12.4: Marine Depositional Environments

Depositional Environments

Broadly, depositional environments along the coast are marine or a transitional zone between terrestrial and marine. 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. These environments form the landforms around the coast.

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.

Table \(\PageIndex{1}\) provides a summary of the processes and sediment types that pertain to the various marine and transitional depositional environments illustrated in the figure above.

Table \(\PageIndex{1}\): 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

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. The table shows the sediment, rocks, fossils and sedimentary structures typically found in marine depositional environments.

Table \(\PageIndex{2}\): Rock record and marine depositional environments.

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, contourites	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

Abyssal

Abyssal sedimentary rocks form on the abyssal plain. The plain encompasses a relatively flat ocean floor with some minor topographical features, called abyssal hills. These small seafloor mounts range from 100 m to 20 km in diameter and are possibly created by extension [38]. Most abyssal plains do not experience significant fluid movement, so sedimentary rocks formed there are very fine-grained [39].

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

There are three categories of abyssal sediment. Calcareous oozes consist of calcite-rich plankton shells that have fallen to the ocean floor. An example of this type of sediment is chalk. Siliceous oozes are also made of plankton debris, but these organisms build their shells using silica or hydrated silica. In some cases such as with diatomaceous earth, sediment is deposited below the calcite compensation depth, a depth where calcite solubility increases. Any calcite-based shells are dissolved, leaving only silica-based shells. Chert is another common rock formed from these types of sediment. These two types of abyssal sediment are also classified as biochemical in origin.

Figure \(\PageIndex{3}\): Diatomaceous earth

The third sediment type is pelagic clay. Very fine-grained clay particles, typically brown or red, descend through the water column very slowly. Pelagic clay deposition occurs in areas of remote open ocean, where there is little plankton accumulation.

Two notable exceptions to the fine-grained nature of abyssal sediment are submarine fan and turbidite deposits [40]. Submarine fans 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. The submarine fan is formed by a network of turbidites that deposit their sediment loads as the slope decreases, much like what happens above-water at alluvial fans and deltas. This sudden flushing transports coarser sediment to the ocean floor where they are otherwise uncommon. Turbidites are also the typical origin of graded Bouma sequences.

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

Continental Slope

Continental slope deposits are not common in the rock record. The most notable type of continental slope deposits are contourites [41]. Contourites form on the slope between the continental shelf and the deep ocean floor. Deep-water ocean currents deposit sediment into smooth drifts of various architectures, sometimes interwoven with turbidites.

Figure \(\PageIndex{5}\): Contourite drift deposit imaged with seismic waves.

Lower Shoreface

The lower shoreface lies below the normal depth of wave agitation, so the sediment is not subject to daily winnowing and deposition. These sediment layers are typically finely laminated and may contain hummocky cross-stratification. Lower shoreface beds are affected by larger waves, such those as generated by hurricanes and other large storms [42].

Figure \(\PageIndex{6}\): Diagram describing wavebase in the lower shoreface.

Upper Shoreface

The upper shoreface contains sediments within the zone of normal wave action but still submerged below the beach environment. These sediments usually consist of very well-sorted, fine sand. The main sedimentary structure is planar bedding consistent with the lower part of the upper flow regime, but it can also contain cross-bedding created by longshore currents [43].

Figure \(\PageIndex{7}\): Diagram of a shoreface profile. (By Dronkers J; via Coastal Wiki.)

Transitional Coastline Environments

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 and underwater. Shoreline environments are an important source of hydrocarbon deposits (petroleum, natural gas).

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

The study of shoreline depositional environments is called sequence stratigraphy. Sequence stratigraphy examines depositional changes and 3D architectures associated with rising and falling sea levels, which is the main force at work in shoreline deposits. These 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.

Littoral

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. Because beach sands, past or present, are so highly evolved, the amount of grain weathering can be discerned using the minerals zircon, tourmaline, and rutile. This tool is called the ZTR (zircon, tourmaline, rutile) index [44]. The ZTR index is higher in more weathered beaches because these relatively rare and weather-resistant minerals become concentrated in older beaches. In some beaches, the ZTR index is so high the sand can be harvested as an economically viable source of these minerals. 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.

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

Tidal Flats

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].

Tidal water carries in sediment, sometimes focusing the flow through a narrow opening called a tidal inlet. Tidal channels, creek channels influenced by tides, can also focus on tidally-induced flow. Areas of higher flow like inlets and tidal channels feature coarser grain sizes and larger ripples, which in some cases can develop into dunes.

Figure \(\PageIndex{10}\): Aerial view of tidal channels draining tidal flat, Everglades National Park, Florida. Gulf of Mexico in background. (By Marli Miller; via Geologic Time Pics.)

Reefs

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 features. Capitol Reef National Park in Utah contains a topographic barrier, a reef, called the Waterpocket Fold.

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

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

Sediment found in coral reefs is typically fine-grained, mostly carbonate, and tends to deposit between the intact coral skeletons. Water with high levels of silt or clay particles can inhibit the reef growth because coral organisms require sunlight to thrive; they host symbiotic algae called zooxanthellae that provide the coral with nourishment via photosynthesis. Inorganic reef structures have much more variable compositions. Reefs have a big impact on sediment deposition in lagoon environments since they are natural storm breaks, wave and storm buffers, which allow fine grains to settle and accumulate.

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. If the reef builds up above sea level and completely encircles the top of the seamount, it is called a coral-ringed atoll. If the reef is submerged, due to erosion, subsidence, or sea-level rise, the seamount-reef structure is called a guyot.

Figure \(\PageIndex{14}\): Seamounts and guyots in the North Pacific.

Lagoon

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]. Lagoons, as well as estuaries, are ecosystems with high biological productivity. Rocks from these environments often include bioturbation marks or coal deposits. Around lagoons where evaporation exceeds water inflow, salt flats, also known as sabkhas, and sand dune fields may develop at or above the high tide line.

Figure \(\PageIndex{15}\): Oludeniz beach with blue lagoon in Turkey. (By Kotangens; education license via Adobe Stock.)

Deltas

Deltas form where rivers enter lakes or oceans and are organized based on the dominant process that controls their shape: wave-dominated deltas, river-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.

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.

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

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.

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

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.

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

References

• 38. Hill, M. N. The Sea, Ideas and Observations on Progress in the Study of the Seas: Physical oceanography. 1, (Wiley-Interscience, 1962).
• 39. Stow, D. A. V. & Piper, D. J. W. Deep-water fine-grained sediments: facies models. Geological Society, London, Special Publications 15, 611–646 (1984).
• 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).
• 41. Stow, D. A. V., Faugères, J.-C., Viana, A. & Gonthier, E. Fossil contourites: a critical review. Sediment. Geol. 115, 3–31 (1998).
• 42. McCubbin, D. G. Barrier-island and strand-plain facies. (1982).
• 43. Reinson, G. E. Barrier-island and associated strand-plain systems. in Facies Models (ed. Walker, R. G.) 119–140 (Geoscience Canada Reprint Series 1, 1984).
• 44. Hubert, J. F. A zircon-tourmaline-rutile maturity index and the interdependence of the composition of heavy mineral assemblages with the gross composition and texture of sandstones. J. Sediment. Res. 32, (1962).
• 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).
• 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).