CHAPTER 3.2: SEDIMENTARY ROCK

Transcription

1 CHAPTER 3.2: SEDIMENTARY ROCK

2 Introduction Second major rock group. Formed from fine constituents of rock usually from mountainous areas which are transported to lower elevation due to certain processes. After traveling at some distance - get deposited over some existing rocks which on consolidation - result in formation of sedimentary rocks.

3 Origin Involves four major processes which are: (a) Weathering (b) Transportation (c) Deposition (d) Lithification (Compaction and Cementation)

4 Weathering Weathering - number of chemical and mechanical processes that act to break up rocks such as an interaction between rocks exposed at the Earth's surface and elements in the atmosphere. Chemical Weathering - produced mineral of increased volume. E.g. Oxidation, hydration, hydrolysis and dissolution. Mechanical Weathering - breakdown of the rocks into smaller fragments without undergoing a change in chemical composition. E.g. frost action, alternate heating and cooling organic activities, etc. The preexisting rocks can disintegrate and decompose either by physically or chemically and forms layer of loose, decayed rock debris or soil. The unconsolidated material can then be transported easily by various agents such as streams, wind, groundwater and glaciers. For example, once surface rocks have been broken up into fragments by weathering processes, erosion (by wind and moving water) can transport the detrital material away from its source region to a new location where these new sediments can be deposited.

5 Transportation Running water - most effective form of sediment transport. Large quantities of sediment carried towards the sea and deltas are formed from sediment transported by rivers. Wind and glaciers also transport sediment although restricted to certain climatic zones. Sorting that occurs during transportation is an important factor in the genesis of sedimentary rock. Water and air are fluids, thus the size of detrital material that can be transported depends on the velocity (i.e. energy) of the fluid. In other words, rapidly moving water or air can transport larger grain size detrital material than more slowly moving water or air. Ice, on the other hand, is a solid. Thus, ice can transport all sizes of sediment independent of the velocity at which the ice is moving. In the case of transport by water or air, sediments are deposited at locations where the velocity of the fluid decreases. For example, consider a river flowing out of the mountains into a lake.

6 Deposition Deposition - process due to settlement of sediments and loose aggregates. The most significant factor in the origin of sedimentary rocks is the environment that exists where the sediment is deposited. The depositional environment determines the characteristics of sedimentary rock formed (physical, chemical and biological condition). E.g. type of transporting agent, geochemical parameters such as pressure, oxygen, temperature, and flow characteristics of depositing fluid (velocity). Distinctive types of texture, composition, internal structure, and fossil assemblages are thus developed in each deposition.

7 Lithification (Compaction and Cementation) Process of converting unconsolidated sediments into sedimentary rocks is called lithification (Compaction and Cementation). Compaction is the process whereby loose sediments are compacted to a denser state by additional stress from accumulated material deposited from time to time or even tectonic forces. The process of expulsion of water from void spaces between particles takes place as they are forced closer together. Clayey-rich sediments can be compacted much better than sands. Cementation is an important process that transforms sediment into solid rock. The process takes place by filling the voids in pore spaces by chemical precipitation. These pore spaces are gradually filled by precipitation from groundwater. This is the most effective lithification process due to the chemical cement that bonds the particles together. Most commonly cementing minerals are: (a) Silica (SiO 2 ) - Rocks cemented by silica are among the hardest and strongest. (b) Calcium Carbonate (CaCO 3 ) (c) Limonite (d) Iron Oxide

8 This whole process of forming Sedimentary Rocks (known as DIAGENESIS)

9 Classification of Sedimentary Rocks Sedimentary rocks are classified according to the derivation of sediment types. The two major sediment types are: (a) Detrital or Clastic Sedimentary Rock (b) Chemical Sedimentary Rocks

10 Detrital or Clastic Sedimentary Rocks Clastic or detrital sedimentary rocks are made up of mineral grains, fragments of other rocks (called lithic fragments), shells and other inorganic (hard) of formerly living organisms. The clastic particles or grains in a sedimentary rock are cemented together by mineral precipitates that form during the process of diagenesis. Also known as fragmental sedimentary rocks that are derived from weathering process of parent rocks. The texture of clastic sediments consists of a fragment which varies in shapes and sizes. e.g. range of various sandstones with different grain sizes.

