Depositional environments

Wiki Write-Off Entry
Student Chapter	Universitas Gadjah Mada
Competition	December 2014

Sedimentary rocks are formed by 5 processes, there are erosion, weathering, transport, deposition, and diagenesis process (Boggs, 1991). These 5 processes with 4 types of rocks make up a cycle that is known as sedimentary cycle. The properties of sedimentary rocks such as sediment textures and structures, are formed by chemical, physics and biological processes. Depositional process and the properties of rock has really close genetic relationship. In sedimentology, determining the depositional environments from the properties of sediment or sedimentary rocks is the main purpose and focus. Thus, depositional environments are important and essential not just in sedimentology, but also in other geological disiplines, such as stratigraphy. To gain the depositional environment, we need to do environmental analysis. Environmental analysis is based on properties of rocks that have environmental significances, which are sediment structures, textures, fossils and sedimentary facies associations. From those informations, we can construct the facies model that’s a general summary of the characteristics of certain depositional system. Then, from the characteristic depositional system, we can know the depositional environments of sediment or sedimentary rock.

Definition of Depositional Environments

Depositional environment is part of earth surface that has certain chemical, biology, and physics characteristics where sediments are laid on. There are 3 kinds of depositional environments, they are continental, marginal marine, and marine environments. Each environments have certain characteristic which make each of them different than others. And different depositional environment, will have different structure and texture of sediments.

Terrestrial (Continental) Environments

There are 4 major kinds of terrestrial environments, they are fluvial, desert, lacustrine, and glacial. Fluvial environment is close-connected to the activity of river and also to alluvial plain. Desert environment is close-connected to the role of wind as the agent of transportation and sedimentation. Lacustrine sediments form in lake and glacial sediments form in ice- covered mountain or slope. Each kinds of terrestrial environments will be explaned more below :

Fluvial Systems

Fluvial deposits, also known as alluvial deposits, involves sediments that are formed by activities of river, stream, and associated with gravity flow process. There are 2 kinds of environmental settings from fluvial deposits, which are alluvial fan and river.

Alluvial Fan and Its Classification

Alluvial deposits has gross – cone-like shape and form a convex-up construction in cross section. These deposits are consist of abudant gravel-sand detritus on mostly fairly steep depositional slope. They are poorly sorted and are common from the base of mountain range with many sediment supplies. Based on depositional processes, there are 2 kinds of alluvial fans, debris-flow-dominated fans and stream-flow-dominated fans. As the flow of sediments from high relief area (mountain range) comes to a lower area, they are free to spread out with help of gravity. Debris-flow deposits are poorly sorted with the lack of sediment structure, except the reverse graded bedding in their basal parts. They contain various sizes of sediment, and mostly impermeable and nonporous because of the high amount of muddy matrix.

There is also mudflow which is similar to debris flow but mostly contains finer size of sediments than debris flow. Debris flow is also associated with landslide, the landslide is commonly as the source of sediments for debris flow. Stream-flow processes are the principal transport mechanism of stream-flow dominated fans. There are sheetflood and incised channel flow. Sheetflood is a broad expanse of unconfined , sediment-laden runoff water moving downslope , commonly proudced by catastrophic discharge. And incised-channel flow takes place through channel, 1-4 m high, incised into the upper fan.

Characteristics of Alluvial Fan

Alluvial fans are cone-shape and have many distributary channels. From the fanhead to the fantoe and also from the cross section, it’s concave upward. Alluvial fan deposits are mostly gravel-sized sediments and show down-fan decreased in grain size and bed thickness and an increase in sediment sorting. The characteristics of debris-flow-dominated fan are the poorly sorted sediment with coarse grain sizes and muddy matrix. But the stream-flow- dominated fans are more sheetlike deposits of finer grain sizes and may have well sorted, laminated, cross-bedded or nearly strcutureless. Mostly, alluvial fan deposits have strongly developed thickening and coarsening-upward succesion, which are caused by active fan progradation or outbuilding. But there are some fan deposits have thinning and finning upward succession which are caused by fan retrogradation.

River Systems

According to Nichols there are 3 major kinds of river based on the sinuosity channels, bars, and the number of channels. They are meandering (single-channel), braided (multiple-channel), and anastomosing rivers. Later on, meandering river will form a characteristic morphology, called oxbow lake. In braided river, deposition of sand and gravel grain sizes happens around the channel.

