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Sedimentary Rocks


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Sedimentary Rocks


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Sedimentary Rocks

  1. 1. Sedimentary Rocks Unit-III
  2. 2. Syllabus Sedimentary Rocks • Introduction, Classification of Sedimentary • Rocks, Common Structures and Textures of Sedimentary Rocks, Descriptive Study of Common Sedimentary Rocks Descriptive Study of Common Sedimentary Rocks (Sandstone, Limestone, Shale, Conglomerate, and Breccias)
  3. 3. Introduction • Sedimentary rocks are those which have formed out of sediments. • Sediments are rock fragments which are product of weathering. Weathering has already been defined as natural processes of disintegration (i.e.. mechanical breakdown) and decomposition (i.e.. chemical decay) of rocks. • Sediments which have formed out of disintegration are loose materials of various sizes like clay, sand and pebbles. Since sediments represent secondary or derived materials from the pre-existing rocks, rocks formed out of them are also called secondary rocks.
  4. 4. Sedimentary rocks on the earth’s crust • The earth is nearly 8000 miles in diameter. The top portion of the earth is called the crust. The crust being less than 40 miles is too thin compared to the size of the earth as a whole.
  5. 5. Sedimentary rocks on the earth’s crust • This crust, is mainly made up of sima and sial. Sima is the underlying portion of the ocean floor and Sial is the overlying granitic portion forming the continental part. • Above these, lies a thin veneer of sedimentary rocks covering large tracts of the surface. This sedimentary layer is discontinuous and of varying thickness. • Quantitatively, sedimentary and related rocks together constitutes only 5 % of the earth’s crust, the remaining 95 % being igneous rocks and their metamorphic equivalents.
  6. 6. Sedimentary rocks on the earth’s crust
  7. 7. Sedimentary rocks on the earth’s crust • Among different sedimentary rocks, shale is the most abundant; sandstone and limestone are of next order. • The three represents approximately 4 %, 0.75 % and 0.25 % of the earth’s crust. Since most of the sedimentary rocks in nature occur as beds, these are also called stratified rocks. Essentially, sedimentary deposits are aqueous, i.e.. formed in the bodies of water such as river, lakes and seas.
  8. 8. Sedimentary rocks on the earth’s crust
  9. 9. Classification of Sedimentary Rock • The classification of sedimentary rock is closely related to their origin. Broadly, they are classified into detrital and non-detrital rocks. Detrital rocks, which are popularly called clastic rocks, are formed out of physically broken and transported rock fragments. Coarser rock fragments are cemented by a finer matrix and give rise to reduaceous and arenaceous rocks. Conglomerates, breccias, tiilites and sandstones represent these kinds of rocks. Still finer sediments like silt and clay mainly undergo compaction and become hard mudstones and shales.
  10. 10. Detrital Rocks
  11. 11. Detrital Rocks
  12. 12. Classification of Sedimentary Rock • Non-detrimental rocks or non—clastic rocks are formed either by precipitation, evaporation or by accumulation of hard parts of plants or animals. Coals, various types of limestone, spring deposits, flint, salt beds, etc, are formed this way.
  13. 13. Classification of Sedimentary Rock
  14. 14. Classification of Sedimentary Rock • Any rock, when subjected to alternating dry and wet climates over a considerable length of time, undergoes through decay and decomposition. This ultimately leaves behind chemically inert and insoluble residual matter. This makes up a group of sedimentary rock known as residual rocks. Usually these rocks are dull looking, porous, spongy and represent a mixture of oxides of ferric iron, manganese, silicon, titanium and hydroxides of aluminum. • Laterite, bauxite and terra rossa and soils are of residual origin.
  15. 15. Residual Rocks.
  16. 16. Residual Rocks.
  17. 17. Classification of Sedimentary Rock • Due to intense and prolonged decomposition of rocks, soluble matter is leached and transported in solution form by surface water or ground water. Due to prolonged disintegration, rocks are broken down into smaller and smaller particles. When large bodies of rocks are reduced to a convenient size in this manner, they are transported mechanically. • Thus, rivers and streams transport rock matter both physically and chemically, of these, mechanically transported sediments undergo deposition whenever favorable condition occur. These are subsequently cemented or compacted to give rise to another group of secondary rocks called detrital rocks. These comprises the most important varieties such as conglomerates, some breccias, sandstones and shales.
  18. 18. Sandstones and Shales.
