Sedimentary Processes Balram Bhadu ONGC Dehradun

Home , Lithification, Sediment transport

1.0 Introduction Sedimentology essentially deals with processes and products of sedimentation. Sediments are formed by disintegration and alteration of pre-existing rocks or by precipitation from solution. Partly particles from volcanism and dust particles of celestial origin also contribute. These sediments are transported to various depositional realms by water, wind or glaciers. Mechanical and chemical processes produce sediments which are lithified as sedimentary rocks by overburden pressure, recrystallisation and cementation. During the process of consolidation from sediments to sedimentary rocks, physical and chemical changes are known as diagenesis or diagenetic changes. This whole process of rock decay, sediment transportation, deposition, precipitation and diagenesis takes place at or near the surface of the earth under normal temperature and pressure conditions contrary to igneous or metamorphic rocks where higher degree of temperature and pressure are involved (Sengupta, 2007).

Fig. 1: Typical rock cycle

Sedimentary rocks cover wide areas on the surface of the earth but they form only about 5% of crustal volume. Average thickness of sediment cover is about 2km on the continental crust and about 1 km on the oceans.

Sedimentary rocks preserve evidences of physical, chemical and biological processes responsible for their formation.

2.0 Processes of sedimentation Processes of sedimentation include physical, chemical and biological processes leading to formation of different types of sedimentary rocks. Main objective is to understand and connect processes and products of sedimentary system.

2.1 Physical processes Sedimentary processes are largely controlled by two natural agencies namely water and wind. Consequently, water and wind cycles are crucial in the process of sedimentation. Physical processes include weathering, erosion, transportation and deposition of material by wind, water and glaciers. Discussion of various physical processes is aimed to understand the physical conditions under which sediments are deposited.

Fig. 2: Physical processes of sedimentation

2.1.1 Weathering Weathering is the breaking down of rocks, soil and minerals through the atmospheric agencies. Weathering occurs in situ and no transportation is involved. Weathering processes include physical and chemical weathering each sometimes involves a biological component. Mechanical or physical weathering is the breakdown of rocks and soils through direct contact with atmospheric conditions such as heat, water, ice and pressure. Chemical weathering involves the direct effect of atmospheric chemicals or biologically produced chemicals also known as biological weathering in the breakdown of rocks, soils and minerals. While physical weathering is accentuated in very cold or very dry environments, chemical reactions are most intense where the climate is wet and hot. However, both types of weathering occur together and each tends to accelerate the other. For example, physical abrasion (rubbing together) decreases the size of particles and therefore increases their surface area, making them more susceptible to rapid chemical reactions. The various agents act in concert to convert primary minerals (feldspars and micas) to secondary minerals (clays and carbonates) and release plant nutrient elements in soluble forms. The materials left over after the rock breaks down combined with organic material creates soil.

Fig. 3: Break down of rock material by weathering Physical weathering  Freeze-thaw: In this type of weathering water percolates along fissures and between the grains of rock. When water freezes, the force of ice crystallization fractures the rock. It is most active in polar climates and most effective during the spring thaw.

 Insolation: This type of weathering occurs where there are large diurnal temperature variations. This is typical of hot arid climates. In desert, the diurnal temperature range in winter is in the order of ~25° C. Rocks expand and contract in response to temperature. As some minerals expand more than others, temperature changes set up differential stresses that eventually cause the rock to crack apart. Because the outer surface of a rock is often warmer or colder than the more protected inner portions, some rocks may weather by exfoliation – the peeling away of outer layers. This process may be sharply accelerated if ice forms in the surface cracks.  Hydration and dehydration: In the regions that experience alternate wet and dry seasons, clays and lightly indurated shales alternatively expand with water and develop shrinkage cracks as they dehydrate. This leads to weakening of physical strength of the rock. Shrinkage cracks increase permeability, thus aiding chemical weathering from rainwater, while waterlogged clays may lead to landslides.  Stress release: Rocks have elastic properties though at varying degree and are compressed at depth by the overburden. With gradual removal of overburden pressure by weathering and erosion, rocks expand and sometimes fracture. Such fracturing is frequently aided by lateral downslope creep. Once stress-release fractures are opened they are susceptible to enlargement by solution from rainwater and other processes.