Sediment Transport Processes in Rivers

Channel Transport

Channels are characterized by the presence of bars. Point bars are deposits that are placed on the side of the river. Erosion happens on the outside of the bends and and deposition happens on the opposite side forming point bars. The characteristics of point bars are cross-bedded structure or fining upward structure. In braided rivers, braid bars are present in midchannel position. Furthermore, the braid bars can form an island.

Floodplain Deposition

Floodplain are strips of land connected to the river and commonly formed in seasonal flood. Floodplains can be present in braided and also meandering rivers. When the stream flood, overtop its banks, deposition happens on natural leeves. This deposition will happen again and again until forming a land along the side of the bank, then formed floodplain. Natural-leeve deposits form on the concave or steep-bank side of meander loops which connected to the channel as a result of sudden lost of competence. Their characteristics are the structure of stratified fine sands overlain by laminated mud.

Characteristics of Fluvial Deposits

The common properties of fluvial deposits are, they consist of sand and gravel grain size and also mud in floodplain deposits of meandering systems, and they have moderate to poor sorting. The deposits of point bars and braid bars are generally fining upward grain size. The repeat of channel shifting and bar migration of braided rivers produces vertical stacking of bar deposits can be separated by mudstones. And the repeat of meander migration produces vertical stacking of fining upward succesion in meandering river deposits. Many sediment structures like planar and trough cross-bedding, upper-flow-regime planar bedding, and ripple-marked surface occur in fluvial deposits. From fluvial deposits, the paleocurrent can be determined especially in meandering rivers. And fluvial deposits can also contain a variaty of fossil hard parts of terrestrial animals and also trace fossils which are created by both plants and animals.

Eolian Desert Systems

Desert areas cover about 20-25 percent of present land surface, within latitudinal belts about 10-30 degrees north and south of equator. These areas have dry climate with low air pressure and low rainfall. These dry areas are dominated by wind activity and covered by sand. There are 3 subenvironments of this desert system, they are aluvial fan, ephemeral river which is present when rain season, and ephemeral lake.

Transport dan Depositional Processes in Desert Systems

As explaned before, desert areas are dominated by wind activity. In these areas, wind plays roles as erosion and also transport agent. Wind is less effective erosion agent than water, but more effective in become transport agent of finer sand sediments. Wind transports sediment almost in the same way as water, with three ways : traction, saltation, and suspension. Effectively, wind transports sediment by suspension. But coarser sediment can also transported by traction or saltation by wind with unusual high velocity. This transport proccess will produce 3 kinds of deposits : dust (silt) deposits, sometimes referred to loess which contains finer sediments from far distance; sand deposits, which is well sorted; and lag deposits, which consist of gravel-size sediments that are too large to be transported by the wind, so it forms deflation pavement.

From this transport, there are many structures and bedforms that are formed. Such as ripples, sand dunes, and cross-bedds. Bedforms that develop during wind transport range from ripple with 0,01 m long and a height of few milimiters to dunes with 500-600 m long and 100 m of height. There’s also draas which is gigantic bedform that has up to 5,5 km long and height of 400 m (Wilson, 1972; McKee, 1982). Environments of deserts are divided into 3 main subenvironments : dunes, interdune, and sand sheet (Ahlbrandt and Fryberger, 1982).

Dunes

Dunes are formed by wind deposition and transport, then the sediments accumulate in a variety of dune forms. Dune morphology is determined by the availability of sand, wind intensity, and the variability of wind directions (Lancaster, 1999; Pye and Tsoar, 1990). Dune consists of well sorted and well rounded grains, mostly quartz rich , but also heavy minerals and rock fragments are present. Coastal dune consists of many ooids , skeletal fragments, and other carbonate grains. Eolian dune is characterized by the presence of large cross bedding.