11 Cont d Clastic sedimentary rocks are classified according to their texture (grain size). Rocks with gravel size detrital grains (Grain size greater than 2 mm) are called conglomerates (if the grains have rounded outlines) or they are called breccias (if the grains are angular in shape). Rocks made up of sand size grains (Grain size 1/16 to 2 mm) are called sandstones. For sandstone, if it was dominated by quartz grains it will be quartz sandstone (also called quartz arenite), if it was dominated by feldspar grains it will be arkose and if dominated by sand - sized rock fragment grains it will be lithic sandstone (also called litharenite or graywacke). Silt is called siltstone (Grain size 1/256 to 1/16 mm (gritty). Clay with grain size less than 1/256 mm (smooth) are called shale (if fissile) or claystone (if massive). Mud is technically a mixture of silt and clay. It forms a rock called mudstone or mud shale if fissile. They have a clastic (broken or fragmental) texture consisting of: (a) Clasts - larger pieces, such as sand or gravel (b) Matrix - mud or fine-grained sediment surrounding the clasts (c) Cement - the glue that holds it all together, such as: calcite, iron oxide and silica

12 Example of Detrital or Clastic Sedimentary Rocks Breccia Conglomerate Quartz Sandstone Arkose Sandstone Greywacke Sandstone Siltstone Shale (fissile) Claystone (massive)

13 Clasts and matrix (labeled) and iron oxide cement (reddish brown color)

14 Types of Sediment and Clastic Sedimentary Rocks Types of Sediment Gravel Sands Silts Clays or Muds Clastic Sedimentary Rock Conglomerate or Breccia Sandstone Siltstone Mudstone or Shale

15 Conglomerate Consist of consolidated deposits of gravel, with variable amounts of sand and mud in the spaces between the larger grains. Cobbles and pebbles usually are well rounded fragments over 2 mm in diameter. Conglomerates are accumulated at base of many mountain ranges, in stream channels and on beaches.

16 Sandstone Sandstones is most familiar sedimentary rock with sand size ranging from mm to 2 mm in diameter. Composed of almost any material thus can be various colors. Quartz grains are usually the most abundant mineral because quartz is a common constituent in many other rock types, not easily broken down by abrasion and chemical action. The particles of sand in most sandstones are cemented by calcite, cilica or iron oxide.

17 Siltstone Siltstone is fine-grained in which the material is mm to mm in diameter (finer than sand but coarser than mud). Silt is a material frequently carried in suspension by rivers and deposited in floodplains and deltas.

18 Shales (Mudstone) Shales (Mudstone) are solidified deposits of mud and clay. The particles that make up the rock are less than mm in diameter and can be seen under microscope. Shale is most abundant in sedimentary rock, usually soft and easily weathered.

19 Chemical Sedimentary Rocks Chemical sedimentary rocks are precipitated from a solution as a result of changing physical conditions or due to the actions of living organisms. The chemical weathering of rocks also lead to the formation of sediments as dissolved matter in solutions. Such sediments are usually identified by their chemical composition. Common dissolved sediments include the bicarbonates of Ca, Mg, Na, and K with calcium and magnesium. Other sediments include dissolved silica in the form of Si(OH) 4, sulphates and chlorides of Na, Mg, Ca, and K. These sedimentary ultimately form sedimentary deposits and sedimentary rocks by two distinct processes known as Organic Sedimentary Rocks and Inorganic Chemical Sedimentary Rocks.

20 Organic Sedimentary Rocks This group consists of rocks composed of organic matter (mainly plant fragments). The most common example is coal, the compacted remains of dead plants that grew in a tropical swamp environment. It composed of accumulations of organic debris.

21 Inorganic Chemical Sedimentary Rocks In organic chemical sediments form by direct precipitation from solution. Typical solutions that chemical sediments form from include: sea water, fresh surface water in rivers and lakes and groundwater. One common class of inorganic chemical sedimentary rocks is called evaporites. Evaporites form by precipitation from sea water or brackish fresh water. The scenario for the formation of an evaporite requires that a batch of sea water becomes isolated from input of additional sea water, for example in a lagoon. The isolated sea water then begins to evaporate, which concentrates the dissolved salts and other components of sea water. As evaporation proceeds, various minerals will be precipitated from the water. Minerals that are formed in this way and are found in evaporites are listed in the Table 3.8. This group includes the evaporites, the carbonates (limestones and dolostone), and the siliceous rocks.

22 Cont d These rocks form within the depositional basin from chemical components dissolved in the seawater. These chemicals may be removed from seawater and made into rocks by chemical processes, or with the assistance of biological processes (such as shell growth). In some cases it is difficult to sort the two out (in carbonates or some siliceous rocks, for example), so they are grouped together as chemical/biochemical.