Chemical weathering Major disintegration of rocks takes place by chemical weathering. This process is more effective in tropical, humid climate where rainy season follows the summer season. Dissolved carbon dioxides ionises rain water into carbonic acid. For this reason rain water is generally acidic with pH ranging from 4 to 7. Acidic water takes natural carbonate into the solution. Humus in the soil also makes limestone unstable and oxidation of organic matter in soil releases CO 2 creating acidic environment with pH as low as 2. - + H2O + CO2= H2CO3 = HCO3 + H Chemical weathering of silicate minerals: In carbonated water, feldspar breaks down into clay minerals, silicic acid and carbonates (Holmes and Holmes, 1978).

6H2O+CO2+2KAlSi3O8= Al2Si2O5(OH)4+4SiO(OH)2+K2CO3 Orthoclase Kaolinite Silicic acid Kaolinite is generally the first mineral to form as a result of chemical weathering. Depending upon the composition of source rock being weathered, smectite, illite, chlorite clay minerals are produced

(Sengupta, 2007). The process of weathering causes considerable changes in the source sediments and degree to which a mineral can resist this change determines its stability.

Fig. 4: Sequence of chemical weathering of different minerals Biological weathering

Biological weathering as such does not contribute much independently but in many cases accelerate and facilitate chemical weathering. A number of plants and animals may create chemical weathering through release of acidic compounds. Mineral weathering can also be initiated and/or accelerated by soil microorganisms. Lichens on rocks are thought to increase chemical weathering rates. The most common forms of biological weathering are the release of organic acids by plants so as to break down aluminium and iron containing compounds in the soils beneath them. Decaying remains of dead plants in soil may form organic acids which, when dissolved in water, cause chemical weathering.

2.1.2 Erosion

Erosion is the process by which soil and rock are removed from one place by exogenic processes such as wind, water and glaciers and then transported and deposited at other places. In

case of weathering, weathered material is not removed from the original place. Erosion is the important component of sedimentary processes. It is the erosion which supplies the sediments for transportation and final deposition.

Fig. 5: Generalised figure showing erosion of rock

Erosion is a natural process and carried out by natural agencies but human activities have increased 10-40 times the rate at which erosion is occurring globally. Excessive erosion causes problems such as increase in desert cover, decreases in agricultural productivity due to land degradation, blocking of waterways, and ecological imbalance due to loss of the nutrient rich soil.

Main physical processes responsible for erosion are-

 Rivers and streams

 Wind

 Glaciers

 Rainfall

 Floods

 Coastal erosion

 Freezing and thawing

 Gravitational

 Exfoliation

Controlling factors of erosion are-

 Rain fall and wind speed

 Strength of water current

 Rock structure and composition

 Topography

 Vegetation cover

Human activities responsible for erosion-

 Deforestation

 Land use pattern

 Climate change

Environmental implications-

 Ecological imbalance

 Land degradation

 Tectonic effects

2.1.3 Transportation and deposition

Sediment transportation is the movement of solid particles i.e. sediments across the earth's surface by water, wind, ice or gravity. It is a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport due to fluid motion occurs in rivers, seas, lakes and other water bodies due to currents and tides. In glaciers sediments flow and on terrestrial surfaces wind is the carrying agent. Sediment transport only due to gravity can occur on sloping surfaces like hill slopes, scarps and cliffs, as well as, on continental shelf- continental slope boundary. Brief mechanisms of sediment transportation and resultant products are narrated here-  Aeolian/ Eolian: In this process sediments are transport by wind and resultant products are ripples and sand dunes. Typically, the size of the transported sediment is fine sand (<1 mm) and smaller, reason that air is a fluid with low density and viscosity and can therefore not exert very

much shear on its bed. Bed forms are generated by aeolian sediment transport in the terrestrial near-surface environment. Aeolian sediment transport is common on beaches and in the arid regions of the world because it is in these environments that vegetation does not prevent the presence and motion of fields of sand. Deposits of fine-grained wind- blown glacial sediment are called loess.