Interdune

Interdune areas occur between dunes and bounded by dunes or other eolian deposits such as sand sheets. Interdunes may be formed by erosion or deposition. Erosional interdunes are present as disconformity overlain by thin, discontinuous, winnowed lag deposits. All interdunes deposits are characterized by low angle stratification (<~10°). Dry interdunes or interdunes that are wet occasionally are more common than wet interdunes. Dry interdunes have ripple-related wind- transport-processes, grainfall in the wind shadow in the lee of dunes, or sandflow from adjacent dunes. These deposits are coarse, bimodal, poorly sorted, and poorly laminated layers. Wet interdune deposits are finer than the dry interdunes deposits. They are usually in silt or clay size which are trapped by semipermanent bodies of water. This deposits contain freshwater organisms such as pelecypod, gastropod, diatom and ostracod. Evaporite interdunes occur where drying of shallow ephemeral lakes or evaporation of damp surface causes precipitation of carbonate minerals : gypsum or anhydrite. This sediments may be characterized by desiccation cracks, raindrop imprints, , evaporite layers, and pseudomorphs.

Sheet Sands

Sheet sands are flat to gently undulating bodies of sand that commonly surround dune fields. They are typically low to moderate dipping (0°-20°) cross stratification. This deposits consist of gently dipping, curved or irregular surfaces of erosion with bioturbation traces from insects and plants , poorly laminated, thin laye of coarse sand intercalated with fine sand and occasional intercalations of high-angle eolian deposits(Ahbrandt and Fryberger, 1982; Korucek and Nelson, 1986; Schwan, 1988).

Lacustrine Systems

1-2 percent of eart surface is covered by lake. Lake deposits are formed by complex chemical, physical, and biological processes. The physical processes are from rivers, waves, streams, and deposition. The chemical processes are from the chemical deposition, and the biology processes are from biological product and organic deposits. The basin of lake can be formed by many mechanisms, such as tectonic movements (rifting, faulting), glacial processes (ice scouring, ice damming, moraine damming), landslide or other mass movements, volcanic activity, deflation, and fluvial activity.

Modern lakes are about a few tens of square meters to ten of thousands of square kilometers. There are 4 variables that determine the lake size and characters, they are basin- floor depth, sill height, water supply, and sediment supply.

Principal Kinds of Lakes

Based on mechanism of deposition, there are 2 kind of lakes, open lakes and closed lakes. Open lakes have outflow of water and a stable shoreline. The deposits are dominated by silisiclastic materials from river, that are deposited along the side of the lake near the river. Closed lakes do not have outflow and have fluctuating shorelines which are controlled by seasonal flood. The deposits are dominated by carbonate mud, silt, and sand which developed together with evaporite sediments, stromatolites, and travertine.

Factors Controlling Lake Sedimentations

Physical Processes

Physical processes are about sediment transport and deposition include wind, river inflow, and atmorpheric heating. Wind creates waves and currents; river inflow may transport fine sediments far into the lake, also may transport sediment by turbidity current and density underflows to the basin. So, there are many depositional mechanisms that operate in lakes.

Chemical Processes

In closed lakes, deposition of chemically processed sediments are common. There are many ions that are abundant in lake, but mostly carbonates are the most present. The evaporite deposits contain many evaporite minerals, such as gypsum, anhydrite, halite, and sylvite. The pH of lake is about 6 to 9, but it can become asidic in some volcanic lakes, and more alkaline in closed desert lakes.

Biological Processes

Many kinds of organisms live in lakes and when they die, their skeletals will be preserved in lake sediments. Siliceous diatoms are common in lake deposits. But there are also pelecypods, gastropods, calcareous algae, and ostracods that give contribution of carbonate materials in lake sediments.

Glacial Systems

Glaciers cover about 10 percent of earth’s surface in high latitude regions. There are about 86 percent of total glaciers on Antartica, about 11 percent on Greenland, and the rest is on Iceland, Baffin Island, and Spitsbergen. Glacial environment exists in high latitude areas at all elevations and at low latitude areas where snow doesn’t melt in summer.

Environmental Setting

There are 4 zones : the basal (subglacial zone) which influenced by contact with the bed, the supraglacial zone which is the upper surface of the glacier, the ice-contact zone around the margin of glacier, and the englacial zone within the glacial interior. Proglacial environment is influenced by melting ice but not with direct contact with ice, this environment includes glaciofluvial, glaciolacustrine, and glaciomarine settings. The periglacial environment is the wider areaand overlapping the proglacial environment. The basal zone is the area where erosion and plucking of underlying bed happens. The supraglacial zone and ice-contact zone are zones of melting where englacial debris carried by glacier accumulates as the glacier melts. The size of glacial environment ranges in various sizes. There are valley glaciers, piedmont glaciers, and ice sheets which have smallest to biggest size of glacial environments.

