Sedimentary rocks you still got itttttttttttttttttt
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Sedimentary rocks you still got itttttttttttttttttt Document Transcript

  • 1. Sedimentary Rocks March 10, 2010 by blessan Sedimentary rocks are formed due to the aggradations and compaction of the sediments. The word sedimentary has been derived from the Latin word which means settling down. The sedimentary rocks are also Called as Stratified or layered rocks and hence Consists of different types of layers and strata of various sediments. Some sedimentary rocks do not consist of layers, ex: loess. Characteristics of Sedimentary rocks Sedimentary rocks are formed of sediments derived from older rocks, Plants and animal remains thus these rocks contains fossils of animals and plants. The sedimentary rocks are widespread about 75 % of the globe consists of sedimentary rocks and the remaining 25% by igneous and metamorphic rocks. Nevertheless the sedimentary constitute only 5 % of the composition of earth’s crust. The deposition of sediments takes place in a certain sequence. The size of the Sediments decreases from the littoral margins to the centre of the water bodies. The sedimentary rocks are consolidated and compacted by cementing elements Such as Silica, iron compounds, Calcite, Clay Etc
  • 2. The consolidation of layers of sedimentary rocks depends upon the nature of cementing materials and the rocks are Seldom found in their original manner because of the lateral compressive and tensile forces Sedimentary rocks are seldom crystalline but are Characterized by Joints perpendicular to the bedding Planes. The connecting Plane between two consecutive beds or layers is called as bedding Plane. Types of formation in Sedimentary rocks When two beds are uniform along the bedding Plane then this type of formation is called conformity. When two beds are not uniform then the structure is called as Unconformity. Types of Unconformity Non-conformity (where sedimentary rocks succeed igneous & metamorphic rocks) Angular unconformity (where horizontal sedimentary beds are deposited over the previously folded strata) Disconformity (where two conformable beds are separated by Changes in sediment type. Paraconfirmity ( where two sets of conformable beds are Separated by same set of sediments) Sedimentation units in the rocks which are more than One centimeter is called BEDS
  • 3. The upper and lower surface of the beds are called BEDDING PLANE or BOUNDING PLANES Sometimes the lower surface of the bed is called SOLE. There are further sedimentary units within a bed. The units having more than one centimeter is called layer or Strata Where as the units below one centimeter are Called laminae. Thus several strata and laminae make up a bed. When beds are deposited at an angle to the depositional surface they are called cross beds and the general phenomena of inclined layers are called cross lamination or cross bedding. Soft mud or alluvial deposits by rivers develop cracks when baked in sun and these cracks are generally of Polygonal Shape, such cracks are Called MUD CRACKS OR SUN CRACKS. Most of the sedimentary rocks are Permeable and Porous but few of them are nonporous and Impermeable Classification of sedimentary rocks ON THE BASIS OF NATURE OF SEDIMENTS o Mechanically formed or clastic rocks  Sandstones  Conglomerates  Clay rock  Shale  Loess o Chemically formed sedimentary rocks  Gypsum  Salt rock o Organically formed sedimentary rocks  Limestone  Dolomites  Coals  Peats ON THE BASIS OF TRANSPORTING AGENTS o Argillaceous or aqueous rocks  Marine rocks  Lacustrine rocks  Riverine rocks o Aeolian sedimentary rocks  Loess o Glacial sedimentary rocks  Till  Moraines
  • 4. Mechanically formed sedimentary Rocks Previously formed rocks are subjected to mechanical or physical disintegration these are called fragmental rock materials or Clastic materials which become the source materials for the formation of mechanical or clastic sedimentary rocks. These materials are further broken down in to fragments during their transportation by the exogenous process such as wind, running water glacier etc, some of the important members of this group are Sandstones, conglomerates, Silt, shale, Clay etc. Sandstones Sandstones are formed mostly due to the deposition, Cementation, compaction of Sand grains. The Sand grains are divided into five categories on the basis of their size Classification of sands by grain size Sand types Grain size (in mm) 1. Very Coarse sand 1.0 to2.0 1. Coarse Sand 0.5 to 1.0 1. Medium sand 0.25 to 0.5 1. Fine sand 0.125 to 0.25 1. Very Fine sand 0.0625 to 0.125 These grains are deposited in the water bodies, and are aggravated and consolidated by Cementing elements such as Silica, Calcium, iron oxide Clay etc. The Colour of the sandstone Varies according to the nature of the cementing elements. Sandstones become red and gray when cemented by iron oxide and in to white when the cementing element is replaced by calcium Carbonate. They become hard and resistant when cemented by Silica. On an average the sandstones are generally porous rocks. Sandstones On the basis of mineralogical and textural Characteristics sandstones are classified in to the following types Quartz Arenites (composed entirely of quartz grains)