Fig. 6: Sand dunes (Eolian deposits) Wind-blown very fine-grained dust is capable of entering the upper atmosphere and moving across the globe. In India, in the state of Rajasthan, Thar Desert is the typical example of eolian deposits.  Fluvial: In this process, sediments are transported by rivers. This is the most important process of sediment transportation. Sediment moved by water can be larger than sediments moved by air because water has both a higher density and viscosity. In the rivers sediments as fine as clay to as large as cobbles and boulders are transported, depending upon the intensity of river current and gradient. Sediment transportation by rivers results in the formation of various sedimentary structures, flood plains and deltas. Once sediment grains are put into motion (above the critical erosional velocity) they are transported downstream with the flow. The mode in which grains are transported depends upon energy of flow and the grain size (Reineck and Singh, 1980). In this process, three types of mode of transportation are distinguished-

a. Bed load: Sediment grains under transportation are in contact with the sediment beds and move by traction etc. Grains move by traction (rolling or sliding along the bottom) if settling velocity is larger than shear velocity of the flow. b. Suspended load: Grains move above the sediment bed but intermittently changed with the bed load. Transportation is either by suspension or saltation (grain movement by bouncing or hopping). When shear velocity is equal or higher than the settling velocity sediment grains is moved in suspension. c. Wash load: Fine grained particles moved in suspension till final deposition.

Fig. 7: Fluvial environment showing river channel and flood plain

Fig. 8: Sandstone deposited in fluvial environment with cross bedding  Coastal: Coastal sediment transport takes place in near-shore environments due to the motions of waves and currents. At the mouths of rivers, coastal sediment and fluvial sediment transport processes interlock to create deltas. It also forms characteristic landforms such as beaches, barrier islands. Waves and currents also distribute sediments in marine depositional realms.

Fig. 9: Sediment transportation at coastal environment

 Glacial: As glaciers move over their beds, they entrain and move material of all sizes. Glaciers can carry the largest sediment and areas of glacial deposition often contain a large number of glacial erratic many of which are several metres in diameter. Glaciers also pulverize rock into fine pieces, often carried away by winds to create ‘loess’ deposits thousands of kilometres afield. Sediment entrained in glaciers often moves approximately along the glacial flow lines causing it to appear at the surface in the ablation zone.  Debris flow: Large masses of material are transported as debris flow comprising mixtures of mud, clasts that range up to boulder-size, and water. Debris flows move as granular flows down steep mountain valleys and washes. Because they transport sediment as a granular mixture, their transport mechanisms and capacities scale differently than those of fluvial systems. Similar mechanism is also applicable in deep water depositional systems.  Hill slopes: In hill slope sediment transport, a variety of processes move regolith downslope. - Soil creep - Tree throw - Movement of soil by burrowing animals - Slumping and land sliding of the hill slope

2.2 Chemical processes In the physical processes of sedimentation weathering, erosion, transportation and deposition of material is involved. In the chemical processes of sedimentation, sediments are precipitated from the solution i.e. chemical changes take place in solutions. The higher content of dissolved carbon dioxide in sea water helps in dissolution of calcium

carbonate. The reduction in CO2 would generate precipitation. The processes that reduces CO2 from normal water (pH = 8.4) tend to change the bicarbonate ions to carbonate ions and encourage lime precipitation. The Ca++ ions present in sea water, react with the bicarbonate

ions giving rise to CaCO3 with expulsion of CO2. ++ - Ca + 2HCO3 <-> CaCO3 + CO2 + H2O

