11.4: Erosion, Transportation, and Deposition

Erosion, Transportation, and Deposition

After sediments are produced via weathering, they then undergo erosion. Erosion Links to an external site. includes both the movement of sediment, transportation via the agents of transportation (wind, water, ice, gravity), and the deposition of sediment in distinct areas referred to as depositional environments Links to an external site..

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Sedimentary Depositional Environments

A depositional environment Links to an external site. is the accumulation of chemical, biological, and physical properties and processes associated with the deposition of sediments that lead to a distinctive suite of sedimentary rocks. Sedimentary environments are interpreted by geologists based on clues within, such as rock types, sedimentary structures, trace fossils, and fossils. Geologists then compare these clues to modern environments to reconstruct ancient environments.

The numerous depositional environments found on earth can be broken into three major categories: 1) terrestrial or continental environments, common environments we find on land, 2) marine environments, those that are in the ocean, and 3) coastal or transitional environments, those at the interface between the first two (Figure 11.7).

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Terrestrial (Continental) Environments

There are many different environments on the continents, but consider how they are separated between those that are dominated by the deposition of sediments and those dominated by the erosion of sediments. Erosion occurs in high altitude areas and, although continents are overall topographically elevated compared to the oceans, there are many consistent areas on the continent with distinctive depositional properties. Continental depositional environments are dominated by clastic sedimentary rocks (see the Clastic Sedimentary Rock section), largely because of their proximity to the source of the sediments.

Glacial Links to an external site. depositional environments are controlled mostly by the weathering and erosion by glaciers and glacial meltwater. Glaciers most commonly occur in areas that are both high elevation and/or high latitudes. Glaciers are fairly slow moving (centimeters a day) and normally travel short distances from their source, but they can cause immense mechanical weathering. Glaciers grind and bulldoze rock and create piles of poorly sorted sediment called moraines Links to an external site.. Glaciers also produce a significant though fluctuating amount of meltwater, which flows through the moraines building a system of braided rivers Links to an external site.. These rivers carry the small sediments further from the end of the glaciers into an area called the outwash plain, which consists of poorly sorted sediment.

Alluvial Links to an external site. depositional environments are controlled primarily by gravity, although ephemeral Links to an external site. river channels may develop during flash flooding. During these flood events extensive sediment will be moved from a high point, generally a mountain range, down to a lower elevation, producing alluvial fan Links to an external site. deposits. These fan shaped deposits are similar to river deltas but occur on land. The sediment closest to the mountains will be larger in size, with smaller-sized sediment deposited towards the edges of the alluvial fan. Sediment here tends to retain a more angular shape, and little chemical weathering occurs.

Sediments that are deposited through the action of rivers are referred to as fluvial Links to an external site. depositional environments. The gradient and discharge of a river can greatly control the shape of the river, how it flows, and how it deposits sediment. Rivers alter sediment both chemically and physically as well as sort the sediment since there is a limit to the size or particles a river can carry. Within a meandering river Links to an external site. we see several different types of sediments, from the pebbles and gravel within the river channel to sandy point bars Links to an external site. along the outer edge of the meander where the water slows. In addition, we see multiple types of sedimentary structures related to the flow of the river as well as those related to flood events.

We also have sediments deposited within lakes, which are called lacustrine Links to an external site. depositional environments. Unlike rivers, lakes do not have rapidly flowing water; thus, there is significantly less movement of sediment and smaller particles. The sediment that accumulates in lakes can come from several sources, including rainwater carrying sediment into the lake from the shores, rivers that flow into the lake, and sediment that is transported by the wind. Once the sediment reaches the lake, it remains undisturbed so we see thin layers of fine sediment, with varying amounts of trace fossils.

Lastly, there are a Links to an external site.eolian Links to an external site. depositional environments, which are dominated by currents of wind rather than water. Since air is less dense than water, only smaller particles can be transported. In addition, wind is not restrained within distinctive channels like water and, therefore, the features of aeolian deposits are more widespread than those of fluvial deposits. Certain areas have predominant wind patterns, such that the wind is fairly consistent in direction and strength, which can generate significant sedimentary structures. Evaporitic Links to an external site. environments are commonly associated in drier climates, where water is only present in the form of seasonal lakes (playa lakes Links to an external site.) that undergo significant evaporation and sometimes leave behind chemical sedimentary rocks (evaporite deposits). See Table 11.3 for an overview of these environments.

Table 11.3: Common characteristics of terrestrial environments. (CC-BY 4.0; Chloe Branciforte, own work)

Environment Terminology	Transport process(es)	Sub-Environments	Sediment types
Glacial	Gravity, moving ice, moving water	Valleys, plains, streams, lakes	Glacial till, gravel, sand, silt, clay
Alluvial	Gravity, intermittent moving water	Steep-sided valleys, talus	Coarse, angular to sub-rounded fragments
Fluvial	Consistent moving water	Streams & rivers	Gravel, sand, silt, organic matter (swampy regions)
Aeolian (Eolian)	Wind	Deserts and coastal regions	Sand, silt
Lacustrine	Moving water (flowing into lake)	Lakes	Sand (near edges), silt, clay, organic matter
Evaporitic	Moving water (flowing into lake)	Lakes in arid regions (playa lake)	Salts, clays

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Transitional (Coastal) Environments

The interface between the continents and oceans are complicated areas that can be influenced by rivers, ocean currents, winds, waves, and tides. The sediments of coastal environments are a mixture of materials derived from the continents (clastic and organic) and those from the ocean (chemical and biochemical).

Shorelines that are influenced by strong daily tidal currents are called tidal mudflat Links to an external site. depositional environments. Tides Links to an external site. are currents that are the result of the gravitational forces exerted by the moon and the rotation of the earth. Shorelines that have strong tidal currents as well as seafloors with low gradients can have large areas that are submerged during high tide and exposed to air during low tide. These areas often have smaller particles than a normal shoreline since the tidal currents can pull fine-grained marine sediments into the area. In addition, the strong bidirectional currents, daily drying out, exposure to the elements, and abundant life create numerous sedimentary indicators of these environments.

Shorelines that are dominated by ocean currents are called beach Links to an external site. depositional environments. Shorelines have constant winds blowing on and off shore that are the result of the difference in the way the land and water heat and cool through the day. These winds produce the waves that are iconic at the beach, but as these waves move onto shore they curve, mimicking the shape of the shore, and result in a longshore current Links to an external site., which runs parallel to the shore itself. This current carries and deposits sand along the beach. In addition, the wind can also produce dunes, which promote a diverse and complicated ecosystem.

Deltaic depositional environments are areas where rivers flow into the ocean, and produce deltas Links to an external site.. As a river that is carrying material empties into the ocean, the water slows and deposits sediment. Most of the sediment is deposited at the mouth of the river with some spilling out into the surrounding areas building a distinctive fan of sediment. Since the sediment is coming from the river, the delta is largely a thick sequence of clastic sediment showing indications of the strong flow of the river. See Table 11.4 for more information regarding these environments.

Table 11.4: Common characteristics of coastal environments. (CC-BY 4.0; Chloe Branciforte, own work)

Environment Terminology	Transport process(es)	Sub-Environments	Sediment types
Tidal	Tidal currents	Tidal flats	Silt, clay
Beach	Waves, longshore currents	Beaches, spits, sand bars, barrier islands	Gravel, sand
Deltaic	Moving water	Deltas	Sand, silt, clay, organic matter (swampy areas only)

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Marine Environments

Marine depositional environments differ in multiple ways, but the resultant rocks are primarily controlled by the proximity and supply of continental sediment, the water depth, and the community of organisms that live in the area. The further an environment is from the shore, the less clastic sediment will be present, and the area will have a higher concentration of the chemical and biological sedimentary rocks that are formed within the ocean. In addition, some organisms in the right environmental conditions can produce huge amounts of skeletal material.

Shallow marine Links to an external site. depositional environments are areas that are close to shore, but always submerged. These areas have a significant amount of mature clastic sediment along with marine algae (like seagrass) as well as skeletal material from animals like coral, echinoderms (sea urchins, sand dollars, sea stars), and mollusks (clams and snails). These areas can have a significant variation in their energy level, from high-energy shallow areas influenced by waves to low-energy deeper areas only influenced by large storms. A better understanding of the relative water depth can often be determined in sedimentary rocks based on their sedimentary structures, in addition to the community of organisms and types of trace fossils found in the rock, since these can be very sensitive to depth.

In warm tropical shallow water, we often find reef Links to an external site. depositional environments. Reefs are formed through the growth of coral colonies building a large three-dimensional structure with their calcite skeletons. Corals can grow in many different marine environments, but they can only produce reefs when their symbiotic algae that live within their tentacles are able to photosynthesize Links to an external site. effectively, resulting in more energy for the coral to grow faster. Reefs also create a barrier between shallow water environments and ocean currents, producing shallow, low-energy environments called lagoons Links to an external site.. These lagoonal environments have thin layers of fine sediment that we would expect in quiet water along with chemical sedimentary deposits that are the result of evaporation.

Most of the ocean consists of deep marine depositional environments. Turbidite flows Links to an external site. are fast-moving currents carrying a variety of sediment sizes down the continental slope Links to an external site. to the abyssal plain Links to an external site. (Figure 11.8). As the current slows, it can no longer carry the largest particles, so these will be deposited first. As the current continues to slow, progressively smaller particles are deposited on top of the bigger particles, forming submarine fans Links to an external site. with graded bedding Links to an external site. (Figure 11.9).

The remainder of the deep ocean contains areas beyond the reach of most clastic sediment other than the dust carried by the wind (red clay deposits Links to an external site.). Therefore, sediment within the deep ocean is primarily produced chemically and biologically. The largest source of sediment in these deep settings is skeletal material from some of the smallest marine organisms (carbonate and siliceous oozes Links to an external site.). Multiple types of single-celled organisms can produce shells composed of either silica or calcite. These shells are mostly produced in the surface waters that are bathed in sunlight permitting photosynthesis. When these organisms die, the shells then rain down into deeper water; this slow accumulation of sediment produces fine layers of biochemical sedimentary rocks. In some cases, these shells are dissolved or altered before they reach the bottom (which can be miles away) and are precipitated as chemical sedimentary rocks. Obviously, there is not a clear boundary between shallow and deep-water environments given the gradient of the ocean floor; however, deep marine depositional environments are normally thousands of feet deep and beyond the influence of even large storms. See Table 11.5 for more information regarding these environments.

Table 11.5: Common characteristics of marine environments. (CC-BY 4.0; Chloe Branciforte, own work)

Environment Terminology	Transport process(es)	Sub-Environments	Sediment types
Shallow marine water	Waves and tidal currents	Continental shelves and slopes, lagoons	Carbonates in tropical climates, sand/silt/clay elsewhere
Reefs	Waves and tidal currents	Reefs and adjacent basins	Carbonates
Lagoonal	Little transportation	Lagoon bottom	Carbonate in tropical climates
Submarine fan	Underwater gravity flows (turbidity currents)	Continental slopes, abyssal plains	Gravel, sand, silt, clay (turbidites)
Deep marine water	Ocean currents	Deep-ocean abyssal plains, trenches	Clay, carbonate mud (carbonate ooze), silica mud (siliceous ooze), red clays

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