23 Evaporation and precipitation forming salt deposits Table 3.8 Inorganic Chemical Sedimentary Rock Rock Type Limestone Dolomite Chert/Flint Gypsum Rock Salt Composition CaCO 3 CaMg (CO 3 ) 2 SiO 2 CaSO 4.2H 2 O NaCI

24 Cont d Evaporites - The evaporites form from the evaporation of water (usually seawater) such as: Rock salt - composed of halite (NaCl) Rock gypsum - composed of gypsum (CaSO 4.2H 2 O) Travertine - composed of calcium carbonate (CaCO 3 ), and therefore, also technically a carbonate rock; travertine forms in caves and around hot springs. Carbonates The carbonate sedimentary rocks are formed through both chemical and biochemical processes. They include the limestone (many types) and dolostones. Two minerals are dominant in carbonate rocks: 1. Calcite (CaCO 3 ) 2. Dolomite (CaMg(CO 3 ) 2 )

25 Carbonate rock names Micrite (microcrystalline limestone) - very fine-grained; may be light gray or tan to nearly black in color. Made of lime mud, which is also called calcilutite. Oolitic limestone (look for the sand-sized oolites) Fossiliferous limestone (look for various types of fossils in a limestone matrix) Coquina (fossil hash cemented together; may resemble granola) Chalk (made of microscopic planktonic organisms such as coccolithophores; fizzes readily in acid) Crystalline limestone Travertine (see evaporites) Others - intraclastic limestone, pelleted limestone

26 Siliceous rocks The siliceous rocks are those which are dominated by silica (SiO 2 ). They commonly form from silica - secreting organisms such as diatoms, radiolarians, or some types of sponges. Chert is formed through chemical reactions of silica in solution replacing limestone. Diatomite - looks like chalk, but does not fizz in acid. Made of microscopic planktonic organisms called diatoms. May also resemble kaolinite, but is much lower in density and more porous). Also referred to as Diatomaceous Earth. Chert - Massive and hard, microcrystalline quartz. May be dark or light in color. Often replaces limestone. Does not fizz in acid.

27 Example of Sedimentary Rocks Limestone Dolostone Rock Salt and Gypsum

28 Limestone Most abundant non clastic rock (chemical). Composed principally of mineral calcite, calcium carbonate (CaCO 3 ) and originates both chemical and organic processes. Many plants and invertebrate animals extract calcium carbonate (limestone) from water in their life processes and use it to construct their shells and hard parts of calcite. When these organisms die, their shells build up deposit of limestone with a texture consisting of shells and shell fragments. In quiet water, calcium carbonate is precipated as tiny needlelike crystals, which accumulate on the bottom. Soon after they are deposited, the grains commonly are modified as they are compacted and become recrystallised. This modification produces microcrystalline limestone, a rock with a dense, very fine grained texture. Its individual crystals can be seen under high magnification. Microcrystalline limestone also is precipitated from springs and from the dripping water in caves.

29 Limestone

30 Dolostone Rock composed of mineral dolomite, a calcium magnesium carbonate (CaMg(CO 3 ) 2. It is similar to limestone in most textural and structural features arid appearance. Can develop by direct precipitation from seawater.

31 Rock Salt and Gypsum Rock salt is composed of mineral halite (NaCI). It forms by evaporation in saline lakes. Gypsum CaSO 4 2H 2 O also originates from evaporation of saline water.

32 Characteristics of Sedimentary Rocks It exhibits variety of distinctive features such as: (1) Texture (2) Sedimentary Structures (3) Fossils (4) Stratigraphy (5) Color (6) Ripple Marks and Mud Cracks

33 Texture Sedimentary rocks are easily recognizable by its texture, i.e. the arrangement of particles or grains that make up the rock. There are two main types of texture of sedimentary rocks: (1) Clastic: These rocks are composed of aggregates of individual mineral or rock fragments. The origin of these types of rocks can be described as detrital since the rock fragments have been eroded, transported and deposited. Detrital rocks with particles larger than sand size are called conglomerate or breccia. Fine grained clastic rocks are called shale or mudstone. Usually shale is easily split into thin slabs parallel to the depositional layering of the sediment. (2) Non Clastic: These rocks are formed by chemical precipitation from aqueous solutions. Both rocks formed either as inorganic and organic process has non clastic texture. E.g. Secretion from organisms that composed of silica or calcium carbonate. However, if a rock is composed of an accumulation of shells or fragments of shells, its texture is considered to be clastic. The formations of chemical sedimentary rocks are usually associated with chemical precipitation from water that is responsible for deposits of limestone, dolomite, salt and gypsum. This process is common in sea water but also occurs in lakes, streams, caves (groundwater) and springs. The most common type of limestone is those of marine deposits.

