Kara-Bogaz Gol lagoon, Turkmenistan.

Lagoons are small bodies of seawater located inland from the shore or isolated by another geographic feature, such as a reef or barrier island. Because they are protected from the action of tides, currents, and waves, lagoon environments typically have very fine-grained sediments [47].

Deltas

Figure \(\PageIndex{16}\): The Nile delta, in Egypt.

Figure \(\PageIndex{17}\): Birdfoot river-dominated delta of the Mississippi River.

Deltas form where rivers enter lakes or oceans and are of three basic shapes: river-dominated deltas, wave-dominated deltas, and tide-dominated deltas. The name delta comes from the Greek letter Δ (delta, uppercase) [48], which resembles the triangular shape of the Nile River delta. The velocity of water flow is dependent on riverbed slope or gradient, which becomes shallower as the river descends from the mountains. At the point where a river enters an ocean or lake, its slope angle drops to zero degrees (0°). The flow velocity quickly drops as well, and sediment is deposited, from coarse clasts to fine sand, and mud to form the delta. As one part of the delta becomes overwhelmed by sediment, the slow-moving flow gets diverted back and forth, over and over, and forms a spread out network of smaller distributary channels.

Figure \(\PageIndex{18}\): Tidal delta of the Ganges River.

Deltas are organized by the dominant process that controls their shape: tide-dominated, wave-dominated, or river-dominated. Wave-dominated deltas generally have smooth coastlines and beach-ridges on the land that represent previous shorelines. The Nile River delta is a wave-dominated type. (see figure).

The Mississippi River delta is a river-dominated delta, shaped by levees along the river and its distributaries that confine the flow forming a shape called a bird-foot delta. Other times the tides or the waves can be a bigger factor and can reshape the delta in various ways.

A tide-dominated delta is dominated by tidal currents. During flood stages when rivers have lots of water available, it develops distributaries that are separated by sand bars and sand ridges. The tidal delta of the Ganges River is the largest delta in the world.

Terrestrial

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).

Fluvial

Figure \(\PageIndex{19}\): The Cauto River in Cuba. Note the sinuosity in the river, which is meandering.

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 floodplain sediment away from the channel is mostly fine grained material that only gets deposited during floods.

Figure \(\PageIndex{20}\): The braided Waimakariri river in New Zealand.

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

Alluvial

Figure \(\PageIndex{21}\): An alluvial fan spreads out into a broad alluvial plain. From Red Rock Canyon State Park, California.

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.

Lacustrine

Figure \(\PageIndex{22}\): Oregon’s Crater Lake was formed about 7700 years ago after the eruption of Mount Mazama.

Lake systems and deposits, called lacustrine, form via processes somewhat similar to marine deposits, but on a much smaller scale. 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 setting during a climate that was relatively wetter and cooler than that of modern Utah. Oxbow lakes, named for their curved shape, originated next to rivers.

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.

Aeolian

Figure \(\PageIndex{23}\): Formation and types of dunes.

Aeolian, sometimes spelled eolian, 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.

Glacial

Figure \(\PageIndex{25}\): Wide range of sediments near Athabaska Glacier, Jasper National Park, Alberta, Canada.

Glacial sedimentation is very diverse and generally consists of the most poorly-sorted sediment deposits found in nature. Many glacial sediments, include very finely-pulverized rock flour along with giant boulders showing the power of ice to crush up rock and transport giant 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 (see the chapter on Glaciers).

References

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• 40. Normark, W. R. Fan valleys, channels, and depositional lobes on modern submarine fans: characters for recognition of sandy turbidite environments. AAPG Bull. 62, 912–931 (1978).
• 45. Eisma, D. Intertidal deposits: River mouths, tidal flats, and coastal lagoons. (Taylor & Francis, 1998).
• 46. Longman, M. W. A process approach to recognizing facies of reef complexes. (1981).
• 47. Nichols, M. M., Biggs, R. B. & Davies, R. A. J. Estuaries. in Coastal Sedimentary Environments 77–173 (Springer-Verlag: New York, 1985).
• 48. Elliott, T. Deltas. Sedimentary environments and facies 2, 113–154 (1986).
• 49. Cant, D. J. Fluvial facies models and their application. (1982).
• 51. 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).