Marginal Marine

The Marginal Marine is located around the boundary between the continental and the marine depositional realms. The Marginal Marine is dominated by wave, river and tidal processes. The characteristics of Marginal Marine are high-energy waves and currents, although some lagoonal and estuarine environments are dominated by quiet-water conditions. The depositional settings in marginal marine sediment are delta, beach, and barrier island, estuarine, lagoonal, and tidal flat.

Deltaic System

Learning about Deltaic System is important because delta told as the host of petroleum, gasses, and coals. Herodotus, a greek philosopher, use the terms of Delta to describe a triangle shape of alluvial plain in the mouth of Nile River. Nowadays, although Delta doesn’t form a perfect triangle, the term Delta is still used to describe the subaerial or subaqueous deposition of fluvial processes in the body of water. Generally, delta is formed by the deposition and sedimentation of materials that are transported through the river in other body of water like sea. This process is called the constructive process. Beside the constructive process, delta is also influenced by the destructive processes like reworking and redistributing of sediment from marine system. The place in which the delta is formed has three requirements. First, it has to have a large and active drainage system. Second, the tectonic activity should be low. Last, it has to have a large supply of sediments.

Based on the sedimentation processes, the morphology of delta can be classified as Delta Plain, Delta Front, and Prodelta.

Delta Classification and Sedimentation Processes

The characteristics of delta are influenced by many complex factors. These factors include:

• Climate
• Water
• Sediment discharge
• River mouth processes
• Nearshore wave power
• Tides
• Nearshore current
• Winds (Coleman, 1981 in Boggs)

Beside the factors mentioned above, there are also some morphological factors:

• Slope of shelf
• Rate of subsidence and other tectonic activity at depositional site
• Geometry of depositional basin

Delta classification on the basis of delta-front regime (Galloway, 1975) appears to be favored by most geologists. Deltas are classified thus as (1) fluvial-dominated, (2) tide-dominated, or (3) wave-dominated.

Vertical Succession of Deltaic System

Deltaic system is formed by the process of progradation. This process is causing fining- upward-succession in sedimentation record.

Beach And Barrier Island

The Beach and Barrier Island are developed on wave-dominated coast where tidal range is small to intermediate. The Beach and Barrier Island are dominated by sea processes and sand transportation by wind with small scale. The Beach and Barrier Island can become one if:

• A beach associated with the mainland.
• A system coastal hills that wider will produce coastal plain.
• The barrier island separates partially or completely from mainland by lagoon and marsh.

Beaches

The Beach environment can be divided into several zones :

• The Backshore, which extends landward from the beach berm above high tide level and commonly includes back-beach dune deposits.
• The Foreshore, called littoral zone or zone between high tide leve to low tide level.
• The Shoreface, called nearshore, located on low tide level to transition zone between beach and sediment on shelf.

Depositional Processes

Erosion, sediment transport, and depositional processes on coast have been studied by engineers interested in coastal processes. Beaches will develop on wave dominated coast where tidal range is small. Beaches associated with wave related processes, which include wave swash, storm waves, and nearshore currents (longshore and rip currents). The cross sand can found on the central part of beach deposit. The main zone of shoreface sediment are multidirectional trough cross-bed sets and trace fossils, such as Skolithos.

Barrier Island Systems

The Barrier Island setting is composite of three separate environments. There are the sandy barrier-island chain itself (the subtidal to subaerial barrier-beach complex), the enclosed lagoon, estuary, or marsh behind it (the back-barrier, subtidal-intertidal region), and the channels that cut through the parrier and connect the back-barrier lagoon to the open sea (the subtidal-intertidal delta and inlet-channel complex). Depositional processes and sediment transport are similar with the mainland beaches.

Figure 1. Barrier Island Systems (Derived from Boggs)



Estuarine

Estuarine is the liquid part and has funnel shape. The estuarine associatwith the ocean. Based on dominant hydrologic characteristics and the kinds of sediment and sediment bodies, it divided into wave dominated, tide dominated, or mixed wave and tide dominated (Dalrymple, Zaitlin, and Boyd, 1992).

Wave Dominated Estuary

High wave occur in the mouth of estuary. Tidal influence does not significantly affect the estuary. Sediment tend to move along the beach and in the beach to the mouth of estuary, which spit subaerial is formed. 