  19. 19. Classification of Sedimentary Rock • The rest of the river-transported matter, carries as solution, is also deposited sometimes, under favorable condition due to chemical processes like precipitation and evaporation. A large varieties of limestone, dolomites, salt deposits, are typical example of this kind are called chemical deposits. • Yet another group of sedimentary deposits are formed due to the accumulation of hard parts of organisms. Under favorable conditions, a part of the dissolved matter is extracted by growing plants and trees and utilized as their food materials. These plants and trees may subsequently form coal deposits. The remaining dissolved material ultimately reach the sea or ocean and get extracted by deposits.
  20. 20. Chemical Deposits
  21. 21. Classification of Sedimentary Rock • Such a group of sedimentary deposits which are formed out of active involvement of plants and other organisms are called “ organic deposits”. • Sedimentary types of deposits are the most abundant, and account for nearly 95 % of all the types put together.
  22. 22. Organic Deposits
  23. 23. Residual Deposits • The residual deposits are the insoluble products of rock weathering which have escaped erosion by geological agents like wind and rivers and which still cover the rocks from which they have been derived, i.e.. they are in situ. • Compositionally, they are made up of unaltered minerals and insoluble products of decomposition of other minerals of the original rock. The insoluble products are usually quartz and muscovite. • Terra Rossa • Laterite • Soils are the chief varieties of residual deposits.
  24. 24. Residual Deposits Terra Rossa • This is reddish clayey soil covering limestone in dry regions. It is formed when argillaceous, limestones are dissolved, insoluble clay content along with other mineral matter is left behind as residual (terra rossa), while calcium carbonate content is carried away in solution form.
  25. 25. Terra Rossa
  26. 26. Residual Deposits Laterite • The term laterite is derived from the greek word later meaning brick laterite was used as a substitute for brick in many places. • When freshly quarried laterite is very soft, wet and can be cut with a knife, but it hardens on drying. • Laterite is formed from a variety of rocks under tropical weathering. • Usually, it shows patches of different color like yellow, brown, red, black and white. It is often porous, concretionary and earthy in appearance.
  27. 27. Laterite
  28. 28. Residual Deposits • As building material, laterite is used as a substitute for brick, in places where it is available, because it can be cut to the required size easily. • As it naturally occurs on the surface, it is very cheap. But as it is not strong, it can be utilized only for small or poor quality works and where heavy loads are not involved. • As it crumbles easily, it cannot makes a good road metal also. Thus laterite is not an important building material, generally.
  29. 29. laterite
  30. 30. Residual Deposits Soil • Soil is the loose porous material made up of sediments (i.e.. rock debris which is inorganic mineral matter) and organic (humus) matter. The associated voids are filled by air or moisture. In soils, generally, sediments are more in proportion than organic matter.
  31. 31. Soil
  32. 32. Residual Deposits Types of soils • Soils are geologically classified into insitu soils and drift soils. • Insitu soils are genetically related to underlying rocks and they are derived from them. Insitu soils are infertile as they have undergone leaching. These soils are also called residual soils or sedimentary soils.
  33. 33. In-situ Soils
  34. 34. Residual Deposits • The drift soils, on the other hand, had formed at some place but occur elsewhere after undergoing considerable transport. • As they occur stratified, they have uniform thickness and by virtue of their mode of formation, these soils are fertile. Such soils are also called as transported soils.
  35. 35. Residual Deposits
  36. 36. Residual Deposits • A soil is also classified on the basis of its dominating constituent i.e.. sand, silt, clay, lime or humus. • Accordingly there are sandy soils, loamy soils (sand + silt or clay), marl (clay or silt + lime), silty soils, clayey soils, calcareous soils, peaty soils, etc. Of these , sandy and loamy soils are light, dry, friable, porous and are devoid of soluble matter. • On the other hard clayey soils are dense, impermeable, wet and possess a considerable amount of soluble matter.
  37. 37. Residual Deposits
  38. 38. Residual Deposits Detrital Rocks • By virtue of their frequency and abundance of occurrence, detrital rocks are treated synonymous to sedimentary rocks and hence these are very important. As already stated, these rocks are the clastic sedimentary rocks and represent those which have been formed from rock fragments, due to geological agencies like wind, river and glaciers. Conglomerates, breccias, sandstones and shales belong to this group.
  39. 39. Detrital Rocks
  40. 40. Residual Deposits Chemically formed Rocks (Chemical Deposits) • During weathering of rocks, some of the soluble constituents are leached and carried away in the form of solution. Such dissolved matter comes out as solid material subsequently, either due to precipitation or evaporation. Which are physicochemical processes. The resulting rocks or deposits are called chemical deposits. • Limestone is a common example.