34 Sedimentary Structures These are characteristic features imparted to the rock during the processes of sediment transport and deposition. The most fundamental sedimentary structure is bedding, i.e. the stacking of sediments in individual layers or beds. Differences in the process of sedimentation can lead to different types of bedding. Three main types of bedding are: (1) Rhythmic Layering (2) Cross bedding (3) Graded bedding

35 Rhythmic Layering Alternating parallel layers having different properties. Sometimes caused by seasonal changes in deposition (Varves). i.e. lake deposits where the coarse sediment is deposited in summer months and fine sediment is deposited in the winter when the surface of the lake is frozen.

36 Cross Bedding Sets of beds that are inclined relative to one another. The beds are inclined in the direction that the wind or water was moving at the time of deposition. Boundaries between sets of cross beds usually represent an erosional surface. Very common in beach deposits, sand dunes, and river deposited sediment.

37 Graded bedding As current velocity decreases, first the larger or more dense particles are deposited followed by smaller particles. This results in bedding showing a decrease in grain size from the bottom of the bed to the top of the bed.

38 Fossils Fossils are remains or evidence of ancient plants and animals that have been preserved in earth's crust, e.g. shell (usually composed of calcium carbonate or silica) and bones (phospatic material) can be preserved for long periods of time. Soft organic remains can be preserved in anaerobic environments although they may be greatly altered from their original state. These environments account for deposits of petroleum, which are derived from the remains of microscopic marine organisms, and coal, the product of terrestrial plants. Fossils which are not necessarily of organic origin can also be sedimented in the rocks, for example petrified wood which composed of silica that has been precipitated by groundwater which replaces the organic material and retaining the original cellular structure.

39 Fossils

40 Stratigraphy Stratigraphy refers to strata (layers) constructed in sedimentary rocks due to the ancient landforms and depositional environments. Usually each stratum is separated by bedding plane and the thickness of the rock and its texture depends on the carrying agent, e.g. wind, water or ice. Geologic maps and cross sections constructed from stratigraphic studies are useful in enabling the prediction of the sequence of rocks that may underlie a particular section. Example laboratory session for mapping (Lab 2).

41 Color The color often indicates the geochemical environment at the time of deposition. Rocks formed in an environment abundant of oxygen are usually in shades of red or brown. The reddish or yellowish color is imparted by the small amount of ferric iron in an oxidized state. Sediments that accumulated in environments lacking of oxygen are usually darker in color, e.g. somber grey and green shades. Petroleum and coal are good examples of sediments rich in organic matter and usually the color will be black. The lack of oxygen is critical to the preservation of these materials because bacteria will decompose the organic matter if oxygen supply exists.

42 Ripple Marks and Mud Cracks Ripples are undulations of the sediment surface produced as wind or water moves across sand. Ripple marks or little waves are usually found at surface of beach, sand dunes or bottom of stream. Ripples which form in unidirectional currents (such as in streams or rivers) tend to be asymmetrical. Crests of asymmetrical ripples may be straight, sinuous, or lobe-like, depending on water velocity. Asymmetrical ripples have a steep slope on the downstream side, and a gentle slope on the upstream side. Because of this unique geometry, its provide information of the condition under which the sedimentary rocks was originally deposited. Mud cracks are also commonly preserved in sedimentary rock and show that the sedimentary environment occasionally was exposed to the air during the time period the sedimentary was deposited. Mud cracks in rocks suggest that the original sediment was deposited in shallow lakes on tidal flats and act as exposed stream banks.

43 Ripple Marks

44 Mud Cracks

45 Sedimentary Rocks in Engineering Works The most abundant rock type over the continents. Well cemented and horizontally bedded sandstones and limestones are suitable and stable for excavation of vertical slopes while weaker rocks are best cut for flatter slopes. Bedding planes are the plane of weakness for sedimentary rocks and the understanding of the direction dipping and amount of slope of these planes are of vital importance to ensure stability. Failure usually occurs at this zone. The strength of sedimentary rocks can be increased by compaction and cementation and are influenced by geologic events that take place long after deposition. These rocks can also be affected by weathering especially when the cementing agent and clay minerals in shales are weakened when exposed to air and water. Rocks such as limestones and evaporate deposits which are soluble under the action of groundwater will pose great danger. Cavities may be formed by chemical dissolution of soils and rocks overlie it may be collapse into the voids and thus destroying the buildings constructed above it. These phenomenon can also be a problem in dams and reservoirs where leakage can occur along bedding planes or through solution cavities in the rock. Massive limestone area in Malaysia can be found in Ipoh, Perak

46 End of the Chapter 3.2 Q & A