Figure 3. Wave Dominated Estuary (top) and Tidal Dominated Estuary (bottom) (Derived from Boggs)

Tide Dominated Estuaries

Tide Dominated Estuaries form in macrotidal beach. The tidal energy is greater than the wave energy. The deposits are characterized by ripple mark structure, cross bedding, and flaser. The estuarine deposits contain brackish water fauna, such as shell, pelecypoda and gastropods.

Mixed Wave and Tide Dominated

There are several estuaries have characteristics intermediate between wave-dominated and tidal-dominated. The location example of mixed wave and tide dominated are St.Lawrence River, Canada.

Lagoon

Lagoonal is shallow morphological that containing water and the location is near with sea. Lagoonal has terrestrial and marine parts. Lagoonal is separated from terrestrial enviroment by reef, barrier island, sandbank, and spit. The lagoonal has low energy, so sediment that dominant is fine grained sediment in lagoonal.

Based on geomorphological and water circulation with the ocean, lagoonal consists three types :

1. Choked, occurs in along coast with high wave energy and drift.
2. Restricted, has two or more entrance channels or inlets, has good water circulation because of tidal circulation, and influenced by wind.
3. Leaky, occurs in along coast where tidal currents are important factor in sediment transport.

Figure 4. Principal Kinds of Coastal Lagoons (Derived from Boggs)



Tidal Flat

The Tidal Flat is transition environment that still affected by tide. The tidal flat divided into 3 zones, supratidal, intertidal, and subtidal.

• Supratidal Zone, the zone is influenced by extreme tides. This zone will evaporation and produce crystal salt. This zone also called Sabkha.
• Intertidal Zone, located on between high and low tide level. This zone occurs the bedload and suspension load transportation.
• Subtidal Zone, is under the water when low tide level. Tide influence in this environment is very important especially at tidal channels, where bedload transport and deposition are dominant.

Sediment characteristics of tidal flat is the dominant of sand in shallow subtidal zone, intertidal bottom zone, and channel. There is cross bedding sand in channel and abundance of mud in supratidal zone.

Deep-Sea System

Deep-sea system are the widest sedimentation area in the world. Despite being the widest area, it’s not well explored because it’s rarely exposed in the surface like the shallow marine , fluvial, delta , or other sedimentation area. This system started from the shelf to the deep-sea basin. Continental shelf is a morphology with slope about 2o-4o. The transportation processes happen in the slope are variative.

Continental rise is a plain that goes all the way to the deep-sea and form a marine-fan. It is located under the continental slope.

Transportation And Deposition Processes

Sedimentation processes in deep-sea takes a long time. Beside transportation by wind, sediment in the deep-sea should be transported by shelf. There are many kind of transportation by shelf, include, creep, gliding/sliding, and slumping.

Principal Kinds Of Deep-Sea Sediment

Kind of deep-sea sediment : Hemipelagic Mud

Hemipelagic mud deposited on very low current. It is mostly found in the basin slope in front of arc.Generally it has the color of grey to green or green to reddish brown. It has mud to shale size. Beside formed from the remains of living things, hemipelagic mud also formed of soft terrigenous quartz, fieldspar mika, mineral lempung, and vilcanin material. Hemipelagic mud can also be found in the back-arc basin, which is formed by the activities of planktonic organism.

Kind of deep-sea sediment : Turbidite

Turbidite is formed in the deep marine fan, shallow-marine canyon, and deeper environtment. It consist of sand, sandy shale , and gavelly sand. It alternates with the pelagic mud and has normal gradation. It is drawn in the Bouma Sequence. It has the structure of sole mark in general. Beside turbidite sandy deposit, mud also appeared in some modern deep-sea sediment. The turbidite is formed in various tectonic settings, active or passive.

Kind of deep-sea sediment : Contourite

Contourite shows inverse-grading or coarsening upward structure with grain size from silt to sand and then followed by the opposite facies (fining upward). Contourite that consist a lot of mud has a homogen composition, a bad layer and shows the intensive bioturbation. Whereas contourite that consist a lot of silt shows bioturbation and mottled. Last, contourite that consist a lot of sand has a thin and random layers, fine grains, bioturbation, and shows cross-lamination.

Sources

• Selley, R. C., 2000. Applied Sedimentology. Florida: Academic Press.