  41. 41. Chemically formed Rocks (Limestone)
  42. 42. Residual Deposits • Precipitation produces very fine material. So, rock formed by this process, like some limestone, are very fine grained. On the other hand, products of evaporation are large crystal or coarsed grained. Salt, calcite and gypsum are typical example are typical example of this kind. • Concretions, secretions, colloids, some siliceous deposits and limestone, dolomites, bog iron ore and different types of salt deposits are representative of this category.
  43. 43. Residual Deposits
  44. 44. Organically Formed Rocks ( Organic Deposits) • Such rocks are sedimentary deposits which have formed with the active involvement of plants and animals. • If these are mainly due to plants, they are called phytogenic and if they are mainly due to animals they are called zoogenic. • Compositionally, the following five types of organic deposits occur in nature: • Calcareous type • Phosphatic type • Ferruginous type • Siliceous type • Carbonaceous type.
  45. 45. Organically Formed Rocks ( Organic Deposits)
  46. 46. Common Structures and Textures of Sedimentary Rocks Stratification • Sedimentary rocks have the general character of occurring in the form of series of beds. This phenomenon is known as “ Stratification” or “Bedding”. When the thickness of individual beds is very small it is called “lamination”. • This is an inherent, primary character of sedimentary rocks is not a continuous process. But occur in stages i.e.. deposition of sediments ceases temporarily and then, after some time, the deposition is resumed. • This short break in sedimentation is represented by the bedding plane. Bedding plane is the plane of contract between any two adjacent beds of strata.
  47. 47. Stratification
  48. 48. Common Structures and Textures of Sedimentary Rocks Cementation • Conglomerates, breccias and sandstones are the important sedimentary rocks which are formed mainly due to cementation, In these, finer material act as the binding medium in between the coarser constituents. • This cementing material is usually secondary silica, calcium carbonate, clay or ferruginous material. Gypsum and barytes also occur rarely as cementing materials. In case of conglomerates, often the cementing material is heterogeneous, though silica, fragments of quartz, occur often in the matrix.
  49. 49. Cementation
  50. 50. Common Structures and Textures of Sedimentary Rocks Fossil Occurrence • Fossils have been defined as “ relicts and remnants of ancient plants and animals preserved inside the rock by natural processes”. • Remains of plants and animals when they get buried under sediments, become fossilized. Of course, all buried creatures or plant materials do not become fossils. Only a few with hard parts become fossils under favorable conditions. In most of the cases, the soft part of buried organisms or plants get decayed and disappear without becoming fossils. • Thus fossils can occur only in sedimentary rocks and remain in them as their integral part.
  51. 51. Common Structures and Textures of Sedimentary Rocks
  52. 52. Common Structures and Textures of Sedimentary Rocks Ripple Marks • In Stagnant and shallow water bodies, the waves and current on the surface of water produce sympathetic impressions in the form of minor undulations on the loose and soft sediments which lie at the bottom. These are known as ripple marks and develop under shallow water condition. Some sandstones, limestones, shales, etc. Show this character. As can be seen from its mode of formation ripple marks can be expected to occur only in sedimentary rocks. The igneous rocks and metamorphic rocks cannot have this features. • Ripple marks are of two types symmetrical and asymmetrical . Symmetrical ripple marks are formed due to wave action and asymmetrical ripple marks are formed due to current action.
  53. 53. Ripple Marks
  54. 54. Common Structures and Textures of Sedimentary Rocks Mud Cracks or Sun Cracks • Water of a sea or lake, depending on different conditions (like tide effect and season effect) covers the gentle sloping sides on and off. When such a wet surface is not covered, it dries up and develops vertical polygonal cracks which are wedge shaped (tapering downwards). Subsequently, when the surface is covered by water, loose sediments may be deposited on it, filling up the underlying cracks. Under favorable conditions, this features of mud cracks is retained in these rocks. Such rocks when broken along the relevant plane, exposes the earlier mud crack like features. Thus mud crack originates and occurs in some sedimentary rocks. Like ripple marks, mud cracks too, when found in unknown rocks, confirm that the concerned rocks are sedimentary rocks.
  55. 55. Mud Cracks or Sun Cracks
  56. 56. Common Structures and Textures of Sedimentary Rocks Rain Print or Rain Marks • Rain prints or rain marks develop under the same conditions as those of mud cracks and are preserved on surface of some rocks. A rain print is a slight shallow depression encircled by a low ridge which is raised by the impacts of rain drop. If the rain falls obliquely on loose soil the ridge on the leeward side is higher than that on the windward side. All this indicates that the rain print can occur only in sedimentary rocks and not in other group of rocks.