  • 5. Arkose Sandstones (feldspar being the dominant Mineral) Lithic Arenites (composed of fine grained rock fragments derived from Shales, schist, slates and volcanic rocks). Greywackle Sandstones (Composed of feldspar, rock fragments, quartz). Note: The English word Arsenite is derived from Latin word Arena-meaning thereby Sand. Conglomerates They are formed due to the consolidation and cementing of pebbles of various Sizes with Sands. Polished round fragments of size with 4 mm of diameter are called Pebbles and those with a diameter 256mm are called boulders. Angular rock fragments are Called breccia. Clay rock and shale They are formed due to the deposition and cementation of fine sediments. Sediment having grain size of 0.3mm to O.4mm are called silts where as Clays are formed When the grain size is between 0.4mm to O. 00012 mm are Cemented or consolidated. Silt and Clay are soft weak rocks but are defiantly impervious. Clay rocks are formed exclusive Kaolin minerals and pure White in colour but they Change to different colours because of being mixed with impurities. Chemically formed sedimentary Rocks When Running Water containing chemicals come in touch with the rocks containing Soluble materials are washed away and come in contact with the Chemicals. The Chemical materials are then settled down, compacted and cemented to form Chemical sedimentary rocks such as gypsum and salt rocks. Organically formed sedimentary rocks The sediments derived from the disintegration and decomposition of Plants and animals deposit and consolidate to form organic sedimentary rocks. The rocks are divided in to three categories on the basis of its carbon and lime content. They are Calcareous rocks Carbonaceous rocks Siliceous rocks Calcareous rocks
  • 6. Calcareous rocks are formed due to the deposition and consolidation of sediments derived from the remnants of animals and plants which contain lime. Limestone is one of the examples of the calcareous rocks. Limestone is formed in the following Manner Calcium oxide (Cao) reacts with water to form Calcium hydroxide Ca (0H) 2. This is given by the following equation, Then Calcium hydroxide reacts with Carbon dioxide (CO2) to give Calcium carbonate CaCo3 this is given by the following equation, The calcareous rocks are collectively called as carbonate rocks or carbonates. Calcium carbonate (Caco3), Magnesium carbonate (Mgco3), dolomite (CaMg (co3)2 are important calcareous rocks. Limestone is both thinly beaded and thickly bedded. The most dominant minerals are calcite (Hexagonal), Aragonite (orthorhombic). Though Limestone are weak rocks in humid regions they become more resistant when exposed to dry climate, because of its uniform or homogenous structure they are not easily affected by differential expansion and contraction due to temperature. The Carbonate rocks after Chemical weathering give birth too karst topography. Chalks another form of Carbonate rocks are formed due to the Precipitation of Carbonate minerals desired from microorganisms like foraminifera.
  • 7. ARGONITE ROCKS (ORTHOTHROMBIC) CALCITE ROCKS (HEXAGONAL) Carbonaceous rocks These rocks contain Carbonic minerals which are mainly remains of vegetation. These rocks are formed when these remains are buried in the ground and undergo Changes because of pressure from overlying deposits and earth movements. The Initial form of carbonaceous rock is Peat which is dark grey in colour, and then subsequently it transforms into rocks with greater Proportion of carbon like lignite, bituminous, anthracite Coals. The coal layers one called coal seams. They are more important economically than geomorphically. Siliceous rocks They are formed due to the dominance of silica Content. These rocks are formed due to the aggradations and compaction of sediments derived from radiolarian organisms (marine organisms that absorb silicon compounds during their lifecycle), sponge, diatom plants. Geyserites are also deposits of Silica around geysers.
  • 8. ON THE BASIS OF TRANSPORTING AGENTS Based on major transporting agents the sedimentary rocks are classified in to three types Argillaceous Aeolian Glacial Argillaceous rocks These are also known as aqueous rocks because they formed in the water areas and are also transported by waters. They are called argillaceous rocks because of the presence of Clay and these rocks are characteristically soft. They are generally impervious rocks and are further sub-divided in to three types Marine argillaceous sedimentary rocks Lacustrine argillaceous sedimentary rocks Riverine argillaceous sedimentary rocks Marine argillaceous sedimentary rocks These rocks are mainly formed due to the deposition and compaction of sediments on the littoral margins of the oceans and seas. The process of sedimentation is well ordered and sequential in Character. The size of the particles decreases progressively as we go from the coastal areas towards the Centre of the sea or oceans some of the examples of these types of rocks are Limestone, sandstone, dolomite and chalk. Lacustrine argillaceous sedimentary rocks These rocks are formed generally when sediments are deposited near water bodies such as Lakes. Generally the rocks are found in the floor of the Lakes. Unlike the argillaceous rocks there is no ordering of size or deposition in-Lacustrine rocks. Riverine argillaceous Sedimentary rocks These rocks are formed due to the deposition of sediments in the Riverine environment. The sediments are deposited on the beds of the rivers and in the flood Plains. These deposits include alluvia which are dominated by Clay. The alluvia are deposited by rivers during floods and are renewed almost every year .These alluvial deposits develop Polygonal cracks due to their exposure to isolation. Aeolian rocks
  • 9. They are formed due to the deposition of Sands brought out by the wind. Pre existing rocks are greatly disintegrated due to mechanical weathering and this result in the formation of large quantity of Sands of different sizes. These Particles are further comminuted in to finer Particles due to the attrition while they are being transported from one place to another. Continuous deposition results in the formation different layers but these layers are not well consolidated and hence are very porous in nature. Sometimes there is complete absence of layers in rocks such as Loess. In India loess are found on the banks of the Paleochannel and the Valley of Narmada River. Glacial rock The materials deposited by the Glaciers are called glacial drifts which are deposited in four conditions and hence they are four types of morainic deposits Lateral moraines- When Glacier Materials are deposited on the either Side of the Glacier. Medial Moraines-When glaciers Materials are deposited along the joining of the Glaciers. Ground moraines- When Glacier materials are deposited on the bed of Glaciers. Terminal Moraines- These formed when glaciers are ablated and materials are deposited there.