In other words, Carbon dioxide dissolved in water produces carbonic acid, which in turn dissolves calcium carbonate (either aragonite or calcite). However, when bicarbonate and calcium are in sufficient supply, calcium carbonate can precipitate. Limestone are common and widespread sedimentary rock formed by chemical process in shallow marine depositional environments. Most of the calcium carbonate that makes up limestone comes from biological sources, ranging from the hard, shelly parts of invertebrates such as molluscs to very fine particles of calcite and aragonite formed by algae. The accumulation of sediment in carbonate-forming environments is largely controlled by factors that influence the types and abundances of organisms that live. Water depth, temperature, salinity, nutrient availability and the supply of terrigenous clastic material all influence carbonate deposition and the buildup of successions of limestone. Some depositional environments are created by organisms, for example, reefs built up by sedentary colonial organisms such as corals. Changes in biota through geological time have also played an important role in determining the characteristics of shallow-marine sediments through the stratigraphic record. In arid settings carbonate sedimentation may be associated with evaporite successions formed by the chemical precipitation of gypsum, anhydrite and halite from the evaporation of seawater. Dolomite, chert, phosphorites are some of the other rocks formed by chemical processes (Nichols, 2009).

2.3 Biological processes

Many reactions in the sediments are biochemical i.e. organisms drive these reactions. Important activities in the biological processes are-

a. Secretion of calcium carbonate skeletons by organisms: Many living organisms deposit

calcium carbonate (CaCO3) as external skeletons to protect and support their soft parts through their metabolic activities. Such organisms are more common in shallow seas and tropical areas. Skeleton of organisms may consist of aragonite and/ or calcite of high- magnesian or low- magnesian variety. Secretion of particular chemical variety depends upon the growth rate, presence of certain inorganic or organic compounds and temperature.

b. Degradation of calcium carbonate skeletons into skeletal debris: Alteration of calcium carbonate skeletons result into formation of skeletal debris, lime sand and lime mud. Broadly predators are responsible for creating sedimentary debris out of skeletal

materials. Like in case of corals, more than 90% sand sized skeletal debris created by degradation of reefs. c. Trapping and baffling by organisms: Trapping and baffling carried out by organismsof lime mud create deposits like mounds, beds, laminae etc. In shallow water, trapping is most commonly evident by filamentous blue green algae. These algae trap the fine grained extraneous particles like lime mud. Laminated lithified deposits consisting of alternating layers of material formed by microorganisms, usually of algae, are known as stromatolite. Stromatolites are common in geologically older rocks like that of Pre- cambrian.

Fig. 10: Cambrian stramatolite d. Pelletisation: Deposit feeding organisms pelletise soft lime mud and excrete as sand size particles. These pellets are usually more organic material rich. e. Burrowing and stirring: Several organisms like worms, molluscs, crustaceans and insects burrow into sediments and find food and shelter. Burrowing activities are very extensive in nature and commonly found in marine, as well as, non-marine environments. Burrowing accelerates weathering and destroys sedimentary structures but gives important clues about the paleoenvironmental conditions. f. Effects of microorganisms: Activities of microorganisms that drive various chemical reactions leading to precipitation of calcite, sulphur and pyrite (Friedman and Sanders, 1978).

3.0 Conclusion In the study of sedimentology, sedimentary processes are important aspect to understand. Before studying the sedimentary rocks and their properties, processes involved in the ir formation provide an insight to the nature of final products. Sedimentary rocks are formed by physical, chemical and biological processes. These different processes may act independently or in combination of all at the given time. Physical processes are more dominant in the formation of clastic rocks but at the time of final conversion of sediments to sedimentary rocks chemical processes play a vital role. Processes involved in the conversion of sediments to sedimentary rock (lithification) like diagenesis, dissolution, compaction, pressure solution are operative at different stage and discussed in separate module of study. Similarly, chemical and biological processes play together in the formation of non-clastic rocks, as well as, in clastic rocks at different stage.