  57. 57. Rain Print or Rain Marks
  58. 58. Common Structures and Textures of Sedimentary Rocks Tracks and Trails • Tracks and trails are the marking indicating the paths of some animals or worms, over a soft sediment, which is able to take and retain the impression.
  59. 59. Tracks and Trails
  60. 60. Common Structures and Textures of Sedimentary Rocks Peculiar Form • These forms occur only in sedimentary rocks. Hence their presence in unknown rocks indicates that they are sedimentary rocks.
  61. 61. Common Structures and Textures of Sedimentary Rocks Conchoidal Fractures • Some massive and compact limestones are extremely fine grained, as they are formed out of participated materials. This type of rocks, when broken, develop conchoidal fractures on the surface. This is because these rocks are very fine grained as well as non-crystalline. Shales, too, sometimes show vague conchoidal fractures by virtue of fine grained character.
  62. 62. Conchoidal Fractures
  63. 63. Descriptive Study of Common Sedimentary Rocks Conglomerate • A conglomerate is a clastic, rudaceous sedimentary rock, which is made up of rounded or surrounded pebbles and gravel. Occasionally, cobbles and boulders also encounter in some conglomerates. Miner logically, the constituent pebble are usually jasper, flint, quartz or rock pieces.
  64. 64. Conglomerate
  65. 65. Descriptive Study of Common Sedimentary Rocks Properties and Uses • (i) The compositional heterogeneity of pebble and cementing material result in difference in their physical character- this contribution to the weakness of rocks. • (ii) The incomplete cementation contributes to the usual porosity and permeability associated with conglomerates this makes them good aquifer but incompetent rocks.
  66. 66. Descriptive Study of Common Sedimentary Rocks
  67. 67. Descriptive Study of Common Sedimentary Rocks • (iii) The rounded shape of pebble of conglomerates does not allow firm grip for cementing material; hence less cohesion in the rock. For all these reasons conglomerates are undesirable at the site of foundation of major civil engineering structures. • One rare but economically very important aspect is that the conglomerates of upper Vindhyans are famous as diamondiferous.
  68. 68. Descriptive Study of Common Sedimentary Rocks Breccias • Like conglomerate, breccias also made up of pebbles, gravels, etc. and the fine grained cementing material provide the binding medium. But unlike conglomerates, these have pebbles and gravels which have sharp and angular edges. This angular character indicates that they had undergone negligible transport and their parent rocks are likely not to be far from the place of their occurrence.
  69. 69. Breccias
  70. 70. Descriptive Study of Common Sedimentary Rocks • The mechanically transported pebbles and gravel, when deposited, give rise to the most common type of breccias. In addition to these there are a few other types of breccias, namely, volcanic breccias, fault breccias etc.
  71. 71. Descriptive Study of Common Sedimentary Rocks
  72. 72. Descriptive Study of Common Sedimentary Rocks Sandstones • Among sedimentary rocks, sandstones are very abundant and are next only to shales in this respect, they represent nearly 15 % of the sedimentary rocks of the earth’s crust. These are very common clastic rocks made up of sand and are described as arenaceous rocks. Unlike conglomerates, sandstones are stratified and sometimes fossiliferous too.
  73. 73. Sandstones
  74. 74. Descriptive Study of Common Sedimentary Rocks • Sand grains in sandstone are mostly quartz, In addition to these the other minerals are mica, magnetite and feldspar. • Nature of Sand Grains and Cementing Materials • Based on the size of sand grains the sandstones may be coarse grained, medium grained or fine grained. But generally the sand grain in any sandstones will be nearly of the same size. • The sand grain present may be rounded or surrounded or angular in shape. • Sandstones occur in different colors such as white, red, yellow grey and green. The color of sand grain is due to cementing material.
  75. 75. Sandstones
  76. 76. Descriptive Study of Common Sedimentary Rocks Porosity Permeability and Uses • By virtue of their mode of formation, like conglomerates, sandstones also generally porous and permeable. The porosity of sandstones is dependent on the size and shape of the sand, Further the degree of cementing also contribute to the porosity. Therefore sandstones vary considerable in their porosity and permeability.
  77. 77. Descriptive Study of Common Sedimentary Rocks • From the civil engineering point of view, however, careful study is necessary before considering a rock as competent or incompetent, useful or useless. When sandstone is considerably porous and permeable i.e.. when the degree of cementation is less, it will not make up a good rock. & if the sand stone are well cemented and if the grains are angular and quartz in composition, the siliceous sandstone are the best and highly satisfactory for all civil engineering purpose.
  78. 78. Sedimentary Rocks
  79. 79. Descriptive Study of Common Sedimentary Rocks Shales • Shales are the most abundant sedimentary rocks. Representing nearly 80 % of them. In other words shales are more abundant than all other sedimentary rocks put together. • Like conglomerates and sandstone shales are also typical elastic rocks i.e. formed out of a mechanically transported and deposited sediments. These are made up of solid particles of extreme fine size i.e. silt and clay. Stratification is best in shales because the individual layers are very thin and differently colored.
  80. 80. Shales
  81. 81. Descriptive Study of Common Sedimentary Rocks • Shales often contain fossil fuels i.e.. flora and fauna. Shales are uniform in composition. They are mostly hydrous aluminum silicates in composition because their constituent clay minerals are product of weathering of feldspar and other primary silicate minerals.
  82. 82. Shales
  83. 83. Descriptive Study of Common Sedimentary Rocks Porosity and Permeability • Being extremely fine grained clastic rocks, shales are highly porous. Since permeability is related to porosity, shales are normally expected to be highly permeable too. But due to the surface tension phenomenon of water and extreme fine intergranular space, shales are impermeable. Rocks of this kind which are porous but not permeable are called aquiclude.
  84. 84. Descriptive Study of Common Sedimentary Rocks Civil Engineering and Other Uses. • Shales when saturated with water, under pressure, are likely to produce lubricating material making a slippery base for overburden; therefore they are unsuitable at the site of foundation of important civil engineering structures.
  85. 85. Descriptive Study of Common Sedimentary Rocks
  86. 86. Descriptive Study of Common Sedimentary Rocks • Shales are the most incompetent rocks in nature. They may yield plastically and also may undergo subsidence. • Shales are unsuitable for civil engineering purposes • But are highly useful in following respect, • On disintegration, they produce soils which are indispensible for agriculture. • They form one of the chief raw material for cement manufacture • They are invaluable as cap rocks in the occurrence of oil and gas deposits.
  87. 87. Descriptive Study of Common Sedimentary Rocks Limestone • Among the sedimentary rocks, in the order of abundance, limestone rank third, next to shales and sandstones. These comprises 5 % of all sedimentary rocks occurring on the earth’s surface. • Limestone are typically non-detrital rocks. They are formed either chemically, due to precipitation of calcium carbonate from surface water. Or organically, due to accumulation of hard part of organisms
  88. 88. Limestone
  89. 89. Descriptive Study of Common Sedimentary Rocks • In hand specimens, limestones show different colors like white, gray, buff, yellow and black. • If the limestone have formed out of chemical precipitation, they are extremely fine grained. In case of organic, limestone have formed out of chemical precipitation, they are extremely fine grained. In case of organic, limestone, the grain size may vary depending on the type of organisms responsible for its formation.
  90. 90. Limestone
  91. 91. Descriptive Study of Common Sedimentary Rocks Types of limestones • Massive compact fine grained limestones with grey or any other light color are common in nature the porous types are yellowish, brownish etc. • The other important types are • Chalk, stalactites, stalagmites and drip stones, travertine, kankar, fossiliferous or shell limestone, flaggy limestone, lithographic limestone, magnesium limestone, argillaceous limestone, siliceous limestone.
  92. 92. Descriptive Study of Common Sedimentary Rocks
  93. 93. Descriptive Study of Common Sedimentary Rocks Porosity and permeability • Limestones are the most deceptive and most unpredictable rocks in terms of porosity and permeability. This is because of the inherent character of limestone, that is getting corroded by carbon dioxide, bearing water at anytime. But inside they may have solution cavities and solution channels of varying magnitudes. Highly irregular and uneven topography formed this way in limestone region is known as Karst topography. • Thus the range of porosity and permeability is unreasonable large in limestone. Therefore they are either one of the best or worst aquifer in nature.
  94. 94. Descriptive Study of Common Sedimentary Rocks Civil Engineering and Other Uses • Massive and compact limestones are reasonable competent to support civil engineering structures. They are suitable as road metal, railway ballast and as construction material but may not be very durable. Further if hollow, their load-bearing strength also will get reduced.
  95. 95. Descriptive Study of Common Sedimentary